Some Features of Annual Variation in the Equatorial Geomagnetic Field

Abstract

Analysis of mean hourly values of H field from Trivandrum (dip 0.3°S) based on data derived for all days and for quiet and disturbed days, restricted separately, reveals the fine structure of the local time dependence of the annual component in the electrojet region. Comparison with the result for Alibag (dip 24°N) shows that (i) the local time variations of amplitude for quiet days at Alibag and Trivandrum are broadly similar; (ii) there is no equatorial enhancement in the amplitude for hours close to local noon, indicating absence of ionospheric conductivity contributions; (iii) at Trivandrum, a significant secondary peak centred at 1700 hrs LT, especially in the data for quiet days, is observed. (This component is nearly in phase opposition to that at 1200 hrs LT, with minimum in June.); (iv) the amplitude of the secondary component, derived from 3-yr data at a time, shows an inverse linear dependence on solar activity whereas the local noon component is nearly independent of solar activity; and (v) local time variation in the amplitude at Trivandrum derived from disturbance field is dissimilar to that observed at Alibag. (The dissimilarity persists even in the semi-annual component of the disturbance field and is indicative of different generating mechanisms).