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 ⟂p / T ∥p > 1, where ⟂ and ∥ denote directions relative to the background magnetic field B o . Theory and simulations predict that enhanced field fluctuations from this instability impose a constraint on proton temperature anisotropies of the form where , and the fitting parameters S p ≲ 1 and α p ≃ 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.