Relative drift between black aurora and the ionospheric plasma

E M Blixt,M J Kosch,J Semeter

doi:10.5194/angeo-23-1611-2005

Abstract

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

Highlights

Black auroras are recognized as relatively small regions with a distinct reduction of auroral luminosity, within large-scale regions of otherwise homogeneous diffuse aurora
Our motivation in this paper is to study how the black patches drift relative to the ionospheric plasma, and whether the drift velocity depends on the energy of the electrons in the diffuse aurora
Each about 1–4 min long, where black auroras have been observed through an image intensified video camera and where the European Incoherent Scatter (EISCAT) radar measured the ionospheric plasma flow and electron density, have been examined in order to study whether black patches are drifting with a velocity consistent with the gradient-B curvature velocity of the hot source plasma or not

Summary

Introduction

Black auroras are recognized as relatively small regions with a distinct reduction of auroral luminosity, within large-scale regions of otherwise homogeneous diffuse aurora. The second type are black vortex streets, which have a clockwise rotation (opposite to that of auroral curls), and commonly have a wavelength of about 1–3 km (Trondsen and Cogger, 1997; Kimball and Hallinan, 1998b). Both types are most common in the midnight sector in the late substorm recovery phase. In this paper we focus on the drift of black patches

Objectives

Findings

Discussion

Conclusion