1. Precipitation of Auroral Electrons Accelerated at Very High Altitudes: Impact on the Ionosphere and a Possible Acceleration Mechanism.
- Author
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Imajo, S., Miyoshi, Y., Kazama, Y., Asamura, K., Shinohara, I., Shiokawa, K., Kasahara, Y., Kasaba, Y., Matsuoka, A., Wang, S.‐Y., Tam, S. W. Y., Chang, T.‐F., Wang, B.‐J., Jun, C.‐W., Teramoto, M., Kurita, S., Tsuchiya, F., Kumamoto, A., Saito, K., and Hori, T.
- Subjects
ELECTRON distribution ,AURORAS ,ION beams ,ELECTRIC fields ,MAGNETOSPHERE - Abstract
The Arase satellite observed the precipitation of monoenergetic electrons accelerated from a very high altitude above 32,000 km altitude on 16 September 2017. The event was selected in the period when the high‐angular resolution channel of the electron detector looked at pitch angles within ∼5° from the ambient magnetic field direction, and thereby was the first to examine the detailed distribution of electron flux near the energy‐dependent loss cone at such high altitudes. The potential energy below the satellite estimated from the observed energy‐dependence of the loss cone was consistent with the energy of the upgoing ion beams, indicating that ionospheric ions were accelerated by a lower‐altitude acceleration region. The accelerated electrons inside the loss cone carried a significant net field‐aligned current (FAC) density corresponding to ionospheric‐altitude FAC of up to ∼3μA/m2. Based on the anisotropy of the accelerated electrons, we estimated the height of the upper boundary of the acceleration region to be >∼2 RE above the satellite. The height distribution of the acceleration region below the satellite, estimated from the frequency of auroral kilometric radiation, was ∼4,000–13,000 km altitude, suggesting that the very‐high‐altitude acceleration region was separated from the lower acceleration region. Additionally, we observed time domain structure (TDS) electric fields on a subsecond time scale with a thin FAC indicated by magnetic deflections. Such a TDS may be generated by the formation of double layers in the magnetotail, and its potential drop could significantly contribute (∼40%–60%) to the parallel energization of precipitating auroral electrons. Plain Language Summary: The auroral arc is produced by electrons in the magnetosphere accelerated downward by a quasi‐static electric field. The region of the electric field, so called the auroral acceleration region, is typically located just above the ionosphere, at altitudes of a few thousand kilometers. However, recent studies have shown that it can be extended to altitudes higher than 30,000 km. The mechanism and impact of such a very‐high‐altitude acceleration region remain a mystery. The high‐angular resolution electron detector on board the Arase satellite successfully measured current and energy flows carried by the electron accelerated at the very‐high‐altitude acceleration region above the satellite altitude of 32,000 km. This acceleration at very‐high‐altitudes contributed significantly to the energy flow into the ionosphere. Based on the anisotropy of the accelerated electrons, the frequency of auroral kilometric radiation, and the presence of spiky electrostatic waves, we estimate that the very‐high‐altitude acceleration region was located more than 12,000 km above the satellite as a thin charge separation layer, apart from the acceleration region below the satellite. Our results suggest that localized electric fields at very high altitudes, which were not previously associated with the aurora, can energize auroral electrons and affect auroral visibility. Key Points: We comprehensively analyzed a very‐high‐altitude (>32,000 km) auroral acceleration eventNet precipitation of accelerated electrons significantly contributes to the field‐aligned current and energy flux into the ionosphereDouble layers in the magnetotail are possible candidates for very‐high‐altitude auroral accelerations [ABSTRACT FROM AUTHOR]
- Published
- 2024
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