Proton temperature anisotropy constraint in the solar wind: ACE observations

The electromagnetic proton cyclotron anisotropy instability may arise in collisionless plasmas in which the proton velocity distribution is approximately bi-Maxwellian with T-perpendicular top/T-parallel top > 1, where perpendicular to and parallel to denote directions relative to the background magnetic field B-omicron. Theory and simulations predict that enhanced field fluctuations from this instability impose a constraint on proton temperature anisotropies of the form T-perpendicular top/T-parallel top - 1 = S-p/beta (alphap)(parallel to)(p) where beta (parallel top) equivalent to 8 pi npk(B)T(parallel top/)B(omicron)(2), and the fitting parameters S-p less than or similar to 1 and alpha (p) similar or equal to 0.4. Plasma and magnetic field observations from the ACE spacecraft reported here show for the first time that this constraint is statistically satisfied in the high speed solar wind near 1 AU, and magnetic power spectra provide evidence that this instability is the source of the constraint.