Proton temperature anisotropy constraint in the solar wind: ACE observations

Abstract

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 Bo. Theory and simulations predict that enhanced field fluctuations from this instability impose a constraint on proton temperature anisotropics of the form T⊥p/T∥p - 1 = Sp/β∥pαp where β∥p ≡ magnetic power spectra provide evidence that this instability is the source of the constraint.