A Model for Dissipation of Solar Wind Magnetic Turbulence by Kinetic Alfven Waves at Electron Scales: Comparison with Observations

In hydrodynamic turbulence, it is well established that the length of the dissipation scale depends on the energy cascade rate, i. e., the larger the energy input rate per unit mass, the more the turbulent fluctuations need to be driven to increasingly smaller scales to dissipate the larger energy flux. Observations of magnetic spectral energy densities indicate that this intuitive picture is not valid in solar wind turbulence. Dissipation seems to set in at the same length scale for different solar wind conditions independently of the energy flux. To investigate this difference in more detail, we present an analytic dissipation model for solar wind turbulence at electron scales, which we compare with observed spectral densities. Our model combines the energy transport from large to small scales and collisionless damping, which removes energy from the magnetic fluctuations in the kinetic regime. We assume wave- particle interactions of kinetic Alfven waves (KAWs) to be the main damping process. Wave frequencies and damping rates of KAWs are obtained from the hot plasma dispersion relation. Our model assumes a critically balanced turbulence, where larger energy cascade rates excite larger parallel wavenumbers for a certain perpendicular wavenumber. If the dissipation is additionally wave driven such that the dissipation rate is proportional to the parallel wavenumber- as with KAWs- then an increase of the energy cascade rate is counterbalanced by an increased dissipation rate for the same perpendicular wavenumber, leading to a dissipation length independent of the energy cascade rate.