Proton temperature anisotropy constraint in the solar wind: ACE observations

The electromagnetic proton cyclotron anisotropy instability may arise in co llisionless plasmas in which the proton velocity distribution is approximat ely bi-Maxwellian with T-perpendicular top/T-parallel top > 1, where perpen dicular to and parallel to denote directions relative to the background mag netic field B-omicron. Theory and simulations predict that enhanced field f luctuations 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 o r similar to 1 and alpha (p) similar or equal to 0.4. Plasma and magnetic f ield observations from the ACE spacecraft reported here show for the first time that this constraint is statistically satisfied in the high speed sola r wind near 1 AU, and magnetic power spectra provide evidence that this ins tability is the source of the constraint.