Geomagnetic solar and lunar daily variations at Alibag, India

Abstract

Geomagnetic solar and lunar daily variations S and L, at Alibag, India are derived, by the well-known Chapman-Miller method, from the series of homogeneous mean hourly magnetic data of the years 1932 to 1972. The data for all the three elements - declination D and horizontal and vertical intensities H and Z - are analysed, by dividing the data suitably for a study of the seasonal variations, the effect of the changes in the solar and magnetic activities on S and L, the oceanic dynamo contribution to L, and their interactions with each other. The main results are as follows. (i) For S the daily pattern and its seasonal progression conform to the type expected from a northern-hemisphere station. On the other hand, the amplitudes of all the four harmonics of L systematically have higher values in winter, and in D and Z the harmonics show large phase differences between summer and winter. The pattern of L in winter suggests that the lunar current system consists of a single set of vortices in the summer hemisphere rather than the conventional vortices, one set in each of the hemispheres. (ii) Solar-cycle modulation on the solar ranges as well as on the amplitudes of the first three harmonics of S is greater than that expected solely from the increase in E-region conductivity, whereas the corresponding modulation on L is comparable to that on the E-region conductivity. (iii) With increasing magnetic activity the first harmonic of S shows an increase, and the first three harmonics of L indicate a general decrease, in amplitude. (iv) Of the variability in S 96%, but in L only 32%, is found to be accounted for by the combined effect of the variations in the solar and magnetic activities.