A study of black aurora from aircraft‐based optical observations and plasma measurements on FAST

Abstract

Black aurora was observed on 30 January 1998 in a narrow‐field camera forty seconds before and a minute after the magnetic footprint of the FAST satellite passed the magnetic zenith of the camera. Electron energy flux measured by FAST provided strong evidence that FAST passed over black aurora. This region was characterized by small‐amplitude wave and DC electric fields (<10 mV m−1) and small upward current densities (∼0.2μA m−2). The electron energy flux dropouts associated with black aurora were associated with spatially restricted regions in which the strong pitch angle diffusion is suppressed at energies greater than ∼2 keV. These dropouts were surrounded by hot (∼1.8 keV) plasma sheet electrons exhibiting strong pitch angle diffusion at all measured energies. Analyzing the electron spectra from six different nights of optical and satellite conjugate data demonstrates a surprising prevalence of electron distributions that showed the suppression of strong pitch angle diffusion at energies above ∼2 keV. What made the electron signature on the night of the black aurora observations unique were the large spatial regions of strong pitch angle diffusion at all energies (corresponding to the diffuse aurora) with only very narrow spatial regions of the partially empty loss cones (corresponding to the black aurora). We suggest that the pitch angle diffusion surrounding the black aurora is due to electron cyclotron waves (ECH) strongly scattering electrons into the loss cone at energies less than ∼2 keV and oblique, upper band whistler mode chorus waves scattering electrons at all measured energies in the plasma sheet. We further suggest that the scattering by the chorus waves is suppressed in localized regions leading to the black auroral features observed.