that showed only a small difference in plasma sheet density under long periods of northward or southward interplanetary magnetic field (IMF). This contrasts studies showing that the plasma sheet is thermodynamically adiabatic and the statistical analysis of plasma sheet density by Nagata et al. found that during substorm growth phases ion density increased while temperature was unchanged, implying that the plasma sheet acts isothermally. As such, increases in the lobe magnetic pressure during substorm growth phases result in changes in the plasma sheet density and temperature. The total pressure of the plasma sheet approximately balances the lobe pressure. Theoretical models of these magnetospheric changes during the growth phase have been validated experimentally for a few events. The increase in flaring of the near-Earth magnetotail increases the solar wind ram pressure transmitted through the magnetopause, which is balanced by an increase in the total pressure exerted from within by the increased magnetic flux in the lobes. Īs magnetopause reconnection occurs, the dayside magnetosphere is eroded, moving inward by 10–20%, the cusps move equatorward and the magnetotail expands to accommodate the increasing open magnetic flux created by the reconnection. The timing of release, the amount of available energy released, how and why it is partitioned into different energy forms and pathways, and the factors controlling these properties are significant unknowns in our understanding of substorm dynamics. This energy is stored as magnetic energy in the magnetotail lobes until released by reconnection in the magnetotail, being roughly equally partitioned between enhanced particle precipitation and Joule heating in the ionosphere energization of the ring current and loss in plasmoids and the postplasmoid plasma sheet. Magnetic reconnection at the magnetopause is estimated to extract ∼10 15 J of energy from the solar wind during the substorm cycle.
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