Depletion of Heavy Ion Abundances in Slow Solar Wind and Its Association with Quiet Sun Regions

The exact coronal origin of the slow-speed solar wind has been under debate for decades in the Heliophysics community. Besides the solar wind speed, the heavy ion composition, including the elemental abundances and charge state ratios, are widely used as diagnostic tool to investigate the coronal origins of the slow wind. In this study, we recognize a subset of slow speed solar wind that is located on the upper boundary of the data distribution in the O7+/O6+ versus C6+/C5+ plot (O-C plot). In addition, in this wind the elemental abundances relative to protons, such as N/P, O/P, Ne/P, Mg/P, Si/P, S/P, Fe/P, He/P, and C/P are systemically depleted. We compare these winds (“upper depleted wind” or UDW hereafter) with the slow winds that are located in the main stream of the O-C plot and possess comparable Carbon abundance range as the depletion wind (“normal-depletion-wind”, or NDW hereafter). We find that the proton density in the UDW is about 27.5% lower than in the NDW. Charge state ratios of O7+/O6+, O7+/O, and O8+/O are decreased by 64.4%, 54.5%, and 52.1%, respectively. The occurrence rate of these UDW is anti-correlated with solar cycle. By tracing the wind along PFSS field lines back to the Sun, we find that the coronal origins of the UDW are more likely associated with quiet Sun regions, while the NDW are mainly associated with active regions and HCS-streamer.

在太阳物理学界，低速太阳风的确切日冕起源几十年来一直备受争议。除了太阳风速度外，重离子组成，包括元素丰度和电荷态比，被广泛用作研究低速太阳风日冕起源的诊断工具。在这项研究中，我们识别出了低速太阳风的一个子集，它位于O7⁺/O6⁺与C6⁺/C5⁺图（O - C图）的数据分布上边界。此外，在这种太阳风中，相对于质子的元素丰度，如N/P、O/P、Ne/P、Mg/P、Si/P、S/P、Fe/P、He/P和C/P都系统性地降低。我们将这些太阳风（以下简称“上部亏损风”或UDW）与位于O - C图主流且碳丰度范围与亏损风相当的低速太阳风（以下简称“正常亏损风”或NDW）进行比较。我们发现UDW中的质子密度比NDW低约27.5%。O7⁺/O6⁺、O7⁺/O和O8⁺/O的电荷态比分别降低了64.4%、54.5%和52.1%。这些UDW的出现频率与太阳活动周期呈反相关。通过沿着PFSS磁力线将太阳风追溯到太阳，我们发现UDW的日冕起源更可能与宁静太阳区域相关，而NDW主要与活动区和日冕物质抛射 - 冕流相关。