Change of solar wind quasi-invariant in solar cycle 23—Analysis of PDFs

Abstract An in situ solar wind measurement which is a very good proxy for solar activity, correlating well with the sunspot number, is the solar wind “quasi-invariant” (QI), which is defined as the ratio between magnetic and kinetic energy densities. Here we use 1-min OMNI data to determine yearly probability density functions (PDFs) for QI. We distinguish between fast and slow solar winds, and exclude interplanetary coronal mass ejections (ICMEs) from the data, since the latter have a different distribution. Fitting the PDFs by a log-kappa distribution, we discuss the variation of QI in the period 1995–2009, encompassing solar cycle 23 and the long, very quiet minimum in 2007–2009. The additional value of kappa allows us to obtain a better description for the tails of the distribution than the log-normal approach. Here we describe for the first time how parameter kappa changes over one solar cycle.