Difference in magnetotail variations between intense and weak substorms

Abstract

Using Geotail plasma and magnetic field data, we have statistically studied the difference in substorm‐associated magnetotail variations between intense and weak substorms. It was found that for intense substorms the plasmoid‐related southward magnetic field and the first decrease in the total pressure occur closer to the Earth. This finding indicates that the magnetic reconnection site at expansion onset is located closer to the Earth in intense substorms, which is consistent with the dependence of the onset latitude in the ionosphere on the substorm intensity. Also, magnetic field lines during the growth phase especially earthward of the magnetic reconnection site are more stretched in intense substorms than in weak substorms. The dipolarization at X ∼ −10 RE is more significant in intense substorms. Furthermore, the Poynting flux in the lobe toward the plasma sheet and the total pressure during the growth and expansion phases are larger at X ∼ −10 RE than at larger distances, and they are larger in intense substorms than in weak substorms. The enhancement of the Poynting flux and the decrease in the total pressure associated with expansion onset are also more pronounced at X ∼ −10 RE in intense substorms. These results suggest that more energy is accumulated and subsequently dissipated in the near‐Earth magnetotail at X ∼ −10 RE during intense substorms.