Solar wind quasi‐invariant as a new index of solar activity

Solar wind parameters, such as magnetic field strength B, plasma density ρ and bulk speed υ correlate with solar cycle measured by sunspot numbers SSN. Using hourly data in the solar wind near the Earth, we show that the energy density ratio QI = (B²/8π)/(ρυ²/2) has a correlation coefficient (cc) with SSN which is appreciably higher than for each of B, ρ and υ taken separately. The proposed quasi‐invariant is related to magnetic Mach number MA (QI = MA−2). The yearly median QI for the last 28 years holds a linear relation with SSN with high cc = 0.98, which makes it the best index of solar activity derived from solar wind parameters so far.