The ionospheric dynamo and equatorial magnetic variations

Abstract

World-wide magnetic variations observed at four universal times on a quiet day are simulated with coupled numerical models of the global ionospheric dynamo and of the equatorial electrojet. The global dynamo model uses a tilted dipole geomagnetic field with realistic conductivity distributions and height-varying tidal winds, and allows for current flow along field lines through the magnetosphere between northern and southern hemispheres. The simulations furnish information about upper atmospheric winds, electric fields and currents. Our results indicate that the winds driving the dynamo are functions not only of geographic latitude and local time, but also have a universal time dependence. In addition to the diurnal tidal wind component, the semidiurnal component appears to play a prominent role in generating currents and electric fields. The daily variation of the electric field and the consequent E × B electrodynamic drift at the magnetic equator show considerable longitudinal variability. Large vertical drifts around sunrise and sunset sometimes appear in the simulations, which are interpreted in terms of polarization charge accumulations in regions of sharp conductivity gradients. Geomagnetic-field-aligned current flow through the magnetosphere is significant, and is associated with part of the observed east-west ground magnetic variations at the magnetic equator. This field-aligned current is driven primarily by winds of the (2, 3) antisymmetric semidiurnal tidal mode.