Investigation of the Formation and Separation of An EUV Wave from the Expansion of A Coronal Mass Ejection

TL;DR

This study uses high-resolution observations to analyze the formation and separation of EUV waves from CMEs, revealing that EUV waves are driven by CME expansion and are composite phenomena.

Contribution

It provides detailed observational evidence showing EUV waves originate from CME expansion and separate after the CME's lateral expansion slows down.

Findings

EUV wave separates from CME after lateral expansion slows.

Diffuse wave front propagates across the solar disk.

Wave and CME fronts have distinct kinematic properties post-separation.

Abstract

We address the nature of EUV waves through direct observations of the formation of a diffuse wave driven by the expansion of a coronal mass ejection (CME) and its subsequent separation from the CME front. The wave and the CME on 2011 June 7 were well observed by Atmospheric Imaging Assembly onboard Solar Dynamic Observatory. Following the solar eruption onset, marked by the beginning of the rapid increasing of the CME velocity and the X-ray flux of accompanying flare, the CME exhibits a strong lateral expansion. During this impulsive expansion phase, the expansion speed of the CME bubble increases from 100 km s$^{-1}$ to 450 km s$^{-1}$ in only six minutes. An important finding is that a diffuse wave front starts to separate from the front of the expanding bubble shortly after the lateral expansion slows down. Also a type-II burst is formed near the time of the separation. After the separation, two distinct fronts propagate with different kinematic properties. The diffuse front travels across the entire solar disk; while the sharp front rises up, forming the CME ejecta with the diffuse front ahead of it. These observations suggest that the previously termed EUV wave is a composite phenomenon and driven by the CME expansion. While the CME expansion is accelerating, the wave front is cospatial with the CME front, thus the two fronts are indiscernible. Following the end of the acceleration phase, the wave moves away from the CME front with gradually an increasing distance between them.