Kinetic Alfvén Waves: Linear theory and a Particle-in-cell Simulation

An Alfvén-cyclotron fluctuation of sufficiently short wavelength has a strong proton cyclotron resonance at propagation parallel to the background magnetic field B_o in a collisionless electron-proton plasma. As θ , the angle between the fluctuation wavevector k and B_o increases, proton cyclotron wave-particle interactions become weaker and the electron Landau resonance becomes effective. Define the critical angle θ _o beyond which the proton cyclotron interaction becomes nonresonant. Here linear Vlasov theory shows that this critical angle is relatively independent of β _p over 0.001 <= β _p <= 0.10, but that θ _o is a sensitive function of the damping rate. Also, a particle-in-cell simulation was carried out in a magnetized, homogeneous, collisionless plasma of electrons and one ion species to study the plasma response to the initial imposition of an Alfvén-cyclotron wave at θ > θ _o. The computation shows strong heating of the electrons in the direction parallel to B_o and the formation of a beam in the direction of parallel wave phase propagation.