超高β等离子体中的无碰撞哨声波热通量不稳定性
Collisionless whistler heat-flux instability in ultra-high-$β$ plasmas
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- Department of Physics, University of Oxford（牛津大学物理系）
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中文总结 AI 辅助
研究超高β等离子体中WHFI调节的热传输，通过理论外推提出假设，用1D3V和2D3V动力学模拟证实，发现热通量传输方式及抑制情况与维度有关，给出不同模拟下平行热通量缩放关系，并发展理论解释现象，扩展了对WHFI的理解。
中文摘要 AI 辅助
动力学不稳定性，特别是哨声波热通量不稳定性（WHFI），在许多天体物理系统相关的中高β等离子体中能显著抑制热传输。本文探索了在一种新状态下由WHFI调节的热传输：即βe≳LT/ρe的超高β等离子体。将先前的WHFI理论外推到超高β等离子体，我们提出不稳定哨声波波动中的磁能在饱和时变得与背景磁场的磁能相当。我们使用粒子模拟代码OSIRIS进行1D3V和2D3V动力学模拟来证实这一假设。发现在超高β等离子体中，热通量是局部化的，不再主要由电子的共振俯仰角散射调节；相反，热能主要通过哨声波相速度处的平流传输。在1D3V和2D3V模拟中观察到热通量抑制；然而，我们表明在超高β状态下，WHFI的饱和及热通量调节对维度敏感。调节热通量的哨声波的振幅和相速度随βe的缩放方式不同，在2D3V和1D3V模拟中，平行热通量（相对于自由流值归一化）分别为qe∥/qfs≈4.7βe−1和qe∥/qfs≈0.3βe−1/2。我们进行了背景磁场倾斜于温度梯度的2D3V模拟，表明横向热传输仍然可以忽略不计。我们从动力学方程发展出一种启发式理论来解释这些现象。我们的工作扩展了我们对WHFI如何修改热传输的理解，使其适用于高能密度物理和再电离的星系际介质。
英文摘要
Kinetic instabilities, notably the whistler heat-flux instability (WHFI), are known to suppress thermal transport significantly in the moderate- to high-$β$ plasmas relevant to many astrophysical systems. This paper explores WHFI-regulated heat transport in a new regime: ultra-high-$β$ plasmas with $β_{e} \gtrsim L_{\mathrm{T}}/ρ_e$. Extrapolating previous theories of the WHFI to ultra-high-$β$ plasmas, we propose that the magnetic energy in unstable whistler fluctuations becomes comparable to that of the background magnetic field at saturation. We corroborate this hypothesis using 1D3V and 2D3V kinetic simulations using the particle-in-cell code OSIRIS. We find that, in ultra-high-$β$ plasmas, the heat flux is localised and no longer regulated primarily by resonant pitch-angle scattering of electrons; instead, thermal energy is transported predominantly by advection at the whistler phase velocity. Heat-flux suppression is observed in 1D3V and 2D3V simulations; however, we show that the saturation of the WHFI and the regulation of heat flux are sensitive to dimensionality in the ultra-high-$β$ regime. The amplitude and phase velocity of the heat-flux-regulating whistler waves scale differently with $β_e$, yielding parallel heat fluxes, normalised to the free-streaming value, of $q_{e\parallel} / q_\mathrm{fs} \approx 4.7 β_{e}^{-1}$ and $q_{e\parallel} / q_\mathrm{fs} \approx 0.3 β_{e}^{-1/2}$ in 2D3V and 1D3V simulations, respectively. We perform 2D3V simulations with background magnetic fields inclined to the temperature gradient, showing perpendicular heat transport remains negligible. We develop a heuristic theory from kinetic equations that explains these phenomena. Our work extends our understanding of how the WHFI modifies thermal transport to regimes applicable to high-energy-density physics and the reionised intergalactic medium.
