Temperature dependence of ultraviolet line parameters in network and internetwork regions of the quiet Sun and coronal holes

Abstract

Aims. We study the temperature dependence of the average Doppler shift and the non-thermal line width in network and internetwork regions for both the quiet Sun (QS) and the coronal hole (CH), by using observations of the Solar Ultraviolet Measurements of Emitted Radiation instrument onboard the Solar and Heliospheric Observatory spacecraft. Methods. We obtain the average Doppler shift and non-thermal line width in the network regions of QS, internetwork regions of QS, network regions of CH, and internetwork regions of CH by applying a single-Gaussian fit to the line profiles averaged in each of the four regions. The formation temperatures of the lines we use cover the range from 10 4 to 1.2 × 10 6 K. Two simple scenarios are proposed to explain the temperature dependence of the line parameters in the network regions. In one of the scenarios, the spectral line consists of three components: a rapid, weak upflow generated in the lower atmosphere, a nearly static background, and a slow cooling downflow. In the other scenario, there are just two components, which include a bright core component and a faint wide tail one.Results. An enhancement of the Doppler shift magnitude and the non-thermal line width in network regions compared to the internetwork regions is reported. We also report that most transition region lines are less redshifted (by 0−8 km s −1 ) and broader (by 0−5 km s −1 ) in CH compared to the counterparts of QS. In internetwork regions, the difference in the Doppler shifts between the coronal hole and the QS is slightly smaller, especially for the lines with formation temperatures lower than 2 × 10 5 K. And the two simple scenarios can reproduce the variation in the line parameters with the temperature very well. Conclusions. Our results suggest that the physical processes in network and internetwork regions are different and that one needs to separate network and internetwork when discussing dynamics and physical properties of the solar atmosphere. The agreement between the results of the observation and our scenarios suggests that the temperature dependence of Doppler shifts and line widths might be caused by the different relative contributions of the three components at different temperatures. The results may shed new light on our understanding of the complex chromosphere-corona mass cycle. However, the existing observational results do not allow us to distinguish between the two scenarios. At this stage, a high-resolution instrument Interface Region Imaging Spectrograph is highly desirable.

Statistical Study of Network Jets Observed in the Solar Transition Region: a Comparison Between Coronal Holes and Quiet-Sun Regions

“…Also in spectroscopic observations, line parameters in the two regions differ only for TR and coronal lines. For instance, Wang et al (2013) used spectroscopic observations of the Solar Ultraviolet Measurements of Emitted Radiation instrument (SUMER, Wilhelm et al (1995)) onboard the Solar and Heliospheric Observatory (SOHO) spacecraft to investigate the doppler shifts and non-thermal line widths of various lines spanning the solar atmosphere from the chromosphere to the TR. It was observed that most of the TR region lines in network regions are broader and more blue-shifted in CH than in QS.…”

Doppler Shift of the Quiet Region Measured by Meridional Scans With the Euv Imaging Spectrometer on Board Hinode

“…In our measurements, the Doppler shifts similarly fluctuated on a scale of 100 ′′ by a few km s −1 . Since this spatial scale is similar but a few times larger than that of the chromospheric network, we cannot attribute this fluctuation to the network structure studied by Wang et al (2013). Therefore, we must compare not only the detailed structures in the transition region and the corona but also the magnetic structures in the chromosphere.…”

“…For further understanding, we could investigate spatially and temporally resolved spectra. Wang et al (2013) studied the Doppler shift in the network and internetwork regions and found enhancements of the Doppler shift magnitude and the non-thermal line width in the network regions than those in the internetwork regions. However, the physical interpretation of this difference remains unclear.…”

Doppler shifts of spectral lines formed in the solar transition region and corona

“…While blueshifts in the quiet Sun corona can be related to true outflows into the solar wind, e.g., from coronal holes (e.g., Hassler et al 1999;Xia et al 2003) or the chromospheric network (e.g., Tian et al 2021), most of the blueshifts in the quiet Sun corona will be related to magnetically closed regions -simply because these blueshifts are everywhere (Peter 1999; Tian et al 2009). Detailed studies also found that the redshifts of lines formed in the lower transition region are larger in the bright network lanes, suggesting a positive correlation between the intensity and Doppler velocity (Brynildsen et al 1998;Curdt et al 2008;Tian et al 2008;Wang et al 2013).…”

“…Many studies also suggested that spicules can be heated to coronal temperatures (e.g., De Pontieu et al 2011;Martínez-Sykora et al 2017;Samanta et al 2019). Considering the mass cycle in the solar atmosphere, the transition region spectral line profiles are the superposition of rapid heated upflows generated in the chromosphere, slow cooling downflows of the previously heated plasma, and a steady background (Wang et al 2013). The varying contributions of these components at different temperatures may cause the change of Doppler velocities for different spectral lines.…”