On the universal time component in the daily variation of the geomagnetic disturbance field

Abstract

The magnitude of the universal time (UT) component in the daily variation of geomagnetic disturbance which depends on the position of the earth's magnetic axis relative to the sunearth system is evaluated at two low-latitude stations, Alibag in the northern hemisphere and Hermanus in the southern hemisphere. To provide better resolution on time scale, mean hourly values of horizontal intensity are used rather than index K of magnetic activity. During solstices the UT component has a mean amplitude of 6.8 γ at Alibag and 3.0 γ at Hermanus with maximum contribution near the predicted time of 0430 UT in June and 1630 UT in December. It is found that the UT component makes a significant contribution to the daily variation of disturbance field during equinoxes but the part dependent on the angle between the earth's magnetic axis and the sun-earth line is found to be larger during September than in March, suggesting an asymmetry in the incident solar wind. This is possibly due to the varying heliographic latitude of the earth and the asymmetry in activity in the two hemispheres of the sun.