  We examine the long-term time evolution (1965-2015) of the relationships
between solar wind proton temperature (Tp) and speed (Vp) and between the
proton density (np) and speed using OMNI solar wind observations taken near
Earth. We find a long-term decrease in the proton temperature-speed (Tp-Vp)
slope that lasted from 1972 to 2010, but has been trending upward since 2010.
Since the solar wind proton density-speed (np-Vp) relationship is not linear
like the Tp-Vp relationship, we perform power law fits for np-Vp. The exponent
(steepness in the np-Vp relationship) is correlated with the solar cycle. This
exponent has a stronger correlation with current sheet tilt angle than with
sunspot number because the sunspot number maxima vary considerably from cycle
to cycle and the tilt angle maxima do not. To understand this finding, we
examined the average np for different speed ranges, and found that for the slow
wind np is highly correlated with the sunspot number with a lag of ~4 years.
The fast wind np variation was less, but in phase with the cycle. This phase
difference may contribute to the np-Vp exponent correlation with the solar
cycle. These long-term trends are important since empirical formulas based on
fits to Tp and Vp data are commonly used to identify ICMEs, but these formulas
do not include any time dependence. Changes in the solar wind density over a
solar cycle will create corresponding changes in the near Earth space
environment and the overall extent of the heliosphere.
