日冕中的焦耳加热和反常电阻率
Joule Heating and Anomalous Resistivity in the Solar Corona
摘要：
最近射电天文学观测到的日冕法拉第旋转可以被解释为日冕电流的证据，其值高达$2.5 \times 10^9$安培（Spangler 2007）。这些估计电流被用来开发一个模型焦耳加热的电晕。它被假定为集中在薄的电流片，建议由二维磁流体动力学湍流理论。电阻率的斯皮策结果被采用作为真电阻率的下限。将计算的体积加热速率与Cranmer等人（2007）的独立理论估计值进行比较。后一种估计解释了日冕在几个太阳半径的日心距离处的动力学和热力学性质。我们计算的焦耳加热速率比Cranmer等人估计的至少小$3 \times 10^5$倍。从斯潘格勒（Spangler，2007）的观测中推断出的电流与日冕加热无关，除非真实电阻率相对于斯皮策值大大增加。然而，用于计算加热速率的湍流电流片的相同模型也给出了电子漂移速度，其可以与电子热速度相当，并且大于离子声速。因此，日冕电流片有可能是不稳定的电流驱动的不稳定性，产生高水平的波，提高电阻率，从而加热率。
英文摘要：
Recent radioastronomical observations of Faraday rotation in the solar corona can be interpreted as evidence for coronal currents, with values as large as $2.5 \times 10^9$ Amperes (Spangler 2007). These estimates of currents are used to develop a model for Joule heating in the corona. It is assumed that the currents are concentrated in thin current sheets, as suggested by theories of two dimensional magnetohydrodynamic turbulence. The Spitzer result for the resistivity is adopted as a lower limit to the true resistivity. The calculated volumetric heating rate is compared with an independent theoretical estimate by Cranmer et al (2007). This latter estimate accounts for the dynamic and thermodynamic properties of the corona at a heliocentric distance of several solar radii. Our calculated Joule heating rate is less than the Cranmer et al estimate by at least a factor of $3 \times 10^5$. The currents inferred from the observations of Spangler (2007) are not relevant to coronal heating unless the true resistivity is enormously increased relative to the Spitzer value. However, the same model for turbulent current sheets used to calculate the heating rate also gives an electron drift speed which can be comparable to the electron thermal speed, and larger than the ion acoustic speed. It is therefore possible that the coronal current sheets are unstable to current-driven instabilities which produce high levels of waves, enhance the resistivity and thus the heating rate.
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