Mass and energy transport in the near and middistant magnetotail around substorm onsets: Geotail observations

Abstract

We have studied mass and energy transport in the magnetotail around substorm onsets by analyzing GEOTAIL data. We selected substorm events from Pi2 pulsations and applied a superposed epoch analysis. We found that particles (ions) are gradually provided from the upstream lobe to the downstream plasma sheet before onset. Energy is transported from the lobe to the plasma sheet in the form of the Poynting flux, which is particularly large and has a large Y component around X ∼ −10 RE continuously before and after onset. The Poynting flux as well as the mass flux toward the plasma sheet starts to increase first around (X, Y) ∼ (−20, 7) RE about 0 ∼ 2 min before onset and then successively increases in the surrounding regions. In the downstream plasma sheet the dominant energy flux is the thermal flux. It is directed duskward before onset as well as the mass flux. However, very large tailward mass and thermal fluxes develop in the region of −23 > X > −30 RE of the premidnight tail after onset in association with the plasmoid. They have a large duskward component in the premidnight sector. Earthward of X ∼ −20 RE, there are not only earthward mass and thermal fluxes but also large duskward fluxes in the plasma sheet after onset. The kinetic energy flux is much smaller than the Poynting and thermal fluxes in any regions before and after onset. These observational results would provide us with significant clues to an understanding of substorm processes, such as the magnetic reconnection and the dipolarization, and their relationship.