Conjugate observation of auroral finger-like structures by ground-based all-sky cameras and THEMIS satellites

摘要

In this study, we analyze the first conjugate observation of auroral finger-like structures using ground-based all-sky cameras and the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites and investigated associated physical processes that are a cause of auroral fragmentation into patches. Two events are reported: one is a conjugate event, and the other is a nearly conjugate event. The conjugate event was observed at Narsarsuaq (magnetic latitude: 65.3°N), Greenland, at 0720-0820 UT (0506-0606 LT) on 17 February 2012. Analysis of the event revealed the following observational facts: (1) variation of parallel electron energy fluxes observed by THEMIS-E shows a correspondence to the auroral intensity variation, (2) plasma pressure and magnetic pressure fluctuate in antiphase with time scales of 5-20 min, and (3) perpendicular ion velocity is very small (less than 50 km/s). In the latter event, observed at Gakona, Alaska, on 2 February 2008, the THEMIS-D satellite passed across higher latitudes of finger-like structures. The data from THEMIS-D also showed the antiphase fluctuation between plasma pressure and magnetic pressure and the small perpendicular ion velocity. From these observations, we suggest that the finger-like structures are caused by a pressure-driven instability in the balance of plasma and magnetic pressures in the magnetosphere.