Pioneer Venus Orbiter observations of a solar wind quasi‐invariant

The recently suggested solar wind quasi‐invariant [QI≡( B²/8π)/(ρυ²/2)] is a combination of magnetic field strength B, plasma density ρ and solar wind speed υ. Measured in the solar wind near the Earth, the median value of QI was shown to be a reliable index of solar activity for the 28 year period holding a linear relation with sunspot numbers (SSN) with a high correlation coefficient (cc) of 0.98 [Osherovich, Fainberg and Stone, 1999]. Here, using hourly data from the Pioneer Venus Orbiter for the period from 1978–1988, we demonstrate that QI near Venus also closely follow SSN with a correlation coefficient of 0.94 for the yearly mean values. We found that the calibration coefficient between QIVenus and QIEarth is slightly larger than unity, namely, QIVenus=a* · QIEarth where a*=1.134±0.001. We suggest that, for any point in the heliosphere, QI can be viewed as a useful measure of solar activity.