Magnetohydrodynamic Wave Mixing in Solar Wind Shear Flows

Details Magnetohydrodynamic Wave Mixing in Solar Wind Shear Flows Magnetohydrodynamic Wave Mixing in Solar Wind Shear Flows

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmsh11a0254w&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #SH11A-0254

Physics

Optics

2149 Mhd Waves And Turbulence (2752, 6050, 7836), 2164 Solar Wind Plasma, 6050 Plasma And Mhd Instabilities (2149, 2752, 7836)

Scientific paper

Magnetohydrodynamic (MHD) wave interactions in a linear shear flow, using a Lagrangian variational approach, are described in terms of the Lagrangian fluid displacement ξ and entropy perturbation Δ S. The equations are used to study MHD wave interactions in the shear between fast, coronal hole solar wind, and slower streamer belt solar wind at lower helio-latitudes, The spatial Fourier harmonics for the waves (SFHs) in the frame moving with the background shear flow (Kelvin's method) satisfy three coupled oscillator equations, with time dependent coupling coefficients, and with source terms proportional to the entropy perturbation Δ S. Normal mode analysis, based on the background flow with no shear, results in a Hamiltonian system of six first order differential equations for the SFHs, corresponding to the backward and forward fast and slow magnetoacoustic and Alfvén modes (this system is referred to as the K-system, since the frequencies and wavenumbers of the modes are constant). An alternative normal mode expansion, based on the background flow including the shear, results in an equivalent Hamiltonian system of six first order differential equations, in which the frequencies and wave numbers evolve in time (the wave number k' in this system evolves according to the ray equations, of geometrical, MHD optics; this system is referred to as the R-system). In the absence of entropy perturbations, both the K-system and the R-system possess the same wave action integral for the eigenmodes (the wave action integral ceases to apply if Δ S=0). We present evidence that the R-system provides a more natural physical description of the wave interactions. For sufficiently large shear parameter, the waves exhibit the phenomenon of over-reflection. The forward propagating modes have positive wave action densities (quasi-particle number densities), whereas the backward propagating modes have negative canonical wave energy densities (action densities). Wave transformation, transmission and reflection processes are discussed. The relationship between our approach and that of Gogoberidze et al. (2004) is discussed.

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