光学薄等离子体中的灾难性冷却不稳定性
Catastrophic cooling instability in optically thin plasmas
AI总结：
本文发现光学薄等离子体中一种独立于热不稳定性的灾难性冷却不稳定性，其判据为Parker等容准则，碰撞电离平衡冷却函数易违反之，可能成为形成冷凝的替代机制。
摘要：
太阳日冕是低密度环境被外部源加热至高温的典型例子。等离子体通过辐射冷却，并且由于其对这种辐射是光学薄的，因此可以通过修改磁流体动力学（MHD）方程，加入近似局部加热和冷却速率的能量源项，来模拟系统的密度、速度和温度结构。由于与这些源项相关的众所周知的线性不稳定性——热不稳定性，在非线性区域会导致等离子体的灾难性加热和冷却，因此解可能高度不均匀，甚至多相。这里我们表明，伴随这些源项还存在一种独立的、更简单的灾难性加热和冷却实例，其在动力学重要性上可与热不稳定性相媲美。线性稳定性判据是Parker（1953）确定的等容判据，我们证明，由碰撞电离平衡导出的冷却函数极易违反该判据。如果灾难性冷却不稳定性能在全局模拟中局部作用，那么它是形成冷凝的另一种机制，并且由于其非平衡特性，它可能与解释热、低密度等离子体中产生较冷气体的一系列现象相关。
英文摘要：
The solar corona is the prototypical example of a low density environment heated to high temperatures by external sources. The plasma cools radiatively, and because it is optically thin to this radiation, it becomes possible to model the density, velocity, and temperature structure of the system by modifying the MHD equations to include energy source terms that approximate the local heating and cooling rates. The solutions can be highly inhomogeneous and even multiphase because the well known linear instability associated with these source terms, thermal instability, leads to a catastrophic heating and cooling of the plasma in the nonlinear regime. Here we show that there is a separate, much simpler instance of catastrophic heating and cooling accompanying these source terms that can rival thermal instability in dynamical importance. The linear stability criterion is the isochoric one identified by Parker (1953), and we demonstrate that cooling functions derived from collisional ionization equilibrium are highly prone to violating this criterion. If catastrophic cooling instability can act locally in global simulations, then it is an alternative mechanism for forming condensations, and due to its nonequilibrium character, it may be relevant to explaining a host of phenomena associated with the production of cooler gas in hot, low density plasmas.
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