Imaging riometer observations of drifting absorption patches in the morning sector

Abstract. Observations by a 7x7-beam imaging riometer in Kilpisjärvi, Finland (~66° MLAT) of the drifting cosmic noise absorption (CNA) structures in the morning sector near the zonal drift reversals are presented. The examination of the absorption intensity images revealed several regions with enhanced CNA (absorption patches) slowly drifting through the riometer field of view (FoV). The absorption patches were found to vary in shape, orientation (for elongated arc-like patches), and drift direction. The latter was calculated from the regression lines to positions of the absorption maxima in the FoV images and compared with the direction of electrojet plasma flow from horizontal magnetic perturbations and (for one event) tristatic ion drift velocities in the F-region. A reasonable agreement was found between these directions both in point-by-point comparisons and in terms of direction reversal timings. The absorption patches of lower intensity appear to have smaller drift velocities and to be associated with weaker magnetic perturbations. These results are consistent with the notion that relatively slow motions of the auroral absorption near the zonal drift reversals are associated with the drift of the entire magnetic flux tube as opposed to the gradient-curvature drift of energetic electrons injected into the ionosphere at the substorm onset. The absorption drift velocity magnitude, on the other hand, was found to be substantially lower than that of the plasma flow based on the results of limited comparison with tristatic ion drift measurements. A comparison of the drift directions with those of the patch elongation showed that a considerable number of patches had these directions close to each other. Using this observation, we demonstrate a satisfactory agreement between the patch drift velocities (both in direction and magnitude) as determined from the absorption images and keograms under the assumption that some patches were propagating in a direction that was significantly different from the perpendicularity to elongation.