MAGNETOSPHERE SIZE AND GEOMAGNETIC ACTIVITY AT EXTREME VALUES OF THE SOLAR WIND PARAMETERS

S.N.Kuznetsov and B.Yu.Yushkov

Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow
State University, 119899, Moscow, Russia

   We used the dynamical pressure P of the solar wind (SW) and Bz 
component of interplanetary magnetic fild as the main SW parameters 
for analysis of the magnetosphere size and geomagnetic activity. The 
most probable value of P is equal to 2 nPa. At such a value and at 
Bz>0 the distance to sub-solar point of the magnetopause Rss is equal 
to 11 Re (Re is the Earth's radius). At Bz<0 the Rss value is 20% 
less. We defined as the extreme the calculated values of Rss<6.6 Re 
and >16 Re. The P values in these cases differ from the mean value by 
a factor of 10 and more. The Rss<6.6 Re corresponds to P>18 nPa at 
Bz>0 or P>4 nPa at Bz<0. The Rss>16Re corresponds to P<0.2 nPa. We 
analysed the Kp-index and Dst during these periods. At low SW 
pressure the weak geomagnetic activity is observed, Dst and Bz are 
near 0. At high pressure especially coupled with Bz<0 the high 
activity and developed geomagnetic storms are observed.