1. 29 ICRC, Pune, India, 2005 Geomagnetic effects on cosmic rays during the very strong magnetic storms in November 2003 and November 2004 Belov (a), L. Baisultanova (a), R. Buetikofer (b), E. Eroshenko(a), E. O. Flueckiger (b), G. Mariatos ©, H. Mavromichalaki ©, V. Pchelkin (d), V.Yanke (a) (a) IZMIRAN), 142190, Troitsk, Moscow region, Russia (b) Physikalisches Institut, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland (c) Nuclear and Particle Physics Section, Physics Department, Athens University, Pan/polis-Zografos, 15771 Athens, Greece (d) Polar Geophysical Institute, 183010 Murmansk, Russia rus-eroshenko-E-abs1-sh35-poster

2. 29 ICRC, Pune, India, 2005 Introduction Despite the CR variations during magnetic storms have already been studied in many papers [1-7] several important problems still remain to be solved: -quantitative relation between magnetic field parameters and dRc during large magnetic storms (Dst<-100nT) [4]; -validation of magnetospheric magnetic field models by comparison with experimentally derived changes in dRc at different stages of the magnetic storm. One of the greatest geomagnetic effects in CR was observed in November 2003 [7] when the CR variation caused by the cutoff rigidity changes during a severe magnetic storm (Dst=-472 nT) reached 6-7% at low latitude stations. The intense burst of solar activity in November 2004 – very close to the solar activity minimum – produced again a series of strong magnetic storms.

3. 29 ICRC, Pune, India, 2005 Data and Method Hourly data from 46 neutron monitors (NMs) of the worldwide network have been used in the analysis: 19 high latitude (Rc 10 GV) stations. The Dst index was taken from: http://swdcwww.kugi.kyoto-u.ac.jp/dstdir/ (WDC-C2). The global survey method described in [5, 7] (GSM) has been utilized to calculate a set of parameters defining the near-Earth galactic CR density and anisotropy from the ground level neutron monitor network, and then, for dRc evaluation. The method takes into account the cosmic ray propagation in the magnetosphere and atmosphere and uses trajectory calculations in the Earth’s magnetic field and the neutron monitor response functions [8, 9]. http://swdcwww.kugi.kyoto-u.ac.jp/dstdir/ The magnetic storm in November 2003 was caused by a strong interplanetary disturbance with Bz =-60nT after the series of M class solar flares on 18 November and was characterized by a record value of the magnetospheric perturbations and their unusual latitudinal distribution with a maximum around cutoff rigidities 7-8 GV.

4. 29 ICRC, Pune, India, 2005 Figure 1. Relative counting rates at different NM stations during the severe magnetic storms in November 2003 (a) and November 2004 (b): AATB-Alma-Ata (Rc=6.7GV), APTY-Apatity (0.65GV), ATHN-Athens (8.7GV), JUNG-Jungfraujoch (4.5GV), MCMD-McMurdo (0.01GV), PTFM-Potchefstroom (7.3GV), SNTG- Santiago (11GV). The magnetospheric effect in 2003 was characterized by a record value of magnetosphere perturbations and reached maximum CR variations of ~7-8%. The effect in November 2004 was a little less, but it occurred on the background of an extremely complex interplanetary structure.

5. 29 ICRC, Pune, India, 2005 Figure 2. Variation of the parameters of interplanetary space, CR, and geomagnetic activity during the disturbed period on November 2004 -CMEs occurred almost every day (from AR10696); -2-3 disturbances were permanently present and produced shocks ; -Five SSC followed by an abrupt increase in the SW velocity ( 700-800 km/s) and in the IMF (up to 40-45 nT); -the Dst index fell down to -373 and -289 nT, and Kp reached 8 and 9; -during ~12 hours on 8.11.04 a CR density increase was observed in the FE minimum Such a peak is usually attributed to the geomagnetic effect in CR, but in this case the enhancement of CR density may be a result of the galactic CR modulation by large-scale structures in the interplanetary space.

6. 29 ICRC, Pune, India, 2005 Characteristic features of the event in November 2004 As a consequence of the interaction of different solar wind streams a compressed region was created, with higher CR density and rather complicated structure, which manifested itself in the sharp, short duration changes in the CR anisotropy (Fig. 2). A somewhat similar effect in the time profiles of NM data during the 1991 March 24 FE has been investigated by Hofer and Flückiger [10]. They attributed the effect to a large- scale interplanetary disturbance passing the Earth, possibly a magnetic cloud. In this event the effects of both interplanetary and geomagnetic origin were present simultaneously.

7. 29 ICRC, Pune, India, 2005 Fig. 3. A regression dependence between obtained dRc and Dst-index for the Athens and Jungfraujoch stations in November 2003 and 2004. Two regions are clearly pronounced: with a small (>-50nT) and with a large (<-50 nT) amplitude of Dst- index. Within the second region there is an approximately linear dependence of dRc on Dst index. This dependence is defined more accurate in the event on November 2003 when the galactic Cosmic Ray modulation was not so strong.

8. 29 ICRC, Pune, India, 2005 The cutoff rigidity variations dRc at different stations were calculated for both events for each hour by the method described in [5, 7]. Fig. 4a. Cut off rigidity variations dRc for the hours of weak and strong disturbed magnetosphere versus Rc [11] in November 2004, obtained by the experimental (points) and model (triangles) calculations. Fig. 4b. Cut off rigidity variations dRc in the maximum of magnetic storm on November 2004 versus Rc referring to the quiescent magnetosphere [11].

9. 29 ICRC, Pune, India, 2005 The maximum changes of Rc in the 2003 event (nearly 1.4 GV) occurred at latitudes corresponding to Rc ≈ 7-8 GV. In the 2004 event the maximum changes in Rc (of the order of 0.7 GV) were observed at cutoff rigidities 5-6 GV. The disturbance of the magnetosphere was therefore weaker in the November 2004 storm as compared to the storm in November 2003. In Fig 3a the dRc calculated using the last “storm” magnetospheric field model T01S of Tsyganenko [12] are plotted by triangles. A comparison of dRc derived from the “model” and experimental data shows a good agreement for rigidities >6 GV. However, below 6 GV there is a significant discrepancy. Possibly, the model T01S still is not adequate enough for the strongest magnetospheric disturbances. This is confirmed also by pictures from the Table, where the magnetosphere current systems obtained from satellite measurements (real data) and from the model calculations for Dst =-50 nT and Dst =-140 nT are compared

10. 29 ICRC, Pune, India, 2005 Currents from the model of Tsyganenko etal. [2003] T01s Currents from the model of Maltsev and Ostapenko[20 01] M01 Currents obtained from the database by Fairfield et al. 1994]. Currents obtained from the database Tsyganen ko et al. [2003]. Currents from the model of Alexeev et al. [2001, 2003] A03 Comparison of the current systems obtained directly from experimental data (database) and from models T01s, M01, and A03 for two values of Dst -70 and -140 nT.

11. 29 ICRC, Pune, India, 2005 Conclusions There was no such gigantic events in November 2004 as those occurred in November 2003. Nevertheless, the series of FEs and SSC is an evidence of the level of interplanetary and geomagnetic perturbations rather high for the solar cycle phase nearly the minimum; A good correlation between cut off rigidity variation dRc and Dst indices is obtained due to applying GSM method that allows a quantitative relation to be defined during the great magnetic storms; During the event on 20 November 2003, the ring current system was located at a closer geocentric distance (~3RE) than is usually observed. Due to this anomaly the maximum dRc (-1.4 GV) was shifted from the usual value of 3-5 GV to 7-8 GV. Maximum dRc in Nov. 2004 reached -0.7 GV at Rc ~5-6 GV; Comparison of dRc derived from CR data and utilizing the last "storm" model T01S of the magnetosphere shows a good agreement between experimental and modeling values for rigidities > 6 GV and great discrepancy for the lower rigidities. One reason for this may be that the “storm” model is not yet an adequate description of the real magnetosphere during the greatest disturbances.

12. 29 ICRC, Pune, India, 2005 Acknowledgements This work is partly supported by Russian FBR grants 03-07-90389, 04-02- 16763, Program BR of the Presidium RAS “Neutrino Physics”, by the Swiss National Science Foundation, grant 200020-105435/1, and by PYTHAGORAS project in Greece. We thank the collaborators and refer to the grants from all CR stations from which data were taken for analysis. REFERENCES [1] H. Debrunner et al., Planet. and Space Sci., 27, 577-581, 1979. [2] E. O. Flückiger et al., Proc. 17th ICRC, Paris. 4, 244-247, 1981. [3] E. O. Flückiger et al., Proc. 20th ICRC, Moscow, 4, 216, 1987. [4] E.O. Flückiger et al., J. Geophys. Res., 91, 7925-7930, 1986. [5] L. Baisultanova et al., Proc 24th ICRC. 4, 1090-1094, 1995. [6] V. M. Dvornikov and V. E. Sdobnov, Intern. JGA, 3, 1-11, 2002. [7] A. Belov et al., J. Geophys. Res., 2005, accepted. Clem, J., and L. I. Dorman, Space Sci. Rev., 93, 1, 335-359, 2000. [9] S. Yasue et al., Report of Cosmic Ray Research Laboratory, N7, 1982. [10] M.Y. Hofer and E.O. Flückiger, J. Geophys. Res., 105 (A10), 23,085- 23,097, 2000. [11] M. A. Shea and D. F. Smart, Proc. 27th ICRC, 10, 4063-4066, 2001. [12] N. A. Tsyganenko et al., J. Geophys. Res., 108(A5), 1209, doi:10.1029/2002JA009808, 2003.