Hellinger and Matsumoto : Oblique Alfvén Fire Hose a 5 - 10 , 521

Two instabilities could take place in plasma with a bi-Maxwellian proton distribution function with Tp‖ > Tp⊥, where Tp‖ and Tp⊥ 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.