New kinetic instability: Oblique Alfvén fire hose

Two instabilities could take place in plasma with a bi‐Maxwellian proton distribution function with T p ‖ > T p ⊥, where T p ‖ and T p ⊥ are proton temperatures, parallel and perpendicular, respectively, to the background magnetic field. The first instability is the fire hose (or whistler fire hose), generating low‐frequency whistler waves at parallel propagation. We found a new, second instability, the Alfvén fire hose, that generates zero‐frequency waves of the Alfvén branch at strongly oblique propagation. The Alfvén fire hose has a linear growth rate comparable to or even greater than that of the whistler fire hose. The two instabilities with the same initial plasma parameters are examined via one‐dimensional hybrid simulations and turn out to have behavior very different from each other. The whistler fire hose has an overall quasi‐linear evolution, while the evolution of the Alfvén fire hose is more complicated: Initially, unstable zero‐frequency waves are gradually transformed into propagating Alfvén waves; during this process the waves are strongly damped and heat protons in a perpendicular direction. Consequently, the Alfvén fire hose is very efficient at destroying proton anisotropy.