Large-scale Bright Fronts in the Solar Corona: A Review of “EIT waves”

Abstract

“EIT waves” are large-scale coronal bright fronts (CBFs) that were first observed in 195 Å images obtained using the Extreme-ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory (SOHO). Commonly called “EIT waves”, CBFs typically appear as diffuse fronts that propagate pseudo-radially across the solar disk at velocities of 100–700 km s⁻¹ with front widths of 50–100 Mm. As their speed is greater than the quiet coronal sound speed (c _s ≤200 km s⁻¹) and comparable to the local Alfvén speed (v _A ≤1000 km s⁻¹), they were initially interpreted as fast-mode magnetoacoustic waves ($v_{f}=(c_{s}^{2} + v_{A}^{2})^{1/2}$). Their propagation is now known to be modified by regions where the magnetosonic sound speed varies, such as active regions and coronal holes, but there is also evidence for stationary CBFs at coronal hole boundaries. The latter has led to the suggestion that they may be a manifestation of a processes such as Joule heating or magnetic reconnection, rather than a wave-related phenomena. While the general morphological and kinematic properties of CBFs and their association with coronal mass ejections have now been well described, there are many questions regarding their excitation and propagation. In particular, the theoretical interpretation of these enigmatic events as magnetohydrodynamic waves or due to changes in magnetic topology remains the topic of much debate.