分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The nonthermal broadening of spectral lines formed in the solar corona is often used to seek the evidence of Alfv\'en waves propagating in the corona. To have a better understanding of the variation of line widths at different altitudes, we measured the line widths of the strong Fe \textsc{xii} 192.4, 193.5, and 195.1 \mbox{\AA} and Fe \textsc{xiii} 202.0 \mbox{\AA} in an off-limb southern coronal hole up to 1.5 $R_\odot$ observed by the Extreme Ultraviolet Spectrometer (EIS) on board the \textit{Hinode} satellite. We compared our measurements to the predictions from the Alfv\'en Wave Solar Model (AWSoM) and the SPECTRUM module. We found that the Fe \textsc{xii} and Fe \textsc{xiii} line widths first increase monotonically below 1.1 $R_\odot$, and then keep fluctuating between 1.1 and 1.5 $R_\odot$. The synthetic line widths of Fe \textsc{xii} and Fe \textsc{xiii} below 1.3 $R_\odot$ are notably lower than the observed ones. We found that the emission from a streamer in the line of sight significantly contaminates the coronal hole line profiles even up to 1.5 $R_\odot$ both in observations and simulations. We suggest that either the discrepancy between the observations and simulations is caused by insufficient nonthermal broadening at the streamer in the AWSoM simulation or the observations are less affected by the streamer. Our results emphasize the importance of identifying the origin of the coronal EUV emission in off-limb observations.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Physical quantities, such as ion temperature and nonthermal velocity, provide critical information about the heating mechanism of the million-degree solar corona. We determined the possible ion temperature $T_i$ intervals using extreme ultraviolet (EUV) line widths, only assuming that the plasma nonthermal velocity is the same for all ions. We measured ion temperatures at the polar coronal hole boundary simultaneously observed in 2007 by the EUV Imaging Spectrometer (EIS) on board the Hinode satellite and the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) on board the Solar and Heliospheric Observatory (SOHO). The temperatures of ions with the charge-to-mass ratio ($Z/A$) less than 0.20 or greater than 0.33 are much higher than the local electron temperature. The measured ion temperature decreases with the $Z/A$ to 0.25 and then increases with the charge-to-mass ratio. We ran the Alfv\'en Wave Solar Model-realtime (AWSoM-R) and the SPECTRUM module to validate the ion temperature diagnostic technique and to help interpret the results. We suggest that the widths of hot lines in the coronal hole (e.g., Fe XII, Fe XIII) are also affected by the solar wind bulk motions along the line of sight. We discussed the factors that might affect the line width fitting, including the instrumental width and non-Gaussian wings in some bright SUMER lines that can be fitted by a double-Gaussian or a $\kappa$ distribution. Our study confirms the presence of preferential heating of heavy ions in coronal holes and provides new constraints to coronal heating models.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Zihao Yang
Hui Tian
Xianyong Bai
Yajie Chen
Yang Guo
Yingjie Zhu
Xin Cheng
Yuhang Gao
Yu Xu
Hechao Chen
Jiale Zhang
摘要： Coronal mass ejections (CMEs) are the largest-scale eruptive phenomena in the solar system. Associated with enormous plasma ejections and energy release, CMEs have an important impact on the solar-terrestrial environment. Accurate predictions of the arrival times of CMEs at the Earth depend on the precise measurements on their three-dimensional velocities, which can be achieved using simultaneous line-of-sight (LOS) and plane-of-sky (POS) observations. Besides the POS information from routine coronagraph and extreme ultraviolet (EUV) imaging observations, spectroscopic observations could unveil the physical properties of CMEs including their LOS velocities. We propose that spectral line asymmetries measured by Sun-as-a-star spectrographs can be used for routine detections of CMEs and estimations of their LOS velocities during their early propagation phases. Such observations can also provide important clues for the detection of CMEs on other solar-like stars. However, few studies have concentrated on whether we can detect CME signals and accurately diagnose CME properties through Sun-as-a-star spectral observations. In this work, we constructed a geometric CME model and derived the analytical expressions for full-disk integrated EUV line profiles during CMEs. For different CME properties and instrumental configurations, full disk-integrated line profiles were synthesized. We further evaluated the detectability and diagnostic potential of CMEs from the synthetic line profiles. Our investigations provide important constraints on the future design of Sun-as-a-star spectrographs for CME detections through EUV line asymmetries.
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