Auroral brightening is one of the most common phenomena that occur during substorm onset and is usually recognized as a projection of the substorm-associated magnetospheric plasma dynamics to the ionosphere. However, electromagnetic fields and plasma features associated with the substorm brightening arc have not been well understood. In this study, we present a comprehensive observation of the source plasma and field variations of a substorm brightening aurora in the inner magnetosphere. We performed a unique conjugate observation of a substorm brightening auroral arc observed by a ground-based camera and by the Arase satellite in the magnetospheric source region at L ∼ 6. The event was observed at Tromsø (69.6°N, 19.2°E), Norway, on 12 October 2017. The brightening arc indicates east-west structures with longitudinal scales of ∼0.5°–2.0°. Field-aligned bi-directional electrons with an energy range between 66 and 1,800 eV were detected by the satellite, simultaneously with the appearance of the brightening arc in the camera. These electrons were probably supplied from the auroral brightening region in the ionosphere, indicating that the satellite was on the same field line of the brightening aurora. The magnetic and electric field data show characteristic fluctuations and earthward Poynting flux around the time that the satellite crossed the aurora. Anti-phase oscillations between the thermal pressure and the magnetic pressure are also reported. Based on these observations, we suggest the possibility that a ballooning instability occurred in the source region of the substorm brightening arc in the inner magnetosphere at L ∼ 6.
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