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arXiv 2408.06155astro-ph.SRphysics.space-ph
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利用太阳轨道器(Solar Orbiter)数据检验通量管膨胀因子与太阳风速度的关系

Testing the flux tube expansion factor -- solar wind speed relation with Solar Orbiter data

J-B. Dakeyo, A. P. Rouillard, V. Réville, P. Démoulin, M. Maksimovic, A. Chapiron, R. F. Pinto, P. Louarn

AI总结:

本研究利用太阳轨道器数据结合PFSS模型检验通量管膨胀因子与太阳风速度的关系,发现反相关仅适用于高纬度冕洞源区,低纬度源区不成立,仅靠膨胀因子无法预测太阳风速度。

摘要:

在日球层中就地测量的太阳风性质,很大程度上由太阳内部的能量沉积过程所调控。既往研究表明,长周期大尺度磁结构中,1天文单位附近就地测量的太阳风速度与太阳大气中磁通量管的膨胀因子呈反比关系。本研究结合太阳轨道器(Solar Orbiter)观测数据、势场源表面(Potential Field Source Surface, PFSS)日冕模型以及太阳风轨迹,评估位于源表面rss处的通量膨胀因子-速度关系。基于约1.5年的太阳观测数据,我们发现就地测量的太阳风整体速度与日冕膨胀因子之间存在统计意义上较弱的反相关关系。按源区纬度对数据分类后,反相关程度呈现差异。我们证实存在起源于低纬度强磁场区域、经历大膨胀因子的快速太阳风,这类风在超径向膨胀阶段(源表面rss以下)达到超声速,理论上遵循速度与膨胀因子的正相关关系。研究发现,平均而言快速风的通量管膨胀发生的半径比慢速风更大。太阳风速度与膨胀因子的反相关关系仅存在于太阳活动极小期高纬度结构起源的太阳风中,这类结构通常与冕洞状结构相关,但该规律无法推广到低纬度源区。因此,仅靠膨胀因子无法用于预测太阳风速度,还必须考虑其他参数,例如膨胀梯度最强的高度。

英文摘要:

The properties of the solar wind measured in-situ in the heliosphere are largely controlled by energy deposition in the solar. Previous studies have shown that long duration and large scale magnetic structures show an inverse relation between the solar wind velocity measured in situ near 1 au and the expansion factor of the magnetic flux tubes in the solar atmosphere. We exploit Solar Orbiter data in conjunction with the Potential Field Source Surface (PFSS) coronal model, and the solar wind trajectory to evaluate the flux expansion factor - speed relation at the solar "source surface" at rss. We find a statistically weak anti-correlation between the in-situ bulk velocity and the coronal expansion factor, for about 1.5 years of solar data. Classification of the data by source latitude reveals different levels of anticorrelation. We show the existence of fast solar wind that originates in strong magnetic field regions at low latitudes and undergoes large expansion factor. We provide evidence that such winds become supersonic during the super radial expansion (below rss), and are theoretically governed by a positive correlation v-f. We find that faster winds on average have a flux tube expansion at a larger radius than slower winds. An anticorrelation between solar wind speed and expansion factor is present for solar winds originating in high latitude structures in solar minimum activity, typically associated with coronal hole-like structures, but this cannot be generalized to lower latitude sources. Therefore, the value of the expansion factor alone cannot be used to predict the solar wind speed. Other parameters, such as the height at which the expansion gradient is the strongest must also be taken into account.

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