Solar wind magnetic fluctuation spectra: Dispersion versus damping

Abstract

Solar wind magnetic fluctuation power spectra at frequencies ƒ < 1 Hz are commonly observed to have the approximate power law dependence ƒ−5/3. These observations may be described by Kolmogorov diffusion in wavenumber space which defines what is called the inertial range. At wavelengths shorter than the inertial range, spectra often are observed to have steeper power laws. This intermediate wavelength regime is sometimes called the dissipation range because it has been suggested that this steeper slope is caused by collisionless damping of Alfvén and magnetosonic waves. Here it is argued that, at intermediate wavenumbers, Alfvén fluctuations are suppressed by proton cyclotron damping, so the observed power spectra are likely to consist of weakly damped magnetosonic/whistler waves which have an increased wavenumber diffusion rate due to their dispersion. Numerical calculations at βp ≲ 2.5 with a model representing this picture yield fluctuation spectra with steep power laws at intermediate wavenumbers and with sharp cutoffs due to electron cyclotron damping at still shorter wavelengths. The term “dispersion range” is more appropriate to describe steep power law spectra in the intermediate wavenumber regime.