Effect of Ionospheric Conductivity Channels on Magnetosphere-Ionosphere Coupling

The electrodynamic magnetosphere-ionosphere (M-I) coupling and the related evolution of field-aligned current systems is strongly controlled by the ionospheric conductivity. In particular confined channels of high conductivity (Cowling channels) or low conductivity (e.g., in SAPS channels) can lead to polarization electric fields and field-aligned currents flowing at the conductivity gradients. A long-standing issue has been to what extent these channels polarize as opposed to being discharged through field-aligned current, which is often parameterized in terms of a "Cowling efficiency" (e.g., Amm et al., 2011), which is zero if the ionospheric current is totally closed by field-aligned currents and one if the channel is completely polarized. This efficiency will depend on the Pedersen and Hall conductivity profiles and the Alfvén speed above the ionosphere and will be affected by the presence of the ionospheric Alfvén resonator as well as by ionospheric feedback due to precipitation. This issue will be addressed by means of a three-dimensional model of M-I coupling due to Alfvén waves that includes a fully height-resolved ionosphere. This model can shed light on the development of polarization fields and field-aligned currents in high-conductivity Cowling channels as well as low-conductivity SAPS channels, and can explain the observations from Swarm of fine-scale field-aligned currents at the boundary between large-scale upward and downward current systems.

Amm, O., et al. (2011), A statistical investigation of the Cowling channel efficiency in the auroral zone, J. Geophys. Res., 116, A02304, doi: 10.1029/2010JA15988