Solar wind parameters, such as magnetic field strength B, plasma density &rgr; and bulk speed v 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=(B2/8&pgr;)/(&rgr;v2/2) has a correlation coefficient (cc) with SSN which is appreciably higher than for each of B, &rgr; and v 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. ¿ 1999 American Geophysical Union