Ionospheric depletion in auroral downward currents

The auroral downward field- aligned current is mainly carried by electrons of ionospheric origin accelerated into the magnetosphere along the Earth's high- latitude field lines. The ionosphere is a finite source of electrons: Thus, if a current is to continue to flow, it is natural to assume that the current region must broaden to access more current carriers. In this paper, we present an Alfven wave model of magnetosphere- ionosphere interaction to describe the evolution of ionospheric E region number density under the influence of a downward current. The behavior of the system falls into two regimes depending upon whether the quantity W = j parallel to 0/alpha en(e)(2)h is greater or less than unity ( where j(parallel to 0) is initial current density, a is the recombination coefficient, ne is background E region number density, and h is E region height): If the current density is smaller than a critical current density, jc = alpha en(e)(2)h ( i. e., W < 1), then the E region only depletes within the original current region, and there is sufficient photoionization to feed the current to the magnetosphere; if the required current density is larger than jc ( i. e., W > 1), then the current region is forced to broaden in order to access sufficient electrons. On the dayside, where a typical E region number density is similar to 10 (11) m(-3), broadening only occurs for very strong current densities similar to 10 mu A m(-2); on the nightside, however, where E region number densities can fall by a factor of 10, broadening occurs for any current density greater than similar to 0.1 mA m(-2). From this model, we derive expressions for the final depletion width ( generally similar to 1 - 10 times the width of the original current region) and for the characteristic timescale of depletion ( typically similar to 10 - 100 s).