Long term variability in solar wind velocity and IMF intensity and the relationship between solar wind parameters & geomagnetic activity

摘要

A study is carried out on the mean monthly values of in situ observations of solar wind velocity (V) and the intensity of interplanetary magnetic field, Bto elucidate their long term variations using the technique of singular spectrum analysis. It is shown that Bexhibits a clear solar cycle signal with progressively deepening minimum and a well-defined longer period variation but Vis marked by a ~9-yr cycle. Time variations in the amplitude of 16-month periodicity, observed sporadically in the velocity earlier by others are clearly brought out, as also the characteristic amplitude changes in a 42-month signal in velocity. It appears that time intervals which show certain significant fluctuations in Bare marked by the absence of similar signal in V.aaindex is next used as a proxy for solar wind velocity, after establishing a good correlation between Ap and observed Vso that we could examine the evolution of different periodicities over 14 solar cycles. A significant trend with least value in 1900 and a near-linear rise up to 1960 is shown to be the main feature of the velocity change. The solar cycle component in Vlags the solar activity peak by ~22 months. Streams emanating from coronal holes in the declining phase seem to be the most dominant contributor to the 11-year variation in velocity. The anomalous pattern of changes in V observed in cycle 20 is not present in any of the other 13 cycles. Statistical relationships between V, B and plasma density (N) with Apare studied and it is shown that over three solar cycles (20, 21 and 22) the patterns are almost the same with a slight change observed in cycle 21. IMF B and Apare linearly related over a wide range of Apvalues from close to 0 up to about 60, whereas density/Aprelation appears insignificant. In case of V, an initial rapid rise in Vcauses moderate changes in Apbut for velocity in excess of 700 km/sec, the enhancement in geomagnetic activity is more rapid. From the statistical relation of several other solar wind/IMF parameters, their variability and combinations with Ap, one sees linear relation for solar wind electric field, n/s component of IMF and variability in the components of B. An estimate is made of the base level of the magnetosphere, corresponding to quiet levels of geomagnetic activity.