ELECTRODYNAMIC MODEL FOR THE FORMATION OF AURORAL IONOSPHERIC CAVITIES

Auroral ionospheric cavities (AICs) are latitudinally narrow, field-al igned density depletions of the wintertime polar F region ionosphere. AICs have been detected in incoherent scatter radar data from the Sond restrom radar facility in a 2-year sample and during two coordinated m ulti-instrument campaigns (Doe et al., 1993, 1994). These data suggest the possibility that AICs are created by the ionospheric closure of f ield-aligned currents (FACs) in the polar ionosphere. In this scenario , the cavity forms in a region where ionospheric electrons are evacuat ed upward as charge carriers for a downward FAG. In order to model thi s process; a two-dimensional (altitude versus latitude) simulation; ha s been constructed that imposes an oppositely directed FAC pair att th e tap of a polar ionosphere that is subject to chemical loss and diffu sive transport; the pertinent equations are solved for the resultant s ystem of closure currents and localized plasma loss; Electrodynamic ev acuation is modeled by solving Ohm's law and del . j = 0 on the ionosp heric grid with an imposed constant topside potential. The sensitivity of the modeled ionosphere to modification from chemistry and diffusio n alone is evaluated by removing the topside potential, and impbsing a region of enhanced ion temperature ion-neutral (slip) velocity at F r egion altitudes. This confirms our earlier conclusion that perturbed t hermospheric temperatures and velocities alone cannot create AICs on o bserved time scales. Modeling results from field-aligned current closu re, on the other hand, indicate that FACs are very efficient at modify ing the polar ionosphere: modest currents of 0.2 to 0.02 mu A m(-2) ca n create cavitylike structure on time scales from 30 to 64 s, respecti vely.