10:15 〜 10:30
[PEM15-06] Simulation of temperature anisotropy instability by a fluid model with non-local closure
キーワード：プラズマシミュレーション、温度異方性不安定
Physical descriptions of collisionless plasmas are given by either fluid models or kinetic models. Fluid models, the most popular one being magnetohydrodynamics (MHD), require relatively less computational resources, so they are suited for multi-dimensional macroscopic simulations. Kinetic models that solve the time evolution of distribution function in six-dimensional phase space are not only too resource-heavy but also make it difficult to understand macroscopic phenomena because of the infinite degree of freedom in velocity space. The common problem with most fluid models is that it does not take into account wave-particle interaction. Although wave-particle interaction is considered a microscopic phenomenon, it often affects the macroscopic dynamics of plasmas in the nonlinear phase through, for example, the relaxation of temperature anisotropy via kinetic instability.
There is a method called Landau closure, which approximates the highest order moment of the Vlasov equation (often the heat flux tensor) by a linear combination of lower-order moments (number density, fluid velocity, and pressure tensor) in the Fourier space. The original Landau closure gives an approximated plasma response for the Landau resonance. We have recently proposed an extension of the non-local closure, which takes into account cyclotron resonance as well. Our model can reproduce not only the cyclotron damping of transverse electromagnetic waves in warm collisionless plasmas but also temperature anisotropy instabilities.
In this study, we compare ion temperature anisotropy instabilities described by a fluid model with local closure (such as MHD with double adiabatic closure), a fluid model with non-local closure, and full kinetic models. Non-local closure that takes into account the cyclotron resonance effect is essential to reproduce electromagnetic cyclotron (EMIC) instability, in which the left-handed polarized wave becomes unstable under the condition that perpendicular pressure is higher than parallel pressure. In parallel firehose instability, on the other hand, the right-handed polarized mode becomes unstable when parallel pressure exceeds the perpendicular pressure. This instability can be described by conventional fluid models such as the Chew-Goldberger-Low (CGL) model. However, it is known that the cyclotron resonance has a critical impact on moderate beta plasmas. We find our non-local closure model can predict a qualitatively correct linear growth rate for both EMIC and firehose instabilities and reproduces the quasilinear isotropization. The isotropization is described by the correlation between off-diagonal pressure and transverse magnetic field. We will discuss possible improvements to the linear growth rate for better qualitative agreement with a fully kinetic model.
There is a method called Landau closure, which approximates the highest order moment of the Vlasov equation (often the heat flux tensor) by a linear combination of lower-order moments (number density, fluid velocity, and pressure tensor) in the Fourier space. The original Landau closure gives an approximated plasma response for the Landau resonance. We have recently proposed an extension of the non-local closure, which takes into account cyclotron resonance as well. Our model can reproduce not only the cyclotron damping of transverse electromagnetic waves in warm collisionless plasmas but also temperature anisotropy instabilities.
In this study, we compare ion temperature anisotropy instabilities described by a fluid model with local closure (such as MHD with double adiabatic closure), a fluid model with non-local closure, and full kinetic models. Non-local closure that takes into account the cyclotron resonance effect is essential to reproduce electromagnetic cyclotron (EMIC) instability, in which the left-handed polarized wave becomes unstable under the condition that perpendicular pressure is higher than parallel pressure. In parallel firehose instability, on the other hand, the right-handed polarized mode becomes unstable when parallel pressure exceeds the perpendicular pressure. This instability can be described by conventional fluid models such as the Chew-Goldberger-Low (CGL) model. However, it is known that the cyclotron resonance has a critical impact on moderate beta plasmas. We find our non-local closure model can predict a qualitatively correct linear growth rate for both EMIC and firehose instabilities and reproduces the quasilinear isotropization. The isotropization is described by the correlation between off-diagonal pressure and transverse magnetic field. We will discuss possible improvements to the linear growth rate for better qualitative agreement with a fully kinetic model.