First Evidence of Coexisting EIT Wave and Coronal Moreton Wave from SDO/AIA Observations

TL;DR

This study presents the first clear observation of a fast-mode wave coexisting with an EIT wave, supporting the magnetic fieldline stretching model over the fast-mode wave interpretation for EIT waves.

Contribution

It provides the first direct evidence of a fast-mode wave preceding an EIT wave, confirming the fieldline stretching model and clarifying the nature of EIT waves.

Findings

Detected a fast-mode wave at 560 km/s ahead of an EIT wave.

Confirmed EIT waves are not fast-mode waves, supporting the fieldline stretching model.

Observed a coronal Moreton wave three times faster than the EIT wave.

Abstract

"EIT waves" are a globally propagating wavelike phenomenon. They were often interpreted as a fast-mode magnetoacoustic wave in the corona, despite various discrepancies between the fast-mode wave model and observations. To reconcile these discrepancies, we once proposed that "EIT waves" are apparent propagation of the plasma compression due to successive stretching of the magnetic field lines pushed by the erupting flux rope. According to this model, an "EIT wave" should be preceded by a fast-mode wave, which however was rarely observed. With the unprecedented high cadence and sensitivity of the {\em Solar Dynamics Observatory} ({\em SDO}) observations, we discern a fast-moving wave front with a speed of 560 km s$^{-1}$, ahead of an "EIT wave", which had a velocity of $\sim 190$ km s$^{-1}$, in the "EIT wave" event on 2010 July 27. The results, suggesting that "EIT waves" are not fast-mode waves, confirm the prediction of our fieldline stretching model for "EIT wave". In particular, it is found that the coronal Moreton wave was $\sim 3$ times faster than the "EIT wave" as predicted.