HERSCHEL & PLANCK missions – L2

Herschel and Planck were launched together on 14th of May 2009and about sixty days later reached orbit around the Sun-Earthsecond Lagrangian point - L2 where they entered different Lissajousorbits in L2, in orbit around the Sun.

The L2 point is very interesting for science as instruments can berelatively easily shielded from sunlight and Earth/moon straylightwith a sunshield covering one side of the spacecraft allowing forvery cool operation on the other exposed side. This minimizesnoise and allows for very precise measurements in multiplewavelengths, like infrared (Herschel) and microwave (Planck).

Both spacecrafts carry on board a Standard Radiation EnvironmentMonitor (SREM) which monitors the particle radiation fromprotons and electrons. This offers valuable data for the analysis ofthe particle radiation environment in L2 which has not been verywell characterized, especially for Solar Proton Events.

HERSCHEL & PLANCK – INTEGRAL Count-rates

We used data taken from three ESA missions, Integral, Herschel,and Planck with SREM modules aboard. We have selectedmeasurements from all the SEP events in the time period of August2010 – March 2013, amounting to more than 140000measurements. This period was selected to compare count-ratesoriginating from SEPs common to all three missions. Below areconcatenated time-series containing all SEPs showing comparisonsbetween Herschel-Integral and Planck-Integral.

The comparisons show that there is a good agreement betweenmeasurements from Herschel/Planck (H/P) and Integral indicatingstrong similarities in the proton fluxes.

HERSCHEL & PLANCK – INTEGRAL Solar Proton Fluxes

We employed two different unfolding methods ANNs and SVD[Sandberg 2012] to produce proton fluxes the from count-rates ofthe considered missions. Following the trend demonstrated incount-rates the proton fluxes show strong similarities between HPand Integral. The two methods produce similar fluxes, especiallyfor energies up to ~120 MeV. Below are shown the flux time-seriesfor the whole Herschel mission unfolded with ANNs at the 10SEPEM energies, results for Planck are very similar.

Below are comparisons between unfolded fluxes from Integral andHerschel at increasing proton energies from ANN and SVD methods

The comparisons show that both methods indicate that protonfluxes from Herschel and Integral are very similar across a widerange of energies, results from Planck are similar. These resultsshow that the radiation environment regarding solar protons in L2

is just as severe as in 1 AU and similar precautions are needed.

Experimental Cumulative Distribution Functions

Using the unfolded fluxes we construct the experimentalcumulative distribution functions (CDF) for Herschel and Planckwith fluxes from both from SVD and ANN. The CDFs are producedby plotting the sorted fluxes for each energy channel over the

probability vector 𝑃 =𝑁+1−𝑖

𝑁, 𝑖 = 1:𝑁, where N is the number of

measurements. Both SVD and ANN methods produce practicallyidentical CDFs for the Herschel and Planck datasets while they alsoshow a quite good agreement between them as shown below.

Flux unfolding with Artificial Neural Networks

Neural Networks are powerful tools that have been used in variousapplications for regression, classification and more. Here, we haveused radial-basis function networks (RBF), a widely used andstudied type of neural networks.

The network consists of layers of neurons (nodes) which areinterconnected by weighted synapses. The input layer linearlyforwards the input and the hidden layer employs non-linearGaussians functions (φN). Finally, the output layer combines theweighted outputs of the hidden layer to produce the final outputs. The networks are trained with virtual datasets consisting of

virtual count-rates and virtual fluxes.The fluxes are derived from analytical exponential-cut off power-law functions 𝑓 𝐸 = 𝑎𝐸𝑏𝑒𝑐𝐸 . The virtual count-rates arecalculated from the virtual fluxes according to the equation:

𝐶𝑖 = 0∞𝑓 𝐸 𝑅𝐹𝑖 𝐸 𝑑𝐸 which gives the counts in the i-th channel

from its response function RFi and the spectrum of the incidentparticle radiation at a given moment in time. The datasets containthousands of virtual fluxes-counts pairs and cover a wide range ofspectral indexes for the analytical functions.

SREM Instrument

The European Space Agency (ESA) Standard Radiation EnvironmentMonitor (SREM) belongs to the 2nd generation of instruments in aprogram which was established by ESA’s European Space Researchand Technology Centre (ESTEC) to provide:• minimum intrusive radiation detectors for space applications• radiation hazard alarm function to instruments on board

spacecrafts• investigation activities on possible radiation related anomalies

observed on spacecraft• in-flight technology demonstration activities

SREM, seen below left, was designed to measure electrons withenergies Ee>500 KeV and protons with energies Ep>10 MeV withfair spectral and angular resolution. The SREM monitor bins themeasurements in 15 counters (channels), the proton responsefunctions are seen right.

SREM units monitor the radiation environment and providesuitable functions related to hazards for the host spacecraft and itspayload. An alarm flag can be set whenever high radiation levelsare reached during the spacecraft's orbit and the payloadinstruments may react accordingly entering, if necessary, in aspecial safe mode that protects them from possible radiationdamages. SREM was co-developed by ESA, the Paul ScherrerInstitute (PSI) for Astrophysics, and Contraves Space A.G. In total,six SREM units have been launched on board, Herschel, PlanckPROBA-I, INTEGRAL, ROSETTA, GIOVE-B. While Herschel and Planckare in L2, INTEGRAL is in a Highly Elliptical Orbit (HEO) orbit whichcrosses the Radiation Belts regularly. Here we use data from themonitors on board these three spacecrafts.

Characterization of the L2 radiation environment using ESA SREM measurements

S. Aminalragia-Giamini1,2, S. Raptis3, I. Sandberg1, A. Anastasiadis2, C. Papadimitriou1, I. A. Daglis3, and P. Nieminen4

1Space Applications & Research Consultancy (SPARC), Greece2National Observatory of Athens, Greece

3National & Kapodistrian University of Athens, Department of Physics, Greece4European Research and Technology Centre, ESA, Noordwijk, The Netherlands

Introduction The radiation environment of the second Sun-Earth Lagrange point (L2) is of high interest for missions that aim to place spacecrafts there, suchas the upcoming X-ray Athena telescope, the James Webb Space Telescope and others. Solar Proton Events are a main source of radiation whichcan have serious and adverse effects on spacecrafts and their onboard equipment. However, relatively little work has been done on theinvestigation of how such Events affect the radiation environment in L2. In this work we analyze data from the ESA Standard RadiationEnvironment monitor (SREM) on-board the two L2 missions Herschel and Planck, as well as from the HEO INTEGRAL mission. A comparison ofthe SREM count-rates and the derived solar proton fluxes is performed. The flux analysis is based on the derivation of proton fluxes usingArtificial Neural Networks (ANN) and Singular Value Decomposition (SVD).

AcknowledgementsThis work received funding through the “AREMBES: ATHENA RadiationEnvironment Models and X-Ray Background Effects Simulators”, ESA ContractNo. 4000116655/16/NL/BW

ReferencesI. Sandberg, I. A. Daglis, A. Anastasiadis, P. Bühler, P. Nieminen, and H. Evans,

Unfolding and Validation of SREM Fluxes, IEEE Trans. Nucl. Sci, vol 9 (4), 2012

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