XXV European Cosmic Ray Symposium, Turin, Sept. 4-9 2016 1

The Solar Orbiter Mission: an Energetic Particle Perspective

R. Gómez-Herrero, J. Rodŕıguez-Pacheco, S. Sánchez-Prieto, M. Prieto, F.

Espinosa Lara, I. Cernuda, J.J. Blanco, A. Russu, and O. Rodŕıguez PoloUniversidad de Alcalá, E-28871, Alcalá de Henares, Spain

R. F. Wimmer-Schweingruber, C. Mart́ın, S.R. Kulkarni, C. Terasa, L. Panitzsch,

S.I. Böttcher, S. Boden, B. Heber, J. Steinhagen, J. Tammen, J. Köhler, C.

Drews, R. Elftmann, A. Ravanbakhsh, L. Seimetz, B. Schuster, and M. YedlaChristian-Albrechts-Universität zu Kiel, D-24118, Kiel, Germany

G.M. Mason and G.C. HoApplied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA

E. Valtonen and R. VainioDepartment of Physics and Astronomy, University of Turku, 20014, Turku, Finland

Solar Orbiter is a joint ESA-NASA mission planed for launch in October 2018. The sciencepayload includes remote-sensing and in-situ instrumentation designed with the primary goal ofunderstanding how the Sun creates and controls the heliosphere. The spacecraft will follow anelliptical orbit around the Sun, with perihelion as close as 0.28 AU. During the late orbit phasethe orbital plane will reach inclinations above 30 degrees, allowing direct observations of the solarpolar regions. The Energetic Particle Detector (EPD) is an instrument suite consisting of severalsensors measuring electrons, protons and ions over a broad energy interval (2 keV to 15 MeV forelectrons, 3 keV to 100 MeV for protons and few tens of keV/nuc to 450 MeV/nuc for ions), providingcomposition, spectra, timing and anisotropy information. We present an overview of Solar Orbiterfrom the energetic particle perspective, summarizing the capabilities of EPD and the opportunitiesthat these new observations will provide for understanding how energetic particles are acceleratedduring solar eruptions and how they propagate through the Heliosphere.

I. THE SOLAR ORBITER MISSION

During the last decades, several space-based obser-vatories have decisively contributed to improve ourknowledge of different aspects of the Physics of theSun and the heliosphere. However, there have beenno missions before Solar Orbiter [1] and Solar ProbePlus (SPP, [2]) specifically conceived to explore thelink between the Sun and the Solar wind. Solar Or-biter is a joint science mission between the EuropeanSpace Agency (ESA) and the National Aeronauticsand Space Administration (NASA) designed with theprimary objective of understanding how the Sun cre-ates and controls the heliosphere. Solar Orbiter isthe first medium-class mission of ESA’s Cosmic Vi-sion program. It is currently planned for launch inOctober 2018, carrying onboard a scientific payloadconsisting of a comprehensive set of remote-sensing(RS) and in-situ (IS) instruments designed to measurefrom the photosphere into the solar wind, working to-gether to answer four interdependent top-level sciencequestions:

• What drives the solar wind and where does thecoronal magnetic field originate from?

• How do solar transients drive heliospheric vari-ability?

• How do solar eruptions produce energetic parti-cle radiation that fills the heliosphere?

• How does the solar dynamo work and drive con-nections between the Sun and the heliosphere?

The third goal above is directly focused on the ori-gin of Solar Energetic Particles (SEP). SEP eventsare of great interest not only from the Space Weatherpoint of view but also because Solar Orbiter with itscomprehensive set of instruments will provide a uniqueopportunity to understand the physics of the injec-tion, acceleration and escape processes of energeticparticles, which is an important and ubiquitous astro-physical process which can only be studied in situ inthe heliosphere.

The three-axis stabilized Sun-pointing spacecraft(s/c) will follow an elliptical orbit around the Sun,with perihelion as close as 0.28 AU, allowing IS mea-surements of the plasma, fields, waves and energeticparticles in a region where much of the crucial physicsrelated with solar activity takes place and is relativelyundisturbed by interplanetary propagation processes[1]. At the same time, high-resolution imaging instru-ments will permit a direct link between the IS obser-vations and the corresponding solar sources. Whileformer pioneering missions such as Helios [3], alreadyexplored the innermost region of the heliosphere andhighlighted the importance of IS measurements close

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to the Sun, the lack of RS instrumentation onboarddid not allow a detailed exploration of the connectionsbetween solar structures and IS observations.

During the late orbit phase the s/c orbital plane willreach inclinations >30 degrees above the heliographicequator, allowing direct observations of the solar polarregions. During limited time intervals the s/c orbitalperiod will be close to the solar rotation period (quasico-rotation), being able to monitor the evolution ofthe same solar region during extended periods.

Solar Orbiter has been optimized in order to reusedesigns and technology from the BepiColombo mis-sion [4] to Mercury. A sophisticated heat shield willprotect the conventional s/c and the payload from theintense direct solar flux when approaching perihelion.The two solar arrays can be tilted in order to con-trol overheating close to the Sun. Multiple planetarygravity assist maneuvers (GAM) at Earth and Venuswill be used in order to reach the final elliptical orbitand to gradually increase the orbit inclination. Af-ter a Near Earth Commissioning Phase (NECP) anda cruise phase lasting more than 2 years, the s/c willstart its 4-year long nominal mission phase (NMP).The first perihelion

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TABLE II: Solar Orbiter scientific payload.

Instrument Acronym Type Principal Investigator

Energetic Particle Detector suite EPD IS J. Rodŕıguez-Pacheco (Spain)

Magnetometer experiment MAG IS T.S. Horbury (UK)

Radio and Plasma Waves experiment RPW IS M. Maksimovic (France)

Solar Wind Analyser instrument suite SWA IS C.J. Owen (UK)

Extreme Ultraviolet Imager EUI RS P. Rochus (Belgium)

Multi Element Telescope for Imaging METIS RS E. Antonucci (Italy)

and Spectroscopy (Coronagraph)

Polarimetric and Helioseismic Imager (Magnetograph) PHI RS S.K. Solanki (Germany)

Solar Orbiter Heliospheric Imager SoloHI RS R.A. Howard (USA)

Spectral Imaging of the Coronal Environment SPICE RS European-led

(Extreme ultraviolet imaging spectrograph) facility instrument

Spectrometer/Telescope for Imaging X-rays STIX RS S. Krucker (Switzerland)

Suprathermal particles with energies above the am-bient plasma in the outer corona and the solar windare known to play an important role as seed popu-lation for acceleration during SEP events. The vari-ability of this seed population may be a key factorto explain the wide range of intensities and compo-sition observed in SEP events. EPD will have theopportunity to perform IS measurements of the com-position and temporal variations of the suprathermalseed population close to the Sun, contributing also tounderstand the origins of the suprathermal ion poolitself.

Energetic particles escaping from the accelerationsites continue their propagation through the turbu-lent interplanetary magnetic field. As shown by He-lios observations [5], SEP events close to the Sun aremuch less disturbed by interplanetary transport ef-fects compared to 1 AU observations. As the observ-ing s/c goes farther from the Sun, the interplanetaryscattering effects become more important and oftenmultiple injections closely spaced in time cannot beresolved. For this reason, Solar Orbiter observationsclose to the perihelion will be crucial to unveil SEP in-jection, acceleration, trapping, release and transportprocesses. These observations will contribute to solvethe controversy about the SEP acceleration sites, dis-entangling the contribution of acceleration at CME-driven shocks and at reconnection sites in solar flaresor behind CMEs.

EPD consists of four instruments measuring ener-getic electrons, protons and ions, operating at partlyoverlapping energy ranges covering from few keV to450 MeV/nuc:

• SupraThermal Electrons and Protons (STEP)

• Suprathermal Ion Spectrograph (SIS)

• Electron Proton Telescope (EPT)

• High Energy Telescope (HET)

FIG. 2: Energy windows covered by the EPD instrumentsfor different species.

The energy intervals covered by the different instru-ments for various particle species are summarized inFigure 2. The four EPD sensors share a common In-strument Control Unit (ICU). EPD instruments andthe ICU have significant heritage from previous mis-sions, improved and optimized for the close approachto the Sun. Figure 3 shows a picture of the wholeEPD instrument suite during the integration tests per-formed in July 2016.

The EPD sensors will measure the composition,spectra and anisotropies of energetic particles withsufficient temporal, spectral, angular and mass res-olution to achieve the mission science goals. The ge-ometric factors are scaled to avoid saturation by highparticle fluxes close to the perihelia. Since Solar Or-biter is a three-axis stabilized s/c, EPD uses multipleapertures and sectoring to cover different pointing di-rections, providing information about the directionaldistribution of energetic particles reaching the s/c.This information combined with the magnetic fielddata will be used to obtain energetic particle pitch

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FIG. 3: EPD instrument suite during the integration testsperformed in July 2016.

FIG. 4: EPD sensors fields of view.

angle distributions, of fundamental importance to un-derstand the interplanetary propagation of SEPs. Fig-ure 4 shows the fields of view of the different EPDsensors in the s/c reference frame. The backgroundis color-coded as a function of the the interplanetarymagnetic field vector distribution observed by the He-lios mission. The different units which constitute EPDare described in the following sections.

B. Suprathermal Electrons and Protons (STEP)

STEP will measure electrons and ions in thesuprathermal energy range, covering 2-100 keV forelectrons and 3-100 keV for protons. It has heritagefrom the Suprathermal Electron Instrument (STE,[6]) onboard the Solar Terrestrial Relations Observa-tory (STEREO), and consists of two identical unitssharing common electronics. Both units have identicalrectangular 54◦×26◦ fields of view, pointing sunwardalong the nominal direction of the Parker spiral. Bothapertures consists of a pinhole in combination withbaffles at the entrance. This configuration reduces theamount of stray-light on the detector. Each unit has asingle layer of silicon solid-state detectors (SSDs) seg-

FIG. 5: STEP pixelated SSD. The top pixel dedicated tomonitor the background produced by galactic cosmic rayand the bottom 3×5 pixel array used to obtain directionalinformation.

mented into 16 2×2 mm2 pixels (see Figure 5). Oneof these pixels is used to monitor the background con-tribution from galactic cosmic rays, while the other 15pixels combined with the pinhole aperture provide di-rectional information. The use of SSDs with ultra-thinohmic contacts provides high sensitivity compared totraditional electrostatic analyzers used for solar windelectron instruments. STEP is able to measure elec-tron and ion fluxes with up to 1 s cadence. One ofthe unit’s sensors is equipped with a magnetic deflec-tion system which rejects electrons, while leaving iontrajectories almost unaffected (see Figure 6). Thisunit will provide ion fluxes while the second unit willmeasure both, electrons and ions. The difference be-tween both measurements will be used to obtain theelectron flux. The nominal geometric factor of eachSTEP unit is 7.5 · 10−3 cm2sr. During periods withvery high fluxes the geometric factor can be reducedto 1.7 · 10−4 cm2sr by reducing the active area of thepixels to 0.3×0.3 mm2. Monte Carlo simulations re-sults showing the angular resolution capabilities of theSTEP proton telescope are shown in Figure 7.

C. Suprathermal Ion Spectrograph (SIS)

SIS is a time-of-flight mass spectrometer that willmeasure all elements from He to Fe, sampling alsotrans-iron elements. The energy window is species-dependent, covering between 50 keV/nuc and 14MeV/n for CNO. The instrument design has her-itage from the Ultra-Low-Energy Isotope Spectrom-eter (ULEIS, [7]) onboard the Advanced CompositionExplorer (ACE) and the Suprathermal Ion Telescope(SIT, [8]) onboard STEREO. SIS consists of two par-ticle telescopes, one looking sunward along the nom-inal Parker spiral direction and the other looking ap-proximately in the anti-sunward direction, 130◦ away

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FIG. 6: Simulation of STEP magnetic deflection system.Two permanent magnets (green rectangles) effectively de-flect electrons (blue trajectories) while protons within theinstrument energy range (green and red trajectories) re-main almost unaffected. The black vertical solid linessymbolize the entrance system of STEP (aperture andpinhole). Since the deflected electrons do not cross thepinhole, they would stop at the entrance system.

FIG. 7: Monte Carlo simulation results of the angular res-olution achieved by the STEP 3×5 pixelated proton de-tector (equipped with magnet).

from the sunward-pointing telescope. Each telescopehas a conical field of view with a full aperture of 22◦

and a nominal geometric factor of 0.2 cm2sr. Bothtelescopes share a single electronics box. A sketch ofone of the SIS telescopes is shown in Figure 8. Timeof flight information is collected when the ion passesthrough the Start-1, Start-2, and Stop detector foilsand secondary electrons are emitted, accelerated to∼1 kV, and directed via isochronous mirrors onto mi-crochannel plate stacks. Finally, they deposit theirenergy in the SSD at the back of the instrument. Thecombination of the total energy and time of fight in-formation permits particle identification. SIS will ac-quire data with a nominal cadence of 30 s, being ableto achieve 3 s during burst mode periods. The veryhigh mass resolution of m/σm ∼ 50 will allow SIS to

FIG. 8: Cross-section view of the SIS sensor.

FIG. 9: SIS mass and energy resolution obtained using a241Am radioactive source.

measure 3He/4He ratios with uncertainty < 1%. Fig-ure 9 shows the mass and energy resolution achievedby one of the SIS telescopes (proto flight model) ob-tained during the calibration tests performed using an241Am alpha particle source. These calibration testsshowed that SIS meets or exceeds the mission require-ments.

D. Electron-Proton Telescope (EPT)

EPT will measure 20-400 keV electrons and 20 keV-7 MeV protons. It has direct heritage from the So-lar Electron and Proton Telescope (SEPT, [9]) in-

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FIG. 10: Cross-section view of an EPT double-ended tele-scope.

strument onboard STEREO, using a magnet-foil tech-nique for particle separation. Each double-ended EPTtelescope has two closely spaced SSDs operating inanti-coincidence. As shown in Figure 10, one SSDlooks through a polyimide foil while the second SSDlooks through a magnetic deflection system. Thefoil stops low-energy protons while leaving electronsmostly unaffected. The magnet system effectivelydeflects electrons leaving protons unaffected. Sincethe magnets corresponding to each pair of double-ended telescopes are closely placed together and forma compensating pair of dipoles, the long-range fieldis strongly attenuated, minimizing the disturbance ofthe measurements by the magnetometer onboard So-lar Orbiter. Figure 11 shows EPT engineering modelcalibration results with a 207Bi radioactive sourcedemonstrating the effective rejection of electrons bythe magnet system. There are two EPT units, eachone consisting of two double-ended telescopes. Thissetup provides a total of four view directions. The firstEPT unit apertures point along the nominal Parkerspiral in sunward and anti-sunward direction. Thesecond unit apertures point 56◦ above and below theorbital plane. The electronics box of each EPT unitis shared with a HET unit. Each EPT aperture hasa conical field of view with a full aperture of 30◦ anda nominal geometric factor of 0.01 cm2sr. The maxi-mum time cadence is 1 s.

E. High Energy Telescope (HET)

HET covers the energy range, which is of specificinterest for space weather, and will perform the mea-surements needed to understand the origin of high-energy SEP events at the Sun. HET will measureelectrons between 300 keV and 15 MeV, protons from10 to 100 MeV and ions from 20 MeV/nuc to 450MeV/nuc (the exact interval is Z-dependent, see [10]).Incident particles will be identified using the dE/dxvs. total energy technique. It has heritage from theRadiation Assessment Detector (RAD, [11]) onboard

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the Mars Science Laboratory (MSL). HET consistsof two double-ended sensor heads, one pointing sun-ward and anti-sunward along the nominal Parker spi-ral, the other pointing above and below the orbitalplane. Thus, HET has a total of four viewing direc-tions (analogous to EPT, see section II D and Figure4). Both HET sensors are identical and consist of adouble-ended set of SSDs and a a high-density Bis-muth Germanate (BGO) calorimeter scintillator (Fig-ure 12). Each double HET sensor unit shares the elec-tronic box with an EPT unit. HET allows separationof the helium isotopes down to a 3He/4He isotope ra-tio of about 1% in a limited energy range. Figure 13show Monte Carlo simulation results illustrating HETresponse to heavy ions, light ions and electrons. HEThas a conical field of view with a full aperture of 43◦

and a nominal geometric factor of 0.27 cm2sr. Thefront detectors of HET (both sides) are protected bylaminated Kapton+Al foil (Kapton is 50 µm and Alis 25 µm thick) to reduce low energy particle flux.In addition, the front detector is divided into concen-tric segments that allow the reduction of proton countrates during high intensity events. In such situations,the thresholds on the larger segment are increased tobeyond the energy deposit of protons. This schemeretains the detection power for the much rarer heavyions while reducing the counting rate for the abundantprotons. The maximum instrument cadence is 1 s.

F. Instrument Control Unit (ICU)

The ICU provides a single point of connection be-tween the s/c and all the EPD sensors, acting as dataand power interface. It is composed of the CommonData Processing Unit (CDPU) and the Low VoltagePower Supply (LVPS). The ICU shares informationwith other EPD instruments to allow synchronizedburst-mode operations following on-board identifica-tion of predefined triggering events in the EPD data.

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FIG. 12: Cross-section view of a HET double-ended tele-scope. Note the large BGO scintillator located in the cen-ter and the two SSDs located at both sides.

FIG. 13: Monte Carlo simulation results of HET responseto light ions and electrons (top) and heavy ions (bottom).Each plot shows the product of the energy loss in the SSDand the total energy versus their ratio.

The ICU has strong architecture heritage from theCommon Data Processing Unit (CDPU) for the Com-prehensive Suprathermal and Energetic Particle An-alyzer (COSTEP, [12]) and the Energetic and Rel-ativistic Nuclei and Electron (ERNE, [13]) instru-ments onboard the Solar and Heliospheric Observa-tory (SOHO). The ICU is designed to manage sen-sor’s control and monitoring, timing clock, and datacollection, compression, and packetization for teleme-try. ICU is also responsible for the s/c telecommandreception and delivery to the sensors if necessary. TheCDPU in based on a LEON2 soft-processor imple-

mented in a RTAX2000 FPGA from Actel, it con-tains external RAM, EEPROM and PROM memo-ries, two hot redundant SpaceWire interfaces, andfour identical serial links (UART-LVDS) with the sen-sors. The CDPU PROM contains the boot code andit can be used to load flight code from either the EEP-ROM, that contains two copies of the application soft-ware, or from the s/c interface via telecommand. TheLVPS board is responsible for filtering, monitoringand switching the s/c primary power. It also providesthe power supply to both CDPUs.

G. EPD data products

EPD has a total telemetry budget of 3600 bit/s(housekeeping + science data). Solar Orbiter teleme-try rate is highly variable during the orbit. Thisimplies that data will be downliked to ground withstrongly varying latencies of up to several months. Aminimal set of science data from all the instrumentswill be downlinked daily (low latency dataset), mainlyfor planning/monitoring purposes, but also with sci-entific value. In order to optimize the science returnfulfilling telemetry limitations, EPD science data sentto ground will vary depending on radial distance, hav-ing higher cadences close to the perihelia.

Following in-orbit commissioning, the Principal In-vestigators of the different instruments onboard SolarOrbiter retain exclusive data rights for the purpose ofcalibration and verification for a period of 3 months af-ter the receipt of the original science telemetry. Upondelivery of data to the ESA Science Operations Cen-ter, they will be made available to the scientific com-munity through the ESA science data archive. The in-strument teams will provide records of processed datawith all relevant information on calibration and in-strument properties to the ESA science data archive,which will be the repository of all mission products[14]. Solar Orbiter’s IS instruments will provide pro-cessed data using the Common Data Format (CDF)standard.

III. MULTIPOINT OBSERVATIONOPPORTUNITIES

Solar Orbiter will offer excellent opportunities formulti-point observation campaigns combining mea-surements by multiple s/c. Synergies with the SPPmission are of particular relevance, since both mis-sions have overlapping timelines and the SPP perihe-lion, reaching up to

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for multi-point measurements in order to optimize thescience return:

• Close approaches of Solar Orbiter to SPP orother s/c providing the opportunity for cross-calibration of the particle instruments onboard.

• Radial alignments enabling the observation ofplasma “packets” from the same solar source re-gion at progressive radial distances as well thestudy of energetic particle radial gradients.

• Alignments along the same interplanetary mag-netic field line allowing the observation of SEPsoriginating at the same acceleration site by twoor more s/c located at different radial distances.

• Combination of RS observations of near-limbsource regions and IS plasma observations bys/c with angular separations close to 90◦.

• Observations of SEP events by multiple s/ccovering wide angular regions (both, in longi-tude and latitude) in order to investigate thespatial distribution of SEPs and the physicalmechanisms producing wide-spread SEPs events(see e.g. [15, 16]). These observations willhelp to understand the possible role played byinterplanetary cross-field diffusion, accelerationat wide shocks and distorted coronal magneticfields with large latitudinal/longitudinal devia-tions from the radial direction.

IV. SUMMARY AND CONCLUSIONS

Solar Orbiter is a unique mission conceived to un-veil the Sun-heliosphere connection. The orbital con-figuration includes a close perihelion, high inclinationintervals allowing the observation of the solar polarregions and quasi-co-rotation periods. These orbitalcharacteristics are combined with a comprehensivecombination of IS and RS instruments and excellentopportunities for multi-s/c studies. The EPD suitewill provide high-quality energetic particle observa-tions over a wide energy range and multiple species,key to understand the seed populations, injection, ac-celeration and transport processes of SEPs.

Acknowledgments

The authors acknowledge the financial support ofthe Spanish MINECO under projects ESP2013-48346-C2-1-R and ESP2015-68266-R (MINECO/FEDER).SIS was funded by ESA as a European-led facility in-strument. CAU acknowledges the financial supportfrom the German space Agency (DLR) under grants50OT1002 and 50OT1202 and by the University ofKiel.

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eConf C16-09-04.3

I THE SOLAR ORBITER MISSIONII THE ENERGETIC PARTICLE DETECTOR (EPD) SUITEA Key science questionsB Suprathermal Electrons and Protons (STEP)C Suprathermal Ion Spectrograph (SIS)D Electron-Proton Telescope (EPT)E High Energy Telescope (HET)F Instrument Control Unit (ICU)G EPD data products

III MULTIPOINT OBSERVATION OPPORTUNITIESIV SUMMARY AND CONCLUSIONS Acknowledgments References