Magnetosphere energetics during substorm events: IMP8 and GEOTAIL observations

Magnetospheric energetics during substorm events is studied in this paper. Three events were selected, a weak substorm, a large isolated one and final ly a prolonged period of substorm activity with multiple intensifications. It is assumed that the energy, that entered the magnetosphere due to electr omagnetic coupling with the solar wind, is described by the epsilon paramet er, proposed by Perreault and Akasofu (1978). High resolution, magnetic fie ld and plasma data from the MGF and LEP experiments on board GEOTAIL were a nalyzed to determine the timing of plasmoid release, its dimensions, its co nvection velocity and finally the energy carried by each plasmoid. Plasmoid s were defined as structures with rotating magnetic fields and enhanced tot al pressure. Tailward plasmoid bulk speed in the distant tail varied from 3 50 to 750 km/s. Their dimensions in X-GSM direction was found to be from 4. 5 to 28Re, and their duration did not exceed 5 min. The average energy carr ied by each plasmoid in the distant tail was estimated to be equal to (2.4 +/- 1.0) x 10(14) J. The thermal energy flux dominates when the plasmoid st ructure is a closed loop. In cases of flux rope plasmoids the kinetic energ y is comparable to or even greater than the thermal energy flux. The study of the overall magnetospheric energetics yielded the result that the input solar wind energy, estimated by the integral of the epsilon parameter over the lifetime of a substorm, represents the energy stored in the magnetotail and the convective dissipation in the ionosphere as well. The energy requi red for the plasmoid release is thought to be provided by the night side re connection in the near-Earth neutral line location. Finally, it was found t hat 20% of the total input solar wind energy is dissipated in the auroral i onosphere. (C) 2001 Elsevier Science Ltd. All rights reserved.