Ion-scale spectral break of solar wind turbulence at high and low beta

Abstract

The power spectrum of magnetic fluctuations in the solar wind at 1 AU displays a break between two power laws in the range of spacecraft-frame frequencies 0.1 to 1 Hz. These frequencies correspond to spatial scales in the plasma frame near the proton gyroradius ρ i and proton inertial length d i. At 1 AU it is difficult to determine which of these is associated with the break, since di=ρi/βi and the perpendicular ion plasma beta is typically β i ∼1. To address this, several exceptional intervals with β i 蠐1 and β i 蠑1 were investigated, during which these scales were well separated. It was found that for β i 蠐1 the break occurs at d i and for β i 蠑1 at ρ i, i.e., the larger of the two scales. Possible explanations for these results are discussed, including Alfvén wave dispersion, damping, and current sheets.