Electrodynamic model for the formation of auroral ionospheric cavities

Abstract

Auroral ionospheric cavities (AICs) are latitudinally narrow, field‐aligned density depletions of the wintertime polar F region ionosphere. AICs have been detected in incoherent scatter radar data from the Sondrestrom radar facility in a 2‐year sample and during two coordinated multi‐instrument campaigns (Doe et al., 1993, 1994). These data suggest the possibility that AICs are created by the ionospheric closure of field‐aligned currents (FACs) in the polar ionosphere. In this scenario, the cavity forms in a region where ionospheric electrons are evacuated upward as charge carriers for a downward FAC. In order to model this process, a two‐dimensional (altitude versus latitude) simulation has been constructed that imposes an oppositely directed FAC pair at the top of a polar ionosphere that is subject to chemical loss and diffusive transport; the pertinent equations are solved for the resultant system of closure currents and localized plasma loss. Electrodynamic evacuation is modeled by solving Ohm's law and ▽ · j = 0 on the ionospheric grid with an imposed constant topside potential. The sensitivity of the modeled ionosphere to modification from chemistry and diffusion alone is evaluated by removing the topside potential, and imposing a region of enhanced ion temperature ion‐neutral (slip) velocity at F region altitudes. This confirms our earlier conclusion that perturbed thermospheric temperatures and velocities alone cannot create AICs on observed time scales. Modeling results from field‐aligned current closure, on the other hand, indicate that FACs are very efficient at modifying the polar ionosphere: modest currents of 0.2 to 0.02 µA m−2 can create cavitylike structure on time scales from 30 to 64 s, respectively.