THE SOLAR-CYCLE VARIABILITY OF LUNAR AND SOLAR DAILY GEOMAGNETIC-VARIATIONS

Hourly mean values of the geomagnetic field from observatories between 6-degrees dip latitude and 60-degrees geomagnetic latitude are analysed for solar and lunar daily variations. Three sets of data are used, from 18 observatories over 20 years (1958-77), from 5 observatories over 75 years (1910-1984) and from about 100 observatories over 4.5 years. The time harmonics of the horizontal components of S and L for each year are expanded into series of spherical harmonics. External and internal parts are separated by use of a spherically symmetric model of the earth's electric conductivity. A linear regression epsilon(n)m (R) = A(n)m (1 + M(n)m R) is evaluated to express the dependence of the external potential coefficients epsilon(n)m of S and L on the sunspot number R. By taking A(n)m and the Wolf ratio M(n)m as complex numbers, the phase shifts between S, L and the sunspot number is considered. The R-dependence of S is found to be about 2.5 times that of L, and as expected, the amplitudes \epsilon(n)m\ increase with R for both S and L. When the sun is active, the foci of the equivalent currents are about 1/2 to 1 hour later in local time for S, but earlier for L, compared to their positions during quiet years. This is a consistent result for both data sets. Theoretical investigations show that the R-dependence of the ionospheric conductivity leads to a phase shift as observed for the lunar variations. An explanation for the different behaviour of S and L is still lacking.