Evidence of a newly-observed turbulent regime at sub-ion scales in space and astrophysical plasmas

Abstract:

How the turbulent energy cascade develops below the magnetohydrodynamic inertial range in space and astrophysical plasmas and how it impacts particle heating are two major open questions. Here, we present a quantitative characterization of the magnetic field spectrum at scales smaller than the ion characteristic scales in both in-situ measurements from the Parker Solar Probe spacecraft as close to the Sun as 36 solar radii, and in 2D and 3D high-resolution hybrid-kinetic numerical simulations of decaying Alfvénic plasma turbulence. Our analysis reveals the existence of a range, just below the ion scales, where the power-law spectrum is systematically steeper than what is typically observed in the solar wind and in the Earth's magnetosheath, and compatible with -11/3. We present a theoretical explanation of such a behavior based on a phenomenological model, according to which this should be regarded as a self-similar regime, rather than just a transient, with different conditions and features with respect to the traditionally predicted -7/3. Our results suggest that the spectral behavior at sub-ion scales may be determined by the dominant contribution to the total energy in such range, regardless of the specific nature of the physical processes occurring. We also investigate ion heating under this plasma regime and the relative contribution of wave-like fluctuations to the plasma dynamics.