Magnetic dipolarizations inside geosynchronous orbit with tailward ion flows

Abstract

Abstract. Electromagnetic field and plasma data from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) near-Earth probes are used to investigate magnetic dipolarizations inside geosynchronous orbit on 27 August 2014 during an intense substorm with AEmax∼1000 nT. THEMIS-D (TH-D) was located inside geosynchronous orbit around midnight in the interval from 09:25 to 09:55 UT. During this period, two distinct magnetic dipolarizations with tailward ion flows are observed by TH-D. The first one is indicated by the magnetic elevation angle increase from 15 to 25∘ around 09:30:40 UT. The tailward perpendicular velocity is V⊥x∼-50 km s−1. The second one is presented by the elevation angle increase from 25 to 45∘ around 09:36 UT, and the tailward perpendicular velocity is V⊥x∼-70 km s−1. These two significant dipolarizations are accompanied with the sharp increase in the energy flux of energetic electron inside geosynchronous orbit. After a 5 min expansion of the near-Earth plasma sheet (NEPS), THEMIS-E (TH-E) located outside geosynchronous orbit also detected this tailward expanding plasma sheet with ion flows of −150 km s−1. The dipolarization propagates tailward with a speed of −47 km s−1 along a 2.2 RE distance in the X direction between TH-D and TH-E within 5 min. These dipolarizations with tailward ion flows observed inside geosynchronous orbit indicate a new energy transfer path in the inner magnetosphere during substorms.