IONIZATION IN ATMOSPHERES OF BROWN DWARFS AND EXTRASOLAR PLANETS. V. ALFVÉN IONIZATION
IONIZATION IN ATMOSPHERES OF BROWN DWARFS AND EXTRASOLAR PLANETS. V. ALFVÉN IONIZATION
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棕矮星和太阳系外行星大气中的电离 V. 阿尔芬电离
DOI：
10.1088/0004-637x/776/1/11
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
Paul B. Rimmer
中科院分区：
文献类型：
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作者：
C. Stark;Ch. Helling;D. Diver;Paul B. Rimmer
对低质量天体的连续无线电和零星X射线发射的观测表明，它们的大气环境中存在局部等离子体。对于低质量物体，热电离的程度不足以将电离成分定性为等离子体，这就提出了一个问题：什么样的电离过程能够有效地产生作为辐射源的所需等离子体？我们提出阿尔文电离作为一种在大气中产生局部电离气体的机制，这些气体具有足够的电离度（10−7），它们构成等离子体。我们概述了阿尔文电离所需的标准，并证明其适用于低质量天体的大气层，如巨大的气体行星，棕矮星和M矮星与太阳和太阳下的金属丰度。我们发现，阿尔文电离是最有效的在中到低大气压力，种子等离子体更容易磁化和驱动所需的中性流的压力梯度是最小的。对于所考虑的模型大气，我们的结果表明，作为阿尔文电离的结果，可以获得10−6-1的电离度。可观察到的结果包括连续韧致辐射发射，与来自等离子体离子种类的谱线叠加（例如，He、Mg、H2或CO线）。禁止线也预计从亚稳态人口。大气等离子体的存在为目前大气模型中尚未考虑的大量等离子体和化学过程打开了大门。阿尔文电离的发生也可能适用于其他天体物理环境，如原行星盘。
Observations of continuous radio and sporadic X-ray emission from low-mass objects suggest they harbor localized plasmas in their atmospheric environments. For low-mass objects, the degree of thermal ionization is insufficient to qualify the ionized component as a plasma, posing the question: what ionization processes can efficiently produce the required plasma that is the source of the radiation? We propose Alfvén ionization as a mechanism for producing localized pockets of ionized gas in the atmosphere, having sufficient degrees of ionization (⩾10−7) that they constitute plasmas. We outline the criteria required for Alfvén ionization and demonstrate its applicability in the atmospheres of low-mass objects such as giant gas planets, brown dwarfs, and M dwarfs with both solar and sub-solar metallicities. We find that Alfvén ionization is most efficient at mid to low atmospheric pressures where a seed plasma is easier to magnetize and the pressure gradients needed to drive the required neutral flows are the smallest. For the model atmospheres considered, our results show that degrees of ionization of 10−6–1 can be obtained as a result of Alfvén ionization. Observable consequences include continuum bremsstrahlung emission, superimposed with spectral lines from the plasma ion species (e.g., He, Mg, H2, or CO lines). Forbidden lines are also expected from the metastable population. The presence of an atmospheric plasma opens the door to a multitude of plasma and chemical processes not yet considered in current atmospheric models. The occurrence of Alfvén ionization may also be applicable to other astrophysical environments such as protoplanetary disks.
DOI：
10.1088/0004-6256/143/4/93
发表时间：
2012-01
期刊：
The Astronomical Journal
影响因子：
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作者：
A. Reiners;N. Joshi;B. Goldman
通讯作者：
A. Reiners;N. Joshi;B. Goldman