Propagation of Alfv\'en waves in the expanding solar wind with the fast-slow stream interaction

TL;DR

This study uses 2D MHD simulations to analyze how Alfvén waves propagate and evolve in the structured, expanding solar wind, revealing the effects of stream interactions on turbulence and wave energy distribution.

Contribution

It introduces a comprehensive simulation approach that self-consistently models fast-slow stream interactions and their impact on Alfvénic turbulence in the solar wind.

Findings

Stream interaction significantly alters Alfvénic turbulence evolution.

Wave energy is depleted in shear regions, reducing cross-helicity.

Kolmogorov-like spectra develop within streams and along the longitudinal direction.

Abstract

We carry out two-dimensional magnetohydrodynamic (MHD) simulations of an ensemble of Alfv\'enic fluctuations propagating in a structured, expanding solar wind including the presence of fast and slow solar wind streams. Using an appropriate expanding box model, the simulations incorporate the effects of fast-slow stream shear and compression and rarefaction self-consistently. We investigate the radial and longitudinal evolution of the cross-helicity, the total and residual energies and the power spectra of outward and inward Alfv\'enic fluctuations. The stream interaction is found to strongly affect the radial evolution of Alfv\'enic turbulence. The total energy in the Alfv\'en waves is depleted within the velocity shear regions, accompanied by the decrease of the normalized cross-helicity. The presence of stream-compression facilitates this process. Residual energy fluctuates around zero due to the correlation and de-correlation between the inward/outward waves but no net growth or decrease of the residual energy is observed. The radial power spectra of the inward/outward Alfv\'en waves show significant longitudinal variations. Kolmogorov-like spectra are developed only inside the fast and slow streams and when both the compression and shear are present. On the other hand, the spectra along the longitudinal direction show clear Kolmogorov-like inertial ranges in all cases.