Upflows in the upper solar atmosphere

TL;DR

This paper reviews the characteristics, formation mechanisms, and roles of systematic upflows in the solar upper atmosphere, highlighting recent observations and unresolved questions in the context of solar wind and heating processes.

Contribution

It provides a comprehensive overview of solar upflows, discusses their potential formation mechanisms, and emphasizes the importance of new observations for future understanding.

Findings

Upflows are prominent in quiet Sun, coronal holes, and active region boundaries.

They are likely related to solar wind formation and atmospheric heating.

Recent observations are expected to clarify their generation and energization.

Abstract

Spectroscopic observations at extreme and far ultraviolet wavelengths have revealed systematic upflows in the solar transition region and corona. These upflows are best seen in the network structures of the quiet Sun and coronal holes, boundaries of active regions, and dimming regions associated with coronal mass ejections. They have been intensively studied in the past two decades because they are highly likely to be closely related to the formation of the solar wind and heating of the upper solar atmosphere. We present an overview of the characteristics of these upflows, introduce their possible formation mechanisms, and discuss their potential roles in the mass and energy transport in the solar atmosphere. Though past investigations have greatly improved our understanding of these upflows, they have left us with several outstanding questions and unresolved issues that should be addressed in the future. New observations from the Solar Orbiter mission, the Daniel K. Inouye Solar Telescope and the Parker Solar Probe will likely provide critical information to advance our understanding of the generation, propagation and energization of these upflows.