D. M. RUST

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Solar Physics at APL

David M. Rust

olar research at APL aims to understand the fundamental physics that govern solaractivity. The tools are telescopes, models, and interplanetary sampling of solar ejecta.The work is relevant to APL’s mission because solar energetic protons disable satellitesand endanger astronauts. Solar activity also causes geomagnetic storms, which can leadto communications disruptions, electric power network problems, satellite orbit shiftsand, sometimes, satellite failure. Predicting storm conditions requires understandingsolar magnetism and its fluctuations. APL scientists have made major contributions tosolar activity research and have taken the lead in developing a variety of new solarresearch tools. They are now starting work on the Solar Terrestrial RelationsObservatory, a major space mission. (Keywords: Magnetic storms, Solar flares, Solarmagnetism, Solar physics, Solar telescopes.)

INTRODUCTIONAt a distance of 150-million kilometers from Earth,

the Sun seems far removed from the earthly mattersthat usually concern APL technologists. Nevertheless,the military and civilian sectors of society have bothcome to rely heavily on space systems for communica-tions, navigation, surveillance, and natural resourcemanagement. As this reliance has grown, the impor-tance of understanding solar disturbances and theireffects on space systems has increased concomitantly.Solar events with potential terrestrial consequencescan happen at any time; however, their frequency peaksabout every 11 years. Near each peak—1969, 1980,1990—have come well-publicized and not-so-well-pub-licized satellite system failures. We can expect moreproblems during the current peak.

One of the earliest recognized hazards to sensitivespacecraft electronics is the proton shower, which is a

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hail of 50-MeV protons launched in a solar flare. Flarestrace their origins to the magnetic strands that arestretched and tangled by the roiling plasma of theunexplored solar interior. In the 1960s and 1970s, APLscientists measured solar protons with Earth-orbitingsatellites and tried to predict their strength from solarflare reports, but by the mid-1970s, it had become clearthat solar flares had to be understood much betterbefore any meaningful proton shower predictions couldbe made. NASA’s Solar Maximum Mission, known asSolar Max (1980–1989), carried a cluster of telescopesthat gave promise of showing where and when protonswere released. As a result, Space Department HeadGeorge C. Weiffenbach and Chief Scientist StamatiosM. Krimigis decided in 1983 to add solar flare researchto the specialties of the Space Physics Group, to try todevelop a capability for remotely sensing the solar

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activity that leads to proton showers and other spacedisturbances.

In this article, we review how the Laboratory’s SpaceDepartment built its capability in solar research andhow APL scientists contributed to the present under-standing of solar flares. The following section reviewscontributions to analyses of X-ray images of flares. Thenext section chronicles how APL efforts have focusedon understanding the fundamental physics of solarflares. We have developed theories and telescopes toshow how the magnetic fields in sunspots and in thesolar atmosphere destabilize and erupt into solar flaresand solar mass ejections. Many solar mass ejectionscontain strong magnetic fields in the form of magneticclouds. We then discuss APL’s role in the developmentof magnetograph technology. The subsequent sectionexplores how magnetic clouds are ejected from the Sunand how they affect Earth. APL scientists helped showhow telescopic measurements of solar eruptions canforecast the direction of the magnetic fields in theejected clouds. Finally, we look to the future, to theSolar Terrestrial Relations Observatory (STEREO) inparticular, with its cluster of telescopes and plasmasensors. STEREO will be launched in 2003 or 2004.Although it is a research mission, it will strengthenreal-time space weather forecasts, and it may arrive intime to reduce the risk that astronauts constructing theInternational Space Station will encounter a protonshower.

SOLAR FLARES AND X-RAYTELESCOPES

The origins of proton showers must be sought in theupper reaches of the solar atmosphere called the coro-na. Strongly accelerated protons could never escape theSun if they had to penetrate the lower, denser atmo-sphere, or chromosphere. The Skylab mission (1973–1974) carried two telescopes that could focus the X-rayemission from the corona (Fig. 1), which has a temper-ature of over 1 million degrees. APL scientists studiedthe Skylab X-ray images, which showed the hot mag-netic arches in the corona above sunspots, but foundthat they provided little detail about the hottest coresof flares. These 100-million-degree features, wethought, could be the sites of proton acceleration.

Images obtained with the Hard X-ray Imaging Spec-trometer (HXIS) aboard the Solar Max1 provided abetter opportunity for finding the acceleration sites.HXIS photographed the 100-million-degree flare plas-mas every 1.5 s. It showed that flares are like twouninsulated high-voltage electric cables that have beenbrought too close together. Where they collide, theplasma is heated rapidly. However, instead of expandingin a shock, as one might expect of such a rapidly heatedgas, the high-temperature flare kernels hardly moved at

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Figure 1. The solar corona as seen from Skylab on 31 May 1974.The bright areas show where X rays are emitted from 1-million-degree ionized plasmas trapped in magnetic fields.

all. This presented a problem if flares were to be thesource of proton showers. The only way we and mostothers could imagine accelerating protons was to trapthem between a shock front and the ambient atmo-sphere. We had to conclude from the HXIS images thatthe magnetic fields in X-ray flares were too strong toallow the gases to break out and form shocks.

The Solar Max telescopes helped us to build a three-dimensional model of a flare, but they did not helpmuch in understanding proton showers. Here, it isimportant to recognize that although our goal is tounderstand and forecast space weather, the path thereis not straight and cannot be forced. When new instru-ments or insights open interesting new lines of inquiry,we, like scientists at other universities and major re-search institutions, are encouraged to pursue them.Thus, we studied the images from the Solar Max tele-scopes and showed that an X-ray-emitting front doessometimes develop in flare loops. Strong magneticfields constrain the fronts, which can only rush fromthe flare site to the points where the magnetic loopsintersect the solar surface. The fronts propagate atabout 1000 km/s, and they carry away most of thethermal energy developed in the flares. We concludedthat the fronts are evidence for electron thermal con-duction. If this interpretation is correct, then the con-duction fronts seen in the HXIS images would be thefirst to have been detected in an astrophysical context.

Solar Max did not carry the sort of full Sun-viewingX-ray telescope that Skylab did, and in the mid-1980sit was recognized that if proton-producing flares were

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to be understood, then a Skylab-type telescope wasneeded. APL scientists worked with their counterpartsfrom other institutions to develop a flare researchmission that would combine the Skylab-type soft X-rayimager, which sees the 1-million-degree ambient solaratmosphere, with a Solar Max–type hard X-ray imager,which sees the 100-million-degree flares. We took ourideas to the Japanese Space Agency because NASAcould not afford a solar research satellite in the budget-lean years of the 1980s. From these discussions camethe immensely successful Yohkoh spacecraft, which waslaunched in 1991 and continues with successful obser-vations today.

The Yohkoh images seen in Fig. 2 are much sharperand more plentiful than those obtained with the Skylabtelescopes. The Yohkoh has acquired millions of imagescompared to only a few thousand from Skylab. Itsimpact on flare research has been impressive,2 and APLscientists have contributed to the scientific accom-plishments of the Yohkoh Soft X-ray Telescope. One ofthe most exciting discoveries was that a type of mag-netic instability well known in laboratory researchapparently takes place during the onset phase in solarflares. This helical kink instability is similar to theinstability or sudden formation of knots that occurs ina rubber band when it is twisted too much.3

Most flare events in the Soft X-ray Telescope imagesevolved from a bright, sharp-edge sigmoid (S-shaped)feature into either an arcade of flare loops or a diffuse

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Figure 2. Representative X-ray images of the Sun, at approxi-mately 6-month intervals since 1991 (time runs left to right). Thecorona is much brighter and filled with detail near the peak of solaractivity in 1991. (Figure courtesy of Lockheed, National Astronomi-cal Observatory of Japan, University of Tokyo, NASA, and ISA.)

cloud. A spectacular example of evolution into a classicloop arcade is shown in Fig. 3. We studied 103 suchtransient, bright sigmoid structures and compared theproportions of 49 of the best-defined ones with the sig-nature of a helical kink instability. The sigmoid struc-tures had an average width of 48,000 km and an averagelength of 200,000 km. For comparison with the kinkinstability, we prepared a histogram of the length-to-width ratio. The histogram has a peak near 5.0 with a

Figure 3. Development of a flare in the solar X-ray corona. The arrow in the leftmost image points out the reverse-S feature that signalsthe onset. In the middle image, which was made with a shorter exposure time, the arrow shows the beginnings of the transformation intoa flare loop arcade. In the final image (right), the arcade is fully developed and very bright. Two black curves were drawn on the image tohelp show the shape of the loops. The white arcs in each panel outline the solar limb.

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slow drop-off toward higher values and a rapid drop-offtoward lower ones. This distribution is consistent withwhat would be expected if most sigmoid features archupward or downward more or less in the line of sight.

We concluded that the length-to-width ratios andthe shapes of the bright features are consistent withhelically kinked magnetic flux ropes. A magnetic fluxrope is a nearly self-contained magnetic field whoselines of force somewhat resemble the strands in a rope.Whereas stability depends on many conditions, thosewith more than a 360° twist are usually unstable. Al-though there are no generally accepted models of howthe instability grows into an eruption, it is likely thatthe twisted magnetic fields inside the flux rope recon-nect and release heat as the rope kinks and stretches.

Previous research had determined flare temperature,density, heating and cooling rate, etc., but never beforewas a distinctive and quantifiable signature of a specificinstability discovered. Now finally, from this analysis ofX-ray images, we have a physical explanation and hardevidence for the kind of solar eruption that mightbecome a shock in the corona and that might, there-fore, have something to do with proton showers. Par-allel work at the Laboratory and other institutions wasalso building a case for focusing on the twisted magneticflux ropes.

APL scientists usually send such research results toThe Astrophysical Journal or the Journal of GeophysicalResearch. That route gets the word out in the long run;however, to speed up the exchange of research results,we organize meetings and lecture at other institutions.For example, when the importance of the twisted fluxtubes became evident, we organized a special session ata meeting of the American Geophysical Union toexplore such helicity in various contexts, in the labo-ratory and in space.

THE CENTRAL ROLE OF SOLARMAGNETIC FIELDS

While images can help us understand quite a lotabout the magnetic fields confining flares, we are un-likely to be able to predict when a flare will occurwithout actually knowing the magnetic field strength.APL scientists had been measuring magnetosphericmagnetic fields since the Transit Program (see the ar-ticle by Williams et al., this issue), but until 1984 therewas no program to remotely sense magnetic fields onthe surface of the Sun. By the mid-1980s it was widelyrecognized that a solar magnetograph program had tobe established, even though it would involve a verydifficult instrumentation development effort. New in-struments, probably using new technology, would beneeded. Consequently, we mounted a small, amateurtelescope on the roof of Building 2 to test new instru-ment ideas. We began development of a high-speed

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imaging system by connecting a video camera attachedto the telescope to the McClure Computing Center inBuilding 3. Those were the days when image processingwas done at considerable expense at central computerfacilities. However, the McClure management wasinterested enough in the project to grant us some freeuse of their facilities.

On the basis of what we accomplished, the Air ForceOffice of Scientific Research selected APL to establishthe Center for Applied Solar Research. The first taskwas to build a solar vector magnetograph, which is aspecialized telescope for three-dimensional mapping ofthe fields in the Sun’s surface. Specifically, our goal wasto measure all three components of the magnetic fieldwith a sensitivity of 0.005 T and a spatial resolution onthe Sun of 500 km. Such specifications were beyond thereach of existing instruments; nonetheless, we hopedthat by using new optical devices, such as ultra-narrowpassband optical filters, liquid-crystal polarization mod-ulators, and a solid-state camera based on charge-coupled device technology, we could break throughthe barriers that confronted the older generation ofinstruments.

So far, we have only partially succeeded. We builta vector magnetograph (Fig. 4) and installed it on a

Figure 4. The APL solar vector magnetograph installed at theNational Solar Observatory in Sunspot, New Mexico.

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mountain ridge at the Sacramento Peak Observatoryoverlooking Alamogordo, New Mexico.4 It was hopedthat there, at an elevation of 9300 ft, the “seeing”(image jitter due to turbulence in the Earth’s atmo-sphere) would be much less than at lower-elevationobservatories. This turned out not to be the case, so thefull resolving power of the telescope was not realized.

Many measurements of the magnetic fields in flare-prone groups of sunspots were made, though, and in onecase we obtained observations before and after a majorflare. Observations of a flare on 2 April 1991 appearedto confirm earlier evidence that eruptive flares weretriggered by measurable magnetic field changes. In the8 hours before the flare, the electric currents flowingparallel to the magnetic fields increased in intensity.The development that likely triggered the flare was theemergence of new magnetic flux (Fig. 5) into the sun-spot region. Data from our vector magnetograph alsoshowed that the motion of the newly emerged spots ledto the development of stronger electric currents. Theflare started near the newly energized fields and spreadto engulf most of the sunspot region. A magnetogramtaken 45 min after flare onset showed possible relax-ation of the sheared fields.

The APL observations appeared to confirm a model5

of solar flare magnetic fields put forward in 1977 by JeanHeyvaerts of the Paris Observatory, Eric Priest of St.Andrews University in Scotland, and the author, thenat American Science and Engineering, Inc., in Cam-bridge, Massachusetts. The idea is that sheared ortwisted magnetic fields in the atmosphere above sun-spots become unstable and erupt when new magneticflux (usually also detectable as sunspots) emerges. Withobservations tending to confirm this 20-year-old two-dimensional model and with much better insight nowinto the physics of flux ropes, current flare research isfocused on making more realistic, three-dimensionalmodels of the effects of emerging flux.

The work on flares illustrates how observations andtheory are intertwined. Although the emphasis at APLis on observations and measurement of the space en-vironment, the context provided by theory allows us tointegrate our experiences and finally understand howphysical forces and energies operate in space.

IMPROVEMENTS INMAGNETOGRAPHS

The primary objective for APL’s first magnetographwas to test new components, particularly those thatcould lead to a lightweight magnetograph in space.Most magnetographs, which require a wavelength se-lection device, had been built around either a spec-trograph or a birefringent filter. Solar spectrographs forresearch at visible wavelengths are simply too large forpractical space missions. Birefringent filters are proven

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instruments, but they are relatively heavy. They havebeen flown only on the Space Shuttle and on the Solarand Heliospheric Observatory (SOHO), which is amuch larger spacecraft than can be considered seriouslyfor today’s solar missions such as STEREO.

APL and the Australian National Measurement Lab-oratory developed a lightweight optical filter6 capable ofisolating the wavelengths in the solar spectrum that areused in measuring the magnetic fields on the visible

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Figure 5. Photograph (top) and maps of the magnetic fields(bottom) of two spots, A and B, that emerged into the group of largerspots and triggered a major solar flare. The magnetic gradientbetween spots A and C steepened as spot A emerged and grew.Similar increases have been noted by others in other sunspotgroups and have been identified with increased flare productivity.

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surface. The filter, a Fabry-Perot etalon made of lithiumniobate, is rugged and much lighter than any birefringentfilter. However, the technology was quite new in 1987when APL proposed it for the SOHO mission; conse-quently, NASA selected a magnetograph based on theestablished technology of birefringent filters.

Lithium-niobate etalons are now in use in severalobservatories around the world. APL scientists used onein the magnetograph in New Mexico and in the StableSolar Analyzer, which measured solar oscillations.7 Wealso worked with the Space Department’s FrequencyStandards Group to demonstrate that the filter couldbe combined with a tunable laser and an atomic ref-erence cell to provide ultrastable solar wavelengthmeasurements.6 Here is an example of the synergy thatoften develops in the Space Department, where manydiverse high-technology skills are available.

APL scientists have continued to develop magneto-graphs for use in space. Having concluded that spaceis the only reasonable venue for a magnetograph, weworked with other interested scientists to develop theOrbiting Solar Laboratory (OSL) for NASA. The cen-terpiece of the OSL would be a large vector magneto-graph. We served on numerous committees formed torefine and promote this mission.

When another NASA budget crisis doomed theOSL in 1991, we started work on the Flare GenesisExperiment (FGE), a balloon-borne solar telescope.8

The FGE would incorporate all the advanced technol-ogy we had developed for a space mission and demon-strate it in the near-space conditions provided by aballoon flight at a 37-km altitude.

On the face of it, the FGE seems a retreat to oldtechnology, because a balloon in-stead of a rocket carries it above theatmosphere. But ballooning inAntarctica is new. It even offersspecial advantages over most satel-lites, because the Sun can be stud-ied without interruption during theAustral summer for periods of usu-ally about 14 days. Twenty-eight-day flights are possible. The firstFGE flight lasted 18 days. Anotherflight is planned for December1999.

The principal science goal ofFGE is to understand how the fi-brous magnetic fields at the solarsurface emerge, coalesce, unravel,and erupt in solar flares. The prin-cipal technology goal is to demon-strate that new APL systems forimage stabilization, wavelengthisolation, polarization analysis,and onboard control can provide

Figure 6. The Flare The solar telescope i1300 W of power, m

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reliable observations in a space environment. Thesesystems must maintain the desired resolution of �0.2arcsec (corresponding to only 140 km on the solarsurface) despite pendulation and wind buffeting of theballoon and its payload.

The FGE benefited from the end of the Cold War.Just as NASA was deciding that it could not afford theOSL, the Strategic Defense Initiative Organization(SDIO) encountered technical difficulties with Starlab,a project to illuminate enemy satellites with laser lightfrom a large telescope in the Space Shuttle bay. SDIOasked APL Space Department scientists how best to usethe valuable Starlab telescope. We showed that withminor modifications it would be ideal for solar researchand, as a result, it was transferred to the FGE Project.

The primary mirror of the Starlab telescope is 80 cmin diameter, making it one of the world’s largest mirrorsfor solar research. Figure 6 shows the telescope in theballoon gondola in Antarctica. The Starlab telescopebody is made of graphite-epoxy fiber, and the mirrors areultra-low-expansion glass. A large solar telescope forspace would probably use the same approach to minimizedimensional changes as the telescope orbits into and outof the sunlight. The other key high-technology elementsof the FGE are the single-crystal silicon secondary mirror,liquid-crystal polarization modulators, the tunable lith-ium-niobate optical filter, an image motion compensator,and a fast electronic camera.

The first FGE flight lasted from 7 to 26 January1996—one of the longest balloon flights yet in Antarc-tica. The telescope remained locked on the Sun for theentire flight. The data collection system obtained over14,000 images. Although a number of problems limited

Genesis Experiment ready for launch in Antarctica, 7 January 1996.s wrapped in a white thermal control blanket. The solar panels supplyost of which is needed to drive the pointing motors.

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the resolution on the first flight to �1 arcsec, the majorFGE systems performed as expected.

The problems on the first flight have been studiedand corrected. High resolution, great stability duringsolar observations, and long runs without interruptionscannot be achieved with any other existing instrument.With these advantages, we expect the next flight of theFGE, scheduled for December 1999, to reveal key fea-tures of the mechanisms of magnetic energy buildupand release in solar flares. The FGE is uniquely capableof observing these processes, and solar activity shouldbe at its peak during the flight.

A NEW TWIST ON SOLARMAGNETISM

Solar research at APL took an unforeseen twist in1993 when Sara Martin at the California Institute ofTechnology and Norbert Seehafer at the University ofPotsdam, Germany, found a global organization in thesolar surface magnetic fields. Magnetic flux ropes canbe twisted in either the right-handed or left-handedsense, depending on whether their electric currentsflow parallel or antiparallel to the magnetic field.What Martin and Seehafer found was a tendency forflux ropes in the Sun’s northern hemisphere to haveleft-handed twists. Those in the Sun’s southern hemi-sphere tended to have right-handed twists. What ifthe magnetic fields in the ejected clouds of materialalso reflect this pattern? It might be a key to under-standing how the kink instability works on the Sun.Using data from many satellites, we were able to showthat when a left-handed magnetic flux rope was eject-ed toward Earth, then the cloud’s field, as measuredby near-Earth spacecraft such as the Advanced Com-position Explorer (ACE) and others, would also beleft-handed.

The magnetic cloud phenomenon is an interestingand practical demonstration of the principle of magnet-ic helicity conservation.9 If left-handed fields emerge inthe north and form sunspots, then shouldn’t the fluxropes that spring from them be left-handed? Shouldn’tthe coronal magnetic arcades be left-handed? Finally,shouldn’t the fields ejected from the northern hemi-sphere be left-handed? We found that more than 80%of magnetic clouds have the chirality (handedness)predicted by the hemispherical segregation rule. Thepicture that emerged is one of helicity buildup in thesolar atmosphere by flux rope mergers and helicitytransfer from the submerged flux rope parts.10 When themagnetic helicity becomes too large for the confines ofthe corona, we get a helical kink instability and con-sequent flux rope eruption. Measurements with ACE,the Near Earth Asteroid Rendezvous (NEAR), andother spacecraft verify that the helicity in the flux ropeis the same as when it leaves the Sun.

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The seemingly academic work on helical magneticfields will have a significant effect on space weatherforecasting. Geomagnetic storms are major spaceweather disturbances. In these storms the Earth’s mag-netosphere and ionosphere change suddenly and strongcurrents surge through the ground, the upper atmo-sphere heats up, and charged particles accelerate in theVan Allen belts. These phenomena lead, in turn, tocommunications disruptions, electric power networkproblems, satellite orbit shifts, and possibly satellitefailure.

The intensity of geomagnetic storms depends,among other things, on the direction of the fields in theclouds that leave the Sun. We can improve forecasts ofindividual storms because the NASA/ESA (EuropeanSpace Agency) SOHO mission and other facilitiesallow us to see the erupting features better and tocorrectly infer their magnetic field structure on theSun. We can predict their pattern of impact at Earthby applying helicity conservation. Moreover, because ofthe global patterns of magnetic fields on the Sun, wecan predict that geomagnetic storms during the presentsolar cycle, 1995–2006, will be more intense than in theprevious cycle. Thus we can make a prediction aboutthe climate in space for this decade.

Why are the helical fields segregated north andsouth? Could the observed pattern change with time?Recent magnetographic research has found that quan-titative changes do occur during the solar cycle. How-ever, a review of old pictures and research has shownthat in each of the past seven solar cycles (about 77years) the magnetic fields remained predominantlyright-handed in the south and left-handed in the north.The classical Coriolis force produces a similar unchang-ing hemispherical segregation in terrestrial cyclones,but the helicity on the Sun does not follow the patternsthat we would expect from the Coriolis force. Instead,the helicity pattern is consistent with a model (Fig. 7)in which the internal field is wound up by rotatingplasmas inside the Sun. The differential solar rotationwill produce left-handed twist in the northern fieldsand right-handed twist in the southern ones for all solarcycles, of course, because the direction of the Sun’srotation never changes.

The model in Fig. 7 is not much better than aninspired sketch, but more sophisticated solar dynamomodels also produce opposite magnetic helicities northand south of the equator. Even these computer modelsleave many important features out, so we really haveno consistent theory of how solar magnetic fields aregenerated and how they destabilize every 11 years. Tomake the next major improvement in understandingthe Sun and space weather, we need a global view ofsolar magnetism. With the STEREO mission, de-scribed in the next section, we are setting out to getthat global view.

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Figure 7. Cross section of the Sun showing one concept of howmagnetic lines of force at the boundary between the interior(radiative zone) and the convective zone twist and rise to thesurface, where they appear as sunspots. Near the equator, mag-netic fields running from north to south will become twisted be-cause the gases in the convective zone rotate faster than those inthe radiative zone. The effect would produce clockwise (CW) twistin the fields in the south and counterclockwise (CCW) twist in thenorth, as is actually observed. Alternatively, Coriolis forces maytwist the fields. The twisted fields will accumulate in the solaratmosphere above the convective zone and then erupt and leavethe Sun as coronal mass ejections (Fig. 8).

THE FUTURE OF SOLAR RESEARCHAT APL

It took a decade from the Space Department’s de-cision to add solar research with telescopes to APL’sestablished strengths in solar particle detection, but itpaid off. Only close familiarity with both types ofmeasurements could have produced our improved un-derstanding of solar magnetism and its application inforecasting space weather.

Although it is impossible to foresee what new ideaswill emerge, we expect that solar activity and its effectswill be a central factor in our work. Last year, APLscientists played a major role in formulating NASA’sSun–Earth Connections Program. Its aims are toimprove mankind’s understanding of the origins of solarvariability: how that variability transforms the inter-planetary medium, how eruptive events on the Sunimpact geospace, and how they might affect climateand weather.

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NASA has just asked us to study how magnetic fluxropes evolve from their departure at the Sun to theirarrival at Earth orbit. The principal data sources are tobe solar images (Fig. 8) from the SOHO telescopes andmagnetometer data (Fig. 9) from APL’s NEAR mission.The SOHO/NEAR constellation has some of the fea-tures of the future STEREO mission (Fig. 10). SOHOsolar telescopes most clearly show eruptions leaving ata 90° angle to the Earth–Sun line. During much of itsflight, the NEAR spacecraft will be in an ideal positionto sample the plasma in these eruptions. Figures 8 and9 provide one example of how telescopic observationsof cloud structure can be used with in situ measurementsto build a more complete picture of magnetic flux ropestructure and its evolution from ejection at the Sun topassage near Earth.

APL scientists played a major role in defining thescience goals and instrument payload of the STEREOmission, and NASA selected APL to provide theSTEREO spacecraft bus and to manage mission oper-ations. STEREO will consist of two spacecraft, oneleading Earth in its orbit and the other lagging. Eachwill carry a cluster of telescopes to study solar activity,and each will carry a magnetometer, a solar wind de-tector, and energetic particle detectors to sample solarejections. Thus, magnetic clouds and other disturban-ces can be tracked from their departure at the Sun totheir arrival at various points at Earth’s orbit.

STEREO will be launched in 2003 or 2004. It is oneof five Solar-Terrestrial Probes called for in NASA’s

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Figure 8. (a) The Sun ejected this bright cloud (right) known as acoronal mass ejection on 13 August 1997 in the direction of theNEAR spacecraft. (b) Edge-enhanced version, which gives a faintindication of a circular feature in this cloud. Many other similarclouds exhibit clear circular arcs that could be signatures of atwisted magnetic flux rope seen end-on.11

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Space Science Enterprise Strategic Plan to accomplishthe goals of the Sun–Earth Connections Program. Theother missions and their likely launch dates are asfollows:

• TIMED, the Thermosphere-Ionosphere-MesosphereEnergetics and Dynamics mission (2000)

• Solar-B, which will obtain high-resolution images ofthe solar magnetic field (2005)

• Magnetospheric Multiscale, which will provide a net-work of in situ measurements of Earth’s magneto-sphere (2006)

• Global Electrodynamics, which will probe Earth’supper atmosphere to determine how variations inparticle flux and solar electromagnetic radiation af-fect it (2008)

Although STEREO will probably dominate our solarresearch efforts in the near future, we are working onother ideas as well: solar neutrinos as probes of the

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Figure 9. NEAR magnetometer record of magnetic field compo-nents, Bx, By, and Bz, for the period 18–23 August 1997. Thedisturbance on 20 August followed the ejection of the cloud shownin Fig. 8 by the expected “time of flight” that we calculated from thecloud velocity and the distance of the NEAR spacecraft fromthe Sun. The expanded view (bottom) of the magnetic field mea-surements shows that the eastward component By (green) ofthe field rotated as one would expect in sampling a twistedmagnetic flux rope.

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Figure 10. An artist’s concept of the two STEREO spacecraftviewing a solar eruption.

internal magnetic fields (some evidence12 show thatneutrinos have a magnetic moment and that the twist-ed magnetic fields modulate their passage through theSun’s interior); Solar Probe, a spacecraft that will fly towithin 4 solar radii of the Sun and sample the coronaand solar wind; a DoD constellation of satellites toimprove space weather forecasts; and telescopes two orthree times bigger than Flare Genesis to give the high-est possible resolution to solar images. These largetelescopes will probably be flown first on balloons andthen possibly on the International Space Station.

Eventually, we hope to develop a mission patternedon STEREO to provide data for routine forecasts onweather throughout the heliosphere. Such a missionwill be needed, particularly some time after 2020 whenman will likely undertake interplanetary travel.

Solar research at APL has been a quest to understandthe origins of energetic solar protons, the causes ofgeomagnetic storms, and the fundamental physics thatgovern solar activity. We will likely continue in thisvein with larger telescopes, more sensitive magneto-graphs, and more detailed computer models. Despiteour efforts and those of hundreds of solar physicistsaround the world, we still cannot agree on even someof the most fundamental issues. We will keep trying tosolve them and benefit the nation’s space enterprise aswell.

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REFERENCES1Rust, D. M., “The Solar Maximum Observatory,” Johns Hopkins APL Tech.Dig. 5(2), 188–196 (1984).

2Bentley, R. D., and Mariska, J. T. (eds.), “Magnetic Reconnection in theSolar Atmosphere, Proceedings of a Yohkoh Conference,” Astron. Soc. PacificConf. Ser. 111 (1996).

3Rust, D. M., and Kumar, A., “Evidence for Helically Kinked Magnetic FluxRopes in Solar Eruptions,” Astrophys. J. Lett. 464, L199–L202 (1996).

4Rust, D. M., O’Byrne, J. W., and Harris, T. J., “An Optical Instrument forMeasuring Solar Magnetism,” Johns Hopkins APL Tech. Dig. 9(4), 349–359(1988).

5Heyvaerts, J., Priest, E. R., and Rust, D. M., “An Emerging Flux Model for theSolar Flare Phenomenon,” Astrophys. J. 216, 123–237 (1977).

6Rust, D. M., Kunski, R., and Cohn, R. F., “Development of Ultrastable Filtersand Lasers for Solar Seismology,” Johns Hopkins APL Tech. Dig. 7(2), 209–216(1986).

7Appourchaux, T., and Rust, D. M., “The Stable Solar Analyzer,” in Proc.Symp. on Seismology of the Sun and Sun-like Stars, European Space Agency, SP-286, 227–234 (1988).

8Rust, D. M., Hayes, J. R., Lohr, D. A., Murphy, G. A., and Strohbehn, K.,“The Flare Genesis Experiment: Studying the Sun from the Stratosphere,”Johns Hopkins APL Tech. Dig. 14(4) 358–369 (1993).

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9Glanz, J., “Does Magnetic Twist Crank Up the Sun’s Outbursts?” Science 269,1517 (Sep 1995).

10Bieber, J. W., and Rust, D. M., “The Escape of Magnetic Flux from the Sun,”Astrophys. J. 453, 911–918 (1995).

11Dere, K. P., Brueckner, G. E., Howard, R. A., Michels, D. J., andDelaboudiniere, J. P., “LASCO and EIT Observations of Helical Structure inCoronal Mass Ejections,” Astrophys. J. 516, 465–474 (1999).

12Rust, D. M., and McNutt, R. L., Jr., “Neutrinos and Helical Magnetic Fieldson the Sun—Results of Observations,” in Proc. Fourth Int. Solar NeutrinoConf., W. Hampel (ed.), Max Planck Inst., Heidelberg, Germany, pp. 380–387 (1997).

ACKNOWLEDGMENTS: The Solar Physics Program at APL has been supportedby NASA’s Solar Physics Program Office, the NSF Polar Aeronomy and Astrophys-ics and Solar Terrestrial Physics Programs, and the Air Force Office of ScientificResearch. Additional support was provided by the APL graduate and postdoctoralfellowship programs and the IR&D programs. At APL, John O’Byrne, GrahamMurphy, Pietro Bernasconi, Ashok Kumar, and Brian Anderson have contributedto the program. In addition, I am grateful for the dedicated support on many projectsof the engineering staffs of the Space Department and the Technical ServicesDepartment.

DAVID M. RUST received his Sc.B. degree in physics from Brown Universityand his Ph.D. in astro-geophysics from the University of Colorado. He joinedAPL in 1983. A member of the Principal Professional Staff since 1985 and SolarPhysics Section Supervisor, Dr. Rust has initiated programs to analyze solar X-rayemissions; build new solar telescopes, optical filters, and detectors; and map solarsurface magnetic fields. He also established the APL Solar Observatory. He haspublished over 100 refereed papers on solar physics. He has also published paperson instruments for solar research. Dr. Rust was granted patents for a vectormagnetograph and an imaging polarimeter. His current interests includedevelopment of a balloon-borne solar telescope and design of the STEREOmission. His e-mail address is david.rust@jhuapl.edu.

THE AUTHOR

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