Wind/SWE observations of firehose constraint on solar wind proton temperature anisotropy

The proton resonant firehose instability may arise in collisionless plasmas in which the proton velocity distribution is approximately bi‐Maxwellian with T ∥ p / T ⊥ p > 1, where ⊥ and ∥ denote directions relative to the background magnetic field B ○ . Linear theory and one‐dimensional simulations predict that enhanced field fluctuations from the proton resonant firehose instability impose a constraint on proton temperature anisotropies of the form 1 − T ⊥ p / T ∥ p = S p /β ∥ p α p where β ∥ p ≡ 8π n p k B T ∥ p / B ○ 2 , and the fitting parameters S p ∼ 1 and α p ≃ 0.7. Observations from the Wind spacecraft are reported here. These measurements show for the first time with a comprehensive plasma and magnetic field data set that this constraint is statistically satisfied in the solar wind near 1 AU, with best‐fit values of S p = 1.21 ± 0.26 and α p = 0.76 ± 0.14.