An external current representation of the quiet nightside geomagnetic field level changes.

The annual and semiannual behavior of the local midnight field for North America, is represented as an external current system display derived from a spherical harmonic analysis. There are two vortices of this current: one is clockwise, centered in the Southern Hemisphere in the June solstitial period; the other is counter-clockwise, centered in the Northern Hemisphere in the December solstitial period. The foci of approximately 19, 000 amperes are located near 8 degrees geomagnetic latitude. The seasonal change in orientation of the solar wind with respect to the Earth's geomagnetic dipole axis explains most of the current pattern appearance. The maximum amplitude of the field components reconstructed from the representation rises to about 5 to 6 gammas at the equatorial and polar latitudes and to about half that size at the middle latitudes.