Quiet Sun and Coronal hole in Transition Region

Abstract

Coronal images of the Sun reveal the presence of dark region on the solar surface. This region of reduced emissivity is known as coronal holes, and they are surrounded by a region of higher emissivity called quiet Sun. Coronal hole intensity is not significantly reduced in the transition region. Many studies suggested that magnetic field is responsible for the unusual appearance of coronal holes in the transition region. This study aims to understand the properties of the coronal hole and quiet Sun in the transition region. We analysed the coronal hole and quiet Sun using Si IV 1394 Å spectral line observations taken by Interface Region Imaging Spectrograph (IRIS) and compared its intensity, Doppler velocity, and non-thermal width for the region with similar magnetic field strength. We found quiet Sun is brighter in transition region with respect to coronal holes and their non-thermal width and Doppler velocity were similar at these heights. Magnetic field is a fundamental quantity to understand various phenomenon occurring on the Sun. The magnetic field can only be measured at photospheric height. Thus extrapolation techniques are necessary to calculate and model coronal magnetic field. The extrapolation methods are based on the force-free assumption in Corona, which means the non-magnetic forces like pressure gradient and gravity are neglected. The underlying mathematical problem behind the force-free field, which is the relationship between electrical current and the magnetic field is non-linear. Thus numerical techniques are necessary to solve this non-linear force-free field. Potential and linear force-free fields are also reviewed in this work, they can be used as the first step to the model corona. These two methods cannot accurately reconstruct the coronal magnetic field because they are derived by linearising the underlying non-linear mathematical problem. Thus non-linear force-free fields are essential for the modelling. Here we reviewed the non-linear techniques including optimization procedure and Aschwanden’s vertical current approximation code. The knowledge of observed loop structure can be used to test the accuracy of these extrapolation methods. The extrapolation techniques shall be applied to the quiet Sun and coronal hole regions to understand the structure of the magnetic field in these regions and their possible effects on the heating shall be studied.