Solar wind proton temperature anisotropy: Linear theory and WIND/SWE observations

We present a comparison between WIND/SWE observations (Kasper et al., 2006) of β ∥ p and T ⊥ p / T ∥ p (where β ∥ p is the proton parallel beta and T ⊥ p and T ∥ p are the perpendicular and parallel proton temperatures, respectively; here parallel and perpendicular indicate directions with respect to the ambient magnetic field) and predictions of the Vlasov linear theory. In the slow solar wind, the observed proton temperature anisotropy seems to be constrained by oblique instabilities, by the mirror one and the oblique fire hose, contrary to the results of the linear theory which predicts a dominance of the proton cyclotron instability and the parallel fire hose. The fast solar wind core protons exhibit an anticorrelation between β ∥ c and T ⊥ c / T ∥ c (where β ∥ c is the core proton parallel beta and T ⊥ c and T ∥ c are the perpendicular and parallel core proton temperatures, respectively) similar to that observed in the HELIOS data (Marsch et al., 2004).