Ionospheric Joule heating, fast flow channels, and magnetic field line topology for IMF By‐dominant conditions: Observations and comparisons with predicted reconnection jet speeds

Abstract

When the interplanetary magnetic field (IMF) is dawnward or duskward, magnetic merging between the IMF and the geomagnetic field occurs near the cusp on the dayside flanks of the magnetosphere. While these periods are usually considered “quiet,” they can lead to intense localized energy deposition into the dayside ionosphere. We analyze two intervals during the geomagnetic storm on 24 August 2005: one with steady duskward IMF and one with steady dawnward IMF. Using outputs from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) and data from the Defense Meteorological Satellite Program, we show that intense Joule heating exists on fast ionospheric flow channels which lie on open field lines. In addition, the flow channels on open field lines have large components in the sunward direction and therefore resist the bulk solar wind and magnetosheath flow. We compare observed velocities with predicted reconnection jet speeds using magnetosheath and cusp parameters from an MHD simulation. Results suggest that the fast ionospheric flow corresponds to portions of the reconnection jet populated by low‐density plasma. The importance of ionospheric conductance in determining the ionospheric flow is also discussed.