Abstract:
This paper briefly surveys the nature of magnetic perturbations at the ground and at low-Earth-orbit (LEO) altitudes produced by ionospheric and geomagnetic-field-aligned currents, with the aid of representative simulation and empirical models. The equatorial electojet, driven by the eastward daytime ionospheric electric field, strongly affects magnetic perturbations near the magnetic equator. Thermospheric winds also influence low-latitude ionospheric currents, and create significant east-west magnetic perturbations at lower LEO altitudes. At midlatitudes, currents flow not only within the ionosphere but also along geomagnetic field lines above the ionosphere. The latter can produe significant large-scale magnetic perturbations above the ionosphere over large regions of space. The largest magnetic perturbations are produced by electric currents in the auroral zones and the geomagnetic-field-aligned currents that connect the ionosphere with the outer magnetosphere. Above the ionosphere, the high-latitude magnetic perturbations tend to be dominated by toroidal magnetic perturbation fields associated with the field-aligned currents. LEO observations of magnetic perturbations can provide valuable information about the drivers of the ionospheric dynamo, the state of the magnetosphere, and the transfer of energy between the magnetosphere and the ionosphere.
