Reimei Satellite Observations of Alfvénic Interaction Modulating Inverted‐V Electrons and Filamentary Auroral Forms at the Poleward Edge of a Discrete Arc

• Published in: Journal of geophysical research. Space physics 2024-10, Vol.129 (10), p.n/a
• Main Authors: Hirahara, Masafumi, Ebihara, Yusuke, Kitamura, Naritoshi, Sakanoi, Takeshi, Asamura, Kazushi, Takada, Taku, Saito, Hirobumi
• Format: Article
• Language: English
• Subjects: Alfven wave; Alfven waves; Altitude; discrete aurora; Electric fields; electron acceleration; Electrons; Energy; energy‐pitch angle distribution; Fluxes; Ionosphere; Ionospheric effects; Lower bounds; Magnetohydrodynamic waves; Magnetospheres; parallel potential drop; Pitch (inclination); Satellite observation; Satellites; Upwelling; Voltage drop
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2024JA032650

We present an event based on Reimei satellite observations in the low‐altitude midnight auroral region, showing that intense and clear energy‐dispersed electron precipitations, repetitively generated by field‐aligned accelerations due to dispersive Alfvén waves, were modulating inverted‐V electrons. These Alfvénic electrons had peak energies equal to or slightly larger than those of the inverted‐Vs and were associated with the filamentary auroral forms rapidly streaming at the poleward edge of a broad discrete arc. This arc was caused by the inverted‐V accompanied by ion depletions produced by quasi‐electrostatic parallel potential drop. Assuming instantaneous electron accelerations over a wide energy range in a single location and a simple time‐of‐flight effect for the energy‐time dispersions, the Alfvénic source distances were estimated 1,500 ± 500 km above the satellite altitude of ∼676 km, a lower bound since the interaction locations are realistically distributed in altitudinally extended regions. The electron characteristics in detailed energy‐pitch angle distributions obtained at high time resolution can be categorized into: (a) original inverted‐V fluxes energized by quasi‐electrostatic upward electric field, (b) accelerated and decelerated/reduced inverted‐V fluxes, (c) field‐aligned energy‐dispersed precipitations accelerated by dispersive Alfvén waves, and (d) upwelling secondary components effectively produced by the field‐aligned precipitations particularly at energies of a few tens of eV. This event is useful to reveal the interactions between the inverted‐V and Alfvénic electrons and their related ionospheric effects in the magnetosphere‐ionosphere coupling processes. The detailed energy‐pitch angle distributions presented here provide constraints for models of these interactions and processes. Key Points Field‐aligned Alfvénic accelerations of electrons repetitively occurred at the poleward edge of broad auroral arc in the nightside oval Energy‐dispersed electron signatures modulated the inverted‐V electron distributions and implied short (1,000–2,000 km) source distances The Alfvénic electrons were associated with repetitive filamentary auroral forms streaming at the edge of the discrete arc