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Substorm Energy Transport From the Magnetotail to the Nightside Ionosphere

The present study addresses the transport of energy from the nightside magnetosphere to the ionosphere during the substorm expansion phase. From the cross‐examination of the estimates of various ionospheric and magnetospheric processes, it is inferred that (1) Poynting flux from the tail lobes to the plasma sheet accounts for the ionospheric energy dissipation during the substorm expansion phase, suggesting that the lobe reconnection is the primary driver of the substorm energy transport to the ionosphere; (2) the area around the duskside poleward boundary of the auroral bulge, where auroral emission is intense throughout the expansion phase, accounts for >10%, likely a few tens of percent, of the ionospheric substorm energy dissipation, suggesting that the area is a crucial energy sink; and (3) kinetic energy carried by the bursty bulk flows is comparable to the energy deposited in association with the auroral streamers, and each bursty bulk flow accounts for ~1% of the total substorm energy deposition, which may sum up to 10% throughout the expansion phase. Points 2 and 3 imply that energy deposited to the ionosphere is transported mostly along, rather than across, the magnetic field lines in the plasma sheet. The responsible field‐aligned plasma flows may be generated as the perpendicular plasma motion creates field‐aligned pressure gradient in the presence of field line curvature. Energy flux associated with enhanced convection in the plasma sheet is statistically examined with the Geotail data set, which confirms that more energy is carried earthward by the field‐aligned flows than by the convection flows.