Auroral signatures of the plasma injection and dipolarization in the inner magnetosphere

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Using auroral TV data and particle precipitation data from low‐altitude satellites, we identify the ionospheric signature of magnetotail dipolarizations and substorm injections measured in the near‐Earth near‐equatorial plasma sheet by Time History of Events and Macroscale Interactions during Substorms (THEMIS). Field line mapping exploits a recently developed time‐dependent adaptive model which minimizes the variance to THEMIS in situ magnetotail observations. We present strong evidence that the equatorward edge of the auroral bulge corresponds to the innermost extent of earthward propagating dipolarization fronts in the magnetosphere, whereas individual equatorward moving auroral enhancements correspond to the motion of individual injection fronts reaching at times distances as close to Earth as 5.5 RE. The region of tail dipolarization corresponds to the auroral bulge, a broad spatial region of enhanced but structured auroral emissions, bounded on the poleward side by discrete auroral forms and on the equatorward side by a sharp drop in auroral luminosity and particle precipitation. Particle precipitation within the bulge is enhanced considerably at the energies above 30 keV. Ionospheric protons are isotropic and electrons are anisotropic but with fluctuating fluxes which are below, but on occasion comparable with, trapped levels. The equatorward edge of the bulge, herein termed the “Equatorward edge of Auroral Bulge” propagates during substorm expansion toward lower latitudes, initially fast (corresponding to 100 km/s in space at r ∼ 7 RE) but with decreasing speed after onset. Our adaptive model mapping suggests that equatorial points at near‐geosynchronous altitude can map to ionospheric magnetic latitudes up to 2°–3° off of predictions using standard T96 models. The offsets can be either toward lower latitudes due to field line stretching before auroral breakup or toward higher latitudes after breakup due to the near‐Earth tail dipolarization.