Kinetic-scale Current Sheets in the Solar Wind at 1 au: Scale-dependent Properties and Critical Current Density

We present analysis of 17,043 proton kinetic-scale current sheets (CSs) collected over 124 days of Wind spacecraft measurements in the solar wind at 11 samples s−1 magnetic field resolution. The CSs have thickness, λ, from a few tens to one thousand kilometers with typical values around 100 km, or within about 0.1–10λp in terms of local proton inertial length, λp. We found that the current density is larger for smaller-scale CSs, J0 ≈ 6 nAm−2 · (λ/100 km)−0.56, but does not statistically exceed a critical value, JA, corresponding to the drift between ions and electrons of local Alvén speed. The observed trend holds in normalized units: ${J}_{0}/{J}_{A}\approx 0.17\cdot {(\lambda /{\lambda }_{p})}^{-0.51}$. The CSs are statistically force-free with magnetic shear angle correlated with CS spatial scale: ${\rm{\Delta }}\theta \approx 19^\circ \cdot {(\lambda /{\lambda }_{p})}^{0.5}$. The observed correlations are consistent with local turbulence being the source of proton kinetic-scale CSs in the solar wind, while the mechanisms limiting the current density remain to be understood.