4.4 What is the nature of CMEs?

We are coming back to the most fundamental question: What is the CME? This question may come up as a surprise to many colleagues since it is often taken for granted that CMEs are coronal mass ejected into the interplanetary space, without thinking too much about what the CMEs really are.

There is a general consensus about the CME core, which can be easily identified to be the erupting filament/prominence based on their kinematics from near the solar limb to high corona (e.g., House et al., 1981; Low, 1990Jump To The Next Citation Point; Gilbert et al., 2000). The only possible correction might be that the upward reconnection outflow, as shown in Figure 34View Image, would keep refilling the CME core in the form of hot plasmas (e.g., Ohyama and Shibata, 2008). However, as for the CME frontal loop, the understanding is rather illusive.

The CME frontal loop, or leading edge, was initially regarded as a wave phenomenon (Nakagawa et al., 1975; Steinolfson and Nakagawa, 1977), i.e., they are fast-mode MHD waves excited by the pressure pulse from the underlying solar flare. This theory was later discarded since many observations, e.g., Howard et al. (1982Jump To The Next Citation Point); Fisher and Munro (1984); and St Cyr et al. (2000), indicate that at several solar radii, the two legs of the CME frontal loop do not expand laterally. An improved model was proposed by Wu et al. (1983), where the MHD waves were produced by moving mass as suggested by the SXR observations.

The second popular model is that the CME frontal loop is a bundle of the background coronal magnetic field lines filled with plasmas (e.g., Poland and Munro, 1976). In particular, it was proposed that the three-part structure of the CME, i.e., the frontal loop, the cavity, and the core, can be identified as the dome, the cavity, and the prominence of a helmet streamer that transits drastically into a mass motion (Low, 1984, 1990; Hundhausen et al., 1984). For example, Gibson et al. (2006b) reported a pre-existing structure (consisting of a cavity and a bright loop outside of it) erupts, producing a typical three-part CME. Similarly, it was proposed that the CME frontal loop could be a shell-like layer draped over the flux rope (e.g., Chen, 1996Jump To The Next Citation Point; Krall et al., 2001Jump To The Next Citation Point; Thernisien et al., 2009), which is roughly perpendicular to the flux rope. In particular, Chen et al. (2000) and Krall and Chen (2005) suggested that the CME bright rim is ahead of the flux rope axis by 2a, where a is the minor radius of the flux rope.

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Figure 43: The flux rope model for the CME frontal loop (from Mouschovias and Poland, 1978Jump To The Next Citation Point).

The third model is that the CME frontal loop is a twisted flux rope expanding and broadening in the background coronal plasma and magnetic field (Mouschovias and Poland, 1978), as shown in Figure 43View Image.

One common feature implied by the above-mentioned two models is that the two legs of the CME frontal loop, whose plasma is frozen in the line-tied magnetic field lines, should be fixed near the solar surface. This might be in contradiction with the observational fact that CMEs expand laterally in the low corona (e.g., St Cyr et al., 1999). Another common feature implied by these two models is that the top part of the CME frontal loop moves radially as a mass motion. However, the spectroscopic observations with the Ultraviolet Coronagraph Spectrometer (UVCS) on board the SOHO satellite revealed that the Doppler velocity of the CME frontal loop is significantly smaller than its apparent propagation velocity, indicating that the frontal loop propagation is not a mass motion, at least for halo CMEs, as discussed in Ciaravella et al. (2006). They suggested that CME frontal loop may correspond to dense coronal plasmas swept up by a shock or compression wave. In the review paper of Forbes (2000), the CME frontal loop is considered to be plasma pile-up, while the detailed physical process of the piling-up needs to be clarified.

In the research aimed to investigate the relationship between CMEs and “EIT waves”, Chen (2009aJump To The Next Citation Point) proposed another idea on the formation of the CME frontal loop. He compared the white-light coronagraph observation of the 1997 September 9 CME event and the EIT base-difference images, and found that the white-light CME frontal loop is cospatial with the EIT wave front. The cospatiality of “EIT waves” and CME leading loops, as confirmed by Dai et al. (2010), led Chen (2009aJump To The Next Citation Point) to extend the magnetic fieldline stretching model for EIT waves (Chen et al., 2002Jump To The Next Citation Point, 2005cJump To The Next Citation Point) to explain the formation of the CME frontal loop, which is described as follows. As illustrated by Figure 44View Image, as the core structure, e.g., a magnetic flux rope, erupts, the resulting perturbation propagates outward in every direction, with a probability of forming a piston-driven shock as indicated by the pink lines. However, different from a pressure pulse, the erupting flux rope continues to push the overlying magnetic field lines to expand, so that the field lines are stretched outward one by one. For each field line, the stretching starts from the top, e.g., point A for the first magnetic line, and then is transferred down to the leg (point D) with the Alfvén speed, by which the first field line is stretched entirely. The deformation at point A is also transferred upward to point B of the second magnetic field line with the fast-mode wave speed. Such a deformation would also be transferred down to its leg (point E) with the local Alfvén speed, by which the entire second magnetic field line is stretched up. The stretching of the magnetic field lines compresses the coronal plasma on the outer side of the field line, producing density enhancements. All the newly formed density enhancements at a given time form a pattern (green), which is observed as the CME frontal loop. Similar to “EIT wave” fronts, the legs of the CME leading loop separate initially, and may stop when they meet with magnetic separatrices such as the boundary of coronal holes. This is why CMEs generally maintain a fixed angular span in their later stages. At the same time, as the field lines are stretched outward, the enveloped volume increases, resulting in coronal dimmings (or the dark cavity) behind the CME leading loop.

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Figure 44: A schematic sketch of the formation mechanism of CME leading loops, where the CME leading loop (green) are apparently-moving density enhanced structure that is generated by the successive stretching of magnetic field lines as the erupting core structure, e.g., a flux rope, continues to push the overlying field lines to expand outward successively. The piston-driven shock is shown as pink lines (from Chen, 2009a).

According to the above fieldline stretching mechanism (Chen et al., 2002, 2005c), both CME frontal loop and the EIT wave front are a dome-like structure, and the propagation of the CME frontal loop is an apparent motion, rather than a mass motion at this stage (the plasma is moving outward, but with a smaller velocity). It is due to the successive stretching of closed magnetic field. Near the footpoint, the lateral expansion speed is the same as that of EIT wave; Near the top part of the CME frontal loop, its radial propagation velocity is the local fast-mode wave speed, significantly larger than the mass motion velocity. Only when the CME propagates to a certain height where the local fast-mode wave speed decreases to the mass motion velocity of the plasma in the frontal loop, the radial expansion of the CME becomes a real mass motion. This model explains why some CMEs were observed to be very fast even in the very low corona (e.g., MacQueen and Fisher, 1983) and why these fast CMEs would decelerate during their propagation, i.e., the local fast-mode wave speed decreases with height.

However, Zhang et al. (2001a) did show a bright loop expands very slowly in the LASCO/C1 field of view, and then accelerates rapidly. Our conjecture is that the slowly-moving bright loop is actually a coronal loop straddling over a flux rope, rather than the CME frontal loop. As magnetic reconnection is excited below the flux rope, the flux rope is accelerated. The erupting flux rope stretches up the coronal loop as well as the overlying magnetic field lines successively, forming a new density-enhanced shell, i.e., the CME frontal loop, with the mechanism illustrated in Figure 44View Image.

On the other hand, the SOHO/LASCO coronagraph also showed some slow CMEs which are accelerating gradually during their passage over several solar radii. For example, the 2002 May 16 CME event accelerates from a velocity of ∼ 20 km s–1 at a height of 4R ⊙ to 279 km s–1 at 18R ⊙. CME events like this are strongly reminiscent of the expansion of coronal loops, especially the interconnecting loops, being dragged out by the ambient solar wind.

Here, I tentatively propose a unified paradigm for CME eruptions: as the CME progenitor is triggered to rise, it may erupt in one of the two ways:

(1) If there is weak or even no magnetic reconnection that can cut some of the line-tied magnetic field lines from the solar surface, the CME is accelerated slowly, probably due to ideal MHD instabilities or by the ambient solar wind. In this case, the CME frontal loop propagation is a mass motion, with the final velocity approaching that of the solar wind;

(2) If there is significant magnetic reconnection that can cut some of the line-tied magnetic field lines from the solar surface, the CME progenitor, e.g., a flux rope, is accelerated rapidly, stretching up the overlying field lines successively. Such a successive stretching compresses the coronal plasma, forming a new density-enhanced pattern, i.e., the CME frontal loop (see Bao et al., 2006, for the observation of the possible formation of a CME frontal loop). In this case, the propagation of the frontal loop is not a mass motion, and its top part moves outward at the local fast-mode wave speed.

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