Modulation of geomagnetic lunar daily variation in H at Alibag with lunar distance

Abstract

The dependence of geomagnetic lunar daily variation (L) on lunar distance in different seasons and for varied epochs of solar activity is investigated using the long series of horizontal intensity observations at Alibag. The close agreement of the observed ratio of L when the moon is at perigee to that when it is at apogee during different seasons and for the total period to that expected from gravitational tide theory clearly indicates that the direct effect of the moon on the upper atmosphere is primarily gravitational and also suggests that the amplitudes ratio N2: M2 in all seasons departs insignificantly from that expected from tidal theory. A systematic increase of the ratio of L at lunar perigee to apogee with change in phase of solar cycle implies that N2: M2 has a measure of dependence on solar activity. Ionospheric and oceanic parts of L are poorly determined and no definite cause for their behaviour could be given.