PERSISTENCE OF QUASI-2-DAY OSCILLATIONS IN THE GEOMAGNETIC-ACTIVITY INDEXES (A(N), A(S), A(M))

Tt is found that quasi-a-day oscillations persist in the geomagnetic a ctivity indices a(n), a(s) and a(m) and time variations of the oscilla tion characteristics are examined. The 3-hourly indices for 1964-1986 are used for analysis. The main results are: 1) the oscillation persis ts almost continuously, 2) there are not significant hemispheric diffe rences of the oscillation period and amplitude, 3) the dominant period is 48 hours, 4) there is expressed a semi-annual variation of the osc illation period with minimum in solstices and maximum in equinoxes, 5) the oscillation amplitude is modulated by semi-annual and long-term g eomagnetic variations. A possible way for inducing such oscillations i n the time variations of geomagnetic activity indices is proposed.