The need to consider ion Bernstein waves as a dissipation channel of solar wind turbulence

Abstract

Kinetic Alfvén waves (KAWs) with highly oblique wave vectors, k⊥ ≫ k∥, are believed to form an integral part of the turbulent energy cascade in the solar wind near the proton gyroradius scale k⊥ρi ∼ 1. At wave numbers k⊥ρi > 1, where linear theory predicts kinetic Alfvén waves undergo strong Landau damping, mode coupling with ion‐Bernstein waves (IBWs) occurs. This mode coupling enables energy exchange between KAWs and IBWs that may be relevant for turbulent dissipation processes in the solar wind and other collisionless plasmas. It is pointed out that for plasmas having Gaussian velocity distributions, also known as Maxwellian plasmas, the dispersion relation of IBWs exhibits a fine structure or splitting into multiple branches that causes the dispersion relation of IBWs to intersect a given branch of the KAW dispersion relation several times, thus providing multiple channels of energy exchange between KAWs and IBWs (for a given angle of wave propagation). It is also shown that the collisionless damping rate of IBWs can exceed that of KAWs in certain regions of parameter space and that IBWs exhibit different ratios of proton to electron heating than KAWs. Consequently, the role of IBWs in the dissipation of solar wind turbulence requires more careful study. Gyrokinetic theory, gyrokinetic simulations, and other physical models of solar wind dissipation processes which ignore the coupling between KAWs and IBWs may be missing important physics.