Sensitivity of Low-Latitude Ionospheric Convection in the Evening to E-Region Conductivity

Modeling of low-latitude ionospheric electrodynamics reveals a sensitivity of ExB convection in the evening to E-region conductivity. This sensitivity is explained in terms of two related but distinct effects. First, meridional E-region currents associated with Pedersen conductivity partially balance meridional F-region dynamo currents. Since the F-region current density depends more on the pressure-gradient force driving the wind than on the E-region conductivity, changes in the latter provoke an inversely related change in the electric field and plasma convection velocity, even though the relative contribution of the E region to the field-line-integrated conductivity may be small as compared with the F region contribution. The second way in which night-time E-region conductivity affects the evening plasma convection is through regulation of the zonal electric field and vertical/meridional plasma convection. In this case it is the E-region Cowling conductance, rather than the Pedersen conductance, that comes into play. Vertical convection through the E region in the early evening, associated with the pre-reversal enhancement of the vertical drift, is associated with zonal Cowling current that dissipates a relatively large amount of electromagnetic energy, and therefore exerts a drag on the evening plasma convection. This presentation quantifies the sensitivity of the convection to the night-time E-region conductivity, and shows how the convection distribution tends to obey a minimization principle.