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Advances in Space Research 38 (2006) 1642–1646

Precipitating electron events in October 2003 as observed in thepolar atmosphere

V.S. Makhmutov a,*, G.A. Bazilevskaya a, L. Desorgher b, E. Fluckiger b

a Lebedev Physical Institute RAS, Leninsky pr. 53, 119991 Moscow, Russiab Physikalisches Institut, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland

Received 27 September 2004; received in revised form 18 July 2005; accepted 17 January 2006

Abstract

Balloon cosmic ray measurements at polar latitudes (Apatity station, geomagnetic vertical cutoff rigidity Rc = 0.6 GV, invariant lat-itude K = 65�, and Mirny observatory, Antarctica, Rc = 0.03 GV, K = �77�) in October 2003 were analysed. During this period, severalenergetic electron precipitation events were recorded via the bremsstrahlung generated by the electrons in the atmosphere. In order todetermine the flux and the energy spectrum of precipitating electrons on the atmospheric boundary the simulation of electron and X-raytransport in the atmosphere was performed using the Geant4 Monte Carlo code. By comparing the results of balloon measurements withthe simulation results, we deduced the precipitating electron flux during the 22 October 2003 event as Je (E) = 7.7 · 106 exp(�E/12 keV),cm�2 s�1 keV�1.� 2006 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Cosmic rays; Electron precipitation; Atmosphere

1. Introduction

This paper deals with the measurements of ionizing radi-ation by a balloon-borne radiosound in the atmosphere(Charakhchyan, 1964; Bazilevskaya and Svirzhevskaya,1998; Stozhkov et al., 2001). Charged particle radiationin the atmosphere at altitudes from the ground level upto 30–35 km has been monitored since 1957 up to nowby the cosmic ray group of Lebedev Physical Institute(LPI), Russia. These measurements allow us to investigatemodulation processes in galactic cosmic rays (GCR), solarproton events and also to study the precipitation of ener-getic electrons into the Earth’s atmosphere. Observationsare carried out with the light radiosounds at several lati-tudes including the polar ones. The balloons are launchedseveral times a week. A balloon flight lasts usually �1.5–2 h so that a radiosound is at altitudes above 20 km during20–40 min (sometimes up to 1.5 h). Information on the

0273-1177/$30 � 2006 COSPAR. Published by Elsevier Ltd. All rights reserv

doi:10.1016/j.asr.2006.01.016

* Corresponding author. Tel.: +7 095 4854263.E-mail address: makhmutv@sci.lebedev.ru (V.S. Makhmutov).

count rate of a Geiger counter, Geiger counter telescopeand residual atmospheric depths is returned every minute.The charged particle fluxes have typical altitude depen-dence in the atmosphere with the Pfotzer maximum dueto galactic cosmic rays (GCR) cascade processes. Thisdependence is violated during sporadic solar or magneto-spheric particle intrusion. Both a single Geiger counterand a telescope respond to the solar proton intrusion. Pre-cipitating magnetospheric electrons are absorbed at alti-tudes above �50 km but they generate thebremsstrahlung X-rays penetrating down to altitudes of15–20 km depending on their initial energy. The countertelescope does not record the X-rays but a single counterresponds although with efficiency �1%. This allows us todetect the electron precipitating events (EPEs) wheneverthey occur while a radiosound is at the relevant altitude.In majority of EPEs, we deal with subrelativistic electrons.

We identified an EPE if the enhancement in the countrate of a single counter (but not of a telescope) for >30%was observed at altitude higher than 20 km for at least dur-ing 10 min. Then subtracting a background caused by the

ed.

Fig. 1. From top to bottom: (A) CR flux (cm�2 s�1) at the Pfotzermaximum in the atmosphere measured at Apatity and Mirny; (B) NMApatity hourly count rate; (C–E) ACE solar wind plasma observations:solar wind speed (C), IMF magnitude B (D) and Bz-component (E); (F)hourly averaged geomagnetic Dst-index. The vertical dashed lines indicatethe times of EPE observations at Apatity.

V.S. Makhmutov et al. / Advances in Space Research 38 (2006) 1642–1646 1643

secondaries of GCR from the data of a single counter wegot count rate due to bremsstrahlung X-rays versus atmo-spheric depth. The basic characteristics of EPEs recordedin the atmosphere, and interplanetary and geomagneticconditions related to their occurrence are given in Bazilevs-kaya and Makhmutov (1999), Makhmutov et al. (2001,2003a,b,c).

Relativistic electrons in the inner magnetosphere com-prise a huge fluxes, their origin and evolution not beingquite understood as yet. Dynamics of this population isof great interest for both scientific (underlying physicalprocesses) and practical (space weather) aspects (Bakeret al., 1997; Baker, 2000). Relativistic electron flux exhibitsstrong variations on the timescales from seconds to days(e.g., Foat et al., 1998) indicative of complex processes ofacceleration and transport. Precipitation may be the dom-inant loss mechanism for this population (Millan et al.,2002). Precipitation of relativistic electrons into the atmo-sphere is observed with both balloon borne and spacecraftinstruments (Brown and Stone, 1972; Imhof et al., 1991; Liet al., 1997). Most of experiments are dedicated to thisproblem and make observations with high angular andtime resolution (e.g., Parks et al., 1993; Lorentzen et al.,2000; Millan et al., 2002; Tan et al., 2004). However, theLPI experiment has an advantage of being in situ atmo-spheric radiation measurement at the same locations andlocal time during several decades. Our data series is homo-geneous in the course of almost 50 years although each dai-ly observation lasts only about half an hour. It is importantto interpret the data of the X-rays balloon observations interms of fluxes and energy spectra of precipitating electronsat the atmospheric boundary.

This paper presents results of EPEs observations in theatmosphere during October 2003. The measurements wereperformed in northern (Apatity, Murmansk region,68�57 0N, 33�03 0E), and southern (Mirny obs., Antarctica,66�34 0S, 92�55 0E) polar regions. Using the Monte CarloATMOCOSMICS code based on Geant4, we have simulat-ed the production of bremsstrahlung gamma rays inducedby the precipitation of energetic electrons in the atmo-sphere (Agostinelli et al., 2003; Desorgher et al., 2003; Des-orgher, 2004). By comparing our simulation results to themeasurements of secondary photon fluxes in the atmo-sphere, we can estimate the primary flux of the precipitat-ing electrons. We present simulation results and theestimation of the primary flux.

2. EPE observations in the Earth’s atmosphere in October

2003

In the second half of October 2003 five balloon launch-ings at Apatity and six launchings at Mirny were per-formed, the ceiling altitudes being above 20 km. ThreeEPEs were registered in the polar atmosphere at Apatityon 15, 17 and 22 October. On 20 October no EPE wasobserved but the ceiling balloon altitude was �22 km, soan EPE might be present at higher altitude.

In Fig. 1, panel (A) represents the measurements of cos-mic rays at the Pfotzer maximum in the atmosphere atApatity and Mirny. Panel (B) shows the ground based neu-tron monitor (NM) Apatity observations during 12–24October 2004. ACE solar wind speed, interplanetary mag-netic field (IMF) magnitude B, and Bz component areshown on the panels C, D and E, respectively (http://sec.-noaa.gov). The geomagnetic Dst index is shown on the bot-tom panel (F). The vertical dashed lines denote the times ofEPE observations in the atmosphere at Apatity. A longlasting geomagnetic disturbance began at the end of 12October and was caused by the arrival of a high speed solarwind stream that reached a maximum speed of�730 km s�1 on 15 October. The IMF increased up to�17 nT and had a negative Bz component over �10 days.A small decrease of GCR intensity was detected by theNM Apatity during 15–17 October. A GCR Forbushdecrease began at the end of 21 October. The EPEs wereobserved during a period when Dst was oscillating arounda value lower than �40 nT. The events were recorded atApatity only and not at Mirny. This could be due to thefacts that, (1) in contrast to Apatity, Mirny is located inthe region of open geomagnetic field lines, and (2) thesource of precipitating electrons in these events is the ter-restrial magnetosphere.

Fig. 2 shows the time profiles of count rates of the omni-directional counter (1) and of the vertical telescope (2) dur-

N (

min

-1)

N (

min

-1)

N (

min

-1)

Fig. 2. Counting rates (N, min�1) of omnidirectional counter (1) and ofvertical telescope (2) during the EPEs on 15, 17 and 22 October 2003.Dashed horizontal lines mark the periods when electron precipitation wasobserved (see text for details).

Fig. 3. The calculated differential energy spectra of photons at atmo-spheric level X = 15 g cm�2. Solid and dashed lines show the resultsobtained for vertical and isotropic electron flux, respectively. The resultsare normalized to 1 primary electron with energy Ee = 50 keV, 500 keVand 5 MeV.

1644 V.S. Makhmutov et al. / Advances in Space Research 38 (2006) 1642–1646

ing the EPEs on 15, 17 and 22 October 2003. The dashedhorizontal lines mark the periods when electron precipita-tion was observed. For the purpose of comparison, the tele-scope count rate was multiplied by a factor eight. It isinteresting to note that the first two events (15 and 17 Octo-ber) show the presence of variations in the flux of second-ary photons with characteristic times of �5–7 min andamplitude up to 100% (lines marked by 1 in Fig. 2) duringthe time periods of electron precipitation. These variationswere only recorded by the omnidirectional counter andtherefore were caused by photon fluxes. At the same timethe telescope recorded GCR fluxes. The joint analysis ofthe omnidirectional counter, telescope records and radio-sonde altitude sensor allows us to conclude that theobserved variations of the photon flux reflect the time vari-ations of the precipitating electron flux during these events.The detailed analysis of the nature of the variations record-ed during these events is out of the scope of this paper. Weonly note that the variations of precipitating electron fluxwere reported earlier with a variation time scale extendingfrom a few seconds to a few minutes (e.g. Lazutin et al.,1982; Foat et al., 1998).

3. Estimation of the precipitating electron flux and energy

spectra

As mentioned before, during the EPEs a flux of second-ary bremsstrahlung photons is generated by a precipitatingelectron flux at the top of the atmosphere. These photonscan penetrate down to altitudes of 15–20 km in the atmo-sphere and can be registered by a radiosound. The maincharacteristic of an electron precipitation event observedin the atmosphere is a photon absorption spectrum during

the event (for details see Makhmutov et al., 2003a). In caseof smooth increasing of the X-ray fluxes with altitude wecan assume a constant flux of precipitating electrons atthe atmospheric boundary (during the balloon flight). Inorder to estimate the energy spectrum of the incident elec-trons we have simulated the production of the secondaryphotons and their transport through the atmosphere, sim-ilarly to other works (e.g., Berger and Seltzer, 1972; Bergeret al., 1974; Lazutin, 1979; Kalinina et al., 1988). However,our consideration was specially directed to simulation asclose as possible of conditions of the LPI balloon measure-ments. For this purpose we have used the Monte CarloATMOCOSMICS code based on Geant4 (Desorgheret al., 2003; Desorgher, 2004). This code simulates thehadronic and electromagnetic interactions of energetic par-ticles at energies E < 100 GeV with the Earth’s atmosphere(atmospheric model MSISE90 or NRLMSISE00 orTABLE; Desorgher, 2004). It computes the resulting fluxof atmospheric shower particles at user defined altitudesand/or atmospheric depths, the energy deposited in theatmosphere versus altitude and/or depth, and the produc-tion of cosmogenic nuclides. The simulation can be carriedout with or without applying the Earth’s magnetic field.The electromagnetic shower is simulated by taking intoaccount the following processes: bremsstrahlung, energyloss by ionisation, multiple scattering, pair production,Compton scattering and photoelectric effect. For the simu-lation of hadronic interactions, different models of Geant4are used, depending on the energy range. The decay of par-ticles is included in the code. Fig. 3 presents results of sim-ulations for monoenergetic incident electrons. It shows thecomputed differential energy spectra of photons at atmo-spheric level X = 15 g cm�2 induced by the precipitationof 50 keV, 500 keV and 5 MeV electrons in the atmosphere.These spectra are normalized to one primary incident elec-tron. The solid and dashed lines correspond to isotropic

V.S. Makhmutov et al. / Advances in Space Research 38 (2006) 1642–1646 1645

and vertical fluxes of primary electrons, respectively. Ourresults are in excellent agreement with those of Bergerand Seltzer (1972).

In order to estimate the spectrum of precipitating elec-trons during EPEs, we made an assumption that the primaryflux of precipitating electrons at the top of the atmosphere ischaracterised by an exponential spectrum, Je � exp (�E/Eo)(e.g., Lazutin, 1979), with Eo in the range 10 keV–1 MeV. Wehave computed the number of secondary photons withE > 20 keV at different atmospheric depths (X = 0, 0.05,0.5, 1, 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 31, 34, 37, 40, 43, 46,49, 51 g cm�2) resulting from the precipitation of such typeof primary flux with different Eo. For each exponential coef-ficient Eo, we obtained a photon absorption profile (photonnumber versus atmospheric depth) that we fit with an expo-nential law Jph � exp(�X/Xo). Fig. 4 shows the dependenceof the parameter Xo of the photon absorption spectrum as afunction of the exponential coefficient Eo of the primary elec-tron spectrum. The solid and dashed lines correspond to ver-tical and isotropic primary electron flux, respectively. Usingthe results shown in Fig. 4, we can deduce the spectral param-eter Eo for a specific EPE from the observed parameter Xo ofthe absorption spectrum.

Among the EPEs observed in October 2003 only eventof 22 October was free of temporal variations and enabledus to proceed to the precipitating electron spectrum. Forthis event, we estimated the photon absorption spectrumas Jph(cm�2 s�1) = 699 exp(�X/Xo), where X is an atmo-spheric depth, Xo = 3.8 g cm�2. Using the above statedprocedure and after normalisation to the observed photonflux we deduced for the 22 October 2003 event (09.21–09.32UT) an electron primary spectrum Je (E) = 7.7 · 106

exp(�E/12 keV), cm�2 s�1 keV�1. Because of experimen-tal uncertainty the accuracy of the spectrum estimation isabout 50% (Bazilevskaya and Svirzhevskaya, 1998). Theprecipitating electron spectrum obtained is similar to thosereported by Imhof et al. (1991) and Lazutin, 1979.

2

5

8

11

14

0.01 0.1 1

Eo, MeV

Xo,

g*c

m-

2

Fig. 4. Results of the simulations: parameter Xo as a function of theparameter Eo. The solid line shows the result obtained for electron flux ofvertical incidence at the top of atmosphere, the dashed line refers to anisotropic angular distribution.

4. Results

Electron Precipitation Events were observed at Apatity(northern polar region) on 15, 17 and 22 October 2003.For the first two events variations of the photon flux ona few minute time scales were observed. An event of 22October was free of significant temporal flux variations.Using the Geant4 ATMOCOSMICS code, we have calcu-lated the interaction of energetic electrons in the Earth’satmosphere and the subsequent transport of the brems-strahlung photons through the atmosphere. By comparingsimulation results with the balloon cosmic ray measure-ments we deduced the energy spectrum of the precipitatingelectrons during the 22 October event as Je (E) =7.7 · 106 Æ exp(�E/12 keV), cm�2 s�1 keV�1.

Acknowledgements

The results were obtained with partial support ofRFBR (Grants 04-02-17380, 04-02-31007 and 05-02-16185), CNPq (300.000/2005-7) and the Swiss NationalScience Foundation (Grant NF 20-67092.01). The devel-opment of the ATMOCOSMICS code was supported bythe European Space Agency through the SEPTIMESSproject under Contract No. 16339/02/NL/FM. We areindebted to reviewers whose comments were veryuseful.

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