Dissipation to the joule heating: Isolated and stormtime substorms Original Research Article

We discuss the energy input from the solar wind and its dissipation in the nightside ionosphere, as a form of Joule heating, during both isolated and storm-time substorms. The energy supplied to the magnetosphere by the solar wind is estimated by computing integrals of Akasofuʹs epsilon parameter determined from WIND satellite measurements. The northern hemisphere Joule dissipation is estimated using the local electrojet index, IL, derived from the IMAGE magnetometer chain observations in the Scandinavian sector. The integrals of the epsilon parameters and the electrojet index are computed from the beginning of enhanced energy input (southward turning of the IMF) to the end of the recovery phase. The main difference between isolated and storm-time substorms is that while the total ionospheric dissipation of storm-time substorms is considerably larger, its relative role is smaller by a factor of two. As there are cases where the ratio between the input and output energies is much larger than in typical cases, we have computed the epsilon parameter also without the IMF X-component. Statistically this does not make large difference but in single events the input energy computed without the X-component can be only half of what the usual way of computing the epsilon parameter yields. We also note that in events where the input seems to remain small as compared to the output there may well have been considerable input prior to the selected start of integration, indicating that the magnetosphere may have stored energy longer than expected.