Iterative and Steady Black Auroral Stripes and Their Possible Causality in Plasma‐Sheet Electron Distributions: Implications From Low‐Altitude Reimei Observations

Iterative black auroral stripes were identified in auroral images obtained by the low‐altitude microsatellite Reimei. These steady and narrow black stripes are a remarkable feature of diffuse auroras generated by energetic electrons precipitating at higher latitudes than pulsating auroras. The Reimei data confirmed that the iterative black stripes were caused by the recurring depletion of energized plasma‐sheet electron fluxes within specific energy and pitch‐angle ranges, as previously reported. Another common feature of the events presented here is coincident electron energization accompanied by well‐developed inverted‐V components produced by upward (magnetic‐)field‐aligned quasi‐electrostatic fields. This feature is frequently observed in and near the higher‐latitude edge of the inner plasma sheet, where intensified fluxes of isotropic electrons are further energized beyond the inverted‐V electrons. Reimei also occasionally observed another type of electron flux depletion signature, similar to that causing the iterative black auroral stripes, in the plasma sheet populated by less energized electrons. In such cases, faint auroral forms without distinct black stripes and small inverted‐Vs were occasionally detected. Although plasma particle and wave data from the equatorial plasma sheet were not available, we hypothesize that ineffective pitch‐angle scattering near the loss cones of inner‐plasma‐sheet electrons could substantially deplete energetic electron fluxes precipitating into the loss cones, thereby producing black auroras. Spatially striped modulations of pitch‐angle scattering efficiency in the plasma sheet would likely induce alternating diffuse auroras and black stripes, consistent with the plasma‐sheet electron flux distributions observed at low altitudes by Reimei.