Auroral, Ionospheric and Ground Magnetic Signatures of Magnetopause
Surface Modes
Abstract
Surface waves on Earth’s magnetopause have a controlling effect upon
global magnetospheric dynamics. Since spacecraft provide sparse in situ
observation points, remote sensing these modes using ground-based
instruments in the polar regions is desirable. However, many open
conceptual questions on the expected signatures remain. Therefore, we
provide predictions of key qualitative features expected in auroral,
ionospheric, and ground magnetic observations through both
magnetohydrodynamic theory and a global coupled magnetosphere-ionosphere
simulation of a magnetopause surface eigenmode. These show monochromatic
oscillatory field-aligned currents, due to both the surface mode and its
non-resonant Alfvén coupling, are present throughout the magnetosphere.
The currents peak in amplitude at the equatorward edge of the
magnetopause boundary layer, not the open-closed boundary as previously
thought. They also exhibit slow poleward phase motion rather than being
purely evanescent. We suggest the upward field-aligned current
perturbations may result in periodic auroral brightenings. In the
ionosphere, convection vortices circulate the poleward moving
field-aligned current structures. Finally, surface mode signals are
predicted in the ground magnetic field, with ionospheric Hall currents
rotating perturbations by approximately (but not exactly) 90º compared
to the magnetosphere. Thus typical dayside magnetopause surface modes
should be strongest in the East-West ground magnetic field component.
Overall, all ground-based signatures of the magnetopause surface mode
are predicted to have the same frequency across L-shells, amplitudes
that maximise near the magnetopause’s equatorward edge, and larger
latitudinal scales than for field line resonance. Implications in terms
of ionospheric Joule heating and geomagnetically induced currents are
discussed.