Quiet-Sun and Coronal Hole in Mg II k Line as Observed by IRIS

Abstract

Coronal hole (CH) regions are dark in comparison to the quiet Sun (QS) at coronal temperatures. However, at chromospheric and transition region temperatures, the QS and CHs are hardly distinguishable. In this study, we have used the Mg ii  2796.35 Å spectral line recorded by the Interface Region Imaging Spectrometer ( IRIS ) to understand the similarities and differences in the QS and CH at chromospheric levels. Our analysis reveals that the emission from Mg ii k3 and k2v that originates in the chromosphere is significantly lower in CH than in QS for the regions with similar magnetic field strength. The wing emissions of Mg ii k that originates from the photospheric layer, however, do not show any difference between QS and CH. The difference in Mg ii k3 intensities between QS and CH increases with increasing magnetic field strength. We further studied the effects of spectral resolution on these differences and found that the difference in the intensities decreases with decreasing spectral resolution. For a resolution of 11 Å, the difference completely disappears. These findings are not only important for mass and energy supply from the chromosphere to the corona but also provides essential ingredients for the modeling of the solar spectral irradiance for the understanding of the Sun–climate relationships.