3.2 Global picture

Many researchers hope to see a unified model which can explain the diversity of CMEs. The observations, however, seem to show that it is necessary to divide CMEs into at least two categories, i.e., narrow CMEs and normal CMEs. The former has an angular width of ∼ 10° and the latter has a larger angular width. Note that there is no dividing line in the angular width between the two groups. This grouping is apparently based on the angular width. However, the essential feature that can distinguish them is that narrow CMEs show an elongated jet-like shape, whereas normal CMEs present a closed (or convex-outward) loop, as shown by Figure 1View Image. It is noticed that many CMEs look diffuse, and have no a well-defined shape that can be directly fit into the two-type classification. One possibility is that they are normal CMEs not seen as such due to the projection effects (e.g., Cremades and Bothmer, 2004). Other possibilities should also be explored. For example, they might be due to the complex magnetic topology, which may be original (Roussev et al., 2007Jump To The Next Citation Point; Zhang et al., 2007Jump To The Next Citation Point; Zhukov and Veselovsky, 2007Jump To The Next Citation Point) or due to interchange reconnection with ambient magnetic structures (Attrill et al., 2006Jump To The Next Citation Point; Gibson and Fan, 2008Jump To The Next Citation Point). Further investigation is definitely required. Based on the fact that narrow CMEs are the extension of EUV jets in the low corona, Wang et al. (1998) proposed that they result from the magnetic reconnection between a small-scale magnetic bipole, e.g., an emerging magnetic flux or a coronal loop, and the unipolar flux, i.e., the open magnetic field in the coronal holes, as depicted by the left panel of Figure 3View Image. Note that such a model is the extension of the reconnection model for SXR jets (Shibata et al., 1992). At the same time, as the result of magnetic reconnection, a compact solar flare appears near the solar surface.
View Image

Figure 3: Schematic models to describe the eruption of narrow CMEs (left) and normal CMEs (right). The right panel is taken from Forbes (2000Jump To The Next Citation Point).

For the normal CMEs, most of them can be considered as an erupting flux rope system, presenting the typical three-part structure, i.e., a convex-outward frontal loop which is immediately followed by a cavity with an embedded core. Note that one or two components might be absent in the white-light images. For instance, the cavity may be missing due to insufficient instrument sensitivity or an unfavorable angle. As depicted by the right panel of Figure 3View Image, the eruption of normal CMEs can be described as follows (see Forbes, 2000Jump To The Next Citation Point, for a review): A flux rope (a twisted or strongly sheared core magnetic field in a more general sense), which may or may not hold a filament/prominence, is kept in equilibrium by the overlying envelope magnetic field lines which are line-tied to the solar surface. The flux rope rises due to some reason, e.g., magnetic rearrangement, the loss of equilibrium, or some instability. Some of the magnetic field lines straddling over the flux rope are stretched up, forming antiparallel magnetic field in the wake of the rising flux rope. In the three-dimensional (3D) case, the upward and the downward magnetic field lines are not strictly antiparallel, with a certain component in the direction along the photospheric magnetic inversion line below the flux rope. As the flux rope rises, the upward and downward magnetic lines below the flux rope approach each other to form a current sheet. Microscopic instabilities of the current sheet enable resistive or collisionless magnetic reconnection. Such a fast reconnection leads to a solar flare, quite often a two-ribbon flare, below the reconnection point. On the other hand, the magnetic reconnection cuts the line-tied magnetic field lines, which removes the constraint for the flux rope and facilitates the rapid eruption of the flux rope (the details of such a process is as follows: the Lorentz force accelerates the reconnection outflows, and the upward outflow pushes the flux rope to move). Note that the toroidal magnetic flux of the flux rope keeps increasing as the reconnection goes on. The erupting flux rope pushes the overlying closed magnetic lines to stretch up, somehow forming a CME frontal loop and a piston-driven shock ahead of it (see Section 4.4 for the discussion on how the frontal loop forms). If fast reconnection is not excited, the flux rope might also have a chance to erupt due to loss of equilibrium or various ideal MHD instabilities (see Section 4.1 for the debate on the role of reconnection in CME eruptions). In this case, no flares or brightenings are visible near the solar surface.

Note that part of the overlying magnetic loops would be stretched up, extending to the interplanetary space along with the flux rope (e.g., Linker et al., 2003). The stretched field lines are not really open field (e.g., Riley, 2007), although they are often called “open field” since the closure takes place far from the low corona. Other parts of the overlying magnetic loops, especially those near the two ends of the flux rope, might slip to the two ends of the flux rope that are anchored to the solar surface (e.g., Low, 1997Jump To The Next Citation Point).

From the above pictures, it is seen that flares and CMEs, when they are associated, can be considered as different manifestations of the same physical process, namely the conversion of magnetic free energy to radiative and kinetic energies, respectively (see Harrison, 1995). Such a model was originally developed to interpret the various faces of solar flares, including the associated plasmoid ejections, by Carmichael (1964), Sturrock (1966), Hirayama (1974) and Kopp and Pneuman (1976) among others (see Švestka and Cliver, 1992, for a review), hence was later called CSHKP model. The model was dubbed “standard model” for CME/flares (Hudson and Cliver, 2001). It is stressed that such a standard model is just a phenomenological model based on observations. It describes the overall evolution of CME/flares, with many details being not included. With new multiwavelength observations, such a theoretical framework needs to be improved in order to cover the various phases of the CME eruptions from their birth to their pilgrimage to the interplanetary space. First, we are still not quite sure what is the progenitor that is ready to be triggered to erupt as a CME. The rough picture is described as follows: magnetic field, which is generated at the tachocline layer, emerges throughout the convection zone and the lower atmosphere into the tenuous corona. In response to the continual flux emergence and photospheric motions, the coronal field keeps adjusting to form a more and more complex magnetic structure in a quasi-static way (actually pervasive magnetic reconnection happens, producing a pool of small brightenings). At a certain stage, the magnetic structure reaches a nonequilibrium state or a metastable state that has the potential to release part of its magnetic energy. In this picture, it is still an open question whether the pre-CME structure should always possess a flux rope. Or, the so-called flux rope is actually an extreme case of the ordinary magnetic flux tube with a certain twist. The second issue is how the progenitor is triggered to deviate from the equilibrium state. In this aspect, the statistical investigations of the correlation between CME onsets and other phenomena are of extreme significance. The third issue is how a CME is accelerated. The related questions involve (1) whether magnetic reconnection is a necessary condition; (2) how important the interaction between the ejecta and the solar wind is; (3) the effect of prominence mass drainage, among others. The fourth issue is how the CME is related to the accompanying phenomena, such as solar flares, type II radio bursts, Moreton waves, EIT waves and dimmings, transient coronal holes, etc. The fifth issue is how the CME evolves to an interplanetary CME (ICME) and how the CME properties affect the geomagnetic activity. In the following subsections, we present the current viewpoints on some of the above-mentioned issues (see also van Driel-Gesztelyi and Culhane, 2009).

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