Abstract
Abstract. We study three-wave resonant interactions among kinetic-scale oblique sound waves in the low-frequency range below the ion cyclotron frequency. The nonlinear eigenmode equation is derived in the framework of a two-fluid plasma model. Because of dispersive modifications at small wavelengths perpendicular to the background magnetic field, these waves become a decay-type mode. We found two decay channels, one into co-propagating product waves (forward decay), and another into counter-propagating product waves (reverse decay). All wavenumbers in the forward decay are similar and hence this decay is local in wavenumber space. On the contrary, the reverse decay generates waves with wavenumbers that are much larger than in the original pump waves and is therefore intrinsically nonlocal. In general, the reverse decay is significantly faster than the forward one, suggesting a nonlocal spectral transport induced by oblique sound waves. Even with low-amplitude sound waves the nonlinear interaction rate is larger than the collisionless dissipation rate. Possible applications regarding acoustic waves observed in the solar corona, solar wind, and topside ionosphere are briefly discussed.
Highlights
Kinetic sound waves (KSWs) are an extension of plasma sound waves in the range of short perpendicular wavelengths λ⊥ comparable to the ion gyroradius ρi = VT i/ i, λ⊥ ∼ ρi
Because of the KSW dispersion modification at small perpendicular wavelengths, these waves become a decay-type mode in the frequency range below the ion cyclotron frequency (Hasegawa, 1976)
The spectral transport induced by the nonlinear interaction among kinetic sound waves can be used for the explanation of wave phenomena in many solar and space plasmas
Summary
Identification of widespread slow (acoustic) modes in different regions of the solar corona have revived interest in their application to coronal seismology (see recent papers by Ofman et al, 2012; Krishna Prasad et al, 2014, and references therein) Observed dissipation of these modes is difficult to explain by linear damping mechanisms (Krishna Prasad et al, 2014) and nonlinear theory is required. Voitenko: Nonlinear coupling of kinetic sound waves almost electrostatic in low-β plasmas, like solar corona. Our lowfrequency KSWs are the collisionless electrostatic counterparts of oblique MHD slow-mode waves, and our results are applicable only in low-β plasmas where the magnetic field is perturbed only slightly by these modes. We use here the two-fluid MHD plasma model, as it gives a far more simple description and is still sufficiently accurate for the low-frequency waves we study
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