Characterising mesoscale magnetopause surface waves within magnetosphere–ionosphere–ground coupling

Disturbances to the magnetopause location driven by upstream pressure variations or flow shear instabilities may be described as surface waves, which act as localised sources of field-aligned currents coupling the magnetosphere to the ionosphere. While global simulations provide semi-quantitative predictions of their large-scale signatures on the ionosphere and ground and, more generally, qualitative features for interpreting observations, how to scale these predictions across the broad possible ranges of wave and system properties are poorly understood. We, therefore, develop a simple numerical model for dispersionless mesoscale magnetopause surface waves within the coupled magnetosphere–ionosphere–ground system to assess possible scaling relations.

In general, the impacts of finite wave packets can be decomposed into periodic fluctuations (with matching wavelength to that in the magnetosphere) along with slowly-varying trends that result from finite wave effects. Finite wave packets act in the far-field like a string of alternating field-aligned currents well described both in the ionosphere and on the ground as a two-dimensional current dipole. In the ionosphere, near-field periodic fluctuations exponentially decay latitudinally away from the open–closed boundary over the reduced wavelength, which may limit how well they can be resolved by radar.

The relationship between the magnetic field above and below the ionosphere becomes more complicated for surface waves than infinite plane Alfvén waves due to the additional spatial structure, which introduces interference across the spectrum of wavenumbers present. This modifies how the ionosphere screens, rotates, and spatially smears magnetic field perturbations across all three components in different ways, importantly resulting in latitudinal scales of amplitude and polarisation variation smaller than typical ground magnetometer spacings, motivating the need for denser networks. A range of effective skin depths in the ground are applicable to surface waves, meaning ground induction can vary between a near-perfect insulator to a good conductor, affecting both observable ground magnetic fields and resulting geoelectric fields. The predicted peak amplitudes of surface waves' impacts suggest they may act as significant sources of ionospheric/thermospheric Joule heating and geoelectric fields in the ground, thereby contributing to space weather impacts though highly localised latitudinally.

Our results provide key predictions for interpreting ground-based observations, of particular timeliness with the rollout of new digital ionospheric radars and the upcoming SMILE mission's planned conjugate ground–space campaigns.