  In a previous paper, we found that proton temperatures are clearly associated
with the proton-scale turbulence in the solar wind, and magnetic helicity
signature appears to be an important indicator in the association. Based on 15
years of in situ measurements, the present paper further investigates the
magnetic helicity of solar wind turbulence and its role in regulating magnetic
energy spectra and proton temperatures. Results show that the presence of the
helicity signature is very common in solar wind turbulence at scales $0.3
\lesssim k\rho_p \lesssim 1$, with $k$ being the wavenumber and $\rho_p$ the
proton gyroradius. The sign of the helicity is mostly positive, indicating the
dominance of right-handed polarization of the turbulence. The helicity
magnitude usually increases with $k$ and $\beta_{{\parallel}p}$ (the proton
parallel beta) when $k\rho_p$ and $\beta_{{\parallel}p}$ are less than unity.
As helicity magnitude increases, the power index of the energy spectrum becomes
more negative, and the proton temperatures $T_{{\perp}p}$ and
$T_{{\parallel}p}$ rise significantly, where $T_{{\perp}p}$ and
$T_{{\parallel}p}$ are the perpendicular and parallel temperatures with respect
to the background magnetic field. In particular, the rise of $T_{{\perp}p}$ is
faster than $T_{{\parallel}p}$ when $\beta_{{\parallel}p} < 1$ is satisfied.
The faster rise of $T_{{\perp}p}$ with the helicity magnitude may be
interpreted as the result of the preferentially perpendicular heating of solar
wind protons by kinetic Alfv\'en wave (KAW) turbulence.
