Proton Temperature Anisotropy in the Solar Wind: Linear Theory, Observations and Simulations

We present a comparison between WIND/SWE observations of the proton temperature anisotropy T\perp p/T∥ p in the solar wind and predictions of the Vlasov linear theory. The observed proton temperature anisotropy seems to be constrained by oblique instabilities, by the mirror one and the oblique fire hose, whereas the linear theory predicts a dominant role of the proton cyclotron instability and the parallel fire hose. The discrepancy between the linear theory and observations is investigated in the case of T\perp p∥ p using the hybrid expanding box simulations. The simulations show that both the parallel and oblique fire hoses constrain the proton temperature anisotropy with the latter one constituting an ultimate anisotropy frontier. These simulation results are in a agreement with the observations and may explain the apparent discrepancy between the linear theory and observations.