Electromagnetic proton cyclotron instability: Proton velocity distributions

Abstract

The hot (Th ∼ 10 keV) protons of the outer magnetosphere are frequently observed to have T⊥/T‖ > 1, where the subscripts denote directions relative to the background magnetic field. This anisotropy leads to the growth of the electromagnetic proton cyclotron anisotropy instability, which implies two concise scaling relations: an upper bound on T⊥h/T‖h and an expression for the apparent temperature of warm (Tω ∼ 10 eV) protons heated by this instability. One‐dimensional hybrid simulations of this instability in a homogeneous plasma have been used to examine the velocity distribution response of both hot and warm protons. Results are that the enhanced fluctuations from this instability not only preserve the initially bi‐Maxwellian character of a hot proton distribution but also drive a hot distribution which is initially nongyrotropic rapidly toward the gyrotropic condition. These results further support the validity of the hot proton temperature anisotropy upper bound. Although wave‐particle interactions yield nongyrotropic ƒω(υ⊥) distributions, the ƒω(υ‖) remain Maxwellian‐like as they are heated by the enhanced fluctuations.