Global Coronal Waves

TL;DR

This paper reviews the history and understanding of global coronal waves observed by SOHO and SDO, emphasizing the two-wave paradigm involving a shock wave and a CME front to resolve past debates.

Contribution

It clarifies the two-wave framework for coronal waves, proposing distinct names for the shock wave and CME front to improve understanding and resolve controversies.

Findings

Identification of two distinct types of coronal waves.

Recognition of the two-wave paradigm in recent research.

Clarification of the driving mechanisms behind coronal waves.

Abstract

After the {\em Solar and Heliospheric Observatory} ({\em SOHO}) was launched in 1996, the aboard Extreme Ultraviolet Imaging Telescope (EIT) observed a global coronal wave phenomenon, which was initially named "EIT wave" after the telescope. The bright fronts are immediately followed by expanding dimmings. It has been shown that the brightenings and dimmings are mainly due to plasma density increase and depletion, respectively. Such a spectacular phenomenon sparked long-lasting interest and debates. The debates were concentrated on two topics, one is about the driving source, and the other is about the nature of this wavelike phenomenon. The controversies are most probably because there may exist two types of large-scale coronal waves that were not well resolved before the {\em Solar Dynamics Observatory} ({\em SDO}) was launched: one is a piston-driven shock wave straddling over the erupting coronal mass ejection (CME), and the other is an apparently propagating front, which may correspond to the CME frontal loop. Such a two-wave paradigm was proposed more than 13 years ago, and now is being recognized by more and more colleagues. In this paper, we review how various controversies can be resolved in the two-wave framework and how important it is to have two different names for the two types of coronal waves.