Origin of density enhancements in the winter polar cap ionosphere

Coherent and incoherent ground-based radar measurements of the winter polar cap ionosphere at Thule and Sondrestrom, Greenland have established the existence of "patches" of enhanced ionization which drift across the polar cap in an antisunward, noon-midnight direction. Associated with these patches is strong radio scintillation activity which severely disrupts ground-to-satellite communication systems and interferes with the operation of space surveillance radar at high latitudes. Several recent studies have shown that the source of enhanced ionization is the sunlit subcusp ionosphere rather than production by precipitating energetic particles. However, the question of what causes the patchiness has not been addressed. We study this problem by solving the time-dependent plasma continuity equation including production by solar ultraviolet radiation, loss through charge exchange, and transport by diffusion and convection E×B drifts. Time and spatially varying, horizontal E×B drift patterns are imposed, and subsequent ionospheric responses are calculated to determine how enhanced plasma densities in the dark polar cap could result from extended transit of relevant flux tubes through regions of significant solar production. This would occur south of the cusp prior to convection as patches across the polar cap. It is found that a density enhancement in N_max from 7×10⁴ to 5×10⁵ el/cm³ occurs at Thule when a time-varying convection pattern is included in the simulation. The patch of ionization is generated when an initial convection pattern characterized by an 80-kV cross-tail potential and a 12° polar cap radius is abruptly changed to a 100-kV cross-tail potential and a 15° polar cap radius. The horizontal extent of the patch is related to the length of time the new convection pattern remains "turned on."