Substorms, storms, and the near-earth tail

More than 50,000 measurements made with the AMPTE IRM satellite in the magnetotail at radial distances between 10 and 19 R(E) during 40 substorms have been used in a superposed epoch analysis, separating storm-time substorms from those occurring during intervals of low D-st levels. It is found that most of the tail signatures reported previously as typical for substorms are strongly influenced by magnetic storm activity. The magnetic field dipolarization and the Earthward convection in the near-Earth tail are much more pronounced for substorms which occur during the main phase of magnetic storms than for non-storm substorms. Moreover, only for storm-time substorms the lobe magnetic pressure decreases during the expansion phase. These findings indicate that not all substorms are alike and that the near-Earth neutral line scenario may apply only to the storm-time substorms.