Energetics of isolated and stormtime substorms

A fundamental question in the physics of solar-magnetospheric-ionospheric (SMI) interaction is to understand how the solar wind energy is transferred, stored and distributed in the magnetospheric-ionospheric (MI) system. In this paper we discuss how well parameterized expressions can estimate the energy sources and sinks in the SMI system. We also present some recent results from two energy budget studies. We find that the overall energy coupling efficiency, defined as the ratio of energy dissipated in the MI system to the total available solar wind kinetic energy (U SW ) is <1%. We also find that the Akasofu ∈-parameter, which approximates the solar wind input due to day-side reconnection, does not always provides enough energy to balance the energy sinks in the MI system. Allowing for viscous interaction between the solar wind and the magnetosphere, such a mechanism only need to transfer 0.17% of U SW to balance the energy budget. The dominant energy sink in the MI system is found to be the Uj during both isolated and stormtime substorms. The particle precipitation energy flux (U A ) is about 1/2 of Uj. Several studies indicate that U R is about the same as U A even during magnetic storms. Isolated substorms are found to be five times as numerous, but half as intense as stormtime substorms. However, over a 2-year period twice as much energy is dissipated during isolated substorms than during stormtime substorms, indicating that isolated substorms are the main process by which solar wind energy is dissipated in the MI energy sinks.