New Insights into the Coronal Heating Problem: Analysis of Spectral Line Widths in the Solar Corona

Abstract

The coronal heating problem is the most challenging and enduring mystery in solar physics research: how does the corona, the outermost atmosphere, maintain a temperature exceeding one million kelvin, in contrast to the surface at 5,770 K? The past eight decades of research developed multiple coronal heating theories, classified into two major categories: the direct current (DC) models, commonly known as the "nanoflare" conjecture, and the alternating current (AC) models, promoting the wave heating scenarios.

This dissertation aims to advance our knowledge of the mysterious coronal heating problem by employing the broadening of spectral lines, which simultaneously reflects the heating of heavy ions and the unresolved motions in the solar corona. Together with the forward modeling of the line broadening from the Alfvén Wave Solar Model (AWSoM), the dissertation yields new constraints on coronal heating models.

The dissertation consists of three independent studies. The first part of the dissertation investigated the variation of line widths as a function of height in a coronal hole. The Fe XII and Fe XIII line widths start to increase below 1.2 solar radii and plateau beyond this height, which differs from the predictions made by AWSoM. In the second part, a non-monotonic, U-shape dependence of ion temperatures on their charge-to-mass ratio was disclosed at the coronal hole boundary, which challenges the classical ion-cyclotron resonance heating models. In the last part, distinct line width variations between the open- and closed-field regions were unveiled, taking advantage of the 2017 total solar eclipse (TSE) observations in the visible light with a large field-of-view (FOV). Ancillary extreme ultraviolet (EUV) and near-infrared observations further fortify the eclipse data. These discoveries shed light on the essential role of MHD waves and turbulence in coronal heating.

In summary, this dissertation presents a comprehensive study of spectral line widths and their variation in the dynamic and inhomogeneous solar corona. The dissertation finds that nonthermal velocity increases from 30 to 80 km/s, and heavy ions are heated 1.5 to 3 times more than electrons in open structures, while the line widths in closed structures are nearly constant, which provides essential limitations for wave heating models. The differences in the width of spectral lines between various coronal structures suggest that wave heating is more dominant in open structures, while localized heating occurs in closed structures.