1 Belov A., Baisultanova L., Eroshenko E., Yanke V. (IZMIRAN, Russia), Mavromichalaki H. (Athens University, Greece), Pchelkin V. (PGI, Russia) Magnetospheric effects in cosmic rays during the unique magnetic storm in November th European Cosmic Ray Symposium 2004, Florence, Italy

2 Abstract Cosmic ray variations of magnetospheric origin during the severe magnetic storm on 20 November 2003 are selected from ground level observation data by means of the global survey method. Planetary distribution of the cut off rigidity variations during this disturbed period was obtained on this basis. For this event a correlation between Dst index and cut off rigidity variation was defined for each cosmic ray station. The most essential shift in cutoff rigidities occurred while Dst index was around -465 nT. Geomagnetic effect in cosmic ray intensity at some stations reached 6-8%. Cutoff rigidity variations were also calculated using the last model of magnetosphere from Tsyganenko This magnetospheric effect seems to be the greatest one over the history of neutron monitor observations. Maximum changes of geomagnetic cutoff rigidities were recorded this time at unusual low latitudes corresponded to about 7-8 GV cutoff rigidity. The results can be used in the neutron monitor data correction for magnetospheric effects and for the more accurate modeling as well of the magnetospheric current system during strong geomagnetic storms.

3 Magnetospheric effect in CR on Counting rate at different NMs uncorrected for magnetospheric effect. FD is moderate despite of severe magnetic storm Aurora in Athens. Foto A. Ayiomamitis htm htm

4 Neutron Monitors Stations used in this analysis DATA and METHOD Neutron Monitors Stations used in this analysis Data from 39 NMs : 15 high latitudinal ( 10GV Global Survey Method (GSM): 1. Coupling coefficients- beyond the atmosphere; 2. Trajectory calculations-beyond magnetosphere ; 3. Spherical analysis-to select different harmonics; Counting rate -> isotropic part+3D anisotropy +magn+err:

5 Comparison of obtained dRc with Dst-index

6 Correlation between cut off rigidity variations dRc and Dst-index

7 Latitudinal dependences of dRc for each hour during event. Maximum in latitudinal distribution of dRc is found around 7-8 GV (usually 3-4 GV) that means ring current to be located at 3R.

8 Comparison experimental results with calculations by the last Tsyganenko model dRc variations in dependence on cut-off rigidity in the maximum of geomagnetic storm , derived from experimental CR data (circles) and calculated by the Tsyganenko (T01storm) model (red). 1. Good agreement under Rc>6 GV; 2. Disagreement at Rc<6GV may be caused by inadequacy of model under great magnetospheric disturbances; 3. Experimental methods applied to other magnetic storms gives the classical latitudinal dRc variations with the maximum around 3-4GV

9 Currents from the model of Tsyganenko etal. [2003] T01s Currents from the model of Maltsev and Ostapenko[2001] M01 Currents obtained from the database by Fairfield et al. 1994]. Currents obtained from the database Tsyganenko et al. [2003]. Currents from the model of Alexeev et al. [2001, 2003] A03 Compare current systems obtained directly from experimental data and current systems obtained from models T01s, M01, A03 for tow values of Dst -70 and -140 nT.

10 Main conclusions. 1) At the beginning of extreme magnetic storm on 20 November 2003 a small magnetosphere effect in cosmic rays was recorded, whereas an exclusively large effect was derived during the main phase of this storm. This allowed the latitudinal distribution of the cut off rigidity variations to be obtained for each hour during the main and recovery phases of this magnetosphere storm very useful for analysis a dynamic of evolving and damping out of the ring current systems. 2) The ring current system during magnetic storm on 20 November 2003 was at more close distance from Earth (apparently about 3 Earth’s radii) than it usually observed. As a consequence, maximal magnetosphere effect in CR was recorded at lower latitudes, but not at the mid latitude stations as usual. Maximum changes of the geomagnetic cut off rigidity were shifted due to this anomalous from 3-4 GV (usually) to 7-8 GV. 3) The calculations of Rc changes performed for the last "storm" model T01s of the magnetospheric magnetic field [12] shows a good agreement between experimental and theoretical values for rigidities > 6 GV. Possibly, the model is not adequate for the greatest magnetospheric disturbances, this causes a discrepancy for lower rigidities. 4) Further developing and extended variant of this work, whether another events are analyzed, can be found on

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