Identification and prediction of prolonged intervals of geomagnetic calm

From the daily index Ap of geomagnetic activity, sequences of consecutive occurrence of Ap≤4 for four days and Ap≤7 for 8 days are identified in the period 1963-1998. A superposed epoch analysis is performed with the onset of these sequences and with eight different solar wind and IMF parameters. A systematic pattern of change from days preceding to the days following the quiet intervals is clearly established. Solar wind velocity and its variability attain their lowest values after the onset of the quiet sequence and lengthen the interval. IMF B, on the other hand, attains the lowest value at the onset of the quiet intervals and remains low till the end of the sequence. Possible threshold values of V~320 km/sec, B ~4 nT, Bz > 0 (~0.8 nT), are found for prolonged intervals of geomagnetic calm. Geomagnetic quiet intervals could be predicted from the solar wind parameters with moderate success.