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.
Highlights
Magnetic dipolarization can be observed at or inside geosynchronous orbit during intense substorms with high Auroral Electrojet (AE) indices (AE > 500 nT) (e.g., Dai et al, 2015; Nagai, 1982; Nosé et al, 2014; Ohtani et al, 2018)
In this paper we present dipolarizations with tailward ion flows inside geosynchronous orbit during an intense substorm expansion phase
The dipolarizations with tailward ion bulk flows inside geosynchronous orbit are investigated in our present paper
Summary
Magnetic dipolarization can be observed at or inside geosynchronous orbit during intense substorms with high Auroral Electrojet (AE) indices (AE > 500 nT) (e.g., Dai et al, 2015; Nagai, 1982; Nosé et al, 2014; Ohtani et al, 2018). Using the conjunction observations of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) multiple probes in the NEPS, Duan et al (2011) pointed out that the dipolarization at inner edge of the near-Earth plasma sheet had no one-to-one relationship with the earthward ion bulk flow. Near-Earth dipolarizations with low-frequency waves are detected with thermal ions and electron energization (e.g., Dai et al, 2015; Liang et al, 2009; Nosé et al, 2014; Ohtani et al, 2018) These energetic particles are the main source of inner magnetosphere during substorms and storms. Nosé et al (2014) proposed that the dipolarizations associated with low-frequency fluctuations were observed in the inner magnetosphere during the storm main phase These lowfrequency electromagnetic waves can accelerate O+ ions in the perpendicular direction. Discussions and conclusions of our observations are displayed in the last section
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