| Presentation Title: | Comparison of coronal hole and quiet Sun chromosphere using IRIS Mg II h & k observations of polar off-limb regions |
| Author(s): | Akiko Tei, Stanislav Gunar, Joten Okamoto |
Abstract:
In the solar atmosphere, quiet regions are broadly divided into two categories, depending on the coronal brightness: quiet-Sun regions (QS, bright) and coronal hole regions (CH, dark). To understand how the solar atmosphere forms, it is interesting to compare these different regions. In the corona, it is known from observations that the temperature, pressure, and density in CH are lower than those in QS. In the chromosphere, jet-like structures that appear to connect the photosphere and the corona, called spicules, and the differences between CH and QS spicules have been investigated, especially by using high-resolution imaging observations with Hinode SOT as well as ground-based spectral-imaging observations. Then, we also have a huge amount of IRIS spectral now, and the differences between CH and QS chromospheres can be statistically investigated using its high-resolution, stable, spectroscopic observations. Hence, to reveal the differences between QS and CH in the chromosphere, from the entire IRIS catalog, we extracted all high-resolution spectral datasets of polar off-limb regions for which the spectral slit was fixed in the north-south direction and analyzed the Mg II h and k lines. Then, we obtained the integrated intensity, hk line ratio, line shift, and line width from the Mg II line profile at each time and height, and made distribution functions of these spectral features as a function of height from the solar limb. We found that the integrated intensities in the Mg II k line show a rapid decrease with the height in QS while they remain bright at higher altitudes in CH. Moreover, we found that the widths of the Mg II k line first increase and then decrease with height in both CH and QS, and are overall larger for CH than for QS. We conclude that the chromosphere is higher and exhibits faster motions than QS chromosphere. In addition, we showed that some chromospheric structures or motions exist in the lower chromosphere both in CH and QS regions.