COORDINATED CONVECTION MEASUREMENTS IN THE VICINITY OF AURORAL CAVITIES

Meridional radar scans of electron density from the Sondrestrom incohe rent scatter radar (Greenland, 66.99-degrees-N, 50.95-degrees-W) have been used to identify latitudinally narrow, field-aligned depletions o f the auroral F region ionosphere. Observations of these so-called ''a uroral cavities' have been reported in earlier case studies in close p roximity to E layer arcs at the poleward edge of the nightside oval (D oe et al. 1993). These radar data indicated that the cavities and arcs remained as collocated pairs for periods as long as an hour, while co ordinated imaging and satellite measurements indicated that the pairs were extended in magnetic local time. These observations suggested at least two causal mechanisms: (1) cavity formation by the convective di stortion of an existing density depletion, or (2) cavity formation by vertical evacuation from a downward field-aligned current. New results from a model developed to examine the convection mechanism suggest th at a distorted polar cap density depletion will elongate parallel to t he local convection streamline when observed in the morning sector (fa r from the location of divergent flow lines or the Harang discontinuit y). In order to establish evidence for these two mechanisms and furthe r refine the physical properties of auroral cavities, a joint imaging/ radar experiment was carried out in February 1991, at the Sondrestrom and Goose Bay (Labrador, 54.4-degrees-N, 60.4-degrees-W) radars with a n emphasis on multispectral imaging, horizontal convection and off-mer idional density measurements. When compared to coincident all-sky imag es, the convection data indicate significant cross-arc flow during cav ity formation sequences on 6 and 10 February, 1991, questioning the ap plicability of mechanism (1). A third example on February 8, 1991, sho ws the alignment of a cavity/arc pair with the local streamline at the edge of an E region auroral arc. This last example illustrates the di fficulty in establishing the relative dominance of either mechanism fo r a particular event. The continued association of cavities with auror al precipitation and failure to detect elongated cavities or those ori ented along the local convection streamline lead to the conclusion tha t cavities are probably created by localized field-aligned currents.