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

Abstract

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, B to elucidate their long term variations using the technique of singular spectrum analysis. It is shown that B exhibits a clear solar cycle signal with progressively deepening minimum and a well-defined longer period variation but V is 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 B are marked by the absence of similar signal in V. aa index is next used as a proxy for solar wind velocity, after establishing a good correlation between Ap and observed V so 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 V lags 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 A p are 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 Ap are linearly related over a wide range of A p values from close to O up to about 60, whereas density/A p relation appears insignificant. In case of V, an initial rapid rise in V causes moderate changes in A p but 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 A p, 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.