Meridional radar scans of electron density from the Sondrestrom incoherent scatter radar (Greenland, 66.99¿N, 59.95¿W) have been used to identify latitudinally narrow, field-aligned depletions of the auroral F region ionosphere. Observations of these so-called ''auroral cavities'' have been reported in earlier case studies in close proximity to E layer arcs at the poleward edge of the nightside oval (Doe et al. 1993). These radar data indicated that the cavities and arcs remained as collocated pairs for periods as long as an hour, while coordinated 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 distortion 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 that a distorted polar cap density depletion will elongate parallel to the local convection streamline when observed in the morning sector (far from the location of divergent flow lines or the Harang discontinuity).

In order to establish evidence for these two mechanisms and further 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¿N, 60.4¿W) radars with an emphasis on multispectral imaging, horizontal convection, and off-meridional density measurements. When compared to coincident all-sky images, the convection data indicate significant cross-arc flow during cavity formation sequences on 6 and 10 February, 1991, questioning the applicability of mechanism (1). A third example on February 8, 1991, shows 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 difficulty in establishing the relative dominance of either mechanism for a particular event. The continued association of cavities with auroral precipitation and failure to detect elongated cavities or those oriented along the local convection streamline lead to the conclusion that cavities are probably created by localized field-aligned currents.