Spectral analysis of the geomagnetic activity index Ap during different IMF conditions (1947-1978)

The spectral analysis of the geomagnetic activity index Ap (1947-1978) has been conducted for intervals associated respectively with two and four sectors of the interplanetary magnetic fields per solar rotation. A recurrent 2-sector structure is typically associated with an emerging spectral peak close to Ts (Ts being the period of solar rotation as seen from Earth), while the T2/2 modulation becomes more important during intervals corresponding to four sectors per solar rotation. The recurrence tendency of two high-velocity streams per solar rotation seems to reinforce the relative importance of the T2/2 modulation