R B Torbert; C J Farrugia;; V K Jordanova
M P Freeman; R P Lepping; J T Steinberg; D J McComas; R D Belian
The passage at Earth of the October 1995 magnetic cloud, monitored by the GGS mission spacecraft WIND, caused 2 geomagnetic storms: a major one (min Dst ~ -120 nT) during the ~ 14-hour-long, strong cloud Bz < 0 nT interval, and a moderate one (min Dst ~ -60 nT) during the intermittent Bz < 0 activity due to Alfven waves in the fast stream overtaking the magnetic cloud. The recovery phases of the two storms lasted ~ 12 hrs and 7 days, respectively. The measured Dst is corrected to remove contributions of the conducting Earth and magnetopause currents. As for the latter, we note that the dynamic pressure in the cloud and in the interaction region at its rear was very variable, reaching values as high as 20 nPa. The contribution of the alpha particles to the dynamic pressure is important and is included. The relative He++/H+ concentration was high (~ 12%) during the cloud Bz < 0 nT interval, dropping to ~ 5% during the succeeding Bz < O nT part and rising again to ~ 15% in the interaction region. Using the drift-loss model of Jordanova et al. (1996), we then assess the relative importance in the 2 storms of 3 mechanisms of ring current decay: charge-exchange, Coulomb collisions and wave-particle interactions. The model follows the time evolution of the ring current distribution function using time-dependent magnetospheric electric fields and measurements made at geostationary heights. This work is intended as a contribution to the studies of the interaction of the October 1995 magnetic cloud with Earth.
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