Co-Proposers

Milan MaksimovicLESIA & CNRSObservatoire de ParisPlace Jules Janssen92195 Meudon cedexFranceTel : 331 4507 7669Fax : 337 4507 2806Email: milan.maksimovic@obspm.fr

Marco VelliJet Propulsion Laboratory, USA& University of Firenze, Italy4800 Oak Grove DrivePasadena, California 91109USATel : 1 818 354 4369Email:mvelli@mail.jpl.nasa.govvelli@arcetri.astro.it

The PHOIBOS missionProbing Heliospheric Origins

with an Inner BoundaryObserving Spacecraft

A proposal to the European Space Agency (ESA) for a probe of the Solar Corona and inner Heliosphere

in response to the call for “Cosmic Vision”

The general scientific objectives of the PHOIBOS mission are being supported by a large scientific community distributed throughout the world. The PHOIBOS mission concept is proposed and sup-ported by the following people : Vassilis Angelopoulos28, Thierry Appourchaux1, Bruno Bavassano2, Stuart D. Bale3, Matthieu Ber-thomier4, Lapo Bettarini33, Douglas Biesecker38, Lars Blomberg5, Peter Bochsler6, Volker Both-mer7, Jean-Louis Bougeret8, Andrew Breen39, Carine Briand8, Roberto Bruno9, Vincenzo Car-bone10, Patrick Canu4, Thomas Chust4, Jean-Marc Defise11, Thierry Dudok de Wit12, Luca Del Zan-na33, Anders Eriksson13, Silvano Fineschi35, Lyndsay Fletcher14, Keith Goetz15, Roland Grappin41, Antonella Greco10, Shadia Habbal36, Don Hassler16, Bernd Heber17 , Petr Hellinger18 , Tim Hor-bury19 , Karine Issautier8, Justin Kasper20, Ludwig Klein8, Craig Kletzing21, Säm Krucker3, Vladi-mir Krasnoselskikh12, William Kurth21, Rosine Lallement22, Philippe Lamy23, Hervé Lamy30, Simone Landi33, Olivier Le Contel4, Fabio Lepreti10, Dominique LeQuéau24, Robert Lin3, Milan Maksimovic8, Francesco Malara10, Ian Mann25, Ingrid Mann26, Lorenzo Matteini33, William Mat-thaeus37, Dave McComas27, Ralph McNutt43, Nicole Meyer-Vernet8, Zoran Mikic42, Michel Mon-cuquet8, Neil Murphy28, Zdenek Nemecek29, Emanuele Pace33, Filippo Pantellini8, Viviane Pier-rard30, Jean-Louis Pinçon12, Elena Podlachikova31, Raymond Pottelette4, Lubomir Prech29, Ondrej Santolik29, Robert Rankin32, Franco Rappazzo28, Marco Romoli33, Alain Roux4, Jana Safrankova29, Fouad Sahraoui4, Edward C. Sittler40, Charles W. Smith44, Luca Sorriso-Valvo10, Jan Soucek18, Pavel Travnicek18, Andris Vaivads13, Marco Velli28,33, Andrea Verdini33, Nicole Vilmer8, Robert Wimmer-Schweingruber17, Gaetano Zimbardo10, Thomas Zurbuchen34

1IAS, France; 2IFSI-INAF, Italy; 3University of California Berkeley, USA; 4CETP, Vélizy, France; 5KTH, Stockholm, Sweden; 6University of Bern, Switzerland; 7University of Goettingen, Germany; 8LESIA, Observatoire de Paris, France; 9IFSI-CNR, Italy ; 10Università della Calabria, Italy; 11CSL, Belgium; 12LPCE, Orléans, France; 13IRFU, Uppsala, Sweden; 14University of Glasgow, UK; 15University of Minnesota, USA; 16SWRI Boulder, USA; 17University of Kiel, Germany; 18IAP-CAS, Prague, Czech Republic; 19Imperial College, London, UK; 20MIT, Cambridge, USA; 21University of Iowa, USA; 22SA-IPSL, France; 23LAM, Marseille, France; 24CNRS, Paris, France; 25University of Alberta, Canada; 26University of Kobe, Japan; 27SWRI, San Antonio, USA; 28JPL, Pasadena, USA; 29Charles University, Prague, Czech Republic; 30IAS Brussels, Belgium; 31Royal Observatory, Brussels, Belgium; 32University of Alberta, Canada; 33University of Firenze, Italy; 34University of Michigan, USA; 35 Istituto Nazionale di Astrofisica, Torino, Italy; 36University of Hawaï, USA; 37University of Delaware, USA; 38NOAA Space Environment Center, Boulder, USA; 39University of Wales, UK; 40GSFC, USA, 41Luth, Observatoire de Paris, France, 42SAIC, San Diego, USA, 43APL, Baltimore, USA,44University of New Hampshire, Durham, USA .

TABLE OF CONTENTS TABLE OF CONTENTS..................................................................................................................... 3 Executive summary.............................................................................................................................. 4 1) Introduction ..................................................................................................................................... 6 2) Scientific Objectives........................................................................................................................ 6

2.1) Explore the fundamental processes underlying coronal heating and solar wind acceleration .7 2.2) Determine magnetic field structure and dynamics in the source regions of the fast and slow solar wind.......................................................................................................................................10 2.3) What mechanisms accelerate, store and transport energetic charged particles? ....................13 2.4) Explore dusty plasma phenomena and their influence on the solar wind and energetic particle formation........................................................................................................................................15

3) Mission Profile .............................................................................................................................. 16 4) Payload description........................................................................................................................ 19

4.1) Overview of all proposed payload elements...........................................................................19 4.2 Baseline Payload......................................................................................................................21

4.2.1 Fast Plasma Instrumentation. ........................................................................................21 4.2.2 Ion Composition Analyzer (ICA)...................................................................................21 4.2.3 Energetic Particle Instrument........................................................................................22 4.2.4 DC Magnetometer...........................................................................................................23 4.2.5 Radio and Plasma Wave Instrument (RPWI). .............................................................23 4.2.6 Neutron/Gamma-Ray Spectrometer (NGS). ................................................................24 4.2.7 Coronal Dust Detector (CD)...........................................................................................25 4.2.8 Hemispheric Imager (HI). ..............................................................................................25 4.2.9 Polar Source Region Imager (PSRI). ............................................................................26 4.2.10 Common Data Processing Unit (CDPU). ....................................................................26

5) Spacecraft description.................................................................................................................... 26 5.1) Spacecraft architecture............................................................................................................26 5.2) Key factors for power management........................................................................................28 5.3) Spacecraft mass budgets .........................................................................................................29

6) Science Operations and Archiving ................................................................................................ 30 7) TRL & Key technology areas........................................................................................................ 30

7.1) Spacecraft TRL & Technology areas .....................................................................................30 7.2) Payload TRL & Technology areas .........................................................................................30

8) International partnership and Costs ............................................................................................... 31 8.1) International partnership.........................................................................................................31

9) Communications and Outreach ..................................................................................................... 31 Acknowledgements :.......................................................................................................................... 31 References.......................................................................................................................................... 31 ANNEXE: SUPPORT LETTERS ..................................................................................................... 33

Executive summary Fifty years after the Sputnik launch and the begin-ning of the Space Physics era the time has come for the in-situ exploration of one of the last frontiers in the solar system – the solar corona and inner helio-sphere. PHOIBOS (Probing Heliospheric Origins with an Inner Boundary Observing Spacecraft) is a mission of exploration and discovery designed to make comprehensive measurements in the never-observed region of the heliosphere from 0.3 AU to as close as 3 solar radii from the Sun’s surface. The primary scientific goal of Phoibos will be to determine how magnetic field and plasma dy-namics in the outer solar atmosphere give rise to the corona, the solar wind and the heliosphere. Reaching this goal is a Rosetta-stone step for all of astrophysics, allowing the understanding not only of the plasma environment generated by our own sun, but also of the space plasma envi-ronment of much of the universe, where hot tenuous magnetized plasmas transport energy and accelerate particles over a broad range of scales. Moreover, by making the only direct, in-situ measurements of the region where some of the deadliest solar energetic particles are energized, PHOIBOS will make unique and fundamental contributions to our ability to characterize and forecast the radiation environment in which future space explorers will work and live. Scientific objectives The existence of the solar wind has been estab-lished for over forty years now, and abundant data has been accumulated concerning its average prop-erties. More recently, a number of satellite mis-sions carried out both in-situ measurements of the solar wind in the region beyond 0.3 AU, as well as remote-sensing measurements probing the solar atmosphere from the photosphere to the outer co-rona in the visible, ultraviolet and x-ray bands, with nearly continuous coverage. This has led to major advances in our understanding of how the solar magnetic field and the magnetic activity cycle shape the solar wind’s structure and influence its overall dynamics. The fundamental mystery of how the solar corona is heated and the solar wind is generated remains unsolved, however, because of the major gap in our knowledge of the sub-mercurean region of the heliosphere. Remote-sensing strategies have probed the coronal properties by analysing the photons emitted, scattered, or absorbed by the

Sun's outer atmosphere, within the first few solar radii. In this regards, there are both theoretical limi-tations (in the understanding of the physics of the coupling between photons and plasma) and experi-mental limitations (limited number of observables such as spectral lines or the hardly solvable inverse problem of the line of sight integration). On the other hand solar wind in-situ measurements have had ac-cess to the very detailed state of the local plasma properties (full particles velocity distribution func-tions, observations of the electromagnetic plasma fluctuations over a huge frequency range ) but at locations far from the corona and the solar wind acceleration region. To understand therefore the engine at the source of the solar wind one must probe the region from the photosphere to about 20 Rs, where internal, electric, magnetic and turbulent energy in the coronal plasma is channelled, through mecha-nisms that are not completely understood, into bulk energy of the supersonic solar wind flow (see Figure 1).

Figure 1 : Model profiles of the solar wind speed (U) and the Alfvén speed (Va) with distance from the sun. The vertical bar separates the source region of the wind from the supersonic wind flow. PHOIBOS will be the first mis-sion to fly in the source region of the wind. Understanding the solar wind is of fundamental significance to all of astrophysics since it is the prototype for all stellar winds and related astrophysical flows. Approaching as close as three solar radii from the surface will allow direct detection of the plasma physical processes at work in coronal heating. The scientific objectives of the PHOIBOS mission therefore fit perfectly within the scientific themes defined in the ESA Cosmic Vision 2015-2025 program and more precisely the question “How does the Solar System work?“ International partnership

The international scientific community has been dreaming for a mission to explore the corona for a long time. In the past the European Space Agency and European scientists have already been involved in several studies such as the Vulcan ESA SCI(88)7 initiated by Giuseppe Colombo or the SCP proposal in 1993 for an M3 mission by E. Marsch and A. Roux. On the NASA side, the num-ber of past studies and reports that have been car-ried out in the last thirty years is even greater. The most accomplished study is the 2005 Solar Probe with a Science & Technology Definition Team (STDT) chaired by D.J. McComas NASA/TM—2005 - 212786; (http://solarprobe.gsfc.nasa.gov/ solarprobe_news.htm). These past efforts have led to the final conclusion that a probe to support mankind’s first mission into the Solar Corona is technically feasible. While the scientific team supporting this proposal strongly hopes that NASA’s STDT/2005 Solar Probe mission will be developed and flown as soon as possible, the current PHOIBOS mission concept is being proposed for a New Cosmic Vision 2015-2025 study to demonstrate and emphasize the strong European scientific community interest in a solar probe mission. With HELIOS, Ulysses and SOHO Europe has been involved in a leading posi-tion with respect to space-based solar and helio-spheric research during the last two decades. The strong European participation in STEREO and a timely launch of Solar Orbiter would keep some of the momentum of the last decades. It must be em-phasized that the top priority for the European solar and heliospheric community is realization of the Solar Orbiter within the 2015 time-frame. The realization of an ambitious mission such as PHOIBOS would complement and build upon Orbiter science to finally resolve one of the fun-damental problems in all of astrophysics: the existence of stellar coronae and the driving of solar and solar-type winds. Such knowledge is also paramount to our understanding of the Earth’s space environment and the influence of space envi-ronment on the Earth and space exploration. The PHOIBOS mission is to be developed in a joint collaboration between NASA, ESA and member states. The international partnership and sharing of responsibilities will be discussed be-tween the concerned space agencies. The overall mission cost is estimated to be around 900 Meuros (see section 8.3). We propose therefore to imple-ment PHOIBOS as an L mission with a cost shar-ing that could range between 1/2 to 2/3 for ESA & member states and consequently between 1/2 to 1/3

for NASA. In such a situation, the PHOIBOS launch could occur in December 2018, as discussed in sec-tion 3, or every 1.6 years later following the mission launch window. As an alternative option, in the case there were an opportunity for an earlier launch and an ESA/NASA agreement, PHOIBOS could be envisage as an M mission with a cost sharing of 1/3 for ESA & mem-ber states and 2/3 for NASA. The launch could occur in this case in October 2015 or May 2017. The PHOIBOS mission profile The mission concept proposed here differs from the present NASA scenario and is not meant to compete with that Solar Probe: its purpose on the contrary is a collaboration with NASA in the event the nature of the solar probe mission were to change. In particular were NASA were not to implement Solar Probe in its present STDT/2005 form, the PHOIBOS concept could provide an interesting alternative scenario, in which: (i) classical solar panels technology combined with heat shield thermal generators are used as power systems instead of RTGs and (ii) a new original trajectory combined to the use of a plasmic electric propulsion system are implemented in order to reach the final scientific orbit instead of the classical Jupiter low altitude swing-by with a very high C3 for Earth escape. This final orbit is highly elliptical with a perihelion at 4 solar radii (3 radii from the surface), an aphelion at 3.5 AU and an inclination of 60 deg. with the ecliptic plane. The important and interesting point considering this alternative scenario is that, de-spite its apparent complexity, it will allow more flexibility in the fine adjustment of the final orbit. It may avoid the possible drawbacks of a Jupiter swing-by such as the high departure V∞, the risky low altitude Jupiter swing-by at 5.5 AU with expo-sure to significant radiation levels and no subsequent ability to change the trajectory. With the proposed final orbit, PHOIBOS will spend up to 10 days making comprehensive measurments in the never-observed region of the heliosphere from 0.3 AU to 3 solar radii from the Sun’s surface and up to about 20 hours at distances closer than 10 solar radii. The Payload concept Concerning the payload necessary to reach our scien-tific objectives, the current proposal has greatly been inspired by previous Solar Probe proposals and espe-cially by the last NASA STDT/2005 report. To meet the PHOIBOS science objectives, we recommend a payload comprising both comprehensive in-situ and

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relevant limited remote-sensing instruments. The PHOIBOS payload is described in section 4.

1) Introduction PHOIBOS, a mission of exploration and discovery to the inner frontier of the solar system, is designed to understand how the solar corona is formed and the solar wind accelerated. This mission will access the source regions of the heliosphere with in-situ measurements and possibly images of the solar photosphere, the corona and the polar magnetic fields. With a perihelion well within the region where the magnetic field still guides the plasma outflow and the solar wind kinetic energy is domi-nated by that of the sun’s magnetic field – the Alfvén critical point, somewhere between 10 and 20 Rs - PHOIBOS will be able to access the regions of the corona where the plasma turbulence is strongest. It will most probably reach, at perihelion, the location of the proton temperature maximum and probe the region where fast solar wind accel-eration is occurring. At perihelion, PHOIBOS falls within helmet streamers at solar minimum, and will probably spend a significant time within closed field line regions at solar maximum. With the three orbits planned, the conditions at solar maximum and solar minimum will be explored and compared. As a result PHOIBOS will determine by which physical mechanisms the outer solar atmosphere gives rise to the corona, solar wind and helio-sphere throughout the sun’s activity cycle. The main scientific objectives of PHOIBOS are to: 1 - Explore the fundamental processes underly-ing coronal heating and solar wind acceleration 2 - Determine magnetic field structure and dy-namics in the source regions of the fast and slow solar wind. 3 - Determine what mechanisms store, acceler-ate and transport energetic particles. 4 - Explore dust and dusty plasma phenomena in the vicinity of the Sun and their influence on the solar wind and energetic particle formation. To address these objectives experiments aboard PHOIBOS will: Measure the plasma and its composition, elec-tromagnetic field, energetic ions and electrons and their properties in the region from 60 Rs to 4 Rs; monitor neutron and gamma ray emission from the sun; image the corona in white light and possibly EUV; determine the photospheric magnetic field at high latitudes from a vantage

point within 30 Rs. These measurements must be done continuously in time during all phases of the near encounter period in order to ensure wave mode identification or not miss brief spatial struc-tures such as discontinuities and shocks.

2) Scientific Objectives The solar wind flow at solar minimum is subdivided into high and low speed streams, with speeds of around 750 km/s and 400 km/s respectively. The Ulysses mission has shown that the fast wind is the basic outflow from the corona at solar minimum, while the much more irregular slow solar wind is confined to the equatorial regions, presumably aris-ing from regions adjacent or inside the streamer belt. As the solar cycle progresses, the streamer belt ex-pands in latitude so that, at activity maximum, the corona appears to be nearly uniformly distributed around the solar disk, while high speed wind streams occur over a much smaller volume. This is illustrated by the ‘dial plot’ in Figure 2.1, (McComas et al. 2003) which depicts solar wind speed measurements as a function of latitude during the first and second Ulysses orbits, near times of solar minimum and maximum activity respectively.

Figure 2.1 : Polar plots of solar wind speed as a function of latitude for Ulysses’ first two orbits, superposed on solar images characteristic of solar min (8/17/96, left panel) and max (12/ 07/00, right panel), EIT , LASCO C2 images from SOHO and Mauna Loa K-coronameter (McComas et al., 2003). These images show how the solar wind characteristics measured in-situ depend strongly on the solar coronal magnetic field structure, fast wind ema-nating from coronal holes and slow wind appearing to originate from the magnetic activity belt. The fast solar wind, with average speed around 750 km/s, originates from regions where the coronal elec-tron temperature is lower. This inverse correlation

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between flow speed and coronal electron tempera-ture where the freezing in of minor ion charge states occurs (Figure 2.2) shows that the foundation of the original theory of the solar wind (Parker, 1958), i.e. that high coronal electron temperatures and electron heat conduction drive the solar wind expansion, needs to be reconsidered. SOHO meas-urements of the very high temperatures of the cor-onal ions, together with the persistent positive cor-relation of in-situ wind speed and proton tempera-ture, suggest that other forces, namely magnetic mirror and wave-particle interactions should also contribute strongly to the expansion of the outer corona.

Figure 2.2 : Anti-correlation of solar wind speed (dot-dashed line) with the freezing in temperatures deter-mined from O7 to O6 abundances (blue line), and mag-nesium to oxygen composition ratios (red line) as a function of time during a low-latitude high-speed low-speed wind crossing period in Ulysses’ first orbit. SOHO observations have also made important contributions to our knowledge of the slow solar wind, which is confined to regions emanating from the magnetic activity belt and seems to expand in a bursty, intermittent fashion from the top of helmet streamers, seen to expand continuously, in X-rays, by Yohkoh. A third type of flow arises from larger eruptions of coronal magnetic structures, or cor-onal mass ejections (CMEs), which also lead to acceleration of high-energy particles. As the solar activity cycle progresses, Ulysses has shown how the simple fast-slow structure gives way to a much more variable, but typically slower, solar wind at activity maximum, apparently originating not only from the much more sparse coronal hole regions and quiet sun, but also from coronal active regions. The energy that heats the corona and drives the wind is believed to come from photospheric mo-tions and is channeled, stored and dissipated by the magnetic fields that emerge from the photosphere and structure the coronal plasma.

Several fundamental plasma physical processes – waves and instabilities, magnetic reconnection, velocity filtration, turbulent cascades – operating on a vast range of temporal and spatial scales are believed to play a role in coronal heating and so-lar wind acceleration, but the lack of magnetic field and detailed plasma measurements in the region inside 70 Rs does not allow their validation or confutation at this time, and though Solar Or-biter and Solar Sentinels will move inside 0.25 AU or 53 RS before 2020, only PHOIBOS will explore the critical regions within 20 Rs. Basic unanswered questions concern the storage, transport, and release of the mechanical energy re-quired for coronal heating; the specific mechanism(s) for the conversion of energy between the magnetic field and thermal particles; the dynamics of photo-spheric and coronal magnetic fields in the source regions of the solar wind; and the sources of high-energy particles and the mechanisms by which they are accelerated. These questions motivate three broadly distinct but interlinked top-level PHOIBOS objectives. A fourth top-level objective of an exploratory nature concerns the source, composition, and dynamics of dust in the inner solar system. In the following sections, these four main objectives are translated into specific sci-entific questions and basic measurement require-ments.

2.1) Explore the fundamental processes underly-ing coronal heating and solar wind acceleration The solar corona loses energy in the form of radia-tion, heat conduction, waves, and the kinetic energy of the solar wind flow. This energy must come from mechanical energy residing in photospheric convec-tion, the solar magnetic field acting both to channel and store this energy in the outer atmospheric layers. However, the mechanisms by which the energy is transferred and dissipated to generate the hot corona, solar wind, and heliosphere throughout the Sun’s activity cycle remain one of the fundamental unan-swered questions in solar and heliospheric physics. Remote-sensing measurements of the solar corona and in-situ measurement of particle distribution func-tions in the fast and slow solar wind streams have shown that the heating process is correlated with magnetic structure. SOHO/UVCS observations using the Doppler dimming technique (Li et al., 1998; Kohl et al., 1998) (Figure 2.3) and interplanetary scintillation measurements (Grall et al. 1996) indi-cate that the high speed solar wind is rapidly acceler-

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ated near the Sun, reaching speeds of the order of 600 km/s within 10 Rs. Observations of comet C/1996Y1 confirm a most probable speed of about 720 km/s for the solar wind at 6.8 RS (Raymond et al., 1998).

Figure 2.3 : Acceleration profiles of the fast solar wind after SoHO: H0 (red) and O+5 (black) flow velocities from Kohl et al. (1998), OVI (green) flow velocities from Antonucci et al. (2004); Full-dashed blue lines : plume and interplume flow velocities from Gabriel et al. (2005) Such rapid acceleration appears to result from the extremely large and anisotropic effective tempera-tures in the lower corona, which have been meas-ured by SOHO/UVCS in coronal holes, though not directly for protons, the main solar wind constitu-ent. These temperatures are much higher perpen-dicular to the magnetic field. The fast solar wind measured in situ shows what may be a relic of this anisotropy, smaller than that inferred from coronal observations, but persisting in the distance range from 0.3 to 5 AU. Proton, alpha-particle, and minor ion distribution functions in the fast wind also pre-sent a non-thermal beam-like component whose speed is comparable to the local Alfvén speed. All these properties suggest that Alfvén or ion-cyclotron waves play a major role in coronal heat-ing and solar wind acceleration in high-speed wind. Measurements close to the sun, within the region where the solar wind becomes supersonic to Alfvén waves, are necessary to remove ambiguity due to in situ evolution and obtain direct measurements where the main acceleration is occurring. The fast solar wind flow is steady, with fluctua-tions in radial speed of order 50 km/s, and the charge-state distributions indicate a low freezing-in temperature. The slow solar wind is variable, with higher but variable freezing-in temperatures. The composition of the fast and slow wind also differ, Mg and Fe being overabundant with respect to O in the slow wind. Solar wind protons and ions are

in the slow wind. The difference between the fast and the slow solar wind extends to the shape of the particle distribution functions. The fast wind exhib-its proton perpendicular temperatures which are slightly higher than the parallel temperatures. Proton distribution functions in the fast wind also present a beam accelerated compared to the main distribution by a speed comparable to the Alfvén speed, a feature shared by the alpha particles. Turbulence is also different in fast and slow streams, with fast streams containing fluctuations in transverse velocity and magnetic fields which are more strongly correlated in what is known as Alfvénic turbulence, a well-developed spectrum of quasi-incompressible waves propagating away from the sun. In the slow wind no such preferred sense of propagation is observed, while larger density and magnetic field magnitude fluctuations are present, revealing a much more stan-dard and evolved MHD turbulent state (Grappin et al. 1990).

however typically hotter in high speed streams than

nti-correlation of wind speed with coronal (freez-

ontrary to proton distributions, observed electron

Aing-in) electron temperature and the heliospheric distribution of the high speed wind at solar minimum (Figure 3) place the origin of the fast wind in coronal holes. Measurement from the CDS -SUMER ex-periments aboard SOHO have ascertained that the electron temperature is bounded by 106 K (David et al 1998), in agreement with the brightness tempera-ture based on radio observations of the corona. This presents a discrepancy with the freezing in tempera-ture for different ion charge states measured in-situ by the SWOOPS experiment on Ulysses, the most direct interpretation of which requires an electron temperature maximum of about 1.5 106 in coronal holes. The discrepancy may be resolved by only by admitting strongly non-maxwellian distribution func-tions for the electrons, or large differential flow speeds between ions of the same charge in the co-rona, which could have strong implications on the structure of the fast solar wind in the acceleration region. Cvelocity distribution functions (eVDFs) exhibit non-Maxwellian features whatever the type of wind, slow or fast, in which they are observed. The eVDFs per-manently exhibit three different components : a thermal core and a supra-thermal halo, which are always present at all pitch angles, and a sharply magnetic field aligned “strahl” which is usually an-tisunward-moving (Rosenbauer et al., 1977). Energy transport and dissipation mechanisms strongly de-pend on the mean free path of particles in the coronal plasma, which varies drastically both with distance from the Sun (from the base of the corona to the

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supersonic solar wind), as well as across coronal structures (coronal holes to helmet streamers). This dependence has led to the suggestion that coronal heating arises from energy stored in non-thermal wings of particle distribution functions generated between the chromosphere and transition region or, more generally, in the region where the solar at-mospheric plasma changes from collisional to col-lisionless. The higher temperatures and subsequent outflows would then arise naturally through veloc-ity filtration by the Sun’s gravitational potential (Scudder, 1994), and may even explain the exis-tence of the fast solar wind (Maksimovic et al., 1997; Zouganelis et al, 2004).

e he different properties of th low-frequency elec-

hether the solar corona is heated by low-

imulations show that, in a highly stratified at-

nly observations close to the solar surface will

Ttromagnetic field and velocity fluctuations ob-served in the fast and slow solar wind are further evidence of the role played by turbulence and wave-particle interactions in coronal heating. Fast streams contain stronger fluctuations in transverse velocity and magnetic fields, and display a higher degree of correlation between the velocity and magnetic fluctuations (often described as a well-developed spectrum of quasi-incompressible Alfvén waves propagating away from the Sun). In the slow wind, this correlation occurs at a much lower level, while larger density and magnetic field magnitude fluctuations are present, indicating a more evolved MHD turbulent state there. This dif-ference in turbulence state between fast and slow wind streams, together with the fact that slow wind distribution functions are much closer to equilib-rium, suggests that the outward propagating wave flux contributes to the heating of the steady fast wind, while the slow wind is heated much more variably. It is not known, however, how the turbu-lent activiy increases toward the Sun, whether it is sufficient to power coronal heating and solar wind acceleration, and how it is driven by time-dependent events in the photosphere, chromos-phere, transition region, and low corona (see e.g., Mathtaeus et al, 1999). Wfrequency waves resulting from motions natu-rally arising in the photosphere or whether the dominant energy source resides in the currents stored via slower field line motions has been the subject of strong debate. Among the MHD waves, only Alfvén waves would appear to sur-vive the strong gradients in the chromosphere and transition region, because slow modes steepen into shocks while fast modes suffer total reflection. Transmitted waves propagate at large angles to the radial direction, due to the large

Alfvén speed, low frequencies and strong structur-ing of the corona (Velli, 1993). Waves reaching the lower corona should therefore be shear Alfvén waves, although discrete coronal structures such as loops and plumes might channel surface waves and propagate energy as global oscillations as well. Smosphere, the nonlinear interactions of Alfvén waves launched from the photosphere are able to generate and sustain an incompressible turbulent cascade, which displays the observed Alfvénicity. The efficiency of turbulence in transporting en-ergy to the dissipative scales is, however, still unclear. The spectral slope at different coronal heights evolves with distance, subject to expansion and driving effects, affecting the radial depend-ence of dissipation. The initial spectrum of Alfvén waves in the photosphere cannot be constrained by in-situ data collected in the far solar wind, since local processes contribute to its shaping there (Verdini and Velli, 2007). Ohelp in constraining the shape of the Alfvénic spectrum with relevant implications on the role of turbulence in the acceleration of the solar wind and the heating of the corona (Figure 2.4).

Figure 2.4 : The rms amplitudes u and b (in veloci

rements of the

tyunits) as functions of heliocentric distance for a photo-spheric Kolmogorov spectrum with u = 40 km s-1 at the

coronal base. Far symbols indicate observational con-straints from in-situ measurements and inter-planetary scintillation, near symbols from remote sensing. PHOI-BOS should reach inside the fluctuation maximum re-gion, measuring spectra and correlations where the gap in the data is (Verdini and Velli, 2007).

By providing the first in situ measudistribution functions, waves, turbulence, and elec-tromagnetic fields from 0.3 AU to 4 Rs, and by cor-relating them with plasma and magnetic field struc-tures, PHOIBOS will be able to answer the basic questions of how the solar corona is powered, how the energy is channeled into the kinetics of particle distribution functions in the solar corona and wind,

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and how such processes relate to the turbulence and wave-particle dynamics observed in the helio-sphere. The comprehensive measurement of plasma and electromagnetic fluctuations in the inner solar wind (<20 RS), will determine how the energy that powers the corona and wind is dissi-pated and what the dominant dissipative structures are as well as the frequency spectrum of electro-magnetic fluctuations. Small-scale magnetic recon-nection occupies an important place in current dis-sipation theories of the coronal plasma (Matthaeus et al. 2003). An important set of investigations on PHOIBOS will therefore be the multi-instrument detection of signatures of small-scale reconnection, such as bi-directional plasma jets, accelerated par-ticles, magnetic field, and velocity gradient correla-tions along the trajectory. PHOIBOS’s measure-ments of the properties of turbulence and nonlinear plasma dynamics in the corona and solar wind will be a watershed for all of astro-physics, where these phenomena are invoked over widely different contexts, from accretion disks to the collisionless shocks occurring in galaxy-cluster formation. Measurement Requirements

Magnetic field, velocity field, and density fluc-

ions of protons, elec-

ld measurements

(electron density,

roaching the

ent, density gradient,

2.2) Determine magnetic field structure and

he geometry of the magnetic field expansion in

he magnetic network in the quiet Sun looks re-

he magnetic field in active regions above sunspots

•tuations and spectra • Particle distribution functtrons, alpha particles, and possibly minor ion spe-cies; suprathermal populations • 3-axis electric and magnetic fie• Coherent structure identification using electricand magnetic wave-form data • Plasma wave measurementstemperature, velocity) and high resolution wave-form data for electromagnetic fields. • High frequency measurements appproton cyclotron frequency. • Electron temperature gradielectric field/ interplanetary potential.

dynamics in the source regions of the fast and slow solar wind. Tthe inner corona, from the photosphere out to a few solar radii, plays a fundamental role in determining density distribution and solar wind speeds in solar wind models, as the field lines define the flow tubes along which mass and energy flux are con-served. Ulysses observations have shown that the radial magnetic field component measured in the fast wind is largely independent of latitude, so that

any latitudinal gradient in the average field at the coronal base must be washed out by transverse non-radial expansion closer to the Sun. Flux tube expan-sion is a natural effect of the combined decrease in magnetic field and currents induced by the accelerat-ing solar wind flow, and models suggest that it oc-curs out to radial distances > 10 Rs. Based on Ulys-ses data, the magnitude of the average polar mag-netic field has been estimated to be 6 G at solar minimum, though values up to 15 G in the photo-sphere may not be excluded. At present, there are no direct measurements of the polar magnetic field be-low 1.5 AU (Sittler and Guhathakurta, 2002). By measuring the radial magnetic field in situ along its trajectory, and remotely sensing the polar photo-spheric field at the same time, PHOIBOS will allow a complete description of magnetic field and solar wind expansion free from unknown parameters. These measurements will provide both a test of exist-ing models of coronal structure and rigorous con-straints on future coronal models. Tmarkably similar to the network in coronal holes in spectral lines formed at lower, transition region tem-peratures, while it is harder to distinguish in lines formed at 106 K. If a similar coronal heating mecha-nism is at work in both the quiet Sun and coronal holes, any difference in their appearance is presuma-bly related to the magnetic field topology, including, perhaps, its time dependence. The larger densities, apparently higher electron temperature, and different chemical composition of the quiet Sun would then be the result of a larger filling factor of closed magnetic field lines compared with that in coronal holes. While the imprint of the coronal holes and of the equatorial helmet streamers in the solar wind meas-ured in-situ is well visible in the form of fast and slow wind streams and embedded plasma sheet, the fate of the quiet Sun corona is unknown. Is the plasma in the quiet Sun confined by closed magnetic field lines, so that the fast wind is entirely of coronal hole origin? Or is there a mass loss from the quiet Sun as well, and if so, what is its speed and how does it merge with the surrounding solar wind? Tprovides the strongest confinement of hot plasma in the corona and is seen as bright x-ray loops, which often end in cusp-like shapes at their summit. At greater heights, these develop into streamers, which at solar minimum are large and elongated and form a belt around the solar magnetic equator. Remote sens-ing observations by SOHO/UVCS of the EUV emis-sion lines of minor ions, combined with multi-fluid models, provide some clues about the source regions

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of the slow solar wind in coronal streamers, but the magnetic field topology in these regions and the role it plays in plasma outflow are unknown. The complexity of the magnetic field structure

creases with increasing activity during the solar

the quiet un, and the active solar corona at distances be-

ctroscopic bservations also show coronal holes to be far from

S telescopes on the SOHO ission have made important contributions to our

incycle. At activity maximum, disk observations show the existence of very complicated loop struc-tures, and images of the extended corona show streamers protruding from the solar surface not only in the equatorial regions but at all latitudes around the disk as well. PHOIBOS will determine where the slow solar wind forms in and around streamers and whether specific magnetic signa-tures, such as embedded current-sheets, are associ-ated with its formation. Further, studies of solar wind sources during periods of solar maximum indicate a contribution to the wind from inside active regions as well. PHOIBOS will determine the topology of magnetic field lines within active regions that give rise to solar wind flow. PHOIBOS will travel over coronal holes,Stween 9 and 4 Rs and under both solar minimum and maximum conditions. It will trace the origin of the fast and slow wind and correlate the flow speed with closed/open magnetic field line topologies, as measured by photospheric field measurements and determined indirectly through the in-situ measure-ment of such parameters as electron and energetic particle bi-directional streaming. Relating the in-situ coronal observations with surface structures will require remote sensing: ecliptic viewing of the white light corona to trace field lines in the plane of the PHOIBOS orbit, tomographic images from the all-sky coronagraph to identify coronal structures in the local spacecraft environment, and a polar view of the photosphere and photospheric magnetic fields from the spacecraft perspective to identify and locate the source region structures. White light and UV coronograph-speofeatureless as well. Bright striations, or plumes, can be traced all the way from the solar surface out to 30 Rs .The relationship of plumes to the fast wind is poorly understood. They appear above X-ray bright points in the coronal holes, and are denser than the surrounding regions. UV lines in the plumes appear to be narrower (i.e., the plumes are cooler) than in the darker lanes separating them, while measure-ments of outflows suggest that the dark lanes are preferential outflow regions (Teriaca et al., 2003). Fine-scale structures are observed in the fast wind as well as in coronal holes, including the so-called

microstreams and pressure-balanced structures. These are fluctuations in radial velocity that last about sixteen hours in the spacecraft frame and have a magnitude on the order of 50 km/s. By flying through coronal holes over a range of distances from 30 to 8 Rs PHOIBOS will observe and cross coronal plumes or their remnants, estimate their filling factor and contribution to the overall solar wind flow, and assess the expansion factors of the flow tubes carry-ing the solar wind flow. These observations will make it possible to clarify how microstreams form and evolve and to determine what their relationship to coronal fine-scale structures is. Achieving this objective will require both in-situ measurement of the magnetic field and plasma velocity and full dis-tribution function (density temperature and composi-tion of solar wind) to identify individual flow tubes and use of the tomographic reconstruction technique of the all sky white-light coronagraph, which will provide information on the filling factor and geomet-rical distribution of plumes. The LASCO and UVCmknowledge of the origins of the slow solar wind streams around helmet streamers. Sequences of LASCO difference images obtained in 1996 (sunspot minimum) give the impression of a quasi-continuous outflow of material in “puffs” from the streamer belt (Sheeley et al., 1997). A quantitative analysis of moving features shows that they originate above the cusp of helmet streamers and move radially outward, with a typical speed of 150 km/s near 5 Rs, increas-ing to 300 km/s at 25 Rs . The average speed profile is consistent with an isothermal corona at the tem-perature T ≈ 1.1 X 106 K (UVCS/SOHO measure-ments indicating a temperature 1.6 X 106 K in the streamer core, at activity minimum) and a critical point near 5 Rs. The ejection of material may be caused by loss of confinement due to pressure-driven instabilities as the heated plasma accumulates or to current-driven instabilities (tearing and or kink-type instabilities) in the sheared field of the streamer. PHOIBOS will cross the paths of these ejecta from streamers and will ascertain whether the ejection of coronal material occurs in a continuous flow or whether the puffs are in fact disconnecting plas-moids. If the latter, PHOIBOS will determine the magnetic field configuration of the plasmoid as well as the magnetic morphology at the point of discon-nection in the corona. Comparison of Galileo radio data with UVCS/SOHO images clearly shows the association of the slow wind with streamer stalks, that is, with the regions above the cusps of helmet streamers that include the current sheet (Habbal et al., 1997). It is not known, however, whether there is

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a single current sheet that runs along the nearly equatorial strip of maximum brightness in the white corona, i.e., along the streamer belt (as sur-mised by Wang et al., 1997), or whether there are a number of stalk/sheet structures of finite longitudi-nal extent. Nor is the structure of current sheets in streamer stalks known. Do they have a simple structure, or are they made up of multiple sheets in a more complex magnetic field morphology, as is suggested in part by UVCS/SOHO measurements (Noci et al., 1997) and multiple current sheet cross-ings in-situ (Smith, 2001). As observed in-situ at large distances from the Sun,

e solar wind appears as a continuous plasma out-

solar wind, evidence in favor of an termittent origin is even more abundant. As men-

structure of the lar wind, of the ion and electron distribution func-

the solar wind, in-tu ion, plasma and magnetic field measurements at

thflow. Its quasi-steady character may be a property of the outflow at the solar source. However, the apparently quasi-stationary wind may also result from a number of spatially limited, impulsive events that are distributed over smaller scales (Neugebauer, 1991, Feldman et al. 1997). There is abundant evidence for the intermittent or “pulsed” (Feldman et al., 1997) character of the high-speed wind: observations of microstreams and persistent beam-like features in the fast wind; interplanetary scintillation measurements of field-aligned density structures having a 10:1 radially-aligned axial ratio and apparent field-aligned speeds ranging from ~400 to ~1280 km/s (Coles et al. 1991; Grall et al. 1996); and remote sensing observations of the chromosphere, transition region, and corona reveal-ing explosive, bursty phenomena, dubbed mi-croflares, associated with magnetic activity over an extremely wide range of energy and time scales. Feldman et al. (1996, 1997) have interpreted the fine-scale structures observed in the fast wind as remnants of spicules, macrospicules, X-ray jets, and H-alpha surges and hypothesize that the fast wind results from the superposition of transient reconnection-generated jets. If this hypothesis is correct, then the heating of the corona leading to its time-dependent acceleration to form an ensemble of outward-going jets could be accompanied by the annihilation of oppositely-directed magnetic flux bundles clustered near the magnetic network, in turn leading to transient hard X-ray and gamma-ray bursts, along with neutron production in the 1 to 10 MeV energy range, which could be detected by PHOIBOS. For the slowintioned above, blobs of plasma appear to be lost by helmet streamer structures overlying active regions and various mechanisms have been proposed for this process. At solar maximum, an important and

definitely intermittent solar wind component is pre-sent in the form of CMEs and the fine-scale structure of the solar wind from active regions supports at least a spatially structured origin for the various flow streams. More generally, smaller CME-like events at all scales could contribute significantly to the solar wind throughout the activity cycle. Direct in-situ measurements of the sotions, as well as elemental abundance variations close to the sun are required to understand the source regions of the wind. PHOIBOS will directly measure both the electron distribution function and flow speeds of minor ions in the coronal hole, and, at peri-helion, may directly sample composition differences on closed and open fields. By continuous direct sam-pling the plasma flow as it moves close to the Sun, PHOIBOS will be able to assess the space and time filling factor of the fast solar wind, while imaging the coronal structures that it will cross in the range above 10 Rs. The time-dependent variability ob-served in the wind might also increase close to the sun, leading to effects of multiple sources observable by PHOIBOS, for example from a multitude of bursty events or micro CME’s. To locate the source regions ofsihigh resolution are necessary covering an extended field of view due to the contribution to aberration of Alfvénic turbulence (max estimated to be 200 km/s, or ∆u/U=0.3), together with images both of the un-derlying photospheric field and the quasi-simultaneous coronograph all sky images as well as context from telescopes in earth orbit. Bi-directional electron streaming could identify position on closed/open field lines. Note that for fast particles, the aberration is essentially due to db/B (max esti-mated in the range 0.2-0.3). In coronal holes, fila-mentary structures such as coronal plumes are ob-served out to 30 Rs, the range of speeds between 300 km/s (perihelion) implies speeds across plumes in the range 100-200 km/s. With an expected size less from 103 - 105 km at 8 Rs, crossing time of an indi-vidual plume should last between 5s to 1 hour. Dur-ing that time one must ensure a sufficient number of plasma, particle and velocity measurements. Radio measurements at the plasma frequency appear also essential to give a separate and independent measure of density, speed and temperature of the core of the electron distribution function. Measurement Requirements.

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• Full distribution function measurements (density

lasma velocity at

ar

ctron distribution function and strahl (bidi-

ns measurements at

oronal

2.3) What mechanisms accelerate, store and

olar energetic particle events (SEPs) come in two

istinguishing the various acceleration processes

imultaneous solar observations from 1 AU it will be

lthough the occurrence rate of SEP events is

temperature and composition of solar wind as a proxy for individual flow tubes) • In situ magnetic field and phigh cadence in inner heliospheric regions (below 20 Rs), continuous, lower cadence below 0.3 AU • Density, temperature and composition of solwind • Elerectional streaming as evidence of closed magnetic field line topology and correlations with composi-tion and wind speed and magnetic field) and high energy tails of proton helium distribution functions at high cadence. Time-dependent neutron and gamma-ray energy spectra. • Energetic electrons and io• Photospheric magnetic fieldhigh latitude and line of sight velocity fields • All sky coronograph measurements of cstructure above 10-20 Rs.

transport energetic charged particles? Sdistinct categories. Gradual events are accelerated by CME-driven shocks and are characterized by roughly coronal abundances and charge states. Impulsive events are generally much smaller events associated with impulsive x-ray flares and are char-acterized by enrichments in 3He, heavy ions such as Fe, and electrons, with charge states characteris-tic of temperatures ranging from ~5 to 10 MK. This paradigm distinguishes between two separate ac-celeration processes and acceleration sites, both driven by eruptive events on the Sun: a) CME-driven shock acceleration starting in the high co-rona and continuing into interplanetary space and b) acceleration at the flare site, presumably driven by magnetic reconnection. Both processes are known to operate in larger SEP events, and studies at 1 AU during solar cycle 23 present a complex picture of events that often exhibit characteristics of both gradual and impulsive SEP events (e.g., Cohen et al., 1999; Cane et al., 2003; Tylka et al., 2004). In addition to such transient energetic events, observations at 1 AU show a continual outflow of intermediate-energy particles from the Sun extending from suprathermal energies to >10 MeV/nucleon. The mechanisms responsible for the acceleration of these particles are not known. Doccurring at the Sun on the basis of data acquired at 1 AU is difficult. Transport through the interplane-tary medium washes out the time structure, reduces

the intensities by orders of magnitude, and leads to mixing of particles from different acceleration sites. PHOIBOS measurements, made at distances as close to the Sun as 4 RS, will not suffer from transport effects because the Probe will sample energetic par-ticles close to their acceleration sites on the Sun and will explore, in situ, acceleration sites in the corona and inner heliosphere. In particular, recent results from ACE, SOHO, and WIND point to the increas-ing importance of the high corona (2RS < r < 20RS) as an acceleration site for energetic ions and elec-trons—a region that PHOIBOS will sample directly. These measurements will address key questions im-portant for understanding solar energetic particle acceleration and transport. Spossible to trace events observed by PHOIBOS to the flare site, to measure the flare properties, and to obtain the underlying magnetic field configuration. In addition to composition measurements, PHOIBOS will measure near-relativistic (V > 0.1 c) electrons from these events within a fraction of a minute of their release. These electrons are particularly impor-tant for untangling acceleration processes because their acceleration sites can be senses remotely by microwave radio emission or hard x-rays. Analo-gously for energetic ions, PHOIBOS may also ob-serve gamma rays and neutrons from these solar flare events, providing information on the accelerated particle components on closed field lines in the solar atmosphere. Agreatly reduced at solar minimum, strong evidence suggests that particle acceleration occurs continu-ously on the Sun or in the inner heliosphere,. All solar wind species that have been measured (H+, He+, and He++) exhibit suprathermal tails that extend up from several times the solar wind speed (~10 keV/nucleon). These tails are more prominent in the ecliptic than over the poles, and they are continu-ously present, even in the absence of solar activity or interplanetary shocks (e.g., Gloeckler et al. 2000). The fact that even interstellar pickup He+ exhibits a suprathermal tail suggests that the acceleration oc-curs in the inner heliosphere (e.g., Ruffolo et al., TBD). However, evidence also indicates that 3He is continuously accelerated at the Sun, even during the quietest periods, suggesting that more or less con-tinuous acceleration may be occurring in microflares such as those reported by RHESSI (Krucker et al. 2002). The small-scale, randomly occurring “com-ponent” reconnection that typifies microflares may be an indicator of a scale-invariant dissipation proc-ess that not only heats coronal plasma, but also pro-

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duces a stochastic component of the electric field that contributes to particle acceleration. Hard x-ray, gamma-ray, and neutron observations by PHOI-BOS can also reveal the occurrence of sporadic and/or continuous particle acceleration on the Sun. Solar neutron observations on PHOIBOS are of special interest because low-energy neutrons that do not survive to 1 AU can only be observed close to the Sun. (~1 MeV (10 MeV) neutron intensities at 5 RS are ~1.5 x 1010 (3.7 x 106) times greater than at 1 AU.) Neutron observations close to the Sun may reveal evidence of small nanoflares, which have been suggested as a principal source of energy for heating the corona.

o forecast large SEP events reliably, it is neces-

iming studies have shown that gradual SEP

he probability that PHOIBOS will encounter parti-

lysses measurements have shown that solar ener-

nergetic electrons are observed in both impulsive

Tsary to determine why some CMEs accelerate par-ticles more efficiently than others. The suggested possibilities include: (1) the presence or absence of a pre-existing population of suprathermal ions, left over either from a previous gradual event (e.g., Kahler 2001) or from small impulsive flares (Ma-son et al., 1999); (2) the presence or absence of successive, interacting CMEs (Gopalswamy et al., 2002); (3) pre-conditioning and production of seed-particles by a previous CME (Kahler, 2001); (4) improved injection efficiency and acceleration rate at quasi-perpendicular (as opposed to quasi-parallel) shocks (Tylka et al., 2004); (5) variable contributions from flare and shock-accelerated particles (Cane et al., 2003), including acceleration of associated flare particles by the shock (Li and Zank, 2004; Cliver et al., 2004); and (6) production of SEPs in polar plumes, where shock formation may be easier (Kahler and Reames, 2003). Tevents are first accelerated at distances between ~3 and 12 RS (Mewaldt et al., 2003). It has therefore been suggested that SEPs originate beyond ~3 RS because there is a peak in the Alfvén velocity at ~3 RS, such that it is only beyond this radius that shocks can be easily formed and sustained for typi-cal CME speeds (e.g., Gopalswamy et al., 2001). In MHD simulations of SEP events driven by coronal shocks (e.g., Zank et al., 2000; Sokolov et al., 2004) it is necessary to assume or model a variety of conditions in the region where gradual SEP events originate, including the magnetic field and density profiles, the solar wind and Alfvén speeds, the density of seed particles, and turbulence levels that determine the particle diffusion coefficient. It remains a mystery, why, for a given CME speed, the peak intensity of >10 MeV protons can vary by a factor of ~104 (Kahler, 2001). PHOIBOS will measure the solar wind and magnetic field close to

the Sun, the density and energy spectrum of su-prathermal seed particles, and the spectrum of mag-netic turbulence directly. It will thus be able to ascer-tain the presence of shocks and discontinuities and determine their role in particle acceleration. Tcle intensity levels characteristic of large SEP events at 1 AU (e.g., >100 particles/cm2sr-s with E >10 MeV) is about 80% during solar maximum condi-tions (Feynman et al., 2000). It is much less likely, however, (~10-20% probability) that the PHOIBOS flyby will take place while a CME-driven shock is accelerating >10 MeV particles inside 100 RS. None-theless, PHOIBOS measurements of the ambient conditions that exist prior to such events will be of enormous value to our efforts to understand SEP acceleration and transport. Ugetic particles (SEPs) can reach high latitudes (McKibben et al., 2001). Three explanations for these observations have been proposed: (a) the CME shocks accelerating the particles extended to high latitudes and crossed the interplanetary magnetic field lines connecting to Ulysses; (b) significant par-ticle cross-field diffusion took place; and (c) mag-netic field lines connecting high latitudes with low latitude active regions existed in the solar corona, allowing particles to reach high latitudes close to the Sun. On the basis of a comparison of onset times at Ulysses with onset times in the ecliptic for events with the same solar origin, Dalla et al. (2002) con-clude that high-latitude events are not compatible with direct scatter-free propagation along a magnetic field line, but rather the large path lengths and late release times suggest that propagation to high lati-tudes requires scattering. By approaching the Sun along a polar trajectory PHOIBOS will encounter energetic particles at all latitudes and determine how scattering properties from the corona into the solar wind vary with magnetic field and turbulence inten-sities. These measurements will also identify large-scale deviations from the Parker spiral configuration (Objective 2) and determine their role in energetic particle scattering. Eand gradual SEP events. Because of the electrons’ near-relativistic velocities, the onset times of elec-tron events at 1 AU are often used to deduce SEP release times near the Sun for comparison with their associated electromagnetic signatures. Surprisingly, the deduced release times almost always appear to be delayed by ~10 minutes with respect to electromag-netic signatures such as soft x-ray and optical emis-

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sions from flares and associated radio emissions (e.g., Krucker and Lin, 2000; Haggerty and Roelof, 2002). This discrepancy has resulted in consider-able debate concerning its cause—whether storage and subsequent release of the electrons, longitudi-nal propagation of the acceleration mechanism from the flare site to the injection site, or radial transport of the acceleration mechanism in the form of a CME-driven shock (Haggerty and Roelof, 2002). Close to the Sun propagation delays will be minimized, and energetic electron measurements combined with interplanetary magnetic field obser-vations will reveal where and how particles are released from the Sun and/or accelerated in inter-planetary space. Measurement Requirements

High-energy ions and electrons

gions, flares, solar

alpha particles) and mag-

s ex-

f ions extending from

ec-

ion spectra, e

2.4) Explore dusty plasma phenomena and their

he origin of dust in the inner solar system is not

he interaction of dust in the inner heliosphere and

espite some valuable observations (e.g., from He-

•• In situ vector magnetic field • Remote sensing of active reradio bursts, and CMEs. • Basic plasma (proton, netic field measurements, and their gradients • Major and minor ion distribution functiontending to high-energy tails. • Composition and spectra oenergies through ~100 MeV/nuc, including 3He • Plasma wave spectroscopy measurements (eltron density, temperature, velocity). • Magnetic field and plasma fluctuat• Correlation with underlying magnetic structur(imaging)

influence on the solar wind and energetic parti-cle formation Twell understood. The ultimate sources of the dust population are thought to be the release of dust from comets and asteroids and the breakup of me-teoroids. Subsequent dust-dust collisions lower the average mass of the dust particles. Dust orbital motion combines with Poynting-Robertson decel-eration to increase the dust number densities to-wards the Sun (Burns et al., 1979). Inward from 1 AU, the fragmentation of cometary meteoroids locally is believed to produce a majority of dust particles (Grün et al., 1985; Ishimoto, 2000; Mann et al., 2004). Dust particles attain electric surface charge through photo-ionization, electron emission, and interaction with the solar wind. While larger (>1 micron) particles move primarily in Keplerian orbits, smaller charged grains are deflected by the

interplanetary magnetic field. The degree of deflec-tion depends on the surface charge, which has not yet been directly measured for dust particles in space, and on the exact magnetic field parameters and their variation in time (Mann et al., 2000). In addition, dust dynamics is likely to be influenced by events such as coronal mass ejections, which may even lead to dust destruction (Misconi, 1993). Tthe solar wind plasma influences not only the dust population but the local plasma and gas environment as well. Notably, dust grains in the inner heliosphere are important as a source of pickup ions, protons as well as heavier species, which differ from the solar wind in their charge state and velocity distribution. These “inner source” pickup ions,. discovered with Ulysses, have provided limited knowledge concern-ing the composition of the gas released from dust and constraints on the spatial distribution and fluxes of dust grains. One of the surprising results has been the detection of noble gases and light elements in the inner source pickup ions having a composition re-markably similar to that of the slow solar wind. Molecular ions in the mass range up to ~40 amu have also been detected. These measurements imply that recycling of solar wind particles through adsorp-tion and desorption constitutes an important mecha-nism for the origin of the inner source pickup ions. However, the fluxes of dust required to account for the amounts of observed pickup ions exceeds by orders of magnitude the fluxes deduced from zodia-cal light observations. Further progress in resolving the origin of inner source pickup ions will require in-situ measurements close to the Sun as well as better models of dust microphysics. Inner source pickup ions are also potential candidates for subsequent acceleration and may contribute to the anomalous cosmic ray population (Cummings et al., 2002). Dlios and Ulysses), many basic questions require de-tailed measurement of the near-Sun dust population. What, are the mass distributions and fluxes of dust particles as a function of distance from the Sun? How are dust fluxes correlated with fluxes and ve-locity distributions of pickup ions? What are the major elemental compositions and bulk density of the dust and how do they vary with distance from the Sun? In-situ observations with PHOIBOS will be crucial in resolving many of the present uncertainties regarding dust origin, its composition, and spatial distribution. Since dust is a common component of interstellar material as well as most likely of other stellar systems, PHOIBOS results will have a direct bearing on certain astrophysical problems, with the

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near-Sun dust cloud serving as an analogue for circumstellar dust clouds, for example. PHOIBOS will characterize the near-Sun dust envi-

addition to its interaction with the quasi-

ust impacting the spacecraft will influence the

ust fluxes are expected to be especially high and

ronment by determining how the mass distribution of dust and impact directions vary along the space-craft trajectory and how the observed impact sig-nals vary with the mass and impact parameters of the dust particles. PHOIBOS dust measurements will likely require substantial revision of the para-digm of a homogeneously distributed dust cloud that is stable in space and time. Instationary wind, the near-Sun dust population also interacts with and is influenced by transient events such as CMEs (Ragot and Kahler, 2002). Colli-sional evaporation, particularly in cometary mete-oroid trails, is expected to influence the solar wind parameters measured locally. For example, a recent study shows that dust collisions in the inner solar system can produce some of the heavy species in amounts comparable to the observed inner source fluxes (Mann and Czechowski, 2005). The material released in such collisions may be responsible for the enhancements of the interplanetary field meas-ured by Ulysses in association with meteoroid trails (Jones et al., 2003. These enhancements—which last for minutes to hours, are clustered in space, and occur more frequently in the inner solar sys-tem—may be the result of mass loading of the solar wind plasma induced by collisional vaporization in the dust trails (Mann and Czechowski 2005). It is still an open question how noble gases observed in the inner source are produced, with the solar wind surface interactions being a distinct possibility. Dplasma environment of the spacecraft and may bias plasma and field measurements. Signals due to impact-generated ion cloudlets have been observed by plasma experiments on several spacecraft in the vicinity of planetary rings (Gurnett et al., 1983; Meyer-Vernet et al., 1986), in the interplanetary medium (Gurnett et al., 1997), and during encoun-ters with the comets Giacobini-Zinner, Halley, and P/Borrelly (Neubauer et al., 1990; Tsurutani et al., 2003). Dtime-variable near Sun. PHOIBOS will measure both the dust fluxes and pickup ion densities and composition as a function of radial distance and latitude with sufficient resolution, sensitivity, and dynamic range to characterize the species gener-

ated from the dust grains and elucidate the mecha-nisms by which material is released from the dust. Measurement Requirements • Spatial variation of dust flux as a function of radial distance and latitude from 4 RS to 5 AU • Distribution functions and composition of inner source pickup ions • Solar wind bulk parameters • Solar wind ion composition • Plasma wave measurements • Energetic particle spectra and composition • Magnetic field orientation and strength.

3) Mission Profile As described in the executive summary, the present proposal is interesting alternative scenario to the NASA STDT/2005 study in which: (i) a classical solar panels technology combined with heat shield thermal generators are used as power systems instead of RTGs and (ii) a new original trajectory combined to the use of a plasmic electric propulsion system are implemented in order to reach the final scientific orbit instead of the classical Jupiter low altitude swing-by with a very high C3 for Earth escape. The PHOIBOS mission profile allows therefore more flexibility in the fine adjustment of the final orbit. It may avoid the possible drawbacks of a Jupiter swing-by such as the high departure V∞, the risky low altitude Jupiter swing-by at 5.5 AU with an im-portant radiation level and no consequent trajectory changing capability. The selected science orbit, pre-sented below, is therefore the main mission require-ment and drives all the mission profile. Launcher and Orbit requirements The PHOIBOS spacecraft will use an Ariane AR5 ECA launcher and a plasmic propulsion stage in order to reach it’s final scientific orbit described in Table 3.1. The required escape conditions are a modulus of the hyperbolic excess velocity v∞ = 3.0km/s and a declination of the hyperbolic excess velocity δ∞ = - 6deg. The launcher performance for these conditions is 5000kg offering 25% of margin wrt the need. Table 3.1 : PHOIBOS final orbit characteristics

perihelion radius 4 Rs aphelion radius 3.5AU sidereal period 853 days

(2.33 years) eccentricity e = 0.9886

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inclination with respect to the ecliptic plane

60deg

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Figure 3.1 : projection of the PHOIBOS trajectory on the ecliptic plane.

The plasma thrusting phases are indicated in green.

Figure 3.2 : Sun-Earth-S/C angle as a function of time for the first two solar encounters.

Mission profile and cruise scenario

The overall mission profile is summarized on Figure 3.1. For a launch in December 2018, the acquisition of the final orbit is planned in June 2027, the first flyby of the Sun at 4 Rs will take place on 29 Sep-tember 2027 and the second one on 29 January 2030. The launch opportunity for such a scenario is a window every 584 days (1.6 years). The earlier

possible launch dates are therefore May 2017 or October 2015. The final launch date should also be chosen so that the first flyby of the Sun occurs dur-ing solar minimum of activity which is approxi-mately the case for a December 2018 launch. In order to reach the final orbit, a total Delta-V of 12.25km/s is necessary. To obtain such a Delta-V, two Earth swing-by (March 2025 and August 2026),

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one Venus swing-by (September 2025) and plasmic propulsion are necessary. The envisaged plasmic engines are PPS5000 with one or two engines work-ing in parallel depending on the available power (specific impulse, Isp = 1900s, thrust magnitude, 185mN < F < 280mN, electrical power, 3kW < P < 4.5kW). The total duration of the cruise phase will be 8.5 years (3100 days). The Sun closest approaches during the cruise phase will be in August 2025 (0.56AU) and December 2029 (0.26AU), which is equivalent to the ESA Solar Orbiter mission. The two most efficient thrust-ing phases in term of Delta-V will occur at helio-centric distances between 1.8 and 3.7 AU. These phases are indicated on Figure d.1 as aphelion thrusts 1 & 2. While the main thrusting axis during the “aphelion thrust 1” period remains in the eclip-tic, for “aphelion thrust 2” the probe will be oriented in such a way, that thrusting will allow not only to reach the necessary Delta-V in order to lower the perihelion, but also to increase the inclination of the final orbit up to 60 deg. from the ecliptic plane. The total Xenon mass for the plasmic engines required for the mission is 1949kg. Therefore the final space-craft composite dry mass will be around 2050kg. Alternative launchers A slight variation to proposed scenario with only one Earth swing-by and a total duration for the cruise phase of 7.5 years could be envisaged with the use of the new generation of Ariane 5 launcher (Ariane 5 ECB). Finally, one should note also that the use of the PROTON M launcher, with its BREEZE M upper stage fulfils also, albeit without performance margin, the mission profile that we have described. Ground segment requirements Since the most valuable scientific measurements are performed during the Sun’s closest approaches, the tracking and command capabilities are mainly driven by the ability to download in real time as most of the data as possible Figure 3.2 displays the Sun-Earth-S/C configurations during the first two solar passes. X-band/Ka-band downlink allows (i) to gain 11.6 dB of S/N ratio wrt the coronal scintil-lation and (ii) a beam of 0.2° allowing a Earth an-tenna tracking at 0.2° from the Sun disc. A BepiColombo configuration allows a telemetry of 45 kbits/s at 1 AU. However, taken into account the 35m Earth antenna temperature noise when turned toward the Sun, a flow of 2 kbits/s can be expected for the TM budget at the most critical point. It will

be necessary therefore to use a large onboard mem-ory to store as much data as possible during the solar passes and process and transmit them after perihelion Onboard communication system The complete on board communication system is based on the Bepi Colombo one. Its features can be estimated to 25 kg in mass and 90 W in consump-tion.

4) Payload description As we already indicated in the executive summary of the proposal, the overall philosophy of the cur-rent proposal in terms of scientific objectives and necessary payload in order to achieve them has greatly been inspired by previous Solar Probe pro-posals and especially by the last NASA STDT/2005 report.

4.1) Overview of all proposed payload elements To meet the PHOIBOS science objectives, we rec-ommend a payload comprising both comprehensive in-situ and relevant remote-sensing instruments. However since we want to emphasis the in-situ measurements, the in-situ payload should have the priority in terms of resources and accommodation trade-offs. The complementary remote sensing ob-servations may be provided by Solar Orbiter, de-pending upon the schedules of the two missions. PHOIBOS in-situ instrumentation consists of a Fast Ion Analyzer, a Fast Electron Analyzers, an Ion Composition Analyzer, an Energetic Particle In-strument, a Magnetometer, a Radio and Plasma Wave Instrument, a Neutron/Gamma Ray Spec-trometer, and a Coronal Dust Detector. The remote-sensing instrumentation comprises a Hemispheric Imager for white-light imaging of coronal structures and a Polar Source Region Imager for EUV and magnetic imaging of the photosphere. The payload is serviced by a common data processing unit (CDPU) and low voltage power supply (LVPS). Table 4.1 summarizes the contribution of the vari-ous instruments to the PHOIBOS scientific objec-tives. The specifications for each instrument and their rationale are discussed in the sections that fol-low. The mass, power and data rate allocations for the baseline payload are shown in Table 4.2. They are based on those of past or existing instrumenta-tion or components.

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Table 4.1: Science Objectives and Contributing Instruments

Scientific Objectives

FI

A

FEA

C EPI

RPW

I

MA

G

NG

S

CD

PSR

I

HI

Explore the fundamental processes underlying coronal heating and solar wind acceleration.

R S R R S S S

Determine magnetic field structure and dynamics in the source regions of the fast and slow solar wind.

R S S R S R R

Determine what mechanisms store, accelerate and transport energetic particles. R R R R S S S

Explore dust and dusty plasma phenomena in the vicinity of the Sun and their influence on the solar wind and energetic particle formation.

R S S S R S

FIA: Fast Ion Analyzer FEA: Fast Electron Analyzer ICA: Ion Composition Analyzer

NGS : Coronal Dust Detector

EPI: Energetic Particle Instrument PSRI : Polar Source Region Imager MAG: DC Magnetometer HI: Hemispheric Imager

RPWI: Radio and Plasma Waves Instrument R : required NGS : Neutron/Gamma-Ray Spectrometer S : supporting

Table 4.2 Instrument Resource Requirements1 Instrument

Mass(kg)

Power(W)

Peak Data Rate (kbps)

Fast Ion Analyzer (FIA) 2.5 3.7 10 Fast Electron Analyzer (FEA) 5.0 7.2 20 Ion Composition Analyzer (ICA) 6.0 6.0 10 Magnetometer (MAG) 1.5 2.5 1.1 Radio and Plasma Wave Instrument (RPWI) 7.0 5.0 4 Energetic Particle Instrument, Low Energy (EPI-Lo) 1.5 2.3 5 Energetic Particle Instrument, High Energy (EPI-Hi) 2.5 1.7 3 Neutron/Gamma Ray Spectrometer 2.5 3.0 0.5 Coronal Dust Detector 1.5 3.8 0.1 Hemispheric Imager (HI) 1.5 4.0 70 Polar Source Region Imager (PSRI) 3.5 4.0 70 Common DPU/LVPS 10 14 N/A Total 45.0 57.2 123.7 1. Source is NASA STDT 2005 Report.

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4.2 Baseline Payload

4.2.1 Fast Plasma Instrumentation. There are several basic requirements for the meas-urement of the coronal thermal plasma. The ion instrumentation should be able to distinguish alpha particles from protons under all conditions. The field of view (FOV) coverage for the distribution functions should be as complete as possible. The basic moments of the distributions, density, velocity and temperature should be obtained fast enough and accurately enough to enable Alfénic and MHD tur-bulence to be analysed The PHOIBOS Fast Plasma Instrumentation will consist of a single Fast Ion Analyzer (FIA) and a pair of Fast Electron Analyzers (FEAs). The FIA and one of the FEAs are mounted, together with the Ion Composition Analyzer (ICA), on a movable arm on the ram side of the spacecraft; the arm is gradu-ally retracted as the spacecraft approaches the Sun. This arrangement provides viewing to near 5° (i.e., includes attitude control margins and finite size of charged particle entrance apertures) inside of the edge of the heat shield umbra. The second FEA is mounted on the anti-ram side of the spacecraft body, pointing 180° away from the first. While the mission-unique aspects of PHOIBOS will require new designs for the FIA and FEA instruments, the basic designs and subsystems can be drawn from a wide variety of previous heritage missions such as Ulysses, ACE, Helios, Wind and Stereo. Fast Ion Analyzer (FIA). The FIA should be capa-ble of measuring two- and three-dimensional distri-bution functions for protons and alphas over the energy/charge range of 50 eV/q to 20 keV/q. This energy range covers the lowest and highest expected speeds for 100 km/s protons and 1400 km/s alpha particles, respectively. The FIA’s 3D temporal reso-lution of 3 seconds and 0.1 second for 2D distribu-tion functions allows identification of boundaries in the solar wind down to ~1000 km near perihelion and wave modes (e.g., the gyrofrequency is ~30 Hz over the poles). The energy resolution (∆E/E) should be approximately 5%, which does a good job of resolving the supersonic solar wind beam out to beyond 1 AU. The sensitivity and dynamic range need to be adequate to measure 2D (energy and one angle) ion distributions in 0.1 s at 20 RS without saturating the detectors all the way into perihelion. The FIA’s field of view (FOV) needs to observe as much of the ram side of the viewing space as possi-ble. To resolve the ion distributions everywhere from 0.3 AU into perihelion, FIA’s angular resolu-

tion needs to be ~5° around the solar wind beam and ~30° over the remainder of its FOV. Fast Electron Analyzer (FEA). The FEAs should be capable of measuring two- and three-dimensional electron distribution functions over the energy range from ~1 eV to 5 keV. This energy range covers from the lowest energy photoelectrons, through the thermal core population and well up into the supra-thermal halo population. The FEAs’ 3D temporal resolution of 3 s (0.1 s for 2D distribution functions of energy and one angle) is matched to the FIA to help resolve plasma conditions and structures on the same scales. The energy resolution (∆E/E) should be approximately 10%, which does a good job of resolving the hot electron distributions. Like the FIA, the FEA requires a sensitivity and dynamic range adequate to measure the 2D distributions in 0.1 s at 20 RS without saturating the detectors all the way into perihelion. Together the FEAs need to observe as much of 4π steradians as possible; all-sky imagers and deflecting top-hat analyzers are both appropriate approaches for achieving the needed FOVs. To resolve possibly very narrow halo electron beams (the strahl), the FEAs need angular resolutions that approach 3° in at least one dimen-sion at higher energies around the magnetic field direction (this information is supplied real-time from the magnetometer via the payload DPU), while ~30° angular resolution is adequate to measure the remainder of the halo population and the core and photoelectron populations at lower energies.

4.2.2 Ion Composition Analyzer (ICA). The ICA is mounted, together with the FIA and one FEA, on the movable ram-looking arm referred to above. The ICA should be capable of measuring two- and three-dimensional distribution functions of He and heavy ions in the solar wind, over an energy range from ~100 eV/q to ~60 keV/q and a mass range from 2 to > ~60 amu. The required energy range covers all major solar wind species that will be observed during the solar encounter. ICA’s 3D temporal resolution of 10 s (at 20 RS) permits tem-poral and spatial effects to be distinguished and allows comprehensive assessment of the non-thermal properties of the distribution functions that are generally expected from various solar wind ac-celeration and heating mechanisms. Furthermore, with the required mass range the ICA will measure species with low ionic charge states (i.e., He+) and high masses (i.e., SiO2), such as those produced from neutral sources in the inner heliosphere or created by the solar wind’s interaction with dust near the Sun (e.g., inner source pickup ions). The energy resolution (∆E/E) should be 4–5%, sufficient

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to resolve the supersonic solar wind beam out to beyond 1 AU. The sensitivity should be sufficient to measure He/O ratios every 10 s at 20 RS which can be achieved scaling from 1 AU observations of solar wind composition and charge states. The dynamic range should be ~104. The ICA FOV needs to ob-serve as much of the ram side of the viewing space as possible due to the large amount of variability expected due to turbulence or waves in the outer corona. This can be achieved, for example, with a top-hat and swept FOV, or with an instrument with large instantaneous FOV as done on MESSENGER, provided that the edge of the FOV extends to close to the heat shield. To resolve the ion distributions everywhere from 1 AU to perihelion, ICA’s angular resolution needs to be ~10° around the solar wind beam and ~20° over the remainder of its FOV.

4.2.3 Energetic Particle Instrument. The PHOIBOS Energetic Particle Instrumentation (EPI) consists of a low-energy sensor (EPI-Lo) and a high-energy sensor (EPI-Hi). Both packages are to be mounted on the spacecraft body, where they view particles incident from both the sunward and anti-sunward hemispheres. EPI Low-Energy Instrument (EPI-Lo). The EPI low-energy instrument is required to measure the composition and pitch-angle distributions of ener-getic particles. The composition includes hydrogen to iron as well as energetic electrons. As a minimum the detector should be able to make the ion meas-urements from ~20 keV/nucleon to ~1MeV/nucleon and the electron measurements from ~25 keV to ~1 MeV. Composition measurements should discrimi-nate protons, 3He, 4He, C, O, Ne, Mg and Si, and Fe. The measurements should have sufficient angu-lar spread and resolution to enable pitch-angle measurements of the differential particle fluxes for a (nominal) radial magnetic field. A “slice” field of view of ~10° wide and >120° and at least 5 angular bins would suffice; at least 120° coverage and an angular resolution of no worse than 30° are re-quired. The wider opening should be aligned with the spacecraft spin axis with the field of view just clearing the thermal protection system. Larger solid-angle coverage and better species resolution are, of course, preferred. The sensitivity should be at least ~1 (cm2 ster s keV)–1. Timing resolution should be no worse than 1 s for e–, 5 s for protons, and 30 s for heavier nuclei. The capabilities described here can be achieved with energetic particle instruments of the type currently being flown on MESSENGER and STEREO.

EPI High-Energy Instrument (EPI-Hi). The EPI high-energy instrument (EPI-Hi) is required to measure the composition and energy spectra of en-ergetic nuclei with 1 ≤ Z ≤ 26 from ~1 to 100 MeV/nucleon, as well as energetic electrons from ~0.3 to 3 MeV. The source of the energetic ions to be observed over the course of the PHOIBOS mis-sion range from quiet-time intensities of cosmic rays, to low-energy ions accelerated in CIRs and transient interplanetary shocks, to ions accelerated in small, impulsive events associated with solar flares, to solar energetic particles accelerated in large gradual events. As a minimum, the charge resolution should be sufficient to measure differen-tial intensities of H, He, C, N, O, Ne, Mg, Si, and Fe, although minor species are also of interest. It would also be very useful to include nuclei with 30 ≤ Z ≤ 83 that are found to be enhanced in some SEP events associated with impulsive solar flares. It is required that 3He and 4He be separately identified whenever the 3He/4He ratio exceeds 1%. Assuming that onboard particle identification is used to sort species into a matrix of species versus energy bins, the energy resolution of these bins should be no worse than six intervals per decade. Near the Sun it can be expected that energetic ions may be highly anisotropic and beamed along the interplanetary magnetic field, which is expected to be on average radial at closest approach, but could be highly vari-able. It is therefore desirable for the EPI-Hi instru-ment to sample as much of 4π steradians as possi-ble, including, in particular, the forward hemi-sphere. As a minimum EPI-Hi should be able to observe particles with pitch angles ranging from 30° to 120° with respect to the spacecraft Zaxis with an angular resolution no worse than 30°. EPI-Hi should have sufficient directional information to be able to determine the magnitude and direction of 3D anisot-ropies. Although not well known, it is expected that the intensity of SEP events will scale with distance from the Sun (R) approximately as R–3 (cf. Reames and Ng, 1998, and references therein). To observe particle populations that range from quiet-time lev-els near 1 AU to solar energetic particle (SEP) events near the Sun requires a dynamic range of ~107. The peak intensity of a typical impulsive event at 1 AU is ~1 to 10 protons/cm2-sr-s >1 MeV. Scaling this to 4 RS by R–3 suggests that intensities up to ~106 protons/cm2sr-s >1 MeV should be measurable. Particle intensities should be measured with a timing resolution that is no worse than 1 s for electrons, 5 s for H, and 30 s for Z = 2 nuclei. There is considerable heritage for energetic particle in-struments in the 1 to 100 MeV/nucleon energy

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range. Instrument designs that could be adapted to meet these requirements (assuming modern, low-power, low mass electronics) have flown on Helios, Voyager, ISEE-3, Ulysses, Wind, ACE, and STE-REO.

4.2.4 DC Magnetometer. The PHOIBOS direct current Magnetometer (MAG) will provide context and definition of local mag-netic structure and low frequency (<10 Hz) mag-netic fluctuations. MAG consists of one or more 3-axis sensors mounted close to the end of a deploy-able, non-retractable axial boom extending from the bottom deck of the spacecraft. (Owing to the size of the Thermal Protection System (TPS), MAG sen-sors can not be placed sufficiently far from the spacecraft body for a dual magnetometer configura-tion to be practical in removing spacecraft fields. A second MAG sensor could be used to provide low-power and low-mass redundancy.) The MAG sensor will be located close to the search coil component of the Plasma Waves Instrument (PWI), making it necessary for both to work together to provide a suitable measurement environment. Close collabo-ration between the two teams is critical to contain-ing cross-contamination of the instruments and pro-viding for the success of the mission. Signatures of plasma processes at the proton inertial scale, which result in the conversion of magnetic energy into heat, fall within the frequency range of the PWI, and only two suitably configured instruments will be able to provide the needed plasma diagnostics. MAG Performance. Photospheric structures with scale sizes of tens of km will have scale sizes of hundreds of km if they are coherent out to the orbit of PHOIBOS. A sample rate of 20 Hz gives a spa-tial resolution of approximately 30 km over the Sun’s pole, which will provide minimal resolution of the magnetic structures. A burst or snapshot mode of higher time resolution is used for compari-son with the PWI. Data telemetry compression will permit adequate retention of measurement resolu-tion. Total telemetry dedication of 960 bps will permit adequate download of continuous 20-Hz vector measurement plus snapshot buffer. Extrapolation of Helios data acquired at distances ≥0.3 AU yields an average interplanetary magnetic field (IMF) of approximately 260 nT at 20 Rs, the distance at which the primary mission begins. Vari-ous measurements and theories suggest that, within some regions and structures, the magnetic field might be as high as 1 to 6 G at 4 Rs. MAG should be capable of switching sensitivity ranges. At least four ranges are needed, with the most sensitive be-ing |B| < 0.1 nT and the high-field range |B| < 8.2 G.

With some adjustment to accommodate the upper range, this requirement could be met with magne-tometers commonly flown on magnetospheric mis-sions today.

4.2.5 Radio and Plasma Wave Instrument (RPWI). The RPWI sensors consist of a 3-axis search coil for detecting magnetic field fluctuations and a 3-element electric field antenna system. The search coil sensor is mounted on the aft spacecraft boom, with the separation from the DC magnetometer and other instruments to minimize contamination of the search coil data to be determined. The electric field antenna system should be designed is such a way to accommodate, if possible, both DC electric field and high frequency Quasi-Thermal Noise (QTN) measurements (see below). The antenna system is mounted on the base of the spacecraft, with the three antenna elements separated by ~120°. Each element is about 1.75 m long. The antenna inclina-tion to the spacecraft axis is varied with distance from the Sun, so as to maintain permanently some portion of it in sunlight, while minimizing heat in-put into the spacecraft. The portion of antenna in sunlight needs to be the same on each element in order to enable low frequency (< ~3 kHz) plasma waves to be sampled. Plasma wave instruments with the necessary capabilities have been implemented on numerous missions, including Ulysses, Wind, Cluster, Polar, FAST, Geotail, Cassini and Stereo. Similar antenna concepts have been used on several of these missions; however, they were not designed to work in the thermal environment expected for PHOIBOS. To be accommodated safely on the spacecraft, the PWI antenna will need to be made from a refractory material that will operate at tem-peratures up to 1400°C. Search Coil Magnetic Field Measurements. The PWI magnetic field experiment should operate in the frequency range ~1 Hz to 80 kHz, allowing overlap with the DC magnetometer at low frequen-cies and to measure fluctuations beyond the ion cyclotron frequency at high frequencies (The sensi-tivities of the search coil and DC magnetometer are expected to be equivalent at approximately 10 Hz). The strawman instrument samples in frequency space at 40 channels per decade, with cross-spectral power between the field components at 20 channels per decade. Bursts of waveform data are also col-lected at a cadence of up to 60 s to allow detailed study of small-scale processes in the near-Sun plasma. Electric Field Measurements. The RPWI electric field experiment should measure fluctuations in the

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electric field from close to DC to above the plasma frequency (1 Hz to 30 MHz was chosen for the strawman instrument) so as to return information on low-frequency wave, turbulence, small scale struc-tures and processes at and below the ion inertial scale. RPWI will be also designed to diagnose accu-rately the electron plasma parameters (density and temperature) using the technique of the quasi-thermal noise (QTN) spectroscopy. QTN requires sampling the electric field fluctuations from low frequency to above the plasma frequency. The strawman instrument has a sampling density of 40 samples per decade and a temporal sampling period of 0.1 s to allow rapid sampling of plasma parame-ters local to the spacecraft. A sensitivity of 2 × 10–17 V/m2/Hz at 10 MHz provides adequate signal to noise for QTN measurements. The strawman in-strument returns 3-axis measurements sampled at 40 samples per decade, and as with the magnetic field, cross spectra between components are returned. In the low frequency regime (<10 kHz), cross spectra between E and B are measured to facilitate identifi-cation of wave modes. Waveform data that allow the study of small-scale phenomena are returned as burst mode data with a 60-s cadence. Electro-Magnetic Cleanliness (EMC). The EMC requirements are driven by the RPWI and MAG instruments, carrying an electric antenna, search coils and a magnetometer. As for magnetic issues, with expected field of ~250 nT, a DC cleanliness requirement at the magnetometer of 10 nT would be reasonable, with a low frequency AC requirement of 1 nT. With enough resources, a twin magnetometer configuration could also be used to detect and eliminate spacecraft fields, reducing the magnetic cleanliness requirements as well as providing re-dundancy. As for electrical issues, several inexpen-sive measures can be taken to ensure that the PHOIBOS spacecraft is clean from the point of view of both conducted and radiated electromag-netic interference. The sensitivity of the RPWI instrument will be approximately 2 × 10–17 V/m2/Hz at 10 MHz. In addition the waveform analyser is sensitive to impulsive interference of duration as short as a fraction of a µs. It is most important that these requirements be considered in the design from day one, with the cooperation of all concerned - spacecraft, payload and AIV. Although these re-quirements may appear to require an excessively “clean” spacecraft, they are not difficult to achieve if good EMC practices are incorporated in the spacecraft design.

4.2.6 Neutron/Gamma-Ray Spectrometer (NGS). The NGS detector should be capable of detecting and positively identifying neutrons and γ-rays from the Sun having energies that range up to 10 MeV. The neutron component should be capable of intrin-sic energy resolution sufficient to separate neutrons having energies below and above 1 MeV, and better than 50% energy resolution for neutron energies between 1 and 10 MeV. This last requirement is needed to separate quasi-steady-state neutron emis-sion from transient neutron emission. The NGS will measure the products of the acceleration of protons (via neutrons and γ-rays) and electrons (via γ-rays) as they interact with the dense low chromosphere and photosphere. If microflares or nanoflares play a significant role in coronal heating, these signatures of particle acceleration will be present. Their spec-trum and time variation provide information on the acceleration process(es). Upward-propagating protons and electrons may be directly detected by PHOIBOS, although the prob-ability of crossing the appropriate field lines at the critical time may be small. The neutron and γ -ray detection suffers no such restriction. Furthermore, PHOIBOS’s close passage to the Sun provides tre-mendous advantage for detection of low energy neutrons because of their short lifetimes, as well as for spectroscopy of faint γ-ray bursts. These obser-vations will, for the first time, provide solid statisti-cal knowledge of frequency of energetic accelera-tion in small solar flares. A detection of a burst of γ -rays would help refine the energy spectrum of transient neutrons through use of the measured time of flight between neutron arrival times and the time of the gamma burst. The detection sensitivity of the NGS should be sufficient to measure neutrons produced by flares that release greater than 1024 ergs. A broad-band analysis of the γ-ray spectrum can provide a measure of the electron and ion compo-nents that will complement the detection of neu-trons. The neutron measurements are most sensitive to the lowest-energy heavy-ion interactions, while the ion-induced gammas sample higher energies that may be present in the ion population. Bremsstrahlung from accelerated electrons will manifest themselves in a continuum spectrum that is distinguishable from that of the ion-induced gam-mas. NGS Performance. The NGS is mounted behind the hydrazine tank. The presence of hydrazine onboard can be used as a separate detection channel via

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moderated neutrons. To adequately resolve the on-set and duration of a γ-ray burst requires a sample period of 4 s in each of 64 energy bins, encompass-ing an energy band of 0.1 to 10 MeV. The neutron channel, because of the moderating influence of the hydrazine, will be detecting degraded neutrons with timing but less spectroscopic information. The neu-tron channel will require a sample period of 16 s over a neutron energy range of 0.05 to 20 MeV, also with 64-channel spectra. The detector must be able to distinguish statistically between fast neutrons and gammas and should possess an unambiguous neu-tron-detection channel. A spectral resolution better than 50% will allow broad-band analysis of the gammas and neutrons, sufficient to resolve the bremsstrahlung, nuclear, and neutron components. Both neutron and gamma spectrometer functions must efficiently reject charged particles.

4.2.7 Coronal Dust Detector (CD). The CD should be compact and lightweight and must be able to cope with the near-Sun thermal and particle environment. The CD assumed for this study is an impact ionization detector based on the Mars Dust Counter (Igenbergs et al., 1998). Such devices measure ions and electrons produced by the impact of dust particles on the detector’s target area to derive particle mass and have been successfully flown on Ulysses, Hiten, Galileo, and Nozomi. The CD is mounted on the +X (ram) face of the PHOI-BOS spacecraft, where it will be exposed to the maximum dust flux. For an aperture area of 140 cm2, the dust model described in Appendix B pre-dicts that 2 × 104 particles of masses larger than 10-

17 g and up to 2 × 106 particles of masses larger than 10-19 g would be detected at the high-impact veloci-ties that PHOIBOS will experience. Independent pointing and special pointing accuracy are not re-quired. The CD will be operated continuously (ex-cept when in direct sunlight). Only modest teleme-try allocation is required. The CD should have an external cover to be removed after launch. No spe-cial cleanliness is required, but purging with N2 should be considered. Issues to be addressed for further development of the CD are the high voltage parts and the influence of the radiation environment and outgassing from the heat shield on the meas-urements. Measurement of particle mass is standard for impact ionization detectors but has not yet been demonstrated for the high impact speeds that the PHOIBOS CD will experience. Although a single sensor has been assumed in the payload design, we have conservatively included enough mass to accommodate a second sensor mounted at a different location on the spacecraft to

provide a second look direction. The allocated power is adequate to two alternately operated sen-sors. Measurements from two sensors on different spacecraft locations can be used to distinguish be-tween particles in prograde and retrograde motion as well as between particles in near-ecliptic and out of ecliptic orbits. If resources permit, an alternative to two sensors would be a single detector with time of-flight (TOF) capability. This would enable TOF measurement of the impact-produced ions, yielding mass spectra and allowing the elemental composi-tion of the dust to be derived. The RPWI instrument will also be sensitive to dust impacts via corresponding plasma cloud and pickup signal on the electric field antenna. This is based on dust impacts observed by similar instrumentation of the Voyager or Cassini spacecraft at Saturn (Gurnett et al., 1983; Meyer-Vernet et al., 1986; Moncuquet et al., 2007).

4.2.8 Hemispheric Imager (HI). HI is a broadband, very-wide-angle, white-light coronagraph with ~160° FOV to image the local coronal environment and provide tomographic im-aging of coronal structures (e.g., polar plumes) as PHOIBOS flies through the corona. HI will also be able to observe coronal mass ejections (CMEs) and other dynamic structures as they evolve. Coronal tomography is a fundamentally new approach to coronal imaging (similar to a medical CAT scan) and is possible only because the imaging is per-formed from a moving platform close to the Sun, flying through coronal structures and imaging them as it flies by and through them. HI observations of the 3D coronal density structure are required to resolve ambiguities in the interpretation of spatial and temporal changes seen in the in-situ measure-ments. HI heritage stems from the all-sky corona-graph on SMEI, the HI wide-angle coronagraph on STEREO, and instrument prototypes developed as part of the 1995 PHOIBOS Instrument Develop-ment Program (Buffington et al., 1998). The HI’s FOV and resolution derive from the need to provide context for the in-situ instruments and to be able to reconstruct the 3D density structure of the corona tomographically. The 160° FOV is sufficiently large to view the corona from near the solar limb to be-yond the zenith. A wide-angle view is particularly important for imaging faint coronal features, be-cause the coronal intensity contrast is greatest along flux tubes and other magnetic structures near the zenith. The spatial resolution required to image small-scale coronal structures is of order 1°. The temporal cadence required to provide continuous

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observations and sufficient data for 3D tomographic reconstruction is ~90 s at perihelion.

4.2.9 Polar Source Region Imager (PSRI). PSRI uses an imaging periscope to view the Sun’s poles above 60° latitude at distances beyond 20 RS. The PSRI consists of two channels, a magnetograph channel to image the polar magnetic fields and an EUV imaging channel to identify small-scale hot coronal plasma structures. These two channels will make it possible to relate the magnetic field struc-ture to the heating of coronal structures at the poles and to establish the linkage between these source region(s) and the plasma flows measured at the spacecraft. PSRI magnetograph channel heritage stems from tunable etalon magnetographs flown on the Flare Genesis Balloon and instrument designs developed as part of the 1995 PHOIBOS Instrument Development Program (Title et al., 1999). PSRI EUV channel heritage stems from SOHO/ EIT, TRACE, STEREO, GOES/SXI, and various rocket programs. The imaging periscope uses two chan-nels, each 1 inch in diameter, to view the solar sur-face at the Sun’s poles. The periscope will be ex-tended for 10 seconds (and then retracted) every 10 min while PHOIBOS is beyond 20 RS and above 50° heliolatitude. This operational sequence is based on a detailed thermal and mechanical analysis by the PHOIBOS Engineering Team. During perihelion passage, inside 20 RS, the periscope will be stowed behind the heat shield. The FOV for the PSRI EUV channel is 3°. This requirement is driven by the need to view the entire polar region below the spacecraft from 20–65 RS. The spatial resolution requirement is driven primar-ily by the need to resolve small-scale EUV struc-tures at the coronal base of polar plumes. This spa-tial resolution can easily be achieved with the cur-rent EUV imaging channel baseline pixel size of 10 arcsec (two pixel resolution = 20 arcsec), corre-sponding to a 2-pixel resolution of ~2 Mm at 20 RS (comparable to a spatial resolution of 2.5 arcsec at 1 AU). This single-pixel angular resolution is derived from the assumption of using a 1024 × 1024 format detector to image the full 3° FOV. A 1-s exposure in the EUV yields a single-pixel signal-to-noise ratio of ~10 for quiet Sun observations and ~30 for bright structures, which is adequate for the science re-quirements. The PSRI magnetograph channel is designed both to probe the overall magnetic structure of the pole and to compare the presence of mixed polarity magnetic structures to the solar wind conditions associated with polar plume footpoints. The magnetograph

channel FOV requirements are identical to those of the EUV channel. The magnetograph channel spa-tial resolution is driven by the need to spatially re-solve small-scale mixed polarity structures (~4 Mm). Because of the differential nature of magnetic flux measurements, the signal-to-noise ratio must be at least 100 in each pixel to achieve quantitative measurements of the magnetic field.

4.2.10 Common Data Processing Unit (CDPU). The CDPU integrates the data processing and low voltage power conversion for all of the payload science instruments into a fully redundant system that eliminates replication, increases redundancy, and reduces overall payload resources. The CDPU provides a unified interface to the payload for the spacecraft. The spacecraft selects which side of the CDPU will be powered, leaving the redundant side off as a cold spare. The payload CDPU communi-cates with the spacecraft over a MIL-STD-1553 bus, accepting commands and producing CCSDS pack-ets ready for final processing by the spacecraft for telemetry to the ground. The current heritage and Technology Readiness Level (TRL) and critical issues for the payload are discussed in section 7. The proposed payload pro-curement approach is discussed in section 8.

5) Spacecraft description The spacecraft is constituted by the Phoibos Trans-fer Module (PTM) and by the Phoibos Solar Probe (PSP), both integrated into the Phoibos Composite Spacecraft (PCS) at launch and during transfer. The mission will commence in December 2018 with the launch of the PCS on an Ariane 5. After a long in-terplanetary cruise phase, during which the PCS is powered by the transfer module (PTM), the probe (PSP) will be delivered to its final solar high ellipti-cal orbit in June 2027.

5.1) Spacecraft architecture

Composite spacecraft architecture

The Phoibos Composite Spacecraft (PCS) is com-posed of the Solar probe, mounted on top of the transfer module, featuring four solar arrays of Rosetta size, mounted by pair on two Solar Array Drive Mechanisms (SADM), and 5 or 6 plasmic engines (see discussion below). Figure 5.1 shows the PCS in launch configuration.

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Figure 5.1 : Phoibos Composite Spacecraft in launch configuration After the launch phase and the early operations, the solar arrays are deployed and orientated so that they are perpendicular to the orbit, facing the Sun. The spacecraft body lies therefore in the orbit plane and can rotate freely thanks to the SADM around the orbit normal direction to orient the plasmic engines during propulsive arcs. Figure 5.2 shows the PCS in deployed configuration during the cruise phase. The control of the PCS is ensured by the Probe data management system and AOCS sensors, but using dedicated actuators (wheels and hydrazine system). The solar panels are orientated with null incidence with respect to Sun for the approach at 0.26 AU during the cruise (after the second propulsive arc), and a dedicated protection system (small heat shield on the side of each panel) will be accommodated. There is no propulsion required below 0.85 AU.

Figure 5.2 : Phoibos Composite Spacecraft in deployed configuration during cruise After the final orbit acquisition, the PSP and the PTM are separated and the Phoibos Solar Probe continues its journey on a ballistic orbit towards the Sun.

Figure 5.3 : Separation of the Phoibos Solar Probe from the PCS.

Phoibos solar probe architecture

The Phoibos Solar Probe (PSP) spacecraft is com-posed of a conical Heat Shield and of an orbiter including the science payloads and the platform. The PSP is separated from the transfer module as soon as the science target orbit is reached. The probe includes 3 solar panels of 1.36 m² each that can be folded behind the heat shield during perihelion passage, using the one axis orientation mechanism on each panel.

The heat shield

The heat shield is based on the strong Solar Orbiter heritage, and from the NASA STDT/2005 study. Promising related tests are also currently being car-ried out at the Odeillo Solar furnace facility in France. These tests are intended to study C/C com-posites behaviours under high temperature, ion and UV irradiations (Paulmier et al., 2001; Eck, 2007). The heat shield has a conical shape with a 15 deg half-cone angle and is built with C/C structure. De-pending on the acceptable conduction in the struts supporting the shield, basically two options are pos-sible regarding the coating of the shield. A white coating will result in an α/ε around 0.6 and a cooler shield. Bare C/C will result in an α/ε around 1 and a hotter shield. For instance, the thermal load received by the heat shield at perihelion being around 21 MW, the heat shield external layer temperature will

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be around 1800°C in the case of white coating. Note however that two problems may occur with the white coating. Firstly the value of α/ε around 0.6 should be measured at higher temperatures is order to check whether it is constant or no. Secondly, the problem with the white coating is that possible cracks can occur and also delamination due to the different coefficients of thermal expansion of the materials. Moreover if the temperature is higher, the

white coating can melt leading to very important damage of the structure. The heat shield is separated in two parts: (i) the conical part and (ii) a secondary flat heat shield that closes the cone at its base, using also Solar Orbiter technology. The total thermal input to the bus from the heat shield is around 50 W, and the spacecraft platform is therefore kept at a temperature of 40°C.

Figure 5.4 : Power management beyond 0.2 AU.

Figure 5.5 : Alternative solution for the power management beyond 0.2 AU.

5.2) Key factors for power management The power management is a key aspect of the Phoi-bos mission for both the transfer module and for the solar probe. In the composite configuration, all the power is ensured by the 130 m² surface of solar panels (4 Rosetta panels), providing the required 33

kW equivalent power at 1 AU. MPPT (Maximum Power Point Tracking) are used to limit currents, and the solar panels are coupled for one SADM.

Power in the Probe configuration at Aphelion

In the probe configuration, the power management is ensured by the three deployable 1.36 m² solar

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panels. However, these panels only provide 80 W at the aphelion of the science orbit. The probe is there-fore put in a very low state of activity during the major part of the science orbit, with a low spin sta-bilization around the Sun direction, and the battery is used at regular interval to perform spacecraft control actions & communications. The battery is charged between two actions using the 80 W power, together with spacecraft thermal control.

Power in the Probe configuration at Perihelion

From the Solar Orbiter and Bepi Colombo devel-opments, it appears that the three panels of the probe can only be used up to 250°C, tilted at ~70°, for heliocentric distances down to 0.2 AU. Below this distance, the panels will therefore be stowed behind the shield and a new power system shall relay this standard one. The required power can be generated by thermo-electrical elements and con-verters, accommodated at the secondary heat shield level inside the cone, to take maximum benefit of the heat generated by the heat shield (Figure 5.4). A plate is used as hot source and the secondary heat shield is used as cold source with a calibrated radia-tor. These thermo elements have to be selected pending on the temperature range experienced by the plate. The provided power has been estimated to more than 200 W below 0.1 AU, but the zone be-tween 0.2 AU and 0.1 AU is still a critical part of the mission: this power sub-system of the probe, working below 0.2 AU is therefore one of the key driver of the probe design. An alternative to this system would be to deploy small mirrors behind the shield, reflecting part of the solar light at 4 Rs to-wards the folded solar panels (Figure 5.5). With very high emissive coating and very low absorption (that can be maintained without ageing effect since they would only be in Sun below 0.2 AU), a small mirror would reach ~1200K, below its fusion tem-perature.

5.3) Spacecraft mass budgets

Phoibos Solar Probe mass budget

The Phoibos Solar Probe (PSP) mass budget has been estimated using the outputs of the NASA STDT/2005 study. It takes into account modifica-tions mainly on the power sub-system which no more uses RTG. Table 5.1 shows this mass budget, which has been estimated to 710 kg, including a 30% system margin and a 50 kg payload. Table 5.1 : Mass budget for the Phoibos Solar Probe. payload 50 kg payload accommodation 18 kg Telecoms 21 kg GNC 38 kg

Power 55 kg Thermal protection system 134 kg thermal control 16 kg DH 20 kg Propulsion 26 kg mechanical 108 kg harness 30 kg first total dry 516 kg System margin (30%) 154 kg TOTAL dry 670 kg propellant 40 kg PSP total mass at launch 710 kg

Phoibos Transfer Module mass budget

For the transfer module, the main driver of the com-posite mass is the power sub-system. For classical technology such as the Rosetta solar panels, the 130 m² is estimated to 460 kg. Adding the MPPT, SADM, Power harness, battery and regulation, a total mass of 640 kg can be found. Relying on po-tential future technologies such as Milard technol-ogy or deployable structure technology, both at ESA R&T level, a less efficient solar panel (330 m² need) but lighter one could be set-up, with a total of 345 kg for the power sub-system. The Power sub-system is therefore estimated somewhere between 345 kg and 640 kg. The needs for the plasmic propulsion are the follow-ing: Far from the Sun, two engines in series are needed for time life reasons. Closer to the Sun, two engines in parallel are used to optimize the trajec-tory taking advantage of the high power capability. At intermediate distances from the Sun, one engine plus very partially a second one in series are needed to guarantee the duration of the thrusting. Therefore for reliability reasons, two cases are considered. They are displayed in Figure 5.6.

Figure 5.6 : Minimal (top) and high (bottom) configura-tions for the plasmic propulsion module. The minimal configuration with no additional re-dundancy consists of five PPS5000 family engines, four of them being mounted by pair on two Thrust Orientation Mechanisms (TOM) and one being

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fixed. The high configuration with additional re-dundancies consists of six PPS5000 mounted by pair on three TOMs. The total propulsion system includes also Xenon tanks which represents 8.5% of the Xenon mass and a small hydrazine system for wheels off-loading and swing-by fine adjustments if needed. The PTM mass budget is given in Table 5.2 where the heavy and light versions of both the power sub-systems and the plasmic propulsion modules are summarized. It can be noticed that the structure mass is taken at 20% of the total dry mass before system margins. Table 5.2: Mass budget for the Phoibos Transfer Mod-ule. Heavy

version Light version

Power system 640 kg 345 kg Propulsion 380 kg 330 kg Attitude Control system 30 kg 30 kg Structure (20%) 260 kg 175 kg Sub-total 1310 kg 880 kg System Margin (30%) 390 kg 260 kg Total dry mass 1700 kg 1140 kg As a result, Table 5.3 summarizes to total mass budget for the Phoibos Composite Spacecraft. Table 5.3 : Total Mass budget for the Phoibos Composite Spacecraft Heavy

version Light version

Phoibos Solar Probe 710 kg 710 kg Phoibos Transfer Module 1700 kg 1140 kg Total end of cruise 2410 kg 1850 kg Xenon for cruise 2291 kg 1756 kg Total at launch 4701 kg 3606 kg The "total end of cruise" values have to be com-pared with the 2051kg from the flight dynamics assumptions. The "total at launch” values have to be compared with the 4000 kg from the flight dynam-ics assumptions. Table 5.3 shows the two bounds of the area of solutions. Even if the heavier solution is not fully compliant with the flight dynamics as-sumptions, this table shows that intermediate solu-tions can be found to validate the mission feasibil-ity. The Technology Readiness Levels for the spacecraft sub-stems are discussed in section 7.

6) Science Operations and Archiving

The Science operations architecture and share of responsibilities need to be discussed after the agree-ment between ESA and NASA. Archive approach: Instrument teams will be coor-dinated by a European and American PI who shall be responsible for providing the Science data (Level 2 products) from the engineering data (Level 1 products), made from the raw data (Level 0 prod-ucts). The production of science data will be funded by National Agencies. The data should be stored in a long-term archive data center at NASA and ESA (ESAC). Proprietary data policy: There shall not be any proprietary data rights for the instrument teams. The Level 2 data products should be freely available to the science community as soon as made available by the instrument teams. Such data policy is now common amongst most of the solar and heliospheric missions.

7) TRL & Key technology areas

7.1) Spacecraft TRL & Technology areas The following critical technologies are all at R&D stage (TRL 2-4): the advance option of power sub-system for the Transfer module (solar arrays) and the power subsystem for the Solar probe close to the Sun (thermo-elements). The conical heat shield is of prime importance for the mission. On the European side there is heritage from the Solar Orbiter studies (TRL 5) and relevant studies are carried out at the French Odeillo Solar Furnace facilities (Paulmier et al., 2001; Eck, 2007). On the US side, extensives studies have been car-ried out by both JPL & APL (NASA STDT/2005). Concerning the propulsion module, a PPS5000 pro-totype (called X1000) has passed 1000 functioning hours on a SNECMA bench. The other parts of the spacecraft can reuse standard technology.

7.2) Payload TRL & Technology areas For most of the instruments, there is a concept that has already been flown in a space environment (Ulysses, SOHO, Wind, ACE, Cluster, Stereo). At a conceptual level, the payload is TRL 9. However, because of the important thermal con-straints, the limited space for accommodation be-hind the shield and the observational configuration at perihelion a whole Technology Development

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Plan (TDP) has to be set-up from the beginning of the pre-implementation phase in order to optimize the quality of both the in-situ and remote-sensing observations. For instance this TDP should explore the actual possibilities for Nadir viewing for both plasma and energetic particle measurements (at least crude approach using possibly high temperature detectors. It should also explore the possibility for the RPWI electric field antennas to be in sunlight and accommodate both DC and high frequency measurements. Finally miniaturization of electron-ics and sensor heads is also a desired feature.

8) International partnership

8.1) International partnership The international partnership and sharing of respon-sibilities will be discussed between ESA and NASA. In section 8.3 the overall mission cost is estimated to be around 900 Meuros. We propose therefore to implement PHOIBOS as an L mission with a cost sharing that could range between 1/2 to 2/3 for ESA & member states and consequently between 1/2 to 1/3 for NASA. In such a situation, the PHOIBOS launch could occur in December 2018, as discussed in section 3, or every 1.6 years later following the mission launch window. As an alternative option, in the case there were an opportunity for an earlier launch and an ESA/NASA agreement, PHOIBOS could be envisage as an M mission with a cost sharing of 1/3 for ESA & mem-ber states and 2/3 for NASA. The launch could oc-cur in this case in October 2015 or May 2017. Concerning the mission management structure needs to be discussed once the share of responsibili-ties between ESA and NASA has been defined. Project scientists and projects managers from both agencies will be involved in the structure. Instru-ment teams will be coordinated by a European and an American PI.

9) Communications and Outreach PHOIBOS' perihelion passage - the first spacecraft to fly within 4 stellar radii of a star – will be such a unique, exciting, and important scientific opportu-nity for the solar, space physics and astrophysics communities that all relevant national and interna-tional scientific assets should be dedicated to pro-viding supporting observations in a coordinated perihelion pass campaign. This campaign will be part of a more general education and outreach com-ponent, which we envision as an “All eyes on our

star” series of public lectures, informal science pro-grams for science centers and museum visitors. This educational effort will focus on magnetism on the sun and its connection to the corona, the solar wind and the influence on the Earth's magnetic field, leading to the questions PHOIBOS scientists hope to answer. An outreach web site will be developed with access to the PHOIBOS orbit and science, remote sensing images from current spacecraft and ground based solar telescopes, providing also for the active participation of amateur solar astronomers from around the world. The solar encounter itself will be treated as a climactic event, analogous to planetary conjunctions or comet approaches to the sun. Indeed, PHOIBOS is a man-made sun-grazing comet: and we plan to develop school challenges at primary and secondary school levels for a logo and story-line leading up to the first perihelion pass.

Acknowledgements : The overall mission scenario presented in this pro-posal is the outcome of a specific CNES/PASO study1 conducted by Régis Bertrand for the mission profile and trajectory, Emmanuel Hinglais for the system analysis and Jean-Yves Prado for the overall concept. This study has been performed in the frame of the CNES support to the French scientific con-tributors to the ESA “Cosmic Vision” call. The spacecraft accomodation has been provided by EADS/Astrium. We thank Ivan Juiz for the realiza-tion of the cover figure. 1DCT/PO/PA/2007-0011299, Solution technique pour la proposition Phoibos à Cosmic vision, E. Hinglais (in english), 2007.

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ANNEXE: SUPPORT LETTERS

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