Sako, T

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29th International Cosmic Ray Conference Pune (2005) 1, 2124Possible   10GeV Particle Detection in Association with the 28 May2003 Solar FlareT. Sako  , Y. Muraki  , K. Masuda  , Y. Matsubara  , K. Watanabe  , H. Menjo  , H. Tsuchiya  ,T. Sakai  , S. Shibata  , R. Ogasawara  , Y. Mizumoto  , M. Nakagiri  , A. Miyashita (a) Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-8601, Japan(b) RIKEN, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan(c) College of Industrial Technologies, Nihon University, Narashino 275-0005, Japan(d) Department of Engineering, Chubu University, Kasugai 487-8501, Japan(e) National Astronomical Observatory of JapanPresenter: T. Sako (sako@stelab.nagoya-u.ac.jp), jap-sako-T-abs1-sh11-posterWe report a short time scale excess observed in the Norikura and Hawaii Solar Neutron Telescopes, theNorikura neutron monitor and the Norikura muon telescope at the beginning of the X3.6 class solar flareoccurred on 28 May 2003. Because the direction of the particles measured with the Norikura SNT was to-ward to the sun, we regard it as a solar neutron signal. To explain the response of different energy channelsof the detector, we recognized that consideration of high energy (  10GeV) primary neutrons are necessary.We performed full Monte Carlo calculations of the solar neutron propagation from the sun to the detector andfound that high energy neutrons with a hard ( 	 ) spectrum can explain the observed counts of differentchannels.1. IntroductionHigh energy neutrons emitted from the solar surface (solar neutrons) are being monitored long time to studythe nature of ion acceleration on the sun. Unlike ions, the path of the neutrons is not bent by magnetic fields.The arrival time profile on the earth therefore reflects the emission/acceleration profile on the sun. On the otherhand, since the masses of both ions and neutrons are comparable to their kinetic energies, the observed arrivaltimes disperses according to their energy spectra. Consequently, observed time profile of solar neutrons is aresult of degeneracy of emission profile and energy spectrum. To solve this degeneracy, the Solar Neutron Tele-scope (SNT) is designed and operated as a world wide network [1][2]. Effort to obtain spectrum informationfrom classical neutron monitor is also in progress [3].In this paper, we present an analysis result of a solar neutron event associated with the 28 May 2003 solarflare. To understand the counting rate of energy dependent channels of SNTs, we performed full Monte Carlocalculations of neutron propagation from the sun to the detectors.2. ObservationsThe flare occurred at 0:17 UT on 28 May 2003. The soft X-ray monitored by GOES peaked at 0:27 UT andthe peak flux reached at X3.6. Because the orbit of the RHESSI satellite was in the night, no gamma-ray datawas obtained. Only weak increase of  MeV proton was observed in space after this flare. At the time ofthe flare onset, Mauna Kea (Hawaii, USA) and Norikura (Japan) are suitable places to observe solar neutronsby SNT. The zenith angles of the sun from the observatories were 26  and 37  , respectively. Column densitiesof the atmosphere to the direction toward the sun were 676 ﬀﬂﬁ and 908 ﬀﬃﬁ , respectively.Counting rates of the different type of the detectors are shown in Figure-1. For the Norikura SNT, count of the22 T. Sako et al.plastic scintillator (64 ﬃﬁ and 20 cm in thickness), which acts as a target of neutron to generate charged particleis plotted (S1 Anti). High energy channel (L4) is also plotted which counts penetrating charged particles, whichhits scintillator and 4 successive layers of proportional counters (PRCs) below scintillators. Scintillators arecovered by proportional counters and ’antied’ data are shown here to enhance signal from neutral particles.In addition, we show counting rates of the muon telescope (U+L) and neutron monitor (NM). U+L recordsthe count of two layers of 36 m ﬁ thin plastic scintillators. These 4 data were obtained at the same place,the Norikura Observatory (137.5  E, 36.1  N, 2770 m a.s.l.), Institute for Cosmic Ray Research, University ofTokyo. At the bottom of Figure-1, data from the Mauna Kea SNT is shown, which is the channel measuring thecount of the plastic scintillator with 8 m ﬁ and 20 cm thickness. The Mauna Kea SNT is located in the buildingof the Subaru telescope at Mauna Kea, Hawaii (156.3  W, 19.8  N, 4200 m a.s.l.).We can find excess counts just before flare onset indicated in the figure. Though each excess is small, the coin-cidence in time, especially among the different type of detectors and detectors in different place is remarkable.3. Puzzle of High Energy ChannelAs the Norikura SNT can measure the direction of particles when they penetrate 4 layers of PRCs, we mappedthe direction at 11:15-11:17 UT. At this time, the sun was to the east from Norikura and we found excess inthis direction. This supports our interpretation that we have observed solar neutrons.Similar SNT events showing asymmetrical excess from the solar direction have been already reported [5] [6].However, we have been puzzled by these events. Because the excess in the direction channel is produced byrelatively high energy particles, comparison with lower energy channel must give us information of the neutronspectrum. Here, we take a ratio L4/S1 Anti as an indicator of the hardness of the incoming particles. Taking thecount at 11:15-11:17 UT, observation gives hardness ratio of 0.61  0.40  "!$#%! , nearly unity. The observationalsituation are similar for the other events introduced in the references above.We performed Mote Carlo calculation assuming neutron spectrum proportional to &('*)+ , where & + is kineticenergy of neutrons. Time dispersion and decay during the flight between the sun and the earth, attenuationin the earth’s atmosphere, detector response are all calculated. The resultant hardness ratio for a conventionalpower law index, 4.5, was , 0.03 when counting rate reaches at the peak at 300 sec after light. This considerabledifference from the observation ( , 1) has puzzled us.As first suspected, we further studied the relation between the hardness ratio and the power law index of theprimary neutrons. For  =2.5, we found hardness ratio evolves from 0.08 to 0.03 in time. Maximum value,0.08, is determined because the highest energy of the Monte Carlo calculation for the atmospheric attenuationis limited at 1700 MeV [4]. Because high energy neutrons arrives in a short time interval, even with a powerlaw spectrum highest energy particles significantly contribute to the time profile when the spectrum is hard.This was not the case for  =4.5 where the maximum count appears at 300 sec meaning that the contributionfrom 250 MeV neutrons is significant.To extend the maximum energy for the calculation of atmospheric attenuation, we decided to use GEANT4.6.2with the QGSP Bert model [7]. The MC package must be able to treat pion generation and must agree withthe basic results of [4] below 1 GeV. GEANT4 satisfied these requirements. When we simulated neutrons with =2.5 and energies between 10 GeV and 50 GeV, the hardness ratio became 0.2. The result agrees well withthe observational value in the statistical error.- 10GeV particle on 28 May 2003 234. DiscussionThough  10 GeV neutrons arrive within 2 seconds, the excess seen in the observations continued for about 2minutes. We can not deduce the evolution of hardness ratio during 2 minutes because of the limited statistics.As explained above, with  =2.5 highest energy neutrons mostly determine the detector’s response. If theemission lasts 2 minutes with  =2.5, it can explain average hardness ratio of , 0.2 during 2 minutes.To explain the observed excess count of S1 Anti, we need a solar neutron emission flux on the sun to be,/.01ﬁ324.65$&+7198;:<>=?'@ﬁﬀA B MeV 'DC sr '*C . Independent calculations to explain the Mauna Kea SNT countrate concludes that we also need /.EFGﬁ32>.65$& + 918;:ﬀ<4=%'HﬁIA B MeV 'DC sr '*C emission. We must note that theMauna Kea SNT suffers ambiguous heavy attenuation from the Subaru telescope structure when the sun is inthe north-west direction as was the case in this event. In the calculation above, we included 1 m thick concretearound the detector.With such high energy primary neutrons, GEANT4 predicts not only neutrons but also comparable numberof muons and gamma-rays to arrive at the detector. Because the SNT has sensitivity to these particles, thereappears a difference from the neutron monitor response, which is mainly sensitive to hadrons. Assuming theneutron flux deduced above and considering the NM efficiency to the hadrons, the expected excess count ofNM at Norikura is calculated to be , 400, that is 1.5 J excess in 2 minutes. This explains why the NM datashows only very marginal bump at that time. Because the efficiency of NM for muons is estimated to be 1% ofthat for hadrons [8], contribution from muons is negligible in the present case.5. ConclusionsWe studied a solar neutron event associated with the 28 May 2003 solar flare. An emission of very high energy(  10 GeV) neutrons with a power index of 2.5 is necessary to explain the high hardness ratio observed inthe Norikura SNT. Because high energy neutrons from such a hard spectrum concentrate in very short timeintervals, 2 seconds, they can dominate the detector response. Consequently, observed time dispersion of 2minutes is attributed to its emission on the sun. From the observations at two different places, Norikura andMauna Kea, we obtained the same flux of neutrons independently. The estimated flux is one order of magnitudeweaker at 100 MeV than so far reported solar neutron events. However total energy emitted between 100 MeVand 100 GeV is almost same. Because the  10 GeV neutrons generates considerable number of non hadroniccomponent in the atmosphere, SNTs are suitable to observe such events than neutron monitors.As repeated, neutrons with hard spectrum results a very short excess in the detector. If the emission is instan-taneous, solar neutron event lasting only 1 second is possible at an observable level with a moderate flux. Wemust pay attention for such missing phenomenon.References[1] Y. Muraki, 22nd ICRC, Dublin (1991) 3, 45.[2] H. Tsuchiya et al., Nucl. Instr. & Meth. A 463, 183 (2001).[3] J. W. Bieber et al., J. Geophysical Res., 109, A12106 (2004).[4] S. Shibata, J. Geophys. Res., 99, 6651 (1994).[5] H. Tsuchiya et al., 27th ICRC, Hamburg (2001), 8, 3056.[6] T. Sako et al., 28th ICRC, Tsukuba (2003), 6, 3175.[7] S. Agostinelli et al., Nucl. Instr. & Meth. A 506, 250 (2003).[8] C. J. Hatton, Prog. Elementary Particle and Cosmic Ray Physics, X, 1, (1971).24 T. Sako et al.UT23:00:00 23:30:00 00:00:00 00:30:00 01:00:00 01:30:00Counts/ 2.0 min813814815816817818310×00:17:00 UTMEAN = 815589S1_antiUT23:00:00 23:30:00 00:00:00 00:30:00 01:00:00 01:30:00Counts/ 2.0 min345.5346346.5347347.5348348.5310×00:17:00 UTMEAN = 346841L4Norikura64 2003/5/28UT23:00:00 23:30:00 00:00:00 00:30:00 01:00:00 01:30:00Counts/ 2.0 min22162218222022222224310×00:17:00 UTMEAN = 2220658U+LUT23:00:00 23:30:00 00:00:00 00:30:00 01:00:00 01:30:00Counts/ 2.0 min655006600066500670006750000:17:00 UTMEAN = 66430neutron_monitorTanshuki 2003/5/2823:00:00 23:30:00 00:00:00 00:30:00 01:00:00 01:30:00Counts/ 2.0 min351.5352352.5353353.5354354.5355355.5310×00:17:00 UTMEAN = 353621PMT_LHawaii 2003/5/27Figure 1. Counting rate prole around the solar are occurred at 0:17 UT on 28 May 2003. Top panel shows count ofscintillator of Norikura SNT (S1 Anti). Second shows count for penetrating particles of Norikura SNT (L4). Third panelshows scintillator count of the Norikura muon telescope. Fourth shows Neutron Monitor at Mt.Norikura. Bottom panelshows counting rate of scintillator of Mauna Kea SNT.

Possible >10GeV Particle Detection in Association with the 28 May 2003 Solar Flare

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Possible &gt;10GeV Particle Detection in Association with the 28 May 2003 Solar Flare

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Possible >10GeV Particle Detection in Association with the 28 May 2003 Solar Flare