Detection of Solar Neutrons and Protons by the ground level detectors Y. Muraki, Y. Matsubara, S. Masuda, T. Sako STE-laboratory, Nagoya University, Nagoya, Japan E.O. Flueckiger, R. Buetikofer Physikaliches Institute, University of Bern, Bern, Switzerland A. Chilingalian and G. Hovsepyan Yerevan Physics Institute, Yerevan, Armenia F. Kakimoto, Y. Tsunesada, H. Tokuno Tokyo Institute of Technology, Tokyo, Japan A. Veralde UMSA, Lapaz, Bolivia T. Sakai College of Industrial Technology, Nihon University, Narashino, Japan @ 20th ECRC Sep. 8th 2006 at Lisbon

contents 1. Experimental purpose--- Why do we study solar neutrons? 2. The detectors and the Criterion of the data selection---list of new events 3. An introduction of an interesting event observed in April 15th, 2001. 4. Tips for the solar cycle 24

1. The experimental Purposes 2. Scientific purposes 1. The experimental Purposes Sun Earth Neutron Proton, Ion Magnetic field Physics aim is to confirm particle acceleration model at the solar surface. . When, How? Positive astronomy = Experimental Astronomy However observation of protons does not give us any message about it. Protons are usually coming a few hours later from the flare. This is the reason why we use neutron channel.

2. The detectors and Data Analysis -- SONTEL in European side -- The detector at Gornergrat 　　　　The detector at Aragatz

Criterion of the data selection 1. We searched the excesses of events during solar flare The intensity of flares detected by the GOES X-ray satellite must be stronger than class >X1.0 . ~100 events were observed during 11years of the solar cycle 23 * at the emission time of hard X-ray ( about 3 minutes) or * during GOES start and Maximum time ( about 20 minutes) 2. The atmospheric depth must be less than 850 gr/cm2 3. Local time of the detector must be within ±2 hours around the noon We have found 4 new events by the Gornergrat detector and 1 new event by Aragatz detector since the report by Flueckiger et al in the 27th ICRC (Hamburg), Vol. 8 (2001) 3044.

List of solar neutron events observed by the Gornergrat detector in in the solar cycle 23 Date Begin Max 1/2 Max class position region Gornergrat(g/cm2) σ comments 980427 0855 0920 0938 X 1.0 S16E50 8210 800 0) 000712 1018 1037 1046 X 1.9 N17E27 9077 710 3.3 ● 000714 1003 1024 1043 X 5.7 N22W 7 9077 715 0 no signal 010329 0957 1015 1032 X 1.7 N20W19 9402 836 4.2 ● 010402 1004 1014 1020 X 1.4 N17W60 9393 824 2.7 ● 010402 1058 1136 1205 X 1.1 Y 0M 0 0 799 3.1 ● 010412 0939 1028 1049 X 2.0 S19W43 9415 787 3.0 ● 010415 1319 1350 1355 X 14.0 S20W85 9415 838 2.5 1)Bolivia event ● 010924 0932 1038 1109 X 2.6 S16E23 9632 879 4.7 2)Tibet event (Sako) 030318 1151 1208 1220 X 1.5 S15W46 314 853 4.0 3) 040716 1032 1041 1046 X 1.1 S10E36 649 712 3.4 4) 050714 1016 1055 1129 X 1.2 Y 0M 0 786 708 0 no signal 050909 0942 0959 1008 X 3.6 Y 0M 0 808 805 3.2 5) ● mark represents that those events were reported in Proceed. 27th ICRC (Hamburg), 8 (2001) 3044. 0) The telescope function has been introduced in the SONTEL after September 27th, 1999. 1) Only at the highest channel, the excess is seen just before the GLE at 14:00UT. There is also minor excess at the flare start time 13:19UT in all channels. Only neutron monitor has shown an excess. A good record of solar activity was observed by Yokoh SXT and HXT imagers. 2) At the south channel and >160 MeV with anti, two peaks are seen at the flare start time (9:32UT) and at 9:54UT. A clear signal was detected at Tibet at 9:32UT . However no strong HXT signal was observed. The event was pointed out first by Sako. 3) At the south channel, signal is observed at 12:04UT. The flare start time was 11:51UT. 4) Signals are observed in three channels >40 MeV, >80 MeV and >120 MeV with anti. 5) Clear excess is seen at 10 UT for >120 MeV and > 160 MeV with anti.

List of solar neutron events observed by the Mt Aragatz detector in Armenia Date Begin Max 1/2Max class position region Armenia (g/cm2) σ comments 980427 855 920 938 X 1.0 S16E50 8210 731 980506 758 809 820 X 2.7 S11W65 8210 732 data handling error 980818 814 824 832 X 2.8 N33E87 8307 754 data handling error 000324 741 752 759 X 1.8 N16W82 8910 846 0 000712 1018 1037 1046 X 1.9 N17E27 9077 716 3.5 ● 000714 1003 1024 1043 X 5.7 N22W07 9077 705 0 ● 010402 1004 1014 1020 X 1.4 N17W60 9393 794 4.7 ● 010410 506 526 542 X 2.3 S23W09 9415 1062 4.5 1) ● 010412 939 1028 1049 X 2.0 S19W43 9415 767 3.4 ● 020718 724 744 749 X 1.8 N19W30 30 725 0 2) 031028 951 1110 1124 X 17.0 S16E08 486 954 3.8 3) 040717 751 757 759 X 1.0 S10E24 649 718 14? noise? 050730 617 635 701 X 1.3 N12E61 792 800 (2.6) fluctuation? 050909 942 959 1008 X 3.6 Y 0M 0 808 805 0 050915 830 838 846 X 1.1 S12W14 808 825 (2.8) fluctuation? ● mark represents that those data were reported in Proceed. 27th ICRC (Hamburg), 8 (2001) 3044. 1) Signals in association with this event are recorded also in Tibet and Norikura detectors at 05:06 UT and 5:15 UT. 2) Signal was not observed also in Tibet detector. 3) Neutrons were observed by the neutron monitor located at Tsumeb and Hermanus at 11:06UT. By the neutron telescopes at Aragatz and Gornergrat, neutrons were observed at 9:51 UT the flare start time.

Probability of chance coincidence Operation : T= 7 years gate width Coincidence with hard X-ray flare : typical 3 min. Coincidence with soft X-ray flare : typical 20 min. Intensity of X-rays > X1.0 Neutron excess frequency probability (hard X) (soft X) 3.0 σ 1620 0.087 0.56 3.5 σ 280 0.015 0.10 4.0 σ 38 0.002 0.013 4.5 σ 4 0.0002 0.0013

3. A detail analysis of an interesting event Date Begin Max 1/2 Max class position region Gornergrat(g/cm2) σ comments 980427 0855 0920 0938 X 1.0 S16E50 8210 800 0) 000712 1018 1037 1046 X 1.9 N17E27 9077 710 3.3 ● 000714 1003 1024 1043 X 5.7 N22W 7 9077 715 0 no signal 010329 0957 1015 1032 X 1.7 N20W19 9402 836 4.2 ● 010402 1004 1014 1020 X 1.4 N17W60 9393 824 2.7 ● 010402 1058 1136 1205 X 1.1 Y 0M 0 0 799 3.1 ● 010412 0939 1028 1049 X 2.0 S19W43 9415 787 3.0 ● 10415 1319 01350 1355 X 14.0 S20W85 9415 838 2.5 1)Bolivia event ● 010924 0932 1038 1109 X 2.6 S16E23 9632 879 4.7 2)Tibet event (Sako) 030318 1151 1208 1220 X 1.5 S15W46 314 853 4.0 3) 040716 1032 1041 1046 X 1.1 S10E36 649 712 3.4 4) 050714 1016 1055 1129 X 1.2 Y 0M 0 786 708 0 no signal 050909 0942 0959 1008 X 3.6 Y 0M 0 808 805 3.2 5) ● mark represents that those events were reported in Proceed. 27th ICRC (Hamburg), 8 (2001) 3044. 0) The telescope function has been introduced in the SONTEL after September 27th, 1999. 1) Only at the highest channel, the excess is seen just before the GLE at 14:00UT. There is also minor excess at the flare start time 13:19UT in all channels. Only neutron monitor has shown an excess. There left a good quality of the record of solar activity by Yokoh SXT and HXT.

The solar position of 2001.4.15 flare S20W85 GOES X-ray level X14 13:19 start 13:50 maximum 13:55 end Yokoh HXT

Top two panels Gornergrat > 40 MeV >160 MeV GLE is seen from 14:00UT Bottom 3 panels Chacaltaya at 13:48UT Neutron signal is seen

Bolivia Chacaltaya 5,250m 5 min value (720g) 13:48 start?

Bolivia Chacaltaya 5,250m 1 min value

Swiss Gornergrat 3,150m 1min value 2001.4.15 Peak at 13:57UT (800g)

Armenia Mt. Aragatz 3250m 1 min value Peak at 13:57UT (1000g)

The April 15th 2005 event　（１） In comparison with a large solar flare observed in September 7th 2005 (X17), the signal of neutrons was 15 times weaker than it. However the statistical of April 15th event, the significance was as large as 8 σ. The detection efficiency of the solar neutron detector at Bolivia (made of the plastic scintillator) is calibrated by the September 7th 2005 event. They turn out as 17.8% , 9.5% and 3.2 % for >40MeV, >80MeV and >160MeV respectively of the total detection efficiency of the neutron monitor. Therefore, the intensity of neutrons to the new neutron detector is just one sigma (< 1 σ) : level of the fluctuation. That is the reason why we could not observe any enhancement of the signals in the lower energy channel, but…

The April 15th 2005 event （2） - - an interpretation -- The enhancement at 13:57 UT must made by the muons. High energy protons arrived at the top of the atmosphere. Those Solar Energetic Particles made nuclear interactions and pion decayed muons can penetrate the atmosphere and arrived at the two detectors located at Gornergrat and Aragatz. The delay time from the flare (13:45UT) is estimated about 12 minutes. We have observed many such events until now.

Oulu Neutron Monitor and GLE start time is 14:00 UT

GOES 8 X-ray data 13:19 start 13:23 C4 13:40 M1 13:43 rapid increase 13:50 X14 maximum

Summary of the Event 1) In the solar flare of April 15th 2001 (X14), solar neutrons were detected by the Chacaltaya neutron monitor (8). 2) Those high energy neutrons were produced at the rapid increase time of the GOES X-ray data when hard X-rays were simultaneously observed. 3) The enhancements detected by two different detectors located at Gornergrat and Mt. Aragatz indicate that high energy protons were coming after 12 minutes later (13:57UT) and produced pions [They arrived at the Earth 12minutes later than the highest energy neutrons (13:45UT)], while low energy GLE starts from 14:00 UT.

4. Tip for the solar cycle 24 1) It is important to make particle identification more rigidly and strengthen the current detectors especially on the anti gondola. It is very important to reject muons and electrons that arrived in the central scintillator. The power of anti is ~ 85%. So it should be 99% in the solar cycle 24. Then we can select weak signals of solar neutrons by the SONTEL to the level of X1 flare.

Bolivia Chacaltaya 5,250m 3 min value