On the Nature of Spectral Breaks in Solar Wind Turbulence from Fluid to Kinetic Scales Using MESSENGER Spacecraft Observations

Abstract:

A fluid-like model, at large scales, describes well the nature of solar wind MHD turbulence. There is plenty of observational evidence that indicates the existence of a Kolmogorov-like spectrum throughout the interplanetary medium. Two spectral breaks occur, one at low and one at high frequency (wavenumber). The high-frequency break indicates the onset of dissipation in the solar wind. It signals the scale where the fluid description fails and requires replacement by a kinetic representation. The underlying physical mechanism that drives the dissipation and the corresponding length and timescales are still unknown. We use 20Hz MESSENGER magnetic field data during four planetary flybys to investigate the fluid to kinetic scale break location. The flybys have revealed the existence of high-frequency whistler waves upstream of each planetary bow shock. Performing wavelet analysis on the magnetic field data, we obtain a solar wind MHD turbulence spectrum. Using the presence of upstream whistler waves as a marker, we identify the possible scale marking the onset of dissipation.