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

Abstract

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 0.1–10 λ p in terms of local proton inertial length, λ p . We found that the current density is larger for smaller-scale CSs, J 0 ≈ 6 nAm −2 · ( λ /100 km) −0.56 , but does not statistically exceed a critical value, J A , corresponding to the drift between ions and electrons of local Alvén speed. The observed trend holds in normalized units: J 0 / J A ≈ 0.17 · ( λ / λ p ) − 0.51 . The CSs are statistically force-free with magnetic shear angle correlated with CS spatial scale: Δ θ ≈ 19 ° · ( λ / λ 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.