Consequences of proton and alpha anisotropies in the solar wind: Hybrid simulations

Alfvén-like fluctuations in the solar corona and solar wind may cascade from lower to higher frequencies where they transfer energy via cyclotron resonances to ions of successively higher charge-to-mass-ratios. This yields T_{perpendicular to }/T{∥} > 1 for each ion species, where the subscripts refer to directions relative to the background magnetic field. If sufficiently large, these anisotropies drive electromagnetic ion cyclotron instabilities. This manuscript describes the use of two-dimensional hybrid simulations of a collisionless, homogeneous, magnetized plasma with both protons and alpha particles to study the consequences of scattering by enhanced field fluctuations from such instabilities. The most important new results are that both helium and proton cyclotron instabilities reduce initial differences between the proton and alpha particle anisotropies, and also reduce initial alpha/proton relative speeds. These simulation results are different from theoretical predictions of ion responses to their direct interaction with cascading Alfvén/cyclotron waves, but are consistent with observations from the Ulysses spacecraft.