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 [Formula: 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.