Propagation of Alfvén Waves in the Expanding Solar Wind with the Fast–Slow Stream Interaction

Abstract

We carry out two-dimensional magnetohydrodynamic (MHD) simulations of an ensemble of Alfvénic 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 Alfvénic fluctuations. The stream interaction is found to strongly affect the radial evolution of Alfvénic turbulence. The total energy in the Alfvén waves is depleted within the velocity shear regions, accompanied by the decrease of the normalized cross helicity. The presence of streamcompression 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 Alfvén 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.

Influence of the heliospheric current sheet on the evolution of solar wind turbulence

“…These newlygenerated waves do not have a preferential propagating direction and thus the initial dominance of the outward wave gradually declines. This shear-driven decrease of cross helicity was confirmed by numerical simulations (Roberts et al 1992;Shi et al 2020). Velocity shear is widely adopted in turbulence models as a source for the wave energies and is able to reproduce the observed decrease of σ c in the models.…”

“…We process the simulation data using the same method as described in (Shi et al 2020). At each time, or radial distance to the Sun equivalently, we first calculate the x -averaged fields, i.e.…”

“…The code we use for simulations is a 2D pseudospectral MHD code used by Shi et al (2020) to study the interaction between SIRs and turbulence. EBM module is implemented so that the radial expansion effect of the solar wind is taken into account (Grappin et al 1993;Grappin & Velli 1996).…”

“…In our code, a finite spiral angle of magnetic field and stream interface is allowed so that compression and rarefaction regions are constructed. A detailed description of the numerical method can be found in Shi et al (2020).…”

“…The size of the simulation domain is L x × L y = 10R s × 2πR 0 where x − y is the corotating coordinate system, i.e. x is parallel to the background magnetic field and y is the quasi-longitudinal direction (Shi et al 2020). L y = 2πR 0 means that the domain is a full circle in the ecliptic plane.…”