ISSN 1062�8738, Bulletin of the Russian Academy of Sciences. Physics, 2013, Vol. 77, No. 5, pp. 493–496. © Allerton Press, Inc., 2013.Original Russian Text © G.A. Bazilevskaya, A.G. Mayorov, V.V. Malakhov, V.V. Mikhailov, O. Adriani, G.C. Barbarino, R. Belotti, M. Boezio, E.A. Bogomolov, V. Bonvicini,M. Bongi, L. Bonechi, S.V. Borisov, S. Bottai, A. Bruno, A. Vacci, E. Vanuccini, G.I. Vasilyev, S.A. Voronov, J. Wu, A.M. Galper, L.A. Grishantseva, I.A. Danilchenko, W. Gillard,G. Jerse, G. Zampa, N. Zampa, V.G. Zverev, M. Casolino, D. Campana, R. Carbone, A.V. Karelin, P. Carlson, G. Castellini, F. Cafagna, A.N. Kvashnin, S.V. Koldashov,S.A. Koldobskiy, S.Yu. Krutkov, L. Consiglio, A.A. Leonov, V. Malvezzi, L. Marcelli, W. Menn, E. Mocchiutti, A. Monaco, N. Mori, N.N. Nikonov, G. Osteria, F. Palma, P. Papini,C. Pizzolotto, M.P. De Pascale, P. Picozza, M. Pearce, M. Ricci, S.B. Ricciarini, L. Rossetto, M.F. Runtso, C. De Santis, R. Sarkar, M. Simon, N. De Simone, R. Sparvoli, P. Spillantini,Yu.I. Stozhkov, V. Di Felice, M.D. Kheymits, Yu.T. Yurkin, 2013, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2013, Vol. 77, No. 5, pp. 557–560.

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INTRODUCTION

The mechanisms responsible for accelerating solarcosmic rays (SCRs) to high energies (more than sev�

eral gigaelectron�volts) have yet to be adequatelyinvestigated. It is currently believed that high�energyparticles are produced in the Sun as a result of power�ful eruptions in which the principal mechanisms of

Solar Proton Events at the End of the 23rd and Start of the 24th Solar Cycle Recorded in the PAMELA Experiment

G. A. Bazilevskayaa, A. G. Mayorovl, V. V. Malakhovl, V. V. Mikhailovl, O. Adriania, b, G. C. Barbarinod, e, R. Belottif, g, M. Boezioh, E. A. Bogomolovi, V. Bonvicinih, M. Bongic,

L. Bonechib, c, S. V. Borisovj, k, l, S. Bottaic, A. Brunop, q, A. Vaccih, E. Vanuccinic, G. I. Vasilyevi, S. A. Voronovl, J. Wum, A. M. Galperl, L. A. Grishantseval, I. A. Danilchenkol, W. Gillardm,

G. Jerseh, o, G. Zampah, N. Zampah, V. G. Zverevl, M. Casolinoj, r, D. Campanae, R. Carbonee, k, A. V. Karelinl, P. Carlsonm, G. Castellinin, F. Cafagnag, A. N. Kvashnina, S. V. Koldashovl, S. A. Koldobskiyl, S. Yu. Krutkovi, L. Consiglioe, A. A. Leonovl, V. Malvezzij, L. Marcellij,

W. Mennp, E. Mocchiuttih, A. Monacof, g, N. Moria, b, N. N. Nikonovi, j, k, G. Osteriae, F. Palmaj, k, P. Papinic, C. Pizzolottoh, M. P. De Pascalej, k, P. Picozzaj, k, M. Pearcem, M. Ricciq, S. B. Ricciarinib, L. Rossettom, M. F. Runtsol, C. De Santisj, R. Sarkarh, M. Simonp,

N. De Simonej, k, R. Sparvolij, k, P. Spillantinib, c, Yu. I. Stozhkova, V. Di Felicej, k, M. D. Kheymitsl, and Yu. T. Yurkinl

aLebedev Institute of Physics, Russian Academy of Science, Moscow, 119991 RussiabDepartment of Physics, University of Florence, I�50019 Sesto Fiorentino, Florence, Italy

cINFN Sezione di Florence, I�50019 Sesto Fiorentino, Florence, ItalydDepartment of Physics, Federico II University of Naples, I�80126Naples, Italy

eINFN Sezione di Naples, I�80126 Naples, ItalyfDepartment of Physics, University of Bari, I�70126 Bari, Italy

gINFN Sezione di Bari, I�70126, Bari, ItalyhINFN Sezione di Trieste, I�34149 Trieste, Italy

iIoffe Physicotechnical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia.jINFN Sezione di Rome Tor Vergata, I�001333 Rome, Italy.

kDepartment of Physics, University of Rome Tor Vergata, I�00133 Rome, Italy.lMoscow Engineering Physics Institute (MEPhI), Moscow, 115409 Russia.

mDepartment of Physics, Oskar Klein Center for Cosmoparticle Physics, AlbaNova University Center, KTH, SE�10691 Stockholm, Sweden

nIFAC, I�50019 Sesto Fiorentino, Florence, ItalyoDepartment of Physics, University of Trieste, I�34147 Trieste, Italy

pDepartment of Physics, Universita@t Siegen, D�57068 Siegen, GermanyqINFN Laboratori Nazionali di Frascati, I�00044 Frascati, ItalyrRIKEN Advanced Science Institute, Wako�shi 351�0106, Japan

e�mail: bazilevs@sci.lebedev.ru

Abstract—The PAMELA magnetic spectrometer was launched into a near�Earth orbit on board the Resurs�DK1 satellite in June 2006; in December 2006, it recorded the last strong solar high�energy particle event ofthe 23rd solar cycle. A deficit was thereafter observed in solar energetic particle events because of the lengthysolar activity minimum and the weak evolution of the next (24th) solar cycle. As a result, only a few solarevents involving protons with energies of more than 100 MeV were recorded between 2010 and 1012. Thiswork presents the preliminary results from measurements of charged particle fluxes in these events, recordedby the Pamela spectrometer.

DOI: 10.3103/S1062873813050109

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particle energy enhancement include stochastic accel�eration, acceleration at shock fronts, and accelerationin electric fields produced upon the reconnection ofsolar magnetic field lines. Acceleration can occur at asolar flare site, in the solar corona, and even in inter�planetary space. The extent of the influence of each ofthe above processes determines the maximum energyof the particles and their energy spectrum. The key tounderstanding the processes responsible for the accel�eration of SCRs lies in the accumulation of experi�mental data, and measuring the intensity and differen�tial energy spectra of solar proton events (SPEs) yieldsimportant information for studying the Sun and parti�cle acceleration in interplanetary space [1–3].

The PAMELA magnetic spectrometer is the onlyinstrument capable of directly observing solar protonevents in the energy range of 80 MeV/nucleon to severalGeV/nucleon. This is especially important, since previ�ous measurements of high�energy particle fluxes associ�ated with ground�level enhancement (GLE) events ofsolar activity were obtained with ground�based instru�ments and depended strongly on the response functionof the latter. Adriani and colleagues [5] presented theresults from measuring solar protons and helium withenergies up to several GeV/nucleon associated with thelast GLE event (December 13, 2006) of solar cycle 23,using the PAMELA instrument.

SOLAR PROTON EVENT ACTIVITYIN SOLAR CYCLE 24

The solar activity minimum of solar cycle 23/24was extraordinarily deep and prolonged. The leastvalue of the smoothed sunspot number Rz was 1.7, thevalue of this parameter remaining below 5.0 for21 months. By way of comparison, the Rz values were9.6, 12.2, 12.3, and 8.0 during minima of the preced�ing cycles from 19/20 to 22/23, respectively [6]. Solaractivity started increasing in 2010, and the first solarproton events of cycle 24 occurred as a result. Duringthe 41 months following the onset of the 24th solarcycle, the GOES instrument [7] recorded 12 solarevents that caused perceptible enhancement of theflux of protons with energies over 100 MeV until May2012, eight of were being recorded by the PAMELAinstrument as well.

Figure 1 illustrates the dynamics of variation forrecent solar activity, including its decline, minimum,and increase in the 24th cycle. The monthly averageand smoothed sunspot numbers [6] point to a pro�longed solar activity minimum and the onset of a freshsolar cycle that is now apparently close to its maxi�mum. The crosses in the bottom picture of Fig. 1 indi�cate the dates of solar proton events (SPEs) involvingprotons with energies over 100 MeV [7]. The figurealso shows the frequency of coronal mass ejections(CMEs) calculated using the CACTUS programpackage [8],along with the monthly numbers of X�and M�class solar flares [9]. It can be seen that SPEsappeared in cycle 24 at a level of solar activity higherthan when they vanished in the preceding cycle. Fromthe figure, it is impossible to say whether coronal massejections or solar flares are the main source of high�energy solar particles.

May of 2012 was the 41st month following the onsetof solar cycle 24, and we had the opportunity to com�pare the number of SPEs recorded during the sameperiod of time in various solar cycles (see Table 1). Sincethe cycles differ in strength, we used the accumulatedvalue of the smoothed number of sunspots Rza, i.e.,the sum of Rz numbers over 41 months following theonset of each cycle. The ground level enhancementevents (column GLE in Table 1) recorded by ground�based installations correspond to SPEs with particleenergies Е ≥ 1 GeV. The notation J10 > 1 denotes thenumber of SPEs with the maximum intensity of protonsof the energy Е ≥ 10 MeV exceeding 1 cm–2 s–1 sr–1.The data for cycles 21 and 22 were borrowed from[10, 11], while those for cycles 23 and 24 are prelimi�nary. We can see that the 24th cycle has an excessivenumber of low�energy SPEs. The ratio betweenJ10 > 1 and Rza in this cycle is substantially higherthan in the preceding cycles. At the same time, not asingle GLE event was recorded in its first 40 months.The first GLE event occurred in May 2012.

355

10

2012201020062005

40

302520151050

2

4

6

3

20072008

2009 2011 2013

Year

CM

Es/

day

X�r

ay fl

ares

/mon

th

100

20

0

2012201020062005

40

80

120

60

20072008

2009 2011 2013

Rz

Fig. 1. Top picture: Monthly average sunspot number; thethin curve shows the smoothed values. Bottom picture:Crosses indicate the dates of recording SPEs involvingprotons with Е ≥ 100 MeV. The heavy curve indicates thenumber of CMEs per day; the gray curve, the number ofM� and X�class solar flares per month.

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SOLAR PROTON EVENTS 495

PAMELA RESULTS

The PAMELA instrument was launched on June15, 2006, on board the RESUSR�DK1 satellite into aquasi�polar elliptical orbit (70° inclination) with aperigee of 350 km and an apogee of 610 km. Theinstrument was designed for making precise measure�ments of spectra of charged particles (including lightnuclei) in cosmic rays (including SPEs) in the energyrange of 80 MeV to several hundred gigaelectronvolts.A core of the PAMELA instrument is a permanentmagnet (~0.4 T), a magnetic spectrometer surroundedby anticoincidence detectors, a position�sensitive cal�orimeter, a time�of�flight system built around threescintillation detectors, a scintillation shower catcher atits bottom, and a neutron detector. The spectrometeris designed for determining the sign of a charge and therigidity of particles from their deflection in the spec�trometer’s magnetic field.

During the period of January 2011 to May 2012,the PAMELA spectrometer recorded several solarevents involving protons with energies over 100 MeV.The strongest one of these occurred on March 7, 2012.Some characteristics of these events are presented inTable 2. Examples of the spectra recorded during these

events are presented in Fig. 2. The spectral features donot differ from those for the solar events of the preced�ing cycles. It should be noted that several strong solarevents were missed (e.g., the event of 4 to 9 August2011) because of a gap in data acquisition due to satel�lite maintenance operations.

Table 1. Comparison of SPE activities during the 41 months following the onsets of solar cycles 21 through 24

Cycle Onset 41st month RzaNumber of SPEs

GLE E > 100 MeV J10 > 1

21 1976.5 1979.79 3030.36 6 11 43

22 1986.8 1990.13 3809.49 7 25 56

23 1996.4 1999.71 1885.46 4 15 23

24 2008.9 2012.38 1182.4 1 12 29

Table 2. Solar proton events recorded by the PAMELA magnetic spectrometer in 2011–2012

No. Date X�ray flares Optical flares CMEs SPEs recorded in PAMELA experiment

1 21.03.2011 None None Halo1341 km/s Protons up to 300 MeV Helium up to 120 MeV/nucleon

2 07.06.2011 M2.5 S21W54 2N Halo 1255 km/s Protons up to 600 MeV

3 04.11.2011 X1.9 N22E63 2B Halo 991 km/s Protons up to 200 MeV

4 23.01.2012 M8.7 N28W21 2B Halo 1178 km/s Protons >100 MeV

5 27.01.2012 X1.7 N27W71 Halo 1133 km/s Protons >100 MeV

6 07.03.2012 X5.4 N17E27 3B 1839 km/s Protons up to 500 MeV Helium up to 150 MeV/nucleon

7 13.03.2012 M7.9 N19W59 1B 746 km/s Protons >100 MeV

8 17.05.2012 M5.1 N11W76 1F Halo 1302 km/s Protons >300 MeV

10–1

10–2

10010–1

100

101

102

Intensity, cm–2 s–1 sr–1 GeV–1

Kinetic energy, eV

Quiet solar period

17.05.12 00:00–12:00 UT

21.03.11 00:00–24:00 UT23.01.12 00:00–12:00 UT07.03.12 06:00–20:00 UT

Fig. 2. Examples of energy spectra recorded in thePAMELA experiment during a quiet solar period and dur�ing SPEs.

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CONCLUSIONS

During the 41 months following the onset of the24th solar cycle, the PAMELA spectrometer recordedeight solar proton events involving particles with ener�gies over 100 MeV. The current solar cycle is charac�terized by rather low solar activity. The first ground�level enhancement (GLE) event was observed in the41st month following the onset of the cycle, and it wasweak. At the same time, the relatively large number oflow�energy SPEs indicates some redistribution ofsolar activity toward low�power events.

ACKNOWLEDGMENTS

The authors are grateful to the Space Council ofthe Russian Academy of Sciences, the RoskosmosFederal Space Agency, the TsSKB Progress FederalSpace Center, the Research Center for Earth Opera�tive Monitoring, the Italian Space Agency (AgenziaSpaziale Italiana), the German Aerospace Center(DLR), the Swedish National Space Board, and theSwedish Research Council.

REFERENCES

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4. Picozza, P., et al., Astropart. Phys., 2007, vol. 27, p. 296.

5. Adriani O et al. // Astrophys. J. 2011. V. 742. P. 102.

6. ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/ SUNSPOT_NUMBERS/INTERNATIONAL

7. www.swpc.noaa.gov/

8. http://sidc.be/cactus/

9. www.wdcb.ru/stp/data/FL_XXIII/Fl_XXIV.txt

10. Katalog solnechnykh protonnykh sobytii 1970–1979(Catalogue of Solar Proton Events 1970–1979),Logachev, Yu.I., Ed., Moscow: IZMIRAN, 1983.

11. Solnechnye protonnye sobytiya. Katalog 1980�1986(Solar Proton Events. Catalogue 1980–1986),Logachev, Yu.I., Ed., Moscow: Mezhduvedomstvennyigeofizicheskii komitet AN SSSR, 1990.