Spatial Development of the Dipolarization Region in the Inner Magnetosphere Journal of Geophysical Research: Space Physics

The present study examines dipolarization events observed by the Van Allen Probes within 5.8REfrom Earth. It is found that the probability of occurrence is signi ficantly higher in the dusk-to- midnight sector than in the midnight-to-dawn sector, and it deceases sharply earthward. A comparison withobservations made at nearby satellites shows that dipolarization signatures are often highly correlated (c.c. >0.8) within 1 hr in magnetic local time (MLT) and 1 R EinRXY, and the dipolarization region expands earthward and westward in the dusk-to-midnight sector. The westward expansion velocity is estimated at0.4 hr (in MLT) per minute, or 60 km/s, which is consistent with the previously reported result for geosynchronous dipolarization. The earthward expansion is apparently less systematic than the westward