Ion-Scale Spectral Break in the Normal Plasma Beta Range in the Solar Wind Turbulence

The spectral break (f(b)) of magnetic fluctuations at the ion scale in the solar wind is considered to give important clue on the turbulence dissipation mechanism. Among several possible mechanisms, the most notable two are related respectively to proton thermal gyroradius rho(i) and proton inertial length d(i). The corresponding frequencies of them are f(rho i)=V-SW/(2 pi rho(i)) and f(di)=V-SW/(2 pi d(i)), respectively, where V-SW is the solar wind speed. However, no definite conclusion has been given for which one is more reasonable because the two parameters have similar value when plasma beta similar to 1. Here we do a statistical study to see if the two ratios f(b)/f(i) and f(b)/f(di) have different dependence on beta in the solar wind turbulence with 0.1 < beta < 1.3. From magnetic measurements by the Wind spacecraft, we select 141 data sets with each one longer than 13h. We find that the ratio f(b)/f(di) is statistically not dependent on , and the average value of it is 0.48 +/- 0.06. However, f(b)/f(i) increases with increasing clearly and is significantly smaller than f(b)/f(di) when beta < 0.8. These new results show that f(b) is statistically 0.48f(di), and the influence of could be negligible in the studied range. It indicates a preference of the dissipation mechanism associated with d(i) in the solar wind with 0.1 < beta < 0.8. Further theoretical studies are needed to give detailed explanation.