Black aurora is a small-scale (typically a few to 10 km) black structure seen in diffuse aurora, and its generation process has been studied with immense interest. We report the precise characteristics of black aurora based on simultaneous image and particle measurement data and possible generation process. Thirteen black auroral events are identified from the Reimei satellite data, and the relationship between particle and auroral images around the satellite's magnetic footprints is investigated in detail. We found that a number of small-scale deficiencies were embedded in precipitating electrons from the central plasma sheet with energies greater than 2-7 keV and that each deficiency corresponded exactly to black arcs and black patches at the magnetic footprint. Therefore, black arcs and black patches are not associated with a field-aligned potential (such as a divergent potential structure) but probably originate from the suppression of pitch angle scattering. In the black auroral region, low-energy (2-5 keV) inverted-V-type downward electrons (spanning channels that are several tens of kilometers wide) often appear to overlap with high-energy (several keV) plasma sheet electrons.
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