Relative drift between black aurora and the ionospheric plasma

Abstract

Abstract. Black auroras are recognized as spatially welldefined 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 1.5-4 km/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-ofmagnitude agreement, which most likely is due to the nightside disturbed magnetosphere being significantly stretched.Correspondence to: E. M. Blixt (marten.blixt@phys.uit.no)

Structure of black aurora associated with pulsating aurora

“…Two black aurora events, one of which is shown in Figure , showed black arc segments or patches drifting eastward with a noticeable velocity, which matches previous studies [ Trondsen and Cogger , ; Peticolas et al , ; Blixt et al , ]. The drift speeds measured are on the order of 15 km/s, much higher than previous reports like Blixt et al [] who reported a maximum drift of 4 km/s.…”

Ground-based observations of diffuse auroral structures in conjunction with Reimei measurements

“…There were several large morphological studies of black aurora in the 1990s, describing its characteristics and features (such as, Schoute-Vanneck et al, 1990;Trondsen and Cogger, 1997;Kimball and Hallinan, 1998). More recent studies, using satellites (Peticolas et al, 2002), and radars (Blixt and Kosch, 2004;Blixt et al, 2005) have linked the generation of black aurora to a magnetospheric origin. Therefore its presence in ground-based auroral images can be used to gain information about in situ processes and assist in interpreting combined in situ data.…”

Fine structure of optical aurora

“…Examples of such systems are the mobile, campaign-oriented instruments Portable Auroral Imager (PAI) of the University of Calgary and the Odin (Optical Digital Imager) of the University of Tromsø (Blixt et al, 2005b). With a 50 mm lens the maximum resolution at 105 km altitude is better than 40 m. When emissions are weak, integration may be needed.…”

“…10. Blixt et al (2005b) then investigated the drift of black patches. In seven of the eight events studied the patches drifted eastward with velocities of 1.5-4 km/s.…”