Estimates of Horizontal Ionospheric Currents on the Dayside of Mars

Abstract:

How the solar wind interacts with a planetary object depends upon the object's properties such as the presence of a magnetic field or an atmosphere. An unmagnetized object cannot stand-off the solar wind unless it possess a substantial atmosphere which can be ionized by solar radiation creating a conductive ionosphere. Currents can then be induced in the ionosphere; these currents act to cancel out the external solar wind magnetic field preventing it from reaching the surface. Here we present simple analytical calculations of such induced currents in the ionosphere of Mars. We consider currents in the ionospheric dynamo region which can be driven by thermospheric winds as well as currents driven by electric fields (i.e., plasma motion through the neutrals). We include in these estimates the effects of "equatorial-type" electrojets due to vertical conductivity gradients in the presence of horizontal magnetic fields. The direction of the external driver is important. Vertical neutral winds (or plasma motion) give rise to electrojets whereas horizontal winds (or plasma motion) lead to no net current. These results can give us insights into how external magnetic fields are effectively screened out by induced currents and how these induced currents can influence ionospheric dynamics around unmagnetized objects.