  We present an analysis of the components of solar wind proton temperature
perpendicular and parallel to the local magnetic field as a function of
proximity to plasma instability thresholds. We find that $T_{\perp p}$ is
enhanced near the mirror instability threshold and $T_{\parallel p}$ is
enhanced near the firehose instability threshold. The increase in $T_{\perp p}$
is consistent with cyclotron-resonant heating, but no similar explanation for
hot plasma near the firehose limit is known. One possible explanation is that
the firehose instability acts to convert bulk energy into thermal energy in the
expanding solar wind, a result with significant implications for magnetized
astrophysical plasma in general.
