Active auroral arc powered by accelerated electrons from very high altitudes

Abstract

Bright, discrete, thin auroral arcs are a typical form of auroras in nightside polar regions. Their light is produced by magnetospheric electrons, accelerated downward to obtain energies of several kilo electron volts by a quasi-static electric field. These electrons collide with and excite thermosphere atoms to higher energy states at altitude of ~ 100 km; relaxation from these states produces the auroral light. The electric potential accelerating the aurora-producing electrons has been reported to lie immediately above the ionosphere, at a few altitudes of thousand kilometres1. However, the highest altitude at which the precipitating electron is accelerated by the parallel potential drop is still unclear. Here, we show that active auroral arcs are powered by electrons accelerated at altitudes reaching greater than 30,000 km. We employ high-angular resolution electron observations achieved by the Arase satellite in the magnetosphere and optical observations of the aurora from a ground-based all-sky imager. Our observations of electron properties and dynamics resemble those of electron potential acceleration reported from low-altitude satellites except that the acceleration region is much higher than previously assumed. This shows that the dominant auroral acceleration region can extend far above a few thousand kilometres, well within the magnetospheric plasma proper, suggesting formation of the acceleration region by some unknown magnetospheric mechanisms.

The Characteristics of EMIC Waves in the Magnetosphere Based on the Van Allen Probes and Arase Observations

“…We applied a spin‐tone noise reduction to the MGF data onboard Arase using a method described in Imajo et al. (2021). Using a method described in Takahashi et al.…”

“…We applied a spin-tone noise reduction to the MGF data onboard Arase using a method described in Imajo et al (2021). Using a method described in Takahashi et al (1990), we then rotated the magnetic field data obtained from the EMFISIS and MGF instrument onboard the RBSP and Arase into a mean-field-aligned (MFA) coordinate system.…”

A Statistical Study of the Solar Wind Dependence of Multi‐Harmonic Toroidal ULF Waves Observed by the Arase Satellite

“…Before we performed spectral analysis of the magnetic field, we first removed the spin‐tone (∼8 s) noise from the 64‐Hz data by sine curve fitting that uses a least‐square method (e.g., Imajo et al., 2021). 1‐s averaged data were calculated from the 64‐Hz data after the noise reduction.…”

Statistical Study of EMIC Waves and Related Proton Distributions Observed by the Arase Satellite

“…We computed the dynamic spectra of the magnetic field to detect the EMIC waves in the frequency domain. The original Level-2 magnetic field data measured by the MGF instrument contained harmonic spin-tone noises, thus we applied the spin-tone reducing method of Imajo et al (2021) to the 64 Hz magnetic field waveform dataset. To obtain clean wave signals, we subtracted the 10 s moving average magnetic field data from the original data.…”