How much curl-free Hall current flows out to the magnetosphere as field-aligned current from Cowling channel?

Abstract:

The Cowling channel is a generic name of a current system flowing inside a high conductivity band, in which a secondary polarization electric field is excited via preventing a divergence part of Hall current to flow out to the magnetosphere as the field-aligned current (FAC). Hereafter as stated by Amm et al., [2011], we refer to the notion of Cowling effect and Cowling channel whenever an electric field tangential to horizontal gradients of conductances occurs and a secondary (polarization) electric field builds up. Since the primary and secondary electric field point to different directions, the total electric field E is different from primary electric field in the region where the primary Hall current is confined in the ionosphere. Therefore, from viewpoint of the Magnetosphere- Ionosphere (M-I) coupling, important questions are: How does the Cowling effect change the ionospheric flow pattern from the magnetospheric convection mapped onto the ionosphere as a result of charge separation at the conductivity edge, and how can such deformed convection and the resulting field-aligned current (FAC), which has ionospheric origin, feed back to the magnetosphere.

To quantify the Cowling effect, we need to know the relative strength of the polarization field and to what extent it cancels (closes) the primary Hall current. This problem is complementary to the question [Amm et al., 2011; Fujii et al., 2011], how much curl-free Hall current flows out to the magnetosphere as FAC.

On the basis of electron and ion continuity equations with their mobility inside ionospheric E-region, we reformulate current closure process between FAC and ionospheric divergent current in a quasi-steady state. A divergence free condition of 3-dimesional electron-flow and 2-dimensional ion-flow results in very fundamental pictures: (1) FAC always closes to the Hall current carried by electrons, (2) Pedersen current divergence always balances to the divergence of Hall current carried by ions and secondary polarization electric field is produced by this process. This scenario is universal and gives significant restriction to the closure condition between ionospheric current and FACs.

In this presentation, we will review the outstanding problem relating to Cowling channel and discuss about their role in a Magnetosphere-Ionosphere coupling.