Frascati Physics Series Vol. XXXVII (2004) pp. 59–6459764 FrontierFRONTIER ScienceSCIENCE 2004,2004. PhysicsPHYSICS andAND AstrophysicsASTROPHYSICS inIN SpaceSPACE Frascati, 14-19 June, 2004

HELIOSPHERIC OBSERVATIONS WITH PAMELA EXPERIMENT

M. CasolinoCASOLINO‘W,b,a, L. MarcelliMARCELLIbva,b,a, V. MikhailovNIIKHAILOVC,c, P. PicozzaPICOZZAbv“,b,a, S. RussoRUSSOZWb,a

b INFN, Sezione didi Roma II,U, via delladella Ricerca Scientifica,Scientifica, RomaRoma,, Italy

a Dipartimento di Fisica, Universit`adiUniversitd di Roma ”Tor”Tor Vergata”,Vergata”, via delladella Ricerca Scientifica,Scientifica, Roma, Italy

cC MEPHI, Moscow Engineering andand Physics institute, Moscow, RRussiaussia

Abstract

PAMELA is a borne experiment to be placed in low EarEarthth on board of the ResursDK1ResursDKl Russian satellite in the year 2005. It consists of a permanent magnet core withWith a silicon microstrimicrostripp detector, a Transition Radiation Detector, a Time of Flight system and a Silicon Tungsten tracking calorimeter. Its main aim is the investiginvestigationation of the cosmic radiation: origin and evolution of matter in the galagalaxy,xy, search for and of cosmological significance,significance, ununderstandingderstanding of origin and acceleration of relativistic particles in the galaxy. The detector can however be used also to address issues relative to the —- terrestrial environment (above(above 50 MeV) such as Solar partiparticlecle events + (isotopic(isotopic composition of H and He, e6’− and e8+ spectrum) and composition and temporal dependence of the trapped and albedo particles component. In this work the observational capabilities of Pamela in thethesese contexts willWill be addressed.

59 60 Frontier Science 200420047, Physics and Astrophysics in Space

1 Introduction

The Pamela spectrometer [1][1] will be launched in a semi-polarsemi—polar orbit (70(70.4O.4o inclination and altitude between 300 and 600 km) in the year 22005.005. Its expected lifetime will be 3 years. The primary scientificscientific objectives ooff Pamela are: 1)1) Measurement of the energy spectrum of (80(80 MeV -— 119090 GeV) and (50 MeV -— 270 GeV); 2) Measurement of the proton (80 MeV -— 700 GeV),GeV)7 electron (50 GeV -— 2 TeV) and light nuclei (up(up to several hundred of GeV/n); 3) Measurement of the antihelium/heliumantihelium/helium ratio with sensitivity of at least 101077.−7. To achieve these goals,goals7 the instrument uses a permanent magnmagnetet (0.4(0.4 T) with six layers of double-sideddouble—sided silicon microstrip detectodetectorsrs to measure particle rigidity and sign. Lepton/Hadron identificationidentification is ensured by a Transition Radiation Detector (TRD) located on top of the magnet and a SiSilicon—Tungstenlicon-Tungsten Immaging Calorimeter (44(44 planes; 16.316.3 Radiation Lenghts). Trigger and albedo particle rejection are given by a series of scintillator coucountersnters placed on top of the detector, above the magnet and below it. A neutron detectdetectoror on bottom ensures lepton/hadronlepton/hadron identificationidentification at high energies. In tthishis work we focus mainly on the observational capabilities of Pamela in resperespectct to solar and heliospheric cosmic rays.

2 Solar Particle Events

The launch of Pamela is expected in the firstfirst half of 2005, abouaboutt 5 years from the last solar maximum (Sept.(Sept. 2000). The number of solar protprotonon events in the three years of operations can be estimated from [2]:[2]: takitakingng into account our minimum energy acceptance of 80 MeV we expect about 10 sigsigninificantficant events during Pamela lifetime. The background particle ratratee on the topmost scintillator could be very high for intense events,events7 therefothereforere a dedicated trigger for Solar active days is currently under study. The observatobservationion of solar particle events with a magnet spectrometer will allow for the firstfirst timtimee to perform:

0 Measurement of the component. Positrons are produproducedced mainly • in the decay of π7PL+ coming from nuclear reactions occurring at the flareflare site and they have up to now only been measured indirectly by rremoteemote sensing of the 511 keV ray line. Using the magnetic spectrometer it will be possible to separately analyze the hhighigh energy tail of electron and positron spectra at 1 AU obtaining informatiinformationon both on the production and propagation in the heliosphere.

0 Measurement of the proton component. Pamela can measure the • energy spectrum of solar protons in a very wide energy range ((fromfrom 80 MeV to some GeV). These measurements will be correlated wiwithth M. CasolinoCasalino Heliospheric cosmic ray observations with Pamela experimeexperimentnt 61

other instruments placed in differentdifferent points of the magnetosmagnetospherephere to give information on the acceleration and propagation mechamechanismsnisms of SEP events. Up to now there has been no direct measurement [3][3] of tthehe high energy ((>> 11GeV)GeV ) proton component of SEPs. The importance of a direct measurement of this spectrum is related to the fact [4][4] that ttherehere are many events where the energy of protons is above the highest ((2 100100111MeV6V)) ≃ detectable energy range of current spacecrafts but is below the detection threshold of ground Neutron Monitors[5]. However, at these energies, is possible to examine the turnover of the spectrum, where we finfindd the limit acceleration processes at the . Pamela has a maximum trigtriggerger rate of 100 Hz and a geometrical factor of 20.5 cmcm22 sr37".. This implies that we will be able to read all events with a fluxfiux ((EE > 80SOJWGV)MeV ) up to 4Zip/cm?p/cm2 s srso".. For such events we expect about 2 X 101066 events/day × (assuming(assuming a spectral index of γ'y = 3 we have 2 X 101033 events/dayevents/day above 11 × GeV). Larger events saturate the trigger, so in this case the number of protons will be of the order mentioned or lower due to dead timtime.e.

0 Measurement of the nuclear component. The same arguments of the • proton component can be applied to the study of the heavy nuclnucleiei component of SEP events. We expect 2 101044 44H6He nuclei and 2 101022 33H6He ≃ ≃ nuclei for gradual events and more for impulsive ones. This sstatisticstatistics will allow to examine in detail the amount of the 33H6He component and understand selective nuclear enhancement processes in the high energy range of impulsive [6][6] events to gather information on the seselectivelective acceleration processes.

0 Neutron component. Neutrons are produced in nuclear reactireactionsons at the • flareflare site and can reach the Earth before decaying. Although ttherehere is no devoted trigger for neutrons, the background counting of the detector will measure in great detail the temporal profileprofile and distribdistributionution of solar neutrons.

o Lowering of the geomagnetic cutoff.cutoff. The high inclination orborbitit of the • satellite Resource will allow to study [7,[7, 8] the variations of cosmic ray geomagnetic cutoffcutoff due to the interaction of the solar particparticlele event with the geomagnetic field.field.

3 Jovian electrons

Pioneer 10 discovered Jovian electrons at 1 — 252511161/MeV at a distance of 2 1 − ≃ AU from Jupiter [10,[10, 11]. Since then Jovian electrons have bebeenen measured in several interplanetary missions. It is known that JupiteJupiterr is the strongest electron source at low energies ((<< 25 MeV) in the heliosphere within an 11 AU radius. Its spectrum has a power law [9][9] of γ'y = 11.65.65 increasing above 62 Frontier Science 20042004,, Physics and Astrophysics in Space $3 "I 4-:.-i-:r. sears/us :mi:

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Figure 1: Left: All electron differentialdifferential spectra at various distances from the Sun. Right: Jovian component. The dots represent Pamela obsobservations:ervations: AB =: Primary component; BC -— CD = Reaccelerated component[13].component[13l.

25 MeV, where the galactic component becomes dominant. At 1 AAUU IMP-8IMP—8 could detect Jovian electrons in the range (0.6(0.6 -— 1616 MeV) and mmeasureeasure its modulation by the passage of Coronal Interaction Regions (C(CIR)IR) with 27 days periodicity [11,[11, 12]. There are also long term modulation effeffectsects related to the Earth-JupiterEarth—Jupiter synodic year of 13 months duration: Jovian elelectronsectrons follow the interplanetary magnetic fieldfield lines so, when the two planets are on the same solar spiral line, their transit from Jupiter to the EarEarthth is eased and the fluxflux increases, whereas when the two planets lie on differentdifferent sspiralpiral lines the electron fluxflux decreases. Pamela is able to detect electrons from a minimum energy of 50 MeV. At this energy, however, geomagnetic shielding will reduce ththee active observation time reducing total counts. It will be however possible to ststudyudy for the firstfirst time the high energy Jovian electron component and test the hhypothesisypothesis of reacceleration at the Solar Wind Termination Shock (TS).(TS). It is known that cosmic rays originating outside the heliosphere can be acceacceleratedlerated at the solar wind termination shock. Also Jovian electrons, transportetransportedd outward by the solar wind, reach the termination shock and can undergo shocshockk acceleration M. CasolinoCasalino Heliospheric cosmic ray observations with Pamela experimeexperimentnt 63

− − Galactic and Jovian e6’ Galactic and Jovian e6’ ((N/3days)N/3days) ((N/3days)N/3days) Interval At 1 AU With Cut-offCut—off AB (50 — 7070MeV)MeV ) 7070i88 55122 − ± ± BC (70 — 600600A16V)MeV ) 1020010200:|:100100 20302030i4040 − ± ± CD (600(600A16VMeV — 2206V)GeV ) 1230012300:|:100100 45704570i7070 − ± ± Total 2257022570:|:208208 66056605i112112 ± ± Jovian e6’− Jovian e6’− ((N/3days)N/3days) ((N/3days)N/3days) Interval At 1 AU With cut-offcut—off AB (50 — 1301301W6V)MeV ) 100100i1010 1010:|:33 − ± ± BC (130 — 600GOOA/ICV)MeV ) 9595:1:1010 2020i55 − ± ± CD (600(GOOA/ICVMeV — 22G6V)GeV ) 9090:1:99 3535i66 − ± ± Total 285285i2929 6565:1:1414 ± ± Table 1: Top: Expected galactic and Jovian electron counts wwithith Pamela detector in the various energy ranges. Bottom: Only Jovian ccontribution.ontribution. Energy ranges: AB =2 Primary component; BC -CD—CD = ReaccelerateReacceleratedd component; The right column represents Pamela expectationexpectationss taking into account the geomagnetic cutoff.cutoff.

increasing their energy; part of these electrons are scattescatteredred back in the heliosphere. The position of the shock (still(still unknown and plplacedaced at about 80 -— 100 AU) can affectaffect the reaccelerated electron spectrum [1[13].3]. In Table 3 are shown the expected electron counts with Pamela detectodetectorr (TS=90(T8290 AU,AU; Heliospheric boundary=120boundary:120 AU) using the spectrum calculatcalculateded in [13].[13]. The firstfirst column contains the fluxesfluxes outside the ,magnetosphere; wwhereashereas the second takes into account the geomagnetic cutoffcutoff along Pamela orbitorbit.. If counts are grouped in three day bins it is possible to accumulate enoenoughugh statistics to extract the signal and monitor short term (CIR)(CIR) variationvariations.s. It will be therefore possible to detect electrons in the three energy rranges:anges: 50-7050—70 MeV,MeV7 at the lower limit of Pamela detection,detection; these electrons reprrepresentesent the primary non-reacceleratednon—reaccelerated component; 70-60070—600 MeV,MeV; in this energy rarangenge the main reaccelerated component will be clearly observable; above 600 MeV,MeV; where electrons of mostly galactic origin will be detected. On the overall the Jovian component amounts to about 1%1% of the total galactic flux.flux. This effecteffect will therefore be measurable with our instrument,instrument; which will be aableble to extract the long and short term modulation effects.effects. In addition it is ppossibleossible that the reacceleration of electrons at the solar wind termination sshockhock is modulated by the : with three years of observations toward tthehe solar minimum 64 Frontier Science 20042004,, Physics and Astrophysics in Space it willWill be possible to detect also this effect.effect. In addition to tthesehese phenomena, charge dependent modulation effectseffects willWill be studied by compacomparingring the temporal dependence of electron and positron spectra.

4 Conclusions

We have discussed some observational capabilities of PamelPamelaa in respect of Solar particle events and other heliospheric processes. In addition also Solar modulation at 1 AU and secondary particle production in the aatmospheretmosphere willWill be studied in parallel to the main physics objectives of this experiment.

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