Abstract:
Diurnal variations of the horizontal component of the geomagnetic field ΔH on International
Quiet days of 1999–2012, measured hourly at two stations in the same longitude zone in the Northern
Hemisphere, near and away from the dip equator, have been subjected to principal component analysis. This
technique is also applied to the difference ΔHEEJ of ΔH at these two stations, which is attributed to the
equatorial electrojet (EEJ). The first three principal components, PC1–PC3, account for 91–96% of the variances
in the data. Maximum contribution to the quiet day variations in ΔH around its peak in the morning hours at
both the stations, and in the EEJ, comes from the day-to-day variation of the amplitude of PC1. Patterns of
day-to-day variations of PC1 amplitudes for the equatorial station and the EEJ are essentially semiannual
modulated by solar EUV flux, superimposed on a longer timescale solar EUV flux-dependent trend. Contributions
from PC2 and to a lesser extent from PC3 are seen to be responsible for the absence of semiannual variations in
ΔH in the afternoon hours at the equatorial station. Distribution of amplitudes of PC2 and PC3 for ΔHEEJ for weak
electrojet days shows seasonal features in accordance with greater occurrence of afternoon (morning) counter
electrojet during June (December) solstice. During the extended solar minimum, PC3 amplitudes for ΔH at the
equatorial station and for the EEJ display annual variation. Possible sources for these seasonal features in the
variations of equatorial ΔH are discussed.
