Semiannual and annual modulation of the magnetic field

Abstract

Autospectrums computed from 24 series of monthly horizontal intensity at Alibag, derived from observations restricted to a single hour of the day at a time, establish the existence of intervals when the semiannual and annual modulations of the field assume large magnitudes. Series generated from observations during a brief interval centred around 0300 UT are noticed to be modulated primarily at a frequency of 2 cy^-1, the power density at this frequency being about 15 γ² per cycle per unit bandwidth. The peak in the density is followed by a rapid decrease to its lowest value of 2.2γ² at 0700 UT. Annual modulation of the field is also exceedingly large during a six hour interval. Power density at 1 cy^-1 which has a low value of 2.7 γ² at 0100 assumes its largest value of 22.4 γ² at 0900 with a secondary peak centred at 1700 UT. The results suggest a two-component model of the generating mechanism for the annual modulation.