The Temperature Anisotropy and Helium Abundance Features of Alfvénic Slow Solar Wind Observed by Parker Solar Probe, Helios, and Wind Missions

Jia Huang, Davin E. Larson, Tamar Ervin, Mingzhe Liu, Oscar Ortiz, Mihailo M. Martinović, Zhenguang Huang, Alexandros Chasapis, Xiangning Chu, B. L. Alterman, Zesen Huang, Wenwen Wei, J. L. Verniero, Lan K. Jian, Adam Szabo, Orlando Romeo, Ali Rahmati, Roberto Livi, Phyllis Whittlesey, Samer T. Alnussirat, Justin C. Kasper, Michael Stevens and Stuart D. Bale
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Abstract

Slow solar wind is typically characterized as having low Alfvénicity, but the occasional occurrence of highly Alfvénic slow solar wind (HASSW) raises questions about its source regions and evolution. In this work, we conduct a statistical analysis of temperature anisotropy and helium abundance in HASSW using data from the Parker Solar Probe (PSP) within 0.25 au, Helios between 0.3 au and 1 au, and Wind near 1 au. Our findings reveal that HASSW is prevalent close to the Sun, with PSP observations displaying a distinct “U-shaped” Alfvénicity distribution with respect to increasing solar wind speed, unlike the monotonic increase trend seen in Helios and Wind data. This highlights a previously unreported population of unusually low-speed HASSW, which is found in both sub-Alfvénic and super-Alfvénic regimes. The observed decreasing overlap in temperature anisotropy between HASSW and fast solar wind (FSW) with increasing heliocentric distance suggests different underlying heating processes. Additionally, HASSW exhibits two distinct helium abundance populations, particularly evident in PSP data, with generally higher helium abundance compared to less Alfvénic slow solar wind. Moreover, the decreasing overlap in temperature anisotropy versus helium abundance distributions between HASSW and FSW with decreasing radial distance implies that not all HASSW originates from the same source region as FSW.
帕克太阳探测器、太阳神号和Wind任务观测到的alfv慢太阳风的温度各向异性和氦丰度特征
慢太阳风的典型特征是具有较低的阿尔夫萨梅变性,但偶尔出现的高阿尔夫萨梅变性的慢太阳风(HASSW)引起了人们对其来源区域和演变的质疑。在这项工作中,我们使用帕克太阳探测器(PSP)在0.25 au范围内,太阳在0.3 au和1 au之间,风在1 au附近的数据对HASSW的温度各向异性和氦丰度进行了统计分析。我们的研究结果表明,HASSW在太阳附近普遍存在,与太阳风速度的单调增加趋势不同,PSP观测结果显示太阳风速度的增加呈明显的“u形”alfvsamicity分布。这突出了以前未报道的异常低速HASSW群体,这在亚alfvsamim和超alfvsamim制度中都有发现。随着日心距离的增加,HASSW和快速太阳风(FSW)的温度各向异性重叠减少,这表明不同的底层加热过程。此外,HASSW表现出两种不同的氦丰度,特别是在PSP数据中,与较少的alfv缓慢太阳风相比,氦丰度普遍较高。此外,随着径向距离的减小,HASSW和FSW之间的温度各向异性与氦丰度分布的重叠减少,这意味着并非所有的HASSW都与FSW来自同一源区。
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