The Nature of Turbulence at Subelectron Scales in the Solar Wind

Abstract The nature of turbulence at subelectron scales has remained an open question, central to understanding how electrons are heated in the solar wind. This is primarily because spacecraft measurements have been limited to magnetic field fluctuations alone. We resolve this by deriving new high-resolution density fluctuations from spacecraft potential measurements of the Parker Solar Probe at scales smaller than the electron gyroradius ( ρ e ). A systematic comparison of the density and magnetic spectra shows that both steepen near the electron scales. Notably, the magnetic spectrum becomes steeper than the density spectrum at scales smaller than ρ e , indicating a transition to electrostatic turbulence. The density spectrum exhibits a slope close to −10/3, consistent with theoretical predictions for an electron entropy cascade, which may explain the irreversible dissipation of turbulent energy at sub- ρ e scales. The magnetic spectrum, however, is not as steep as expected for the electron entropy cascade, which may be due to limited signal-to-noise ratio and the presence of weakly damped electromagnetic fluctuations near ρ e .