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

Abstract

[1] Kinetic Alfvén waves (KAWs) with highly oblique wave vectors, k ? ≫ k k , are believed to form an integral part of the turbulent energy cascade in the solar wind near the proton gyroradius scale k ? r i $ 1. At wave numbers k ? r i > 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.

On the Fourier Contribution of Strong Current Sheets to the High‐Frequency Magnetic Power SpectralDensity of the Solar Wind

“…In a cascade picture the high‐frequency breakpoint of the solar wind magnetic power spectrum is associated with the onset in the MHD cascade of kinetic plasma wave damping (e.g., Bruno & Trenchi, ; D'Amicis et al, ; Gary, ; Gary & Borovsky, , ; Howes et al, ; Leamon et al, , ; Podesta, ; Podesta et al, ) or with the onset of dispersion of plasma waves (Sahraoui et al, , ; Saito et al, ; Stawicki et al, ; TenBarge et al, ). Finding #2 in Table points out that the spectral breakpoint is associated with the thicknesses of current sheets in the solar wind.…”

“…In a cascade picture the high‐frequency magnetic power spectrum of the solar wind is associated with plasma wave modes (e.g., whistler waves; Narita & Gary, ; Gary et al, ; Narita et al, , kinetic Alfvén waves; Podesta & TenBarge, ; Smith et al, ; Podesta, ; Salem et al, ; Chen et al, ; Pitna et al, ), both (Boldyrev et al, ; Gary & Smith, ; Mithaiwala et al, ), and perhaps Bernstein waves (Podesta, ; Verscharen et al, ), the wave‐wave interactions of these plasma modes, and the kinetic damping of these wave modes. In the cascade picture the high‐frequency spectrum represents the amplitudes of plasma waves with various wavenumbers.…”

Kinetic scale turbulence and dissipation in the solar wind: key observational results and future outlook

“…Cyclotron damping and/or Landau damping may well be important, but dispersion due to finite Larmor radius effects, as reflected in either Hall MHD or gyrokinetic treatments [45], also leads to spectral breaks at these same ion scales. (For related ideas on the relationship between plasma distribution functions and dissipation, the reader is referred to [46][47][48].) It is clear, however, that to determine the physics of what is actually happening will require more and higher-time-resolution data in addition to detailed analyses.…”

Electric and magnetic contributions to spatial diffusion in collisionless plasmas

“…It is noticeable that the electric field associated to ion Bernstein modes contains an electromagnetic component. These two points have recently been pointed out 22 as well as the absence of ion Bernstein modes close to x $ 0. The mode observed at x $ 0 in Fig.…”