Comparing turbulent cascades and heating vs spectral anisotropy in solar wind via direct simulations

TL;DR

This study compares two types of turbulent cascades in the solar wind, showing how they influence heating and temperature profiles, and highlights the importance of anisotropy and cascade regimes in interpreting solar wind data.

Contribution

It demonstrates the efficiency of radial-slab turbulence in producing observed solar wind temperature profiles and reveals the dominance of transverse cascade structures.

Findings

Radial-slab cascade can produce a 1/R temperature decay.

Turbulent spectra are dominated by transverse structures with k^{-5/3} scaling.

Sampling radial spectra alone may misrepresent the true cascade regime.

Abstract

In a previous work (MGV18), we showed numerically that the turbulent cascade generated by quasi-2D structures (with wave vectors mostly-perpendicular to the mean magnetic field) is able to generate a temperature profile close to the one observed in solar wind ($\simeq 1/R$) in the range 0.2 $\le R \le$ 1 au. Theory, observations and numerical simulations point to another robust structure, the radial-slab, with dominant wave vectors along the radial: we study here the efficiency of the radial-slab cascade in building the $1/R$ temperature profile. As in MGV18, we solve the three-dimensional MHD equations including expansion to simulate the turbulent evolution. We find that an isotropic distribution of wave vectors with large cross helicity at 0.2 au, along with a large wind expansion rate, lead again to a temperature decay rate close to $1/R$ but with a radial-slab anisotropy at 1 au. Surprisingly, the turbulent cascade concentrates in the plane transverse to the radial direction, displaying 1D spectra with scalings close to $k^{-5/3}$ in this plane. This supports both the idea of turbulent heating of the solar wind, and the existence of two different turbulent cascades, quasi-2D and radial slab, at the origin of the heating. We conclude that sampling the radial spectrum in the solar wind may give but a poor information on the real cascade regime and rate when the radial slab is a non-negligible part of turbulence.