A QUANTITATIVE ASSESSMENT OF ENERGY-STORAGE AND RELEASE IN THE EARTHSMAGNETOTAIL

Magnetospheric substorms represent a global interaction between the so lar wind, the magnetosphere, and the ionosphere. Energy extracted from the solar wind is episodically stored in the magnetosphere, with larg e fraction of this energy being in the form of excess magnetic flux in the magnetotail lobes. The stored energy is periodically dissipated i n an explosive instability that occurs in the near-Earth plasma sheet at substorm expansive phase onset. Methods are discussed to estimate t he amount of energy transferred from the solar wind to the magnetosphe re during substorm growth phases. Observational and modeling constrain ts are then used to assess quantitatively the total amount of energy s tored in the plasma sheet and magnetotail lobes. Possible energy sourc es to drive substorm evolution are evaluated in one particularly well- observed case (May 3, 1986). By examining the major avenues of energy dissipation, the energy that is released in the form of plasma sheet h eating, ionospheric Joule heating, etc, is assessed during the substor m. It is found that stored tail-lobe energy is sufficient (by a factor of 3 or more) to drive observed substorm dissipation processes. On th e other hand, energy in the closed field lines of the plasma sheet is insufficient (by a substantial margin) to supply the substorm energy.