Ion Heating Resulting from the Deceleration of Alpha Particles by a Proton-alpha Drift Instability in a Nonuniform Solar-wind Plasma

The deceleration of alpha particle observed in the fast solar wind can contribute to the plasma heating between 0.3 and 1 au. The observational data suggest that the energy released from the deceleration has to be channeled to perpendicular heating of the protons. A possible mechanism of the energy conversion is a proton-alpha drift instability. We present hybrid numerical simulations of this instability in a warm plasma with particle-in-cell ions and a neutralizing electron fluid. The parallel temperature of the alpha particles is assumed to be larger than the perpendicular temperature. This sense of the anisotropy makes parallel-propagating fast magnetosonic waves the most easily excited modes. For typical ion beta values at 0.3 to 1 au, we find that the instability does not produce evident perpendicular heating of the protons if the initial background plasma is uniform. The lack of the heating is related to inefficient cyclotron interaction of the protons with the parallel-propagating fast modes. However, the background plasma in the solar wind is unlikely to be uniform. We consider the background variations across the mean magnetic field in the form of single or multiple equilibrium structures. The inhomogeneity modifies the unstable waves by making them oblique. Furthermore, their wavenumber spectrum extends to perpendicular wavenumbers of the order of the inverse proton gyroradius. Such waves can interact with the protons more efficiently. We show that significant and preferentially perpendicular heating of the protons is present in the nonuniform plasma.