Abstract
As a minimal phenomenological model of unidirectional Alfvénic fluctuations in the solar wind, a non-equilibrium Alfvénic state (NAS) in the Langevin system for single particles is studied. The NAS is a nonequilibrium steady state of the Langevin system including both friction force and random force. It is shown that the overdamped Langevin system gives the NAS without the assumption of the monochromatic wave. The resultant NAS gives the relationship between the energy dissipation rate, the cross-helicity, and the residual energy with a single phenomenological parameter. It is shown that the energy dissipation rate can be evaluated using the non-equilibrium work, which satisfies the fluctuation dissipation-like relation.
Highlights
The non-equilibrium Alfvénic state (NAS) is studied as a minimal model for solar wind Alfvénic fluctuations
The Alfvénic solution in the overdamped Langevin system corresponds to the low-frequency limit of the NAS
The friction coefficient γ can be given by cross helicity and residual energy, which can be evaluated by using spacecraft observation
Summary
Solar wind magnetohydrodynamic (MHD) fluctuations have intensively been investigated by using data of in situ spacecraft observations.[1–25] It is clarified that important characteristics of the solar wind plasma such as anisotropic power-low scaling,[7,10,12,14,24] cross-helicity/residual energy,[11,13,17,23] intermittency,[4,5,6,18] and energy cascade (dissipation) rates[8,21,22] depend on the radial distance from the sun and/or solar wind conditions. Solar wind Alfvénic fluctuations, which are MHD fluctuations with high (normalized) cross-helicity, are often regarded to be composed of uni-directionally propagating Alfvén waves.[19,45,46]. While the physical interpretation of the friction and random forces is not given in the present study, the present phenomenological model is motivated by the recent attention to isotropic (or “collisional” or “thermal”) nature of solar wind. Note that the present model is for uni-directional, circularly polarized Alfvén waves and does not describe turbulent cascade through nonlinear wave–wave interactions. In this sense, the present model includes phenomenological parameters, this is different from the phenomenology of MHD turbulence,[30,32,33,35,36] in which nonlinear cascade caused by the counter-propagating waves is usually presupposed.
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