陈鹏飞，学者凯发k8国际首页主页-凯发网

the effect of heat conduction on 2.5d magnetic reconnection, similar to that in kopp-pneuman model, is numerically studied. it is shown that the heat conduction accelerates the reconnection, increases the amount of shrinkage of the closed field lines, and increases the average rise speed of the sxr loop. mhd slow shocks contribute to the sxr loop heating. when the timescale of heat conduction is shorter than the alfv

a class of pseudo-reconnection caused by a shifted mesh in mhd simulations is reported. in term of this mesh system, some non-physical results may be obtained in certain circumstances, e.g., magnetic reconnection occurs without resistivity. after comparison, another kind of mesh system is strongly recommended.

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.

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.

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.

observations indicate that reconnection-favored emerging flux has a strong correlation with coronal mass ejectons (cmes). motivated by this observed correlation and based on the flux rope model, an emerging flux trigger mechanism is proposed for the onset of cmes, using two-dimensional magnetohydrodynamic (mhd) numerical simulations : when such emerging flux emerges within the filament channel, it cancels the magnetic field below the flux rope, leading to the rise of the flux rope (owing to loss of equilibrium) and the formation of a current sheet below it. similar global restructuring and a resulting rise motion of the flux rope occur also when reconnection-favored emerging flux appears on the outer edge of the

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.

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.

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.

ellerman bombs and type ii white-light flares share many common features despite the large energy gap between them. both are considered to result from local heating in the solar lower atmosphere. this paper presents numerical simulations of magnetic reconnection occurring in such a deep atmosphere, with the aim to account for the common features of the two phenomena. our numerical results manifest the following two typical characteristics of the assumed reconnection process: (1) magnetic reconnection saturates in ～600-900 s, which is just the lifetime of the two phenomena; (2) ionization in the upper chromosphere consumes quite a large part of the energy released through reconnection, making the heating effect most significant in the lower chromosphere. the application of the reconnection model to the two phenomena is discussed in detail.