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

[1] The proton resonant firehose instability may arise in collisionless plasmas in which the proton velocity distribution is approximately bi-Maxwellian with T-parallel top/T-perpendicular top > 1, where perpendicular to and parallel to denote directions relative to the background magnetic field B-o. 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-perpendicular top/T-parallel top = Sp/beta(parallel top)(alphap) where beta(parallel top) = 8 pin(p)k(B)Tparallel to(p) /B-o(2), and the fitting parameters S-p similar to 1 and alpha(p) similar or equal to 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 alpha(p) = 0.76 +/- 0.14.