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

Auroral dynamics is a manifestation of plasma dynamics in the magnetosphere and auroral emissions are caused by electrons precipitation from the magnetosphere. Investigation of auroral structure helps deepening our knowledge of dynamical variation of magnetospheric plasma and their connection to the Earth's atmosphere. In this study, we succeeded the first conjugate observation of auroral finger-like structures using a ground all-sky camera and the THEMIS satellite and investigated physical processes associated with finger-like structures. Two events are reported, one is the conjugate event and the other is the nearly-conjugate event. The conjugate event was observed at Narsarsuaq (MLAT: 69.3N), Greenland, at 0720UT-0820UT (0506LT-0606LT) on 17 February 2012. Analysis of the event produced the following observation facts: 1) variation of parallel electron energy fluxes observed by THEMIS-E shows correspondence to the auroral intensity variation, 2) plasma pressure and magnetic pressure fluctuate in anti-phase with time scales of 5-20 min, 3) perpendicular ion velocity is very small (less than 50 km/s). The development speed of the finger-like structure showed correspondence with the perpendicular ion velocity obtained from the THEMIS-E satellite. In the latter event observed at Gakona, Alaska, on 08 February 2008, the THEMIS-D satellite passed across higher latitude of finger-like structure. The data from THEMIS-D also showed the anti-phase fluctuation between plasma pressure and magnetic pressure, and small perpendicular ion velocity. These observations are consistent with the idea that the finger-like structures are caused by pressure-driven instabilities in the inner plasma sheet. Further investigation of various pressure variations in the inner magnetosphere will be needed to discuss the cause of the auroral finger-like structures.