New views on the emission and structure of the solar transition region

TL;DR

This paper reviews recent high-resolution observations of the solar transition region, highlighting its complex magnetic structures, plasma properties, and dynamic flows, which are crucial for understanding solar wind and coronal heating.

Contribution

It synthesizes recent observational findings on the structure, magnetic topology, and plasma dynamics of the solar transition region, emphasizing its nonuniformity and magnetic complexity.

Findings

TR is highly nonuniform and magnetically structured.

Flows in the TR can last from hours to days, indicating plasma circulation.

Lyman line profiles are significantly affected by TR flows and dynamics.

Abstract

The Sun is the only star that we can spatially resolve and it can be regarded as a fundamental plasma laboratory of astrophysics. The solar transition region (TR), the layer between the solar chromosphere and corona, plays an important role in solar wind origin and coronal heating. Recent high-resolution observations made by SOHO, TRACE, and Hinode indicate that the TR is highly nonuniform and magnetically structured. Through a combination of spectroscopic observations and magnetic field extrapolations, the TR magnetic structures and plasma properties have been found to be different in coronal holes and in the quiet Sun. In active regions, the TR density and temperature structures also differ in sunspots and the surrounding plage regions. Although the TR is believed to be a dynamic layer, quasi-steady flows lasting from several hours to several days are often present in the quiet Sun, coronal holes, and active regions, indicating some kind of plasma circulation/convection in the TR and corona. The emission of hydrogen Lyman lines, which originates from the lower TR, has also been intensively investigated in the recent past. Observations show clearly that the flows and dynamics in the middle and upper TR can greatly modify the Lyman line profiles.