Conservation of Total Wave Action in the Expanding Solar Wind

Abstract

The conservation of wave action in moving plasmas has been well-known for over half a century. However, wave action is not conserved when multiple wave modes propagate and coexist close to degeneration condition (Sound speed equals Alfvén speed, i.e. plasma $\beta \sim 1$). Here we show that the violation of conservation is due to wave mode conversion, and that the total wave action summed over interacting modes is still conserved. Though the result is general, we focus on MHD waves and identify three distinctive mode conversion mechanisms, i.e. degeneracy, linear mode conversion, and resonance, and provide an intuitive physical picture for the mode conversion processes. We use 1D MHD simulations with the Expanding Box Model to simulate the nonlinear evolution of monochromatic MHD waves in the expanding solar wind. Simulation results validate the theory; total wave action therefore remains an interesting diagnostic for studies of waves and turbulence in the solar wind.

Accepted by ApJ

References in corpus (5)

• Evolution of switchbacks in the inner Heliosphere
• Propagation of Alfvén waves in the expanding solar wind with the fast-slow stream interaction
• A New Look at Mode Conversion in a Stratified Isothermal Atmosphere
• MHD mode conversion in a stratified atmosphere
• MHD Mode Conversion around a 2D Magnetic Null Point

Cited by in corpus (5)

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• Evolution of MHD turbulence in the expanding solar wind: residual energy and intermittency
• On stellar hotspots due to star-planet magnetic interactions: How much power can actually be transmitted to the chromosphere?
• A Local Model for the Spherical Collapse/Expansion Problem