  Electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency
are frequently observed in the solar wind, yet their generation mechanism is
still an open question. Based on the Wind data during the years 2005$-$2015,
this paper carries out a statistical study on plasma characteristics associated
with the occurrence of ECWs. The probability density distributions (PDDs) of
proton temperature anisotropy ($T_\perp/T_\parallel$) and proton parallel beta
($\beta_\parallel$) are investigated, where $\perp$ and $\parallel$ refer to
perpendicular and parallel to the background magnetic field, respectively. The
PDDs depend on solar wind types as well as wave polarizations, and those for
ECWs with left-handed (LH) polarization exhibit considerable differences from
the PDDs for ambient solar winds. The distributions of occurrence rates of LH
ECWs in ($\beta_\parallel$, $T_\perp/T_\parallel$) space show a tendency that
the occurrence rates increase with proton temperature anisotropies. The
$\beta_\parallel$ with maximum of occurrence rates is near 0.1 when
$T_\perp/T_\parallel > 1$ while it is around 1 when $T_\perp/T_\parallel < 1$.
The presence of alpha$-$proton differential flow with large kinetic energy
corresponds to a much high occurrence rate as well as the domination of LH
polarization of ECWs. Based on these observations and existing theories, we
propose that the proton cyclotron and parallel firehose instabilities with
effects of alpha$-$proton differential flow are likely responsible for the
local generation of LH ECWs in the solar wind. The generation mechanism of
right-handed ECWs seems to be complicated and more discussions are needed in
future researches.
