Kosch, M. J. and Gustavsson, B. and Blixt, E. M. and Pedersen, T. and Senior, A. and Kavanagh, A. J. and Semeter, J. (2006) EISCAT radar and optical studies of black aurora: A signature of magnetospheric turbulence? In: Proceedings of the Eighth International Conference on Substorms (ICS8), 1900-01-01.
Abstract
Black auroras are recognised as spatially well-defined regions within a uniform diffuse auroral background where the optical emission is significantly reduced, or possibly totally absent. Black auroras typically appear post-magnetic midnight and during the substorm recovery phase, but not exclusively so. Their horizontal size is typically 1x5 km, elongated in the east-west direction, and they move predominantly in an eastward direction with a speed of 1-4 km/s. There is no accepted theory for the phenomenon of black aurora, although they seem associated with substorms. We report on the first incoherent scatter radar observations of black aurora by EISCAT, coupled to white-light TV recordings of the phenomenon. From a 2002 observation, we show that non-sheared black auroras are most probably not associated with field-aligned currents. From 2002 and 2003 observations, we show that the apparent motion of the black aurora is most probably controlled by the drift of particles in the magnetosphere and not ExB drift in the ionosphere. The drift speed is therefore dependent on the energy of the precipitating particles forming the diffuse background. From 2005 bi-static observations, we attempt to confirm this by relating the height and propagation speed of the black aurora to precipitating particle energy within the surrounding background diffuse aurora. Hence, the mechanism for black aurora is most probably active within the magnetosphere and substorm associated plasma turbulence within the magnetosphere may account for the optical morphology of the black aurora, in particular the lack of pitch angle diffusion into the loss cone.