ISSN 1062-8738, Bulletin of the Russian Academy of Sciences: Physics, 2009, Vol. 73, No. 3, pp. 325–327. © Allerton Press, Inc., 2009.Original Russian Text © V.M. Dvornikov, M.V. Kravtsova, A.A. Lukovnikova, V.E. Sdobnov, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009,Vol. 73, No. 3, pp. 342–344.

325

Primary and Secondary Effects in Cosmic-Ray Variationsat Solar Proton Events

V. M. Dvornikov, M. V. Kravtsova, A. A. Lukovnikova, and V. E. Sdobnov

Institute of Solar–Terrestrial Physics, Siberian Branch of the Russian Academy of Sciences, Irkutsk, 664033 Russiae-mail: dvornikov@iszf.irk.ru

Abstract

—The variations of the cosmic-ray rigidity spectrum in the energy range from 0.8 MeV to severaldozen GeV at solar proton events in January 2005 and December 2006 have been analyzed. A comparison ofthe observed and model spectra revealed the power range of direct detection of solar cosmic rays and momentsof their observations.

DOI:

10.3103/S1062873809030137

INTRODUCTION

Anomalous behavior of cosmic-ray (CR) anisotropyat Forbush effects and solar proton events (SPEs) andthe simultaneous arrival of solar CRs (SCRs) in a wideenergy range (from a few MeV to several dozen GeV),with velocities differing by almost an order of magni-tude, at the Earth was noted in [1, 2]. On the basis onthese results, is was suggested that, along with theinduced electric field, polarization and vortex electricfields can be generated in the heliosphere. Polarizationfields can be generated during propagation of particlebeams (accelerated at the Sun) in inhomogeneous mag-netic fields, because protons and electrons drift inopposite directions. As a result, charges are separatedand, with a spatial inhomogeneity of the acceleratedparticle density, a potential difference arises betweenthe beam boundaries along the magnetic drift paths,which leads to the generation of a polarization electricfield increasing in time and, consequently, polarizationdrift of background particles of the solar wind plasma,solar corona, and galactic CRs (GCRs) along this field,i.e to the acceleration of the corona and interplanetaryparticles, whose Larmor radius is smaller than the beamtransverse size. The occurrence of depolarization longi-tudinal currents leads to the formation of a current sys-tem and generation of a magnetic field, and, therefore,a vortex electric field, accelerating particles accordingto the betatron mechanism, etc. Indications to theoccurrence of such fields were obtained by Lindberg inthe laboratory experiments on the motion of acceleratedcollisionless plasma beam in a curved magnetic field(see [3] and references therein). In particular, it wasshown in [3] that charge separation occurs in a certainrange of beam parameters when plasma enters a curvedfield (due to the drift of electrons and protons in oppo-site directions) and a polarization electric field arises,which is orthogonal to the magnetic field and depolar-izes longitudinal currents and electric fields. Thesefields cause anomalous deviation of the beam, which is

transformed from cylindrical into planar, while thedepolarizing currents distort the initial magnetic field.

Thus, these fields implement energy exchangebetween the accelerated particles and background solarcorona plasma, solar wind, and GCR particles, whichleads to the formation of current structures of the helio-sphere and generation of structures interplanetary mag-netic field (IMF). In other words, acceleration andpropagation of particles in the heliosphere is self-con-sistent with electromagnetic fields; this fact, in particu-lar, causes the transformation of the GCR rigidity spec-trum.

An analysis of the SPEs of January 2005 [4] andDecember 2006 showed that specifically the transfor-mation of the GCR rigidity spectrum (with rare excep-tion) is responsible for the observed increase in the par-ticle intensity in the energy range from 4 MeV to sev-eral GeV at SPEs. Exceptions are the readings of thesecond channel on the GOES-11 satellite (energy range4–9 MeV) in the initial stage of increases in the CRintensity of individual SPEs.

On the basis of this fact, it was concluded that thevariations in the energetic particle intensity in theenergy range under consideration are due to the GCRenergy variation induced by the electromagnetic fieldsarising in the heliosphere due to the SCR propagation(secondary SPE effect). The propagating SCRs, in turn,due to the energy exchange with GCRs and solar windplasma particles, lose energy and are detected only atshort time intervals in the energy range 4–9 MeV (pri-mary effect).

To check this suggestion, we investigated the varia-tions in the CR intensity recorded in the first channel onthe GOES satellite (energy range 0.8–4.0 MeV), whichwas not used to determine the model spectrum parame-ters; the probability of detecting particles of solar originin this channel is much higher than in the second one.

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DATA AND METHOD

The analysis was performed using the observationdata on the proton intensity, obtained on the GOES-11satellite, in energy ranges of 0.8–4.0, 4–9, 9–15, 15–40,40–80, 80–165, and 165–500 MeV [5]. The data on thevariations in the higher energy CR intensity areobtained by spectrographic global survey [1, 2] fromground-based measurements on the world-wide net-work of neutron monitors. The observation data wereaveraged over hour time intervals.

To reveal increases in the CR intensity in the firstchannel that were caused by solar sources, we analyzedthe ratio of the observed intensity to the calculatedvalue [6]. The calculated intensity was obtained byextrapolating the model spectrum to lower energies andcorresponded to the expected value in the absence ofSCR. The parameters of the calculated spectrum pereach observation hour were calculated in the energyrange

≥

4 MeV, where the secondary SPE effects(caused by the GCR modulation by induced, polariza-tion, and vortex electric fields) dominate.

10

6

0 4

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8 12 16 20 24 28

December 2006

10

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January 2005

Fig. 1.

Ratios of the experimental and calculated CR intensities in energy ranges of 0.8–4.0 MeV (bold line) and 4.0–9.0 MeV (thinline): the data of January 2005 (upper plot) and December 2006 (lower plot).

Number of particles, (cm

2

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GV

Fig. 2.

Model (solid lines) and observed (circles) CR rigidity spectra.

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PRIMARY AND SECONDARY EFFECTS IN COSMIC-RAY VARIATIONS 327

RESULTS AND DISCUSSIONFigure 1 shows the observed/calculated intensity

ratios in two energy ranges: 0.8–4.0 and 4.0–9.0 MeV.The upper and lower plots are the data of January 2005and December 2006, respectively. When these ratiosare close to unity, the observed intensities in the chan-nels under consideration are consistent with the calcu-lated data, i.e., are due to the secondary SPE effect. Theplots indicate that the ratio of the observed intensities tothe calculated data for the second channel is mainlyclose to unity, except for individual instants in the ini-tial SPE stages. Concerning the first channel, one canclearly see the presence of a low-energy (0.4–4.0 MeV)particle source, since the observed/calculated intensityratios significantly differ from unity. Apparently, theenergy range

≤

4 MeV is a peculiar “reservoir” accumu-lating SCRs after the energy transfer from them tobackground particles and interplanetary magnetic fieldsduring their propagation from the Sun to the Earth.However, it is interesting that in some periods (whenthe particle intensity increases at SPEs) the secondaryCR effects (the analyzed ratios are close to unity) dom-inate also in the first channel. In these periods, SCRsare observed only in short ranges (in the initial stagesduring the GLE at the beginning of December 20, 2005and at the beginning of December 13, 2006), where theratios synchronously increase in the first and secondchannels. Figure 2 shows the model (solid lines) andobserved (circles) spectra at the instants indicated in theplots. An analysis of Figs. 1 and 2 suggests that thespectrum used adequately describes the observed CRintensities, beginning with the third channel on GOES-11(i.e., the intensities of CRs caused by secondary SPEeffects). The discrepancies between the calculated andobserved intensities in the second and first GOES-11

channels are explained by the presence of particlesources of solar origin (primary SPE effects).

CONCLUSIONS

The analysis in this study showed that the CR inten-sity increases in SPE periods are due to two reasons(primary and secondary CR effects). In the energyranges where the used model spectrum describes wellthe observed dependence of the CR intensity on the CRhardness in the analyzed time intervals (Fig. 2), second-ary SPE effects (GCR modulation caused by the changein the GCR energy in the electromagnetic fields formedin the heliosphere due to the SCR propagation) aredominant. In the energy ranges characterized by signif-icant discrepancies between the model and observedintensities, primary effects caused by the arrival of par-ticles of solar origin (after their energy transfer to back-ground particles and interplanetary magnetic fields dur-ing propagation from the Sun to the Earth) dominate.

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