The Solar Wind Quasi-Invariant Observed by STEREO A and B at Solar Minimum 2007 and Comparison with Two Other Minima
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- Leitner, M., Farrugia, C.J., Galvin, A. et al. Sol Phys (2009) 259: 381. doi:10.1007/s11207-009-9412-z
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The solar wind quasi-invariant (QI) is defined as the ratio of the solar wind magnetic energy density to the plasma kinetic energy density (i.e., the inverse square of the Alfvén Mach number). Previous work has found this quantity to be a good proxy for solar activity, correlating very well with the sunspot number at various heliospheric distances. It has the advantage of being locally determined from in situ measurements and can thus function as a heliospheric index of solar activity. Using STEREO A, STEREO B, and Wind data we obtain the distribution of QIs during the current solar activity minimum (March to December 2007). (1) We investigate whether this minimum is indeed weaker than previous ones by comparing this QI distribution with those during two other solar activity minima: 1995, using Wind data, and 1974, using Helios data at 1 AU. We find that, on average, QI(2007) is lower than during the previous two minima, indicating weaker solar activity. It also implies weaker MHD effects in solar wind flow around planetary magnetospheres, which, in turn, alters the solar wind’s interaction with them. (2) In all three solar cycle minima considered we find that the QI distributions are reasonably well represented by a log-normal distribution, for which we give the respective mean and standard deviations. These values are used in comparing the QIs over the three solar minima.