Magnetospheric source region of auroral finger-like structures observed by the RBSP-A satellite

Auroral finger-like structures appear equatorward of the auroral oval in the diffuse auroral region and contribute to the auroral fragmentation into patches. A previous report of the first conjugate observation of auroral finger-like structures using a THEMIS GBO camera and the THEMIS-E satellite at a radial distance of \~8 RE showed anti-phase oscillations of magnetic and plasma pressures in the dawnside plasma sheet. In the present study, we report another simultaneous observation of auroral finger-like structures at Gillam, Canada at \~0900 UT (0230 magnetic local time) on November 14, 2014 with the RBSP satellites at 5.8 RE in the inner magnetosphere. From this simultaneous observation event, we obtained the following observations. (1) Auroral finger-like structures developed poleward in the equatorward-moving auroral arc at the equatorward edge of the auroral oval. (2) Both the electron and ion OMNI fluxes measured by HOPE increased at \~0900 UT as the satellite footprint entered the auroral region, indicating that the satellite was crossing the observed auroral finger-like structures. (3) The absolute value of magnetic pressure was several times that of the plasma pressure, and no systematic phase relationship was identified between the magnetic and plasma pressures, unlike that in the THEMIS case. Based on these observations, we discuss two possible causes of the observed finger-like structures, namely, pressure-driven instability in the magnetosphere and gradient-drift instability in the ionosphere. In this paper, the latter possibility is newly suggested to develop in the equatorward-moving aurora associated with the westward electric field in the equatorward ionospheric density gradient.