Variations in the Earth's surface magnetic field from the magnetopause current system

Abstract

The magnetic field at the Earth's surface produced by the magnetopause current system has been calculated with solar wind conditions (density, speed and direction) taken as constant throughout the year. The strength and shape of this current system (and its associated field) depend upon the orientation of the geomagnetic dipole axis with respect to the direction of the solar wind. The magnetopause currents therefore produce seasonal as well as daily variations in the Earth's surface magnetic field. The daily variations display several of the major features of the observed S q pattern, including seasonal changes and dependence on tongitude. Also in agreement with the observations, the daily field has a maximum at about 11:30 a.m. (local time) owing to the direction of the solar wind (8° west of the optical Sun-Earth line).The magnetopause currents produce both annual and semiannual variations similar in form to the observed patterns. When the contribution from Earth induced currents is taken into account, the magnetopause current system is found to be the source of about 20 per cent of the daily and seasonal variations in the Earth's surface magnetic field. It is concluded that the ionospheric dynamo is not the only source of the S q variations and that at least two other magnetospheric current systems, the neutral sheet and ring currents, may also make contributions to the quiet magnetic variations at the surface of the Earth.