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

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 [Image: see text] and the perpendicular ion plasma beta is typically β(⊥i)∼1. To address this, several exceptional intervals with β(⊥i)≪1 and β(⊥i)≫1 were investigated, during which these scales were well separated. It was found that for β(⊥i)≪1 the break occurs at d(i) and for β(⊥i)≫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.