Nonequilibrium ionization and ambipolar diffusion in solar magnetic flux emergence processes
Nonequilibrium ionization and ambipolar diffusion in solar magnetic flux emergence processes
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DOI：
10.1051/0004-6361/201936944
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发表时间：
2019-12
影响因子：
6.5
通讯作者：
D. N'obrega-Siverio;F. Moreno-Insertis;J. Mart'inez-Sykora;M. Carlsson;M. Szydlarski
中科院分区：
文献类型：
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作者：
D. N'obrega-Siverio;F. Moreno-Insertis;J. Mart'inez-Sykora;M. Carlsson;M. Szydlarski
语境。太阳内部出现的磁通量已被证明是引发多种现象的关键机制。然而，关于磁化等离子体在大气中的上升仍然存在悬而未决的问题，主要是在色球层，在那里等离子体偏离局部热力学平衡（LTE）并被部分电离。目标。我们的目的是研究氢的非平衡（NEQ）电离和重组以及分子形成以及双极扩散对通量出现过程的动力学和热力学的影响。方法。使用辐射磁流体动力学 Bifrost 代码，我们进行了从对流区到日冕的磁通量出现的 2.5D 数值实验。实验包括原子氢的 NEQ 电离和重组、H2 分子的 NEQ 形成和解离以及广义欧姆定律的双极扩散项。结果。我们的实验表明，LTE 假设大大低估了大多数出现区域的电离分数，导致双极扩散人为增加，因此与考虑 NEQ 对氢离子群影响时发现的加热和温度相比。我们发现，LTE 还高估了出现区域内 H2 分子的数量密度，从而错误地放大了通量出现过程中 H2 分子形成对热能的放热贡献。我们发现双极扩散不会显着影响总的无符号出现磁通量，但它在穿过出现区域的冲击中很重要，在 0.1 到 100 s 的特征时间范围内加热等离子体。我们还简要讨论了在状态方程中包含比氢重的元素的重要性，以免高估大气中双极扩散的作用。
Context. Magnetic flux emergence from the solar interior has been shown to be a key mechanism for unleashing a wide variety of phenomena. However, there are still open questions concerning the rise of the magnetized plasma through the atmosphere, mainly in the chromosphere, where the plasma departs from local thermodynamic equilibrium (LTE) and is partially ionized. Aims. We aim to investigate the impact of the nonequilibrium (NEQ) ionization and recombination and molecule formation of hydrogen, as well as ambipolar diffusion, on the dynamics and thermodynamics of the flux emergence process. Methods. Using the radiation-magnetohydrodynamic Bifrost code, we performed 2.5D numerical experiments of magnetic flux emergence from the convection zone up to the corona. The experiments include the NEQ ionization and recombination of atomic hydrogen, the NEQ formation and dissociation of H2 molecules, and the ambipolar diffusion term of the generalized Ohm’s law. Results. Our experiments show that the LTE assumption substantially underestimates the ionization fraction in most of the emerged region, leading to an artificial increase in the ambipolar diffusion and, therefore, in the heating and temperatures as compared to those found when taking the NEQ effects on the hydrogen ion population into account. We see that LTE also overestimates the number density of H2 molecules within the emerged region, thus mistakenly magnifying the exothermic contribution of the H2 molecule formation to the thermal energy during the flux emergence process. We find that the ambipolar diffusion does not significantly affect the amount of total unsigned emerged magnetic flux, but it is important in the shocks that cross the emerged region, heating the plasma on characteristic times ranging from 0.1 to 100 s. We also briefly discuss the importance of including elements heavier than hydrogen in the equation of state so as not to overestimate the role of ambipolar diffusion in the atmosphere.