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arXiv 2104.04016astro-ph.SR
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小尺度通量涌现、冕洞加热和通量管膨胀:一个混合太阳风模式

Small-scale Flux Emergence, Coronal Hole Heating, and Flux-tube Expansion: A Hybrid Solar Wind Model

Y. -M. Wang

摘要:

太阳动力学天文台的极紫外线图像经常显示在磁图中看不到少数极性通量的地方存在环状精细结构,这表明在“单极”区域内短暂区域(ER)出现的速度被低估了。假设这一速率是相同的日冕洞内的安静的太阳,我们表明,ER和开放的场线之间的交换重联引起的太阳风能通量超过10 $^5$ erg厘米$^{-2}$ s $^{-1}$和规模作为在日冕底部的场强,与观测一致。除了在低日冕中提供欧姆加热外,这些重连事件可能是Alfv{é}n波的来源,其周期范围从$\sim$ 10分钟的颗粒时间尺度到许多小时的超颗粒/羽流时间尺度,其中一些较长周期的波在外日冕中被反射和消散。渐近风速取决于加热的径向分布,这在很大程度上是由通量管膨胀率控制。沿着与慢风相关的快速发散的通量管,加热很好地集中在音速点内部(1)因为向外传导的热通量密度和外部日冕温度降低,(2)因为净波能通量以与当地阿尔夫速度成比例的速率耗散。在这个“混合”太阳风模型中,重联加热了较低的日冕并驱动了质量通量,而波在更远的距离上将能量和动量传递给外流。

英文摘要:

Extreme-ultraviolet images from the Solar Dynamics Observatory often show looplike fine structure to be present where no minority-polarity flux is visible in magnetograms, suggesting that the rate of ephemeral region (ER) emergence inside "unipolar" regions has been underestimated. Assuming that this rate is the same inside coronal holes as in the quiet Sun, we show that interchange reconnection between ERs and open field lines gives rise to a solar wind energy flux that exceeds 10$^5$ erg cm$^{-2}$ s$^{-1}$ and that scales as the field strength at the coronal base, consistent with observations. In addition to providing Ohmic heating in the low corona, these reconnection events may be a source of Alfv{é}n waves with periods ranging from the granular timescale of $\sim$10 minutes to the supergranular/plume timescale of many hours, with some of the longer-period waves being reflected and dissipated in the outer corona. The asymptotic wind speed depends on the radial distribution of the heating, which is largely controlled by the rate of flux-tube expansion. Along the rapidly diverging flux tubes associated with slow wind, heating is concentrated well inside the sonic point (1) because the outward conductive heat-flux density and thus the outer coronal temperatures are reduced, and (2) because the net wave energy flux is dissipated at a rate proportional to the local Alfv{é}n speed. In this "hybrid" solar wind model, reconnection heats the lower corona and drives the mass flux, whereas waves impart energy and momentum to the outflow at greater distances.

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