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arXiv 1908.06666physics.plasm-phastro-ph.SRphysics.space-ph
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太阳风条件下哨声热通量不稳定性的粒子模拟

Particle-in-cell simulations of the whistler heat-flux instability in the solar wind conditions

R. A. López, S. M. Shaaban, M. Lazar, S. Poedts, P. H. Yoon, A. Micera, G. Lapenta

摘要:

例如,在来自太空的缺乏碰撞的等离子体中,在太阳风或恒星外流中,由strahl或电子束携带的热通量预计将由自生不稳定性调节。最近,同步场和粒子观测确实揭示了在存在反束电子的情况下增强的哨声式波动,将这些波动与哨声热通量不稳定性(WHFI)联系起来。这种不稳定性仅在有限的电子束等离子体条件下预测,并且尚未在数值模拟中捕获。在这封信中,我们报告的第一个模拟WHFI在粒子在细胞(PIC)的设置,现实的太阳风的条件下,没有温度梯度或各向异性触发的不稳定性在初始阶段。的速度分布有一个复杂的反应,以增强哨声波动条件的不稳定性饱和的相对漂移相结合的诱导(有效)温度各向异性(加热核心电子和俯仰角和能量散射的strahl)的减少。这些结果与最近的准线性方法是一致的,因此,支持一个广泛接受的信念,WHFI饱和在中等幅度。在反向阳方向上,随着电子能量的增加,散射波的倾斜角分布宽度逐渐减小,这似乎是自生哨声的特征,而不是小尺度湍流的特征。

英文摘要:

In collision-poor plasmas from space, e.g., solar wind or stellar outflows, the heat-flux carried by the strahl or beaming electrons is expected to be regulated by the self-generated instabilities. Recently, simultaneous field and particle observations have indeed revealed enhanced whistler-like fluctuations in the presence of counter-beaming populations of electrons, connecting these fluctuations to the whistler heat-flux instability (WHFI). This instability is predicted only for limited conditions of electron beam-plasmas, and was not captured in numerical simulations yet. In this letter we report the first simulations of WHFI in particle-in-cell (PIC) setups, realistic for the solar wind conditions, and without temperature gradients or anisotropies to trigger the instability in the initiation phase. The velocity distributions have a complex reaction to the enhanced whistler fluctuations conditioning the instability saturation by a decrease of the relative drifts combined with induced (effective) temperature anisotropies (heating the core electrons and pitch-angle and energy scattering the strahl). These results are in good agreement with a recent quasilinear approach, and support therefore a largely accepted belief that WHFI saturates at moderate amplitudes. In anti-sunward direction the strahl becomes skewed with a pitch-angle distribution decreasing in width as electron energy increases, that seems to be characteristic to self-generated whistlers and not to small-scale turbulence.

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