e‐POP and Red Line Optical Observations of Alfvénic Auroras

Abstract Suprathermal electron bursts (STEBs), characterized by a board energy spectrum and a field‐aligned pitch angle distribution, have been well recognized to be associated with electron acceleration by inertial Alfvén waves and are thus conventionally termed as “Alfvénic aurora.” In this study, we report joint Enhanced‐Polar‐Outflow‐Probe (e‐POP) and ground‐based optical observations of Alfvénic auroras. In particular, we highlight the prominence of 630‐nm red line emissions under low‐energy Alfvénic auroral precipitation. During the event interval, e‐POP traverses two arcs. One bright arc dominated by green line emissions is clearly seen by all optical instruments; it is embedded in upward field‐aligned currents (FACs) yet leaves little imprint on the e‐POP suprathermal electron imager (SEI), likely due to that the precipitation is well above the upper energy limit of SEI. On the other hand, there is a red line arc that is pronounced only in 630‐nm images. Such a red‐line‐only arc is located in a transition from large‐scale upward FACs to downward FACs and is associated with a prominent STEB structure detected by e‐POP SEI. The STEB features an inverse energy time dispersion, namely, that lower‐energy electrons are seen earlier while higher‐energy electrons appear later. The red‐line‐only arc and its separation from the green line arc evolve in a repeatable fashion, each stemming from a poleward auroral intensification (PAI) propagated from higher latitudes. Following each poleward auroral intensification the green line arc progressively moved southward, while the red‐line‐only arc is quasi‐stationary and stayed relatively stable in latitude. We propose tentative interpretations of the above features based upon stationary inertial Alfvén waves. Key Points We report the first joint optical and e‐POP particle observations of Alfvenic auroras; the Alfvenic aurora is seen only in red line emission e‐POP data show that the STEB is located in a transition between upward and downward FACs and features an inverse energy time dispersion The red‐line‐only arc (Alfvenic aurora) is quasi‐stationary and stays relatively stable in latitudes again southward convection enhancement