Title: Proton fire hose instabilities in the expanding solar wind,
Abstract: Using two-dimensional hybrid expanding box simulations we study the
competition between the continuously driven parallel proton temperature
anisotropy and fire hose instabilities in collisionless homogeneous plasmas.
For quasi radial ambient magnetic field the expansion drives
$T_{\mathrm{p}\|}>T_{\mathrm{p}\perp}$ and the system becomes eventually
unstable with respect to the dominant parallel fire hose instability. This
instability is generally unable to counteract the induced anisotropization and
the system typically becomes unstable with respect to the oblique fire hose
instability later on. The oblique instability efficiently reduces the
anisotropy and the system rapidly stabilizes while a significant part of the
generated electromagnetic fluctuations is damped to protons. As long as the
magnetic field is in the quasi radial direction, this evolution repeats itself
and the electromagnetic fluctuations accumulate. For sufficiently oblique
magnetic field the expansion drives $T_{\mathrm{p}\perp}>T_{\mathrm{p}\|}$ and
brings the system to the stable region with respect to the fire hose
instabilities.