Solar coronal mass ejections (CMEs) are associated with many dynamical phenomena, among which EIT waves have always been a puzzle. In this Letter MHD processes of CME-induced wave phenomena are numerically simulated. It is shown that as the flux rope rises, a piston-driven shock is formed along the envelope of the expanding CME, which sweeps the solar surface as it propagates. We propose that the legs of the shock produce Moreton waves. Simultaneously, a slower moving wavelike structure, with an enhanced plasma region ahead, is discerned, which we propose corresponds to the observed EIT waves. The mechanism for EIT waves is therefore suggested, and their relation with Moreton waves and radio bursts is discussed.

EIT waves are often observed to be propagating EUV enhancements followed by an expanding dimming region after the launch of CMEs. It was widely assumed that they are the coronal counterparts of the chromospheric Moreton waves, though the former are three or more times slower. The existence of a stationary "EIT wave" front in some events, however, posed a big challenge to the wave xplanation. Simulations are performed to reproduce the stationary "EIT wave" front, which is exactly located near the footpoint of the magnetic separatrix, consistent with observations. The formation of the stationary front is discussed in the framework of our model where "EIT waves" are supposed to be generated by successive opening of the field lines covering the erupting flux rope in CMEs.

This paper reviews our recent progress in the numerical study of coronal mass ejections (CMEs) based on flux rope model, which shows that when the reconnection-favored emerging flux appears either within or ABSTRACT on the outer edge of the filament channel, the flux rope would lose its equilibrium, and be ejected, while a current sheet is formed below the flux rope. For the case with emergence within the filament channel, even small flux is enough to trigger the loss of equilibrium, however, there is a threshold for the emerging flux on the outer edge of the filament channel. Given that anomalous resistivity sets in (e.g. when the current density exceeds a critical value), fast reconnection is resulted in, leading to fast eruption of the flux rope and localized flare (either impulsive-type or LDE-type depending on the height of the reconnection point) near the solar surface. The numerical results can well explain why CMEs are not centered on flares and provide hints for CME-flare spatial and emporal relationships.

In the flare event of 1999 March 18, a threadlike structure observed in EUV Imaging Telescope images was found to move inward and collapse to an X-shaped configuration below the ejecta, strongly suggestive of the occurrence of magnetic reconnection. On the basis of the numerical results of a coronal mass ejection (CME) flare model, a similar threadlike structure in the Fe xii 195 image is reproduced in this Letter. It is found that,

Early observations by the EUV Imaging Telescope (EIT) on the Solar and Heliospheric Observatory indicated that propagating diffuse wave fronts, now conventionally referred to as "EIT waves," can often be seen on the solar disk with a propagation velocity several times smaller than that of H Moreton waves. They are almost always associated with coronal mass ejections. We have previously confirmed the existence of such a wave phenomenon with numerical simulations, which indicate that there does exist a slower moving "wave" much behind the coronal counterpart of the H Moreton wave. Further observations have disclosed many new features of the EITwaves: the waves stop near the separatrix between active regions, sometimes they experience acceleration from the active region to the quiet region, and so on. Here we report onMHD simulations performed to demonstrate how the typical features of EITwaves can all be accounted for within our theoretical model, in which the EITwaves are thought to be formed by successive stretching or opening of closed field lines driven by an erupting flux rope. The relationship between EITwaves, H Moreton waves, and type II radio bursts is discussed, with an emphasis on reconciling the discrepancies among different views of the "EIT wave" phenomenon.