Reimei observation of highly structured auroras caused by nonaccelerated electrons

[1] We present observations of highly structured, thin auroras cased by precipitation of nonaccelerated electrons on the basis of optical and particle measurements performed by the Reimei satellite near the equatorward edge of the main auroral oval. The aurora has the following characteristics: (1) A full width at half maximum (FWHM) value is as low as only ∼1.8 km from optical measurements, and ∼0.6 km from particle measurements at the ionospheric altitude, which is much smaller than previously determined. (2) The FWHM value of 0.6 km corresponds to 9 km in the equatorial plane, which is ∼10 times smaller than the gyroradius of typical protons trapped in the near-Earth plasma sheet. (3) At high energies greater than ∼1 keV, the velocity distribution function of precipitating electrons is comparable to that of the trapped ones and does not demonstrate any plateau or positive gradient in the distribution. (4) The aurora was observed in geomagnetically quiet condition. (5) A geosynchronous satellite observed a significant increase in the plasma pressure of hot electrons in comparison with that of hot ions. The structuring of the aurora may be attributed to scattering processes of hot electrons. If this were the case, hot electrons would be scattered by whistler mode chorus, or electrostatic electron cyclotron harmonic waves, and the structured aurora would be a visual manifestation of the highly structured, cold plasma that determines the growth of the waves scattering the hot electrons.

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