Magnetosphere energetics during substorm events: IMP8 and GEOTAIL observations

Abstract

Magnetospheric energetics during substorm events is studied in this paper. Three events were selected, a weak substorm, a large isolated one and finally a prolonged period of substorm activity with multiple intensifications. It is assumed that the energy, that entered the magnetosphere due to electromagnetic coupling with the solar wind, is described by the epsilon parameter, proposed by Perreault and Akasofu (1978). High resolution, magnetic field and plasma data from the MGF and LEP experiments on board GEOTAIL were analyzed to determine the timing of plasmoid release, its dimensions, its convection velocity and finally the energy carried by each plasmoid. Plasmoids were defined as structures with rotating magnetic fields and enhanced total pressure. Tailward plasmoid bulk speed in the distant tail varied from 350 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 carried by each plasmoid in the distant tail was estimated to be equal to (2.4±1.0)×10 14 J . The thermal energy flux dominates when the plasmoid structure is a closed loop. In cases of flux rope plasmoids the kinetic energy 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 required for the plasmoid release is thought to be provided by the night side reconnection in the near-Earth neutral line location. Finally, it was found that 20% of the total input solar wind energy is dissipated in the auroral ionosphere.