Long term variability in solar wind velocity and IMF intensity and the relationship between solar wind parameters & geomagnetic activity
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概要
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A study is carried out on the mean monthly values of in situ observations of solar wind velocity (<I>V</I>) and the intensity of interplanetary magnetic field, <I>B</I> to elucidate their long term variations using the technique of singular spectrum analysis. It is shown that <I>B</I> exhibits a clear solar cycle signal with progressively deepening minimum and a well-defined longer period variation but <I>V</I> 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 <I>B</I> are marked by the absence of similar signal in <I>V</I>.<BR><I>aa</I> index is next used as a proxy for solar wind velocity, after establishing a good correlation between Ap and observed <I>V</I> 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 <I>V</I> 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.<BR>Statistical relationships between <I>V</I>, B and plasma density (<I>N</I>) with <I>Ap</I> 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 <I>B</I> and <I>Ap</I> are linearly related over a wide range of <I>Ap</I> values from close to 0 up to about 60, whereas density/<I>Ap</I> relation appears insignificant. In case of <I>V</I>, an initial rapid rise in <I>V</I> causes moderate changes in <I>Ap</I> 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 <I>Ap</I>, one sees linear relation for solar wind electric field, n/s component of IMF and variability in the components of <I>B</I>. An estimate is made of the base level of the magnetosphere, corresponding to quiet levels of geomagnetic activity.
- 公益社団法人 日本地震学会、地球電磁気・地球惑星圏学会 、特定非営利活動法人 日本火山学会、日本測地学会、日本惑星科学会の論文
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- Long term variability in solar wind velocity and IMF intensity and the relationship between solar wind parameters & geomagnetic activity
- Long term variability in solar wind velocity and IMF intensity and the relationship between solar wind parameters & geomagnetic activity