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arXiv 1407.5941astro-ph.SR
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日冕中的超热电子:非定域输运能解释日球电荷态吗?

Suprathermal Electrons in the Solar Corona: Can Nonlocal Transport Explain Heliospheric Charge States?

Steven R. Cranmer

摘要:

有几种观点被提出来解释日冕是如何被加热的,以及太阳风是如何被加速的。一些模型假设开放的磁场线被光球运动驱动的阿尔芬波加热,并在经历湍流级联后消散。其他的模型认为太阳风的质量和能量大部分是通过闭合的日冕环的磁重联注入的。后一种想法的动机是重新连接射流的观察,也由闭合环路和慢风之间的离子组成的相似性。波浪/湍流模型也成功地再现了观测到的离子成分特征与风速的关系趋势。然而,与观测结果相比,这些模型预测的电荷状态比的绝对值往往太低。这封信通过更好地考虑日冕电子的弱库仑碰撞来完善这些预测,日冕电子的热力学性质决定了低日冕中的离子电荷状态。非局域电子输运的微扰描述适用于现有的一组波/湍流模型。由此产生的电子速度分布在低电晕表现出温和的超热尾特征的“卡帕”指数在10和25之间。这些超热电子被认为是足够的能量,以提高氧离子的电荷状态,同时保持相同的相对趋势与风速时发现的分布被假定为麦克斯韦。更新后的波/湍流模型与太阳风离子成分测量结果非常吻合。

英文摘要:

There have been several ideas proposed to explain how the Sun's corona is heated and how the solar wind is accelerated. Some models assume that open magnetic field lines are heated by Alfven waves driven by photospheric motions and dissipated after undergoing a turbulent cascade. Other models posit that much of the solar wind's mass and energy is injected via magnetic reconnection from closed coronal loops. The latter idea is motivated by observations of reconnecting jets and also by similarities of ion composition between closed loops and the slow wind. Wave/turbulence models have also succeeded in reproducing observed trends in ion composition signatures versus wind speed. However, the absolute values of the charge-state ratios predicted by those models tended to be too low in comparison with observations. This letter refines these predictions by taking better account of weak Coulomb collisions for coronal electrons, whose thermodynamic properties determine the ion charge states in the low corona. A perturbative description of nonlocal electron transport is applied to an existing set of wave/turbulence models. The resulting electron velocity distributions in the low corona exhibit mild suprathermal tails characterized by "kappa" exponents between 10 and 25. These suprathermal electrons are found to be sufficiently energetic to enhance the charge states of oxygen ions, while maintaining the same relative trend with wind speed that was found when the distribution was assumed to be Maxwellian. The updated wave/turbulence models are in excellent agreement with solar wind ion composition measurements.

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