We present a theoretical model for the shock formation that is related to coronal and interplanetary type II radio bursts associated with coronal mass ejections on the basis of the magnetic reconnection model of eruptive solar flares. Coronal type II bursts are usually observed in the metric wavelength range (metric type II bursts), and interplanetary bursts are usually observed in the decametric-hectometric wavelength range (decametrichectometric bursts). Our research shows that the decametric-hectometric type II radio bursts are produced by the piston-driven fast-mode MHD shock that is formed in front of an eruptive plasmoid (a magnetic island in the two-dimensional sense or a magnetic flux rope in the three-dimensional sense), while the metric radio bursts are produced by the reverse fast-mode MHD shock that is formed through the collision of a strong reconnection jet with the bottom of the plasmoid. This reverse shock apparently moves upward as long as the reconnection jet is sufficiently strong and dies away when the energy release of the reconnection stops or weakens significantly. On the other hand, the piston-driven fast shock continues to exist when the plasmoid moves upward. Our model succeeds in explaining the observational result that the piston-driven fast shock that produces decametrichectometric type II bursts moves faster and survives longer than the other shock.

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.

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,

Magnetohydrodynamic (MHD) equations are numerically solved to study 2.5-dimensional magnetic reconnection with field-aligned heat conduction, which is also compared with the adiabatic case. The dynamical evolution starts after anomalous resistivity is introduced into a hydrostatic solar atmosphere with a force-free current sheet, which might be similar to the configuration before some solar flares. The results show that two jets (i.e., the outflows of the reconnection region) appear. The downward jet col-lides with the closed line-tied

We performed 2.5-dimensional numerical simulations of magnetic reconnection for several models, some with the reconnection point at a high altitude (the X-type point in magnetic reconnection), and one with the reconnection point at a low altitude. In the high-altitude cases, the bright loop appears to rise for a long time, with its two footpoints separating and the field lines below the bright loop shrinking, which are all typical features of two-ribbon flares. The rise speed of the loop and the separation speed of its footpoints depend strongly on the magnetic field B0, to a medium extent on the density P0, and weakly on the temperature To, the resistivity η, and the length scale L0, by which the size of current sheet and the height of the X-point are both scaled. The strong B0 dependence means that the Lorentz force is the dominant factor; the inertia of the plasma may account for the moderate P0 dependence; and the weak η dependence may imply that "fast reconnection" occurs; the weak L0 dependence implies that the flaring loop motion has geometrical self-similarity. In the low-altitude case, the bright loops cease rising only a short time after the impulsive phase of the reconnection and then become rather stable, which shows a distinct similarity to the compact flares. The results imply that the two types of solar flares, i.e., the two-ribbon flares and the compact ones, might be unified into the same magnetic reconnection model, where the height of the reconnection point leads to the bifurcation.