1Department of Physics, University of Tromsø, N-9037 Tromsø, Norway
2Space Vehicles Directorate, Air Force Research Laboratory, Hanscom AFB, Massachusetts, USA
3Department of Electrical and Computer Engineering, Boston University, USA
4On leave from Communication Systems, Lancaster University, UK
5Honorary Research Fellow, University of Kwazulu-Natal, Durban, South Africa
Abstract. Black auroras are recognized as spatially well-defined regions within uniform diffuse aurora where the optical emission is significantly reduced. Although a well studied phenomenon, there is no generally accepted theory for black auroras. One theory suggests that black regions are formed when energetic magnetospheric electrons no longer have access to the loss cone. If this blocking mechanism drifts with the source electron population in the magnetosphere, black auroras in the ionosphere should drift eastward with a velocity that increases with the energy of the precipitating electrons in the surrounding aurora, since the gradient-B curvature drift is energy dependent. It is the purpose of this paper to test this hypothesis. To do so we have used simultaneous measurements by the European Incoherent Scatter (EISCAT) radar and an auroral TV camera at Tromsø, Norway. We have analyzed 8 periods in which a black aurora occurred frequently to determine their relative drift with respect to the ionospheric plasma. The black aurora was found to drift eastward with a velocity of 1.5–4km/s, which is in accordance with earlier observations. However, one case was found where a black patch was moving westward, this being the first report of such behaviour in the literature. In general, the drift was parallel to the ionospheric flow but at a much higher velocity. This suggests that the generating mechanism is not of ionospheric origin. The characteristic energy of the precipitating electron population was estimated through inversion of E-region plasma density profiles. We show that the drift speed of the black patches increased with the energy of the precipitating electrons in a way consistent with the gradient-B curvature drift, suggesting a magnetospheric mechanism for the black aurora. As expected, a comparison of the drift speeds with a rudimentary dipole field model of the gradient-B curvature drift speed only yields order-of-magnitude agreement, which most likely is due to the nightside disturbed magnetosphere being significantly stretched.
Keywords. Auroral ionosphere; MI interaction; Energetic particles, precipitating
Blixt, E. M., Kosch, M. J., and Semeter, J.: Relative drift between black aurora and the ionospheric plasma, Ann. Geophys., 23, 1611-1621, doi:10.5194/angeo-23-1611-2005, 2005.