QUASI-BIENNIAL AND QUASI-TRIENNIAL OSCILLATIONS IN GEOMAGNETIC-ACTIVITY INDEXES

Data for geomagnetic activity index aa for 1868-1994 were subjected to spectral analysis for 12 intervals each of 11 consecutive years. In e ach interval, QBO and QTO (quasi-biennial and quasi-triennial oscillat ions) were observed at similar to 2.00, 2.15, 2.40, 2.70 y and similar to 3.20, 3.40 y, but not all in all intervals. These fluctuations are absent near (2-3 y before and after) the sunspot minima and are prese nt only as 2 or 3 peaks in aa indices, one near or before the sunspot maximum and the other (one or two, generally the larger ones) in the d eclining phase of the sunspot cycle. Comparison with the solar wind (1 965 onwards) showed a fairly good match, indicating that the aa variat ions were mostly due to similar variations in the solar wind, which mu st have their origin in solar physical processes. A few aa variations did not match with solar wind. When compared with terrestrial phenomen a, no match was found with stratospheric low-latitude zonal wind QBO; but some QTO in aa matched QTO in ENSO (El Nino/ Southern Oscillation) . This may or may not be a chance coincidence and needs further explor ation.