  Plasma kinetic waves and alpha-proton differential flow are two important
subjects on the topic of evolution of the solar wind. Based on the Wind data
during 2005-2015, this paper reports that the occurrence of electromagnetic
cyclotron waves (ECWs) near the proton cyclotron frequency significantly
depends on the direction of alpha-proton differential flow Vd. As Vd rotates
from anti-Sunward direction to Sunward direction, the occurrence rate of ECWs
as well as the percentage of left-handed (LH) polarized ECWs decreases
considerably. In particular, it is shown that the dominant polarization changes
from LH polarization to right-handed polarization during the rotation. The
investigation on proton and alpha particle parameters ordered by the direction
of Vd further illustrates that large kinetic energies of alpha-proton
differential flow correspond to high occurrence rates of ECWs. These results
are well consistent with theoretical predictions for effects of alpha-proton
differential flow on proton temperature anisotropy instabilities.
