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1. Title 21-st ECRS Košice, Slovakia, 9-12 September, 2008 Relativistic Solar Cosmic Ray Dynamics in Large Ground Level Events E.V.Vashenuyk, Yu.V.Balabin, B.B.Gvozdevsky Polar Geophysical Institute Apatity, Russia

2. INTRODUCTION The neutron monitors (NMs) long since and down to the present time remain the basic means of relativistic solar cosmic rays study. These particles are observed in rather rare Ground Level Enhancement (GLE) events. The rate of GLEs occurrence is ~ 1 per year. For 66 years from the first GLE registered on 28 February, 1942, only 70 events occurred up to now. The worldwide network of neutron monitors can be considered as a multidirectional cosmic ray spectrometer. The author’s GLE modeling technique employing the optimization methods and modern magnetosphere models allows obtaining characteristics of relativistic solar protons (RSP): rigidity (energy) spectrum, anisotropy axis and pitch angle distribution in the primary solar proton flux. Two distinct populations of RSP: the prompt and delayed ones probably having different origins on the Sun have been revealed.

3. OUTLINE • History of the GLE study with neutron monitors • Neutron monitor network as instrument for relativistic solar cosmic ray studies and GLE modeling technique • Results of relativistic solar cosmic ray events study with the GLE modeling

4. Variations of cosmic ray intensity as recorded by the neutron monitors with solar activity. By black triangles are shown GLE occurrences.

5. Apatity (67.55N 33.34E) Barentsburg (78.08N 14.12E) PGI NMs Neutron monitors computer and electronics racks Two neutron monitor stations of the Polar Geophysical Institute SERVER http://pgia.ru/cosmicray INTERNET 4

6. Asymptotic Cone of Acceptance is formed by trajectories of particles contributing into response of NM Effect of magnetosphere on cosmic rays Barentsburg is a high-latitude station and accepts radiation from high latitudes of the selestial sphere and a subpolar station Apatity from equatorial latitudes 5

7. Anisotropy Solar cosmic rays anisotropy effect during the GLE on December 13, 2006 Apatity (10 s data) Barentsburg (1 min) 6

8. Set of NMs The worldwide network of neutron monitors as a multidirectional cosmic ray spectrometer 7

9. Method: 8 direct. Scheme of asymptotic cones calculations: To account the contribution of oblique incident particles we calculate 8 trajectories of particles launched at zenith angle 20o and 8 azimuths Asymptotic directions at magnetopause SCR GCR ~20° Starting directions at a launching point Calculated asymptotic directions are then used inthe following modeling of a NM response 9

10. GLE modeling techniqueof deriving the characteristics of relativistic solar protons (RSP) from the neutron monitor network data consists of a few steps: 1. Definition of asymptotic viewing cones (taking into account not only vertical but also oblique incident on a detector particles) by the particle trajectory computations in a model magnetosphere (Tsyganenko 2002) 2. Calculation of the NM responses at variable primary solar proton flux parameters. 3. Application of a least square procedure for determining primary solar proton parameters (namely, energy spectrum, anisotropy axis direction, pitch-angle distribution) outside the magnetosphere by comparison of computed ground based detector responses with observations

11. The response function of a i-th neutron monitor to anisotropic flux of solar protons. • (dN/N)i is percentage increase effect at a given neutron monitor i • J(R) = JoR-*is rigidity spectrum of RSP fluxwith changing slope • * =  +  ·(R-1) where  is increase per 1 GV (Cramp et al., 1997) • S(R) is specific yield function (Debrunner et al., 1984), • θ(R) is pitch angle (angle between the anisotropy axis given • by;  parameters) • F(θ(R )) ~ exp(-θ2/C)is pitch-angle distribution in a form of Gaussian (Shea&Smart, 1982) Formula 8

12. Increase profilesat some NM stations: Oulu, Apatity, Moscow, Barentsburg, Fort Smith GLE 70 13.12.2006 GLE 70 The asymptotic cones (1-20 GV), for the above NM stations and Th-Thule, McM-McMurdo, SA-SANAE, Ma-Mawson, No-Norilsk, Ti-Tixie, CS-Cape Shmidt, In-Inuvik, Pe-Pewanuk. The derivedanisotropy axis and pitch angle grid lines for solar proton flux at 03.00 UT are shown. The cross is the IMF direction (ACE data). 9

13. Fitting Observed and modeled responses at a number neutron monitor stations ───increase profiles at neutron monitors ●●● modeling responses

14. Dynamics of pitch angle distributions (PAD) derived from neutron monitors data 5 to Sun 1 3 3 1 2 Numbers mark the moments of time 4 PAD demonstrates an initial highly collimated beam of particles (prompt component) followed by a delayed quasi-isotropic population (delayed component) 5 6

15. Dynamics of energetic spectra of relativistic solar protons Direct solar proton data ■GOES-11 TOM intensities ●balloons, 10 UT Spectra derived from NM data 03:05 03:30 04:00

16. GLE 20.01.2005 Increase profiles as registered by a number of NM stations and EAS array “Carpet”(Baksan, North Caucasus) The spectrum derived in moment (1 ) when the promptcomponent was dominated is exponential in energy: J= 1.5105exp(-E/0.92), and spectrum of delayedcomponent (2) has a power-law form: J = 7.5104 E-4.9. (Vashenyuk et al.2006, 2007, Perez-Peraza et al., 2007)

17. Exponential spectrum of the prompt component was a cause of a giant increase effect at McMurdo neutron monitor and power law spectrum of delayed component produced rather moderate effect at this and other NM stations during the GLE 20.01.2005 SYF- specific yield function Debrunner et al., 1984 a c b d Increase profiles at the McMurdo and Mawson neutron monitors (a), rigidity spectra derived at the moments 07:00 (1) and 08:00 (2) UT (b), SYF and spectra 1 and 2 (c); differential responses (d) of the McMurdo neutron monitor to the exponential spectrum at the moment 1 (blue shading) and to the power-law spectrum at themoment 2 (red shading).

18. Two relativistic solar proton components in the GLE 23 February, 1956 (a)Increase profiles at the Leeds and Ottawa neutron monitors; (b) energy spectra derived at the moments 04:00 (1) and 06:00 UT (2), (c) SYF and spectra (1 and 2) and differential responses of the Leeds neutron monitor to the exponential spectrum (1,blue) and to the power-law spectrum (2,red). By numbers are marked, respectively,the moments when the prompt component (1) or delayed one (2) were dominating. One can see comparable responses of both neutron monitors to the power-law spectrum at moment (2).

19. Results of modeling analysis of 20 major GLEs showing existence of two RSP components Spectrum of prompt component: J=J0exp(E/E0), E (GeV); J0, J1 (m2 s st GeV) -1 Spectrum of delayed component J=J1E-γ E.V. Vashenyuk, Yu.V. Balabin, L.I. Miroshnichenko J. Perez-Peraza , A. Gallegos-Cruz3, 30 icrc, Merida, Mx, paper 0588

20. Spectra of prompt and delayed solar proton components derived from neutron monitor data for a number of GLEs Delayed component Prompt component GLE No GLE No Points are direct solar proton data from spacecrafts and balloons Spectra of the prompt component as a rule have exponential dependence upon energy Spectra of the delayed component have close to the power law dependence upon energy

21. Spectra of prompt and delayed solar proton components derived from neutron monitor data for a number of GLEs Delayed component Prompt component GLE No GLE No Points are direct solar proton data from spacecrafts and balloons Spectra of the prompt component as a rule have exponential dependence upon energy Spectra of the delayed component have close to the power law dependence upon energy

22. Prompt and delayed components of relativistic solar protons (RSP) • The modeling analysis of 20 large GLEs occurred in the period 1956-2006 on the data of the worldwide neutron monitors carried out by us revealed two distinct RSP populations (components): • Prompt Component (PC): the early collimated impulse-like intensity increase with exponential energy spectrum, • Delayed component (DC): the late quasi-isotropic gradual increase with a softer energy spectrum of the power law form. • The exponential spectrum may be an evidence of the acceleration by electric fields arising in the reconnecting current sheets in the corona. The possible source of delayed component particles can be stochastic acceleration at the MHD turbulence in expanding flare plasma. • E.V. Vashenyuk, Yu.V. Balabin, L.I. Miroshnichenko J. Perez-Peraza , A. Gallegos-Cruz,J ASR, V.38 (3), 411; (2006); 30 icrc, Merida, Mexico, paper 0658(2007)

23. Results World wide neutron monitor network is an effective tool for the relativistic solar cosmic ray study. The modeling technique employing the optimization methods and modern magnetosphere models allows obtaining characteristics of high energy solar cosmic rays: rigidity (energy) spectrum, anisotropy axis and pitch angle distribution of the primary solar proton flux. There is a good agreement of these characteristics with direct measurements in adjacent energy intervals on balloons and spacecrafts. The presence of the prompt and delayed components (PC and DC) of relativistic solar protons almost in all studied GLEs (20) as well as in superevents 23.02.1956 and 20.01.2005 has been shown. Moreover, the huge increases in both superevents on a limited number NM stations were caused by the prompt component having an exponential energetic spectrum.