On the Mechanism of Chromospheric Network Heating and the Condition for Its Onset in the Sun and Other Solar-Type Stars
On the Mechanism of Chromospheric Network Heating and the Condition for Its Onset in the Sun and Other Solar-Type Stars
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太阳及其他太阳型恒星色球网络加热机制及其发生条件
DOI：
10.1086/308635
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发表时间：
2000
期刊：
影响因子：
--
通讯作者：
M. L. Goodman
中科院分区：
文献类型：
--
作者：
M. L. Goodman
提出了色球网络加热的机制及其发生的充分必要条件。加热机制由质子 Pedersen 电流的电阻耗散组成，质子 Pedersen 电流与弱电离色球等离子体中的磁场正交流动。电流由对流电场驱动，该对流电场是由色球层下层频率为 ν ≲ 3.5 mHz 的线性慢速纵向磁声波的速度振荡产生的。加热发生在细磁通量管中，并在光球场强度较高的磁通量管中色球层较低处开始。下部色球层发出网络中大部分净辐射损耗，由光球场强度约为 700-1500 G 的通量管加热。下部色球层通量管的典型场强和核心直径（核心加热速率为 107 ergs cm-2 s-1）为 170 G 和 10 km。该核心区域包含在直径约 100 km 的区域中，其中加热速率要小一个数量级。大约 N ~ 102 个这些通量管分布在直径 ~103 km 的颗粒的边界区域上，提供整个颗粒 ~107 ergs cm-2 s-1 的平均加热速率。如果堆芯加热速率改变 f 倍，则 N ~ f-1/2102。加热开始的条件是质子回旋频率与质子-氢碰撞频率之比等于1。该比率随着高度的增加而增加，并且在给定通量管中的单一高度处满足该条件。在这个高度，质子动力学的控制开始由磁场主导，而不是与氢的碰撞，并且电导率的各向异性开始在电阻耗散中发挥关键作用。质子被磁化。重离子耗散以及质子佩德森电流（在较小程度上）导致温度开始升高。加热增加氢离子化。随着高度的增加以及质子磁化强度的增加，Pedersen电流密度通过正反馈随着氢电离而迅速增加，并且质子数密度迅速达到并超过重离子数密度，导致加热速率仅在1个压标高度上增加一个数量级。在此过程中，质子迅速主导佩德森电流。磁场定向电流耗散产生的热量是微不足道的。在满足起始条件的大气高度以下，任何与磁场正交的电流必须主要是霍尔电流，这是非耗散的。通过这种机制的加热必须在某种程度上发生在所有太阳型恒星的色球网络中。它被认为是色球网络加热的主要机制，尽管如果核心加热速率远大于 ~107 ergs cm-2 s-1 或者如果这里研究的线性 MHD 波演变成高度增加的冲击波，粘性耗散也可能很重要。预计安静色球层中的通量管有两种可能的核心直径：~10 km，对应于发生网络加热的通量管，以及~104-105 km，可能对应于可能发生活跃区域加热的通量管。该模型在声学截止频率处有一个奇点，对应于 3 分钟左右的周期。因此，除非粘性耗散和热传导等非电阻阻尼机制提供足够强的阻尼，否则色球磁通量管中周期接近3分钟的MHD振荡必定是非线性的。
A mechanism for chromospheric network heating and a necessary and sufficient condition for its onset are presented. The heating mechanism consists of resistive dissipation of proton Pedersen currents, which flow orthogonal to the magnetic field in weakly ionized chromospheric plasma. The currents are driven by a convection electric field generated by velocity oscillations of linear, slow, longitudinal magnetoacoustic waves with frequencies ν ≲ 3.5 mHz in the lower chromosphere. The heating occurs in thin magnetic flux tubes and begins lower in the chromosphere in flux tubes with higher photospheric field strength. The lower chromosphere, which emits most of the net radiative loss in the network, is heated by flux tubes with photospheric field strengths ~700-1500 G. A typical field strength and core diameter for a flux tube in the lower chromosphere with a core heating rate of 107 ergs cm-2 s-1 are 170 G and 10 km. This core region is contained in a region with a diameter ~100 km in which the heating rate is an order of magnitude smaller. About N ~ 102 of these flux tubes distributed over the boundary region of a granule with a diameter ~103 km provide an average heating rate over the entire granule ~107 ergs cm-2 s-1. If the core heating rate is changed by a factor f, then N ~ f-1/2102. The condition for the onset of heating is that the ratio of the proton cyclotron frequency to the proton-hydrogen collision frequency equal unity. This ratio increases with height, and the condition is satisfied at a single height in a given flux tube. At this height, control of the proton dynamics begins to be dominated by the magnetic field rather than by collisions with hydrogen, and the anisotropic nature of the electrical conductivity begins to play a critical role in resistive dissipation. The protons become magnetized. Heating by dissipation of heavy ion and, to a lesser extent, proton Pedersen currents causes the temperature to start increasing. The heating increases hydrogen ionization. With increasing height, and hence proton magnetization, the Pedersen current density rapidly increases with hydrogen ionization via positive feedback, and the proton number density rapidly reaches and exceeds the heavy ion number density, resulting in an increase in heating rate by an order of magnitude over only 1 pressure scale height. During this process the protons rapidly dominate the Pedersen current. Heating by dissipation of magnetic field aligned currents is insignificant. Below the height in the atmosphere at which the onset condition is satisfied, any current orthogonal to the magnetic field must be primarily a Hall current, which is nondissipative. Heating by this mechanism must occur to some degree in the chromospheric network of all solar-type stars. It is proposed to be the dominant mechanism of chromospheric network heating, although viscous dissipation may also be important if the core heating rate is much larger than ~107 ergs cm-2 s-1 or if the linear MHD waves studied here evolve into shock waves with increasing height. Flux tubes in the quiet chromosphere are predicted to have two possible core diameters: ~10 km, corresponding to flux tubes in which network heating occurs, and ~104-105 km, perhaps corresponding to flux tubes in which active region heating might occur. The model has a singularity at the acoustic cutoff frequency, corresponding to periods near 3 minutes. Therefore, unless nonresistive damping mechanisms such as viscous dissipation and thermal conduction provide sufficiently strong damping, MHD oscillations with periods near 3 minutes in chromospheric magnetic flux tubes must be nonlinear.