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.

Small-Scale Dynamic Aurora

“…Continuous international effort is therefore needed for systematic monitoring observations of both global and local auroras at the same time, which would be necessary to build a firm empirical relationship among global-scale, mesoscale, and the small-scale dynamic auroras as reviewed in this article. Systematic surveys have also been conducted to identify the large-scale context of the small-scale structures, such as large-scale upward current or downward current (Liang et al 2019;Wu et al 2020aWu et al , 2020b. Next generation high-resolution global imaging of aurora from spacecraft would be a dream, but for the time being, rapidly increasing resolution of ground-based all-sky cameras, the networked distributions of imagery and small spacecraft, and timely data sharing combined with realistic simulations will renovate this aspect.…”

Potential Evidence of Low‐Energy Electron Scattering and Ionospheric Precipitation by Time Domain Structures

“…For our first conjunction event on 27 August 2014, auroral white‐light images from THEMIS ground‐based all‐sky imagers (Donovan et al, 2006; Mende et al, 2008) are also used to show auroral activity during intervals of wave‐scattered electron precipitation. Note that low‐energy (100–300 eV) electron precipitation cannot, alone, lead to white‐light auroral emissions but is usually associated with red‐line auroras (Liang et al, 2019). However, the presence of white‐light auroral emissions related to such electrons indicates precipitating electrons with energies above 325 eV but with a low‐energy portion measurable by e‐POP SEI.…”

Energetics and Alfvénic Coupling of a Poleward Boundary Intensification: A Polar Case Study

“…The last process, item (d) above, is the auroral excitation by the precipitating, bursty electrons. We refer to the study by Liang et al (2019) to provide some insight into our observations. The authors simulated the temporal effect on auroral luminosity due to small‐scale Alfvénic electron bursts.…”

“…Figure reproduced from Liang et al (2019). (a) Temporal variation pattern of two precipitation inputs: a stationary STEB (black curve) and successively recurring STEB pules (red curve).…”

“…In an attempt to fill in some of the missing data in the photon flux (Figure 3a), we invoke model results from Liang et al (2019), who simulated the temporal effect on luminosity of the Alfvénic aurora, driven by Alfvénic electron bursts. Our justification is that below we provide evidence that supports the Alfvénic driving of the PBI modulations.…”