For narrow CMEs, the progenitor is the open magnetic field, usually the coronal hole. When new magnetic flux emerges (or a coronal loop moves to) near or inside the coronal hole, it reconnects with the open field of the coronal hole. The elongated reconnection outflow may appear as a jet-like narrow CME.
For normal CMEs (hereafter, “CMEs” refer to the normal CMEs except otherwise specified), the progenitor should be a strongly twisted or sheared magnetic structure, which has stored a lot of nonpotential energy. On the other hand, the structure should have a potential to erupt while being kept in a metastable equilibrium or being close to a nonequilibrium state, presumably by the line-tying effect at the footpoints and the overlying closed magnetic field. In the 2.5D case, a simple force-free magnetic arcade with only shear but no twist might be too stable to be ready for eruption (Hood and Priest, 1980). However, in the 3D case, a simply sheared magnetic structure might be eligible for the CME progenitor. The closed magnetic fields on the Sun typically consist of active regions and bipolar magnetic field straddling over quiescent filaments, which are often the source regions for CMEs. Their eruption may take away the overlying large-scale magnetic loops, e.g., the interconnecting loops or transequatorial loops (Khan and Hudson, 2000).
As mentioned before, it has been known that CMEs are strongly associated with eruptive filaments. Since some active region filaments and almost all quiescent filaments are of the inverse polarity type (Leroy et al., 1983; Bommier and Leroy, 1998), which are well described by the flux rope model (Kuperus and Raadu, 1974), it was frequently assumed that the flux rope system is the ideal model for the progenitor of CMEs, i.e., a sheared and twisted field embedded in a less-sheared magnetic system. Considering many CMEs originate from helmet streamers, Low and Hundhausen (1995) describe the CME progenitor as a flux rope embedded under a coronal streamer, with a filament being supported at the bottom, as shown by Figure 4. In such a paradigm, the high-density streamer, i.e., the shaded area, evolves to the frontal loop of the CMEs, the prominence corresponds to the bright core of the CME. In between, there exists a low density cavity, which might be due to stronger magnetic field (Low and Hundhausen, 1995) or the plasma evacuation related to the prominence condensation. Such a 3-part structure was frequently found in streamers before eruption (Gibson et al., 2006b).
From the theoretical point of view, there are several advantages to have a flux rope in the CMEs progenitor as depicted in Figure 4:
(1) The simple but fundamental model for twisted field lines, which carry electric current and therefore magnetic free energy, is a flux rope (Low and Berger, 2003).
(2) The magnetic flux rope system gracefully matches the three-part structure of CMEs, as mentioned above.
(3) As long as the flux rope rises somehow, e.g., due to magnetic rearrangement or a certain instability, a current sheet naturally forms near the separator or a hyperbolic flux tube below the flux rope (Titov et al., 2002). The reconnection in the current sheet leads to the eruption of the flux rope and the overlying magnetic loops, as well as a solar flare near the solar surface, i.e., it fits the classical CSHKP model very well. Even without reconnection, the strongly-twisted flux rope may also erupt due to its intrinsic instabilities (see Section 4.1 for details).
Since the coronal magnetic field cannot be measured directly, the flux rope model can only seek for indirect evidence from observations. Magnetograms showed that the magnetic field is already twisted as it emerges out of the photosphere (Kurokawa, 1987; Lites, 2005). Through Yohkoh satellite observations, Rust and Kumar (1996) found that the geometry of the sigmoidal coronal loops before eruption is in accord with the flux rope model. Although the sigmoidal structure was later confirmed to provide evidence for the flux rope model, their idea, i.e., the sigmoid is just the flux rope itself, was later criticized (Gibson et al., 2006a). Canfield et al. (1999) further found that sigmoidal active regions are significantly more likely to be eruptive. Note, however, that some sigmoidal structures consist of many isolated structures, which appear to be a sigmoid or double J-shaped loops due to projection or poor resolution (Glover et al., 2002), and many CMEs are born in active regions without sigmoidal loops. Besides the helical structure, cavity patterns were also observed in the pre-CME structures in SXR (Hudson et al., 1999). In addition, radio imaging observations also showed a depressed region overlying an erupting filament, which was explained as a flux rope by Marqué et al. (2002). However, it should also be kept in mind that a flux rope might not be the only possibility for the pre-CME cavities. Any plasma dilution process, e.g., a slowly stretching loop, may produce the emission depletion.
On the other side, the nonlinear force-free magnetic extrapolation based on the photospheric vector magnetogram on 2000 July 14 shows a strongly twisted flux rope structure embedded in a simple bipolar magnetic arcade, as seen from Figure 5 (Yan et al., 2001). It is noted that the number of the twist is sensitive to the treatment of the 180° ambiguity of the horizontal magnetogram. For example, with the same vector magnetogram as in Yan et al. (2001), He and Wang (2008) obtained a less-twisted flux rope. With the state-of-the-art extrapolation technique (Schrijver et al., 2008; Guo et al., 2010) and temperature tomography (Tripathi et al., 2009), the existence of a flux rope prior to some eruptions was further confirmed. Before these efforts, the linear force-free extrapolation already showed flux rope structures, with its magnetic dips being in great accordance with the H filament structures (Aulanier and Demoulin, 1998). Concave-outward features behind the CME leading loop as found by Illing and Hundhausen (1983) in some CME events were considered to be consistent with the flux rope model (Chen et al., 1997). Further statistics by Dere et al. (1999) and St Cyr et al. (2000) indicates that 25% – 50% of the CMEs observed by the SOHO/LASCO coronagraph contain a helical flux rope. It is probable that many other events also possess similar helical magnetic structures, which did not show up in the white-light images due to low emissions. Some of these helical flux rope structures may exist before the eruption as discussed above. However, it was also argued that some of the helical flux ropes observed by coronagraphs might be formed during the eruption of the CME (Dere et al., 1999; Gosling, 1999; Amari et al., 2003b).
It should be kept in mind that a flux rope is not the only configuration that can carry strong electric current. Strongly sheared field with weak twist can also contain enough magnetic free energy (e.g., Antiochos et al., 1994; Amari et al., 1996). Furthermore, it might be misleading for some literature to illustrate the flux rope model in a two-dimensional (2D) plane. For example, in all 2D models, the flux rope is described to be detached from the solar surface, which means that all the field lines in the flux rope extend to infinity in the Cartesian coordinates (when the third component of the magnetic field is present) or are twisted annules circling the Sun in the spherical coordinates. It was proposed that such a detached flux rope makes the whole system possess more magnetic energy that the open field with the same flux distribution in the photosphere (see the discussion about the Aly–Sturrock constraint in Section 4.1). However, such a detached structure is certainly contradictory with observations. Considering that an observed CME occupies only a part of the solar corona, detached magnetic structure in the 3D space can only be an isolated magnetic island, with magnetic field lines self-closed within a limited volume. It rules out the possibility to form a prominence through chromospheric injection (Priest et al., 1989). It is also hard to believe that such a structure exists and is stable in the corona. A possible stable system might be some detached field lines interwound with other simply connected field lines, although such an analytical solution has not been obtained.
Therefore, a flux rope structure might be like that an arcade of twisted field lines, coming out of the positive polarity in the photosphere, wind one or more turns in the corona making magnetic dips with the inverse polarity, and then go back to the negative polarity in the photosphere (Priest et al., 1989). What remains unknown is the number of the winding. Through either MHD numerical simulations or coronal field extrapolations, some colleagues showed a number larger than 2 (Yan et al., 2001; Roussev et al., 2003), whereas others suggest a number less than 2 (Amari and Luciani, 1999; Aulanier and Demoulin, 1998; Régnier and Amari, 2004; van Ballegooijen, 2004; Su, 2007; Savcheva and van Ballegooijen, 2009) as illustrated in the left panel of Figure 6. In the latter case that a filament is present, the dip of each magnetic field line holds a thread of the filament. More importantly, through MHD simulations, Antiochos et al. (1994) demonstrated that the non-uniform shearing motion can also inject weak twist into the coronal field, leading to an inverse polarity configuration with far less than a half turn. Therefore, a flux rope configuration is even not necessary for the inverse-polarity filaments.
It is noticed that many active region filaments, which are often of the normal polarity type (Leroy, 1989), may also erupt to form CMEs. For the normal polarity filaments, the magnetic structure may or may not contain a flux rope, as modeled by Malherbe and Priest (1983), where their Figure 4c corresponds to the magnetic configuration with a flux rope, and their Figure 4b to the configuration without a flux rope. The magnetic configuration in the case with a flux rope has a potential to erupt and can also be fit into the CSHKP model with slight revisions as schematically shown by Low and Zhang (2002, see also Figure 37 in this paper). The magnetic configuration in the case without a flux rope is the classical Kippenhahn and Schlüter (1957) model, which seems difficult to be fit into the CSHKP model. However, we emphasize here that the difficulty also results from the misleading 2D plot, which can be resolved in the 3D plot as explained in the next paragraph. Considering that the magnetic field in a filament is always directed with a small angle relative to the magnetic inversion line (Leroy et al., 1983), the magnetic configuration of the CME progenitor with a normal polarity filament (in the case without a flux rope) might be an arcade of sheared magnetic loops with the normal polarity dips, which are all confined below less-sheared bipolar loops, as depicted in the right panel of Figure 6.
Therefore, corresponding to two types of filaments, i.e., with inverse and normal polarities, there might be two types of magnetic configurations for the CME progenitors, one is the twisted field model, and the other is just an arcade of sheared arcades as illustrated by Figure 6, which is the 3D extension of the 2D illustration of Figure 3 in Low (1996). It is also possible to have both inverse and normal polarities in one magnetic configuration (Antiochos et al., 1994; Aulanier et al., 2002). It has already been shown that as the twist increases, a sheared field line would form a dip with the normal polarity, and the normal polarity can transit to the inverse polarity either due to further twist or reconnection below the dip (Priest et al., 1989). Note that non-uniform shearing motions contain a twist component (Antiochos et al., 1994). Therefore, neither the magnetic dips nor the polarity type is crucial for the progenitor to be ready for eruption. The crucial point, which is common for both cases shown in Figure 6, is that an arcade of strongly-sheared flux tubes, which can be called core field, is restrained by the overlying less-sheared bipolar magnetic loops, which can be called envelope field following the terminology in Moore and LaBonte (1980). Such a configuration enables the existence of one long bald patch, two bald patches with one separator (Figure 6a, or see Titov and Démoulin, 1999), a hyperbolic flux tube (e.g., Titov et al., 2002), which degrades to a magnetic null point in a 2D case, or a quasi-separatrix layer (Figure 6b) in the core field. The merit for such a configuration to be the CME progenitor is that whenever the strongly-sheared core field rises due to either magnetic rearrangement or instability, the envelope field is stretched up, and the magnetic separator would collapse into a current sheet under the core field, similar to the 2D case where a magnetic null point can collapse to form a current sheet. The magnetic reconnection of this current sheet gradually removes the magnetic tension force of the overlying field lines, and facilitates the rapid eruption of the core field into the interplanetary space.
Regarding the debate whether the flux rope exists before eruption or is formed during eruption, with the above discussions we come to a conclusion that in some CME/flare events, especially those associated with quiescent filament eruptions, the flux rope may exist before eruption as shown in the left panel of Figure 6, however, for some normal-polarity filament eruptions, the flux rope may be formed during eruption via reconnection, e.g., as the core field lines in the right panel of Figure 6 (red) reconnect with each other.
Regarding the CME progenitor, i.e., the strongly sheared and/or twisted core field restrained by the overlying envelop field, two issues are worthy to be clarified by future MHD numerical simulations:
(1) The helical flux rope in the left panel of Figures 6 was sometimes plotted as twisted field lines winding many times all the way through the whole spine of the associated filament, which would be difficult to explain why only a segment of a long filament corridor erupted in many events. The probable situation would be, as mentioned above, that the flux rope consists of an arcade of weakly twisted magnetic field lines as shown in Figure 6, and each bundle of field lines holds an H thread of the filament at the magnetic dip.
(2) The SXR sigmoids could be the SXR signature of the CME progenitor, which may correspond to the upper part of the core field, with the filament threads lying on the dips of the lower part of the core field. Generally, the entire core field erupts, which is accompanied by the disappearance of both sigmoids and filaments (Canfield et al., 1999). However, the sigmoid occasionally erupted leaving the filament almost unchanged below the resulting postflare loops (Pevtsov, 2002; Liu et al., 2007). One possibility is that the thick core field experienced a pinch instability in the middle between the upper part (i.e., the SXR sigmoid) and the lower part (i.e., the filament), which led to a current sheet between the sigmoid and the filament. The ensuing reconnection results in the sigmoid erupting upward and flaring loops overlying the filament (see also Gilbert et al., 2000; Gibson and Fan, 2006).
Living Rev. Solar Phys. 8, (2011), 1
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