Persistence of quasi-2-Day oscillations in the geomagnetic activity indices (a_n, a_s, a_m)

It is found that quasi-2-day oscillations persist in the geomagnetic activity indices a, a_s and a_m and time variations of the oscillation characteristics are examined. The 3-hourly indices for 1964-1986 are used for analysis. The main results are: 1) the oscillation persists almost continuously, 2) there are not significant hemispheric differences of the oscillation period and amplitude, 3) the dominant period is 48 hours, 4) there is expressed a semi-annual variation of the oscillation period with minimum in solstices and maximum in equinoxes, 5) the oscillation amplitude is modulated by semi-annual and long-term geomagnetic variations. A possible way for inducing such oscillations in the time variations of geomagnetic activity indices is proposed.