Multiple arcs: Evidence for an active ionospheric role in the M-I coupling

Observations of high-latitude auroral-electrodynamic structures have provided evidence that the M-I coupling is more complex than typically assumed in large scale magnetohydrodynamic (MHD) models. The observational evidences from multiple arcs, their widths and multiplicity, will be discussed in the context of solar, ionospheric, and magnetospheric conditions. These relationships, along with more detailed knowledge of the electrodynamics (currents, electric fields, and conductivities), lead to MHD model constraints. Since several different types of MHD models have been developed to include the M-I coupling at high latitudes, a comparison of the constraints and their implementation in these models will be undertaken. The crux of the problem being addressed is whether the magnetosphere simply imposes on the ionosphere multiple arcs, i.e., a pure driver, or whether it is possible for the M-I coupling itself to have the capability of creating this structure. At this time, MHD models that use an ionospheric Ohms Law for M-I coupling and the Knight relationship to modulate ionospheric conductivity simply map magnetospheric electrodynamics into the ionosphere. Studies of multiple arcs with these types of models need to impose the “multiple arcs” empirically into the magnetosphere. By way of contrast the USU M-I coupling model self-consistently generates multiple arcs with realistic spacings and widths as well as dependences on solar, ionospheric, and magnetospheric conditions without externally imposing them. This leads us to believe the active form of the M-I coupling is present at high latitudes.