Since the discovery of CMEs in the early 1970s, the spectacular phenomenon has attracted wide interest, and has been extensively investigated through observations, theoretical analysis, and numerical simulations. The related research keeps being a hot topic in solar physics, not only because CMEs are associated with many other solar eruptive phenomena of different scales, but also because they are the major driving source of the hazardous space weather environments near the Earth. With the improving ground-based and space-borne observations along with the more and more sophisticated modelings, much progress has been obtained in the understanding of CMEs, yet with controversies in every aspect, including the progenitors, the triggering mechanisms, their propagation, the interaction between CMEs with the background solar wind, and in particular, how CMEs are related to other phenomena. The improving knowledge on these aspects directly determines how well we can predict the commencement of CMEs and their impact on the space weather environment. Here we summarize the key issues described in this review paper, with the hope that readers can grasp the general consensuses and controversies about the CME models, which might unavoidably be biased on the author’s personal viewpoints. Detailed reviews on each issue are distributed in the main text.
Morphologically, CMEs can be distinguished as narrow and normal CMEs. The essential difference between them is not the angular width, since some normal CMEs also have a small angular width, e.g., 10°. The meaningful difference might be that “narrow CMEs” are jet-like, whereas normal CMEs are loop-like. The physics of narrow CMEs is quite clear, i.e., they correspond to the outflow as emerging flux or a coronal loop reconnects with the open magnetic field. Therefore, the contents hereafter refer to the normal CMEs.
(1) Classification: Kinematically, the apparent CME velocity covers a wide range from tens to more than 3000 km s–1. Some are very fast, whereas some are very slow. It was once proposed that they can be divided into impulsive CMEs that are associated with solar flares and gradual CMEs that are associated with erupting prominences. Later observations tend to discard such a classification in the sense that there is no a dividing line between the velocity distributions of these two types of CMEs and many CMEs are associated with both solar flares and prominence eruptions.
(2) Progenitors: The pre-CME structure might be strongly sheared or weakly twisted magnetic field (i.e., core field) that is restrained by less-sheared envelope field, and a flux rope is not necessarily required in the progenitor. Filaments and sigmoids might correspond to the inner and outer parts of the core field, respectively. Whether a closed magnetic structure can erupt to form a CME depends not only on how sheared the core field is, but also on how fast the envelope field decreases with height. These two factors need to be combined and investigated quantitatively in the future.
(3) Triggering mechanisms: The triggering process is the most important part of CME models, giving the observational fact that the characteristic time of the gradual free energy accumulation is much longer than that of its impulsive release. About 7 mechanisms have been proposed, and many of them are physically the same, e.g., the flux cancellation, the tether-cutting, and the breakout models (the breakout model is a kind of external tether cutting). Some triggering mechanisms involve resistive MHD processes (e.g., reconnection), and others involve only ideal MHD process. It is generally believed that magnetic free energy has been stored in the CME progenitors, and the triggering process, which can be ideal or resistive, does not supply much energy to the eruptions.
(4) Eruption: After the CME progenitor is triggered to rise, the core field moves up, which is characterized by the slowly ascending filament and/or overlying coronal loop. The ensuing evolution may bifurcate into two cases: (a) If a current sheet is formed below the core field and magnetic reconnection is excited in the current sheet, the evolution becomes eruptive, with flaring loops being formed below the reconnection site and the core field being ejected above the reconnection site. The ejection of the core field and the magnetic reconnection are experiencing a positive feedback interaction, leading to the impulsive phase of the flare and the strong acceleration of the core field. The fast ejection of the core field would stretch up the overlying magnetic field successively, forming the CME frontal loop above. As depicted by the standard CSHKP model, magnetic reconnection plays a crucial role in these, fast generally, events. In the case when the triggering of the CME is also due to reconnection, the whole process can be called two-step reconnection, as proposed for solar flares (Wang and Shi, 1993); (b) If magnetic reconnection is not sufficiently excited below the core field, the magnetic loop may stop or may have a chance to expand, either by ideal MHD instabilities (Chen, 1989; Török and Kliem, 2007; Rachmeler et al., 2009) or being slowly accelerated by the drag force of the ambient solar wind. In this case, the low corona would not show significant brightenings.
(5) Nature of the CME frontal loop: It was proposed that the CME frontal loop is a fast-mode shock wave front in the early time, which was later discarded, and replaced with the idea that they are erupting bright coronal loop or flux rope and the plasma pile-up immediately ahead. However, the very rare spectroscopic observations indicate that the CME frontal loop presents a mass velocity several times smaller than the apparent moving velocity, suggesting that the CME frontal loops might be plasma pile-up swept by waves, rather than being an erupting structure. A new idea was recently proposed based on the observational fact that CME frontal loops are cospatial with EIT waves. The new mechanism claims that the frontal loop of fast CMEs might neither be an expanding magnetic loop nor material swept by waves. It might be the density-enhanced pattern caused by the successive stretching of the field lines (see Section 4.4). If this is confirmed, the top part of the frontal loop in the early stage actually propagates outward with the local fast-mode wave speed, which is not a bulk velocity. Detailed modelings are strongly in need.
(6) Is magnetic reconnection necessary? Theoretically, no in the 3D situations. Practically, in many events, especially those that are interesting in the space weather context, reconnection is the crucial mechanism that enables the fast CME eruption and the associated solar flare. Note that there might be also some CME events in which magnetic reconnection plays a trivial role, compared to other effects like the solar wind and the intrinsic ideal MHD instability.
After finishing this review paper, I have one caution for the readers when scrutinizing the CME modelings in the past decades. When deriving the analytical solutions for CME eruptions, some details are missing, e.g., the reconnection outflow, and the frozen-in effect might be violated; When doing MHD numerical simulations, two factors are tricky and might significantly affect the conclusions: one is the numerical resistivity, and the other is the boundary conditions. The numerical resistivity makes it difficult to distinguish ideal MHD processes from resistive MHD processes, and the different treating of the boundary conditions, especially at the top boundary, may lead to completely different results in terms of whether an eruption can occur or not.
Finally, I have some comments for the future research:
(1) In order to understand and predict the CME initiation, the internal cause, e.g., the magnetic nonpotentiality, and the external cause, e.g., the emerging flux, should be considered together.
(2) Little attention was paid to the nature of the CME frontal loop, which is not fully understood yet. The improper fitting of a CME evolution to a flux rope model might be misleading in many cases in the literature. With the archived imaging and spectroscopic data and the soon-coming Solar Dynamics Observatory (SDO) observations with high spatio-temporal resolutions, we are close to disclosing the veil of the spectacular phenomenon, CMEs.
(3) It might be of great importance to predict how much energy and magnetic helicity can be released from a source region if it erupts as a CME.
Living Rev. Solar Phys. 8, (2011), 1
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