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

摘要

Using auroral TV data and particle precipitation data from low-altitude satellites, weidentify the ionospheric signature of magnetotail dipolarizations and substorm injectionsmeasured in the near-Earth near-equatorial plasma sheet by Time History of Eventsand Macroscale Interactions during Substorms (THEMIS). Field line mapping exploits arecently developed time-dependent adaptive model which minimizes the variance toTHEMIS in situ magnetotail observations. We present strong evidence that theequatorward edge of the auroral bulge corresponds to the innermost extent of earthwardpropagating dipolarization fronts in the magnetosphere, whereas individual equatorwardmoving auroral enhancements correspond to the motion of individual injection frontsreaching at times distances as close to Earth as 5.5 RE. The region of tail dipolarizationcorresponds to the auroral bulge, a broad spatial region of enhanced but structuredauroral emissions, bounded on the poleward side by discrete auroral forms and on theequatorward side by a sharp drop in auroral luminosity and particle precipitation. Particleprecipitation within the bulge is enhanced considerably at the energies above 30 keV.Ionospheric protons are isotropic and electrons are anisotropic but with fluctuating fluxeswhich are below, but on occasion comparable with, trapped levels. The equatorwardedge of the bulge, herein termed the "Equatorward edge of Auroral Bulge" propagatesduring substorm expansion toward lower latitudes, initially fast (corresponding to 100 km/sin space at r ~ 7 RE) but with decreasing speed after onset. Our adaptive model mappingsuggests that equatorial points at near-geosynchronous altitude can map to ionosphericmagnetic latitudes up to 2°-3° off of predictions using standard T96 models. The offsetscan be either toward lower latitudes due to field line stretching before auroral breakup ortoward higher latitudes after breakup due to the near-Earth tail dipolarization.