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This study aims to quantify the solar wind charge-exchange (SWCX) X-ray emission factors (denoted as α-value) and their dependence on solar wind parameters, solar activity cycles, and solar wind origins. By analyzing 13 years (1998–2011) of in situ measurements from the Advanced Composition Explorer (ACE) spacecraft, we investigate the statistical correlations between solar wind ionization states, elemental abundances (particularly oxygen), and bulk plasma parameters (proton speed, density). The derived α-values are critical for interpreting the data from Solar wind and Magnetosphere Interaction Linker Explorer (SMILE), and disentangling SWCX foreground emissions from diffuse astrophysical X-ray sources observed by Einstein Probe (EP) and proposed DIffuse X-ray Explorer (DIXE) payload on Chinese Space Station. This work analyzed high-resolution solar wind ion composition data and plasma parameters from ACE. Events were categorized by solar wind origin (coronal holes, streamers, interplanetary coronal mass ejections (ICMEs)) and solar cycle phase (minimum vs maximum). α-value, defined as the total soft X-ray photon emission cross section per solar wind proton, was computed using an updated charge-exchange model incorporating state-resolved cross-sections for highly charged ions. The model accounts for velocity-dependent cross-section of solar wind-neutral interaction. Statistical method and bin-averaging techniques were applied to extract correlations between α, solar wind speed (vsw), proton density (np), and oxygen abundance. The main results are: 1. Ionization state dynamics: A strong anti-correlation exists between solar wind ionization degree and bulk speed: high-speed wind (>500 km/s) exhibit lower ionization states compared to slow wind (<400 km/s). 2. Elemental abundance trends: Oxygen abundance ([O/H]) anti-correlates with np: denser solar wind plasmas (np > 13 cm–3) exhibit 30–50% lower [O/H], suggesting fractionation processes during plasma acceleration. No significant speed dependence of [O/H] was observed, contrasting with earlier studies. 3. Emission factor (α-value) behavior: α-value decreases rapidly with increasing np and stabilizes for np > 13 cm−3. Conversely, α-value increases gradually with vsw up to 430 km/s, beyond which it plateaus ( Fig. 5 ). ICME-associated α exceeds streamer and coronal hole values by 35–60%, attributed to higher averaged ionic state in transient ejecta. Solar maximum α (2000–2002) is 1.3–2.7 times higher than solar minimum (2008–2010), reflecting cycle-dependent ion composition changes.By bridging in situ solar wind measurements and X-ray emission physics, this work advances the capability to diagnose both solar wind-magnetosphere coupling and the diffuse X-ray background. The validated α-value will benefit data analysis for China's space projects in the 2020s, for examples SMILE, DIXE and EP. -
Keywords:
- solar wind /
- charge-exchange /
- X-ray emission factor
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图 1 美国先进成份空间探测器(Advanced Composition Explorer, ACE)1998年至2011年监测的太阳风粒子状态数据, 除时序间隔外, 该图与Koutroumpa[23]文中图2基本一致. 从上到下: (a) 质子数密度 (cm–3), 灰线是1天平均, 绿线是1周平均; (b) 质子速度(km/s, 蓝线)和太阳表黑子计数(右纵轴, 灰线, 1天平均数据), 其来源于比利时太阳黑子指数和长期太阳观测中心[31]; (c) He2+ 离子数密度(cm–3), 灰线是1天平均, 红线是1周平均; (d)和(e) 不同元素(He、C、Ne、Mg和Si)与O元素丰度比; (f) 高电荷态的C6+C5+和O7+O6+离子比. 竖直点划线分隔的2个区域(2000—2002年和2008—2010年)分别是太阳活动最大和最小时间段
Figure 1. Status of solar wind particles from Advanced Composition Explorer (ACE) during 1998 and 2012. This figure is basically similar with Fig. 2 of Koutroumpa[23] with exception of time step. From top to bottom: (a) Proton density (cm–3), gray line is 1-day averaged, while green line is 1-week averaged; (b) Proton bulk velocity (km/s, blue line) and sun splot number (SSN, righ-axis) with 1-day averaged from the World Data Center SILSO[31]. (c) He2+ density (in cm–3), gray and red lines refer to 1-day and 1-week averaged. (d) and (e) Element (He, C, Ne, Mg and Si) abundance ratios relative to oxygen. (f) Ion fraction ratio of C6+C5+ and O7+O6+. Vertical dashed-dot lines separate two regions (i.e. 2000–2002 year and 2008–2010 year) for solar maximum and minimum, respectively.
图 2 ACE卫星13年的太阳风粒子监测数据二维直方图或点分布图, 数据采用ACE网站的2小时平均数据, 其中太阳风质子速度和密度采用12分钟平均数据重新分组获得. 最上行是O7+O6+与C6+C5+关系的柱状(左)和点(右)分布, 左图中斜蓝线是来源于冕洞数据的分隔线, 即$ \dfrac{{\rm O}^{7+}}{{\rm O}^{6+}}\times \dfrac{{\rm C}^{6+}}{{\rm C}^{5+}}\leqslant 0.01 $, 红色的斜线是斜率是1.32的线性拟合线, 右图中按Koutroumpa[23]和Zhao等人[33]的方法分类了来源于冕洞(红)、冕流(蓝)和星际冕物质抛射物(深绿)的不同数据. 中图给出了O7+O6+与太阳风速度(左)和密度(右)关系的二维直方图. 下图给出了O元丰度与太阳风速度(左)和密度(右)关系的二维柱状分布
Figure 2. 2D histograms and scatter distribution plots of ACE mission data from Feb. 1998 to Aug. 2011. Element and ion fraction data are from the ACE science center with 2-hour averaged, while proton bulk velocity and density are obtained by rebinning the 12-minute data available from the website. Top: O7+O6+ vs C6+C5+ in 2D histogram (left) and scatter (right). The oblique blue line in left panel refers to the criterion value of $ \dfrac{{\rm O}^{7+}}{{\rm O}^{6+}}\times \dfrac{{\rm C}^{6+}}{{\rm C}^{5+}}\leqslant 0.01 $ for coronal hole (CH) and streamer sources. Red line is a linear fit line with a slope of 1.32. Right panel shows the ACE data points of solar wind from different source in Sun by using the distinguise methods of Koutroumpa[23] and Zhao et al.[33], e.g. coronal holes, streamters and interplanetary coronal mass ejections (ICME).
图 3 左: 1998年至2011年ACE监测到的不同太阳风离子(C6+, 5+, O8+, 7+, 6+和Ne9+, 8+)电荷交换X射线辐射在0.1—2.0 keV能带的辐射因子α-值的点分布图(左)和统计百分比分布(右). $ \star $形点来源于Whittaker和Sembay的结果[22, 标记为W16], 水平虚线是经典结果($ 6\times10^{-16} $eV cm2), 带误差棒的实心圆点是来源于Koutroumpa的不同太阳风起源的结果[23], 即冕流(红色点)、冕洞(蓝色点)和星际冕物质抛射(绿色点), 其能带宽是0.1—2.0 keV. 右: 所有记录点统计的平均辐射因子中各元素贡献百分比
Figure 3. Left: Scatter plot and statical distribution of the charge-exchange emission factor α-value of the ACE solar wind particles (C6+, 5+, O8+, 7+, 6+ and Ne9+, 8+) in the energy ranges of 0.1–2.0 keV. $ \star $ symbol points are from the results of Whittaker & Sembay[22, marked as W16]. Horizontal dashed line is the empirical value of $ 6\times10^{-16} $eV cm2. Filled circles with errorbar refers to the calculation by Koutroumpa[23]for different solar wind (e.g. streamers (red symbol with errorbar), CH (blue symbol), and ICMEs (green symbol)). Right: Percentage contribution from the different elements to the mean emission factor α-value of all recored data.
图 4 不同观测时间段和不同太阳风起源的电荷交换辐射因子α值的统计分布, 及其与Koutroumpa结果[23, K24]的比较. 冕流: 红色阶梯线和带误差棒的实心圆点, 冕洞: 蓝色, 星际冕物质抛射: 绿色. 由于冕洞和星际冕物质抛射的统计数值小, 为便于比较, 均乘了6倍的任意数值. 上图是1998—2011年全时间段; 中图是太阳活动强周期2000—2002年; 下图中太阳活动弱周期2008—2010年
Figure 4. Statistical distribution of solar wind charge-exchange emission factor α-value during different observational period and for different solar wind sources, as well as its comparison with the results of Koutroumpa[23, K24]. Streamer: red step lines and filled circles with errorbars, CH: blue lines and symbols, ICMEs: green lines and symbols. For comparison, the statistical distribution of CH and ICMEs are multiplied by an arbitrary value of six. Top: full period of 1998–2011; Middle: 2000–2002 of solar maximum; Bottom: 2008–2010 of solar minimum.
图 5 1998—2011年2小时平均的不同太阳风离子电荷交换X射线总辐射因子α-值与质子数密度(左)和速度(右)的二维直方图. 带误差棒实心圆点表示质子数密度和速度相应网格辐射因子的平均值和其方差, 折线分别表示冕物质抛射(青色)、冕流(黄色)和冕洞(蓝色)的辐射因子平均值
Figure 5. 2D histograms of the charge-exchange emission factor α-value of all particles in solar wind versus proton density (left) and velocity (right) for 2-hour averaged data in 1998–2011. Filled symbols with errorbars refer to mean and standard variance of the α distribution at the grids of the proton density and velocity. Broken curves denotes mean α-value for ICMEs (cyan), Streamer (yellow) and CH (blue).
表 1 不同太阳风离子与中性氢原子碰撞电荷交换截面数据来源.
Table 1. Charge exchange cross-setion sources of different solar wind ions with neutral hydrogen.
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