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 <i>ρ</i>_i and proton inertial length <i>d</i>_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 <i>β</i>_⊥i∼1. To address this, several exceptional intervals with <i>β</i>_⊥i≪1 and <i>β</i>_⊥i≫1 were investigated, during which these scales were well separated. It was found that for <i>β</i>_⊥i≪1 the break occurs at <i>d</i>_i and for <i>β</i>_⊥i≫1 at <i>ρ</i>_i, i.e., the larger of the two scales. Possible explanations for these results are discussed, including Alfvén wave dispersion, damping, and current sheets.