It may be natural to think that the nearly circular front of halo CMEs is just the face-on view of the dome-like CME frontal loop. For simplicity, we assume that the CME frontal loop is represented as the shell of a cone-shaped dome as illustrated by Figure 45. When it is observed edge-on as a limb event, it would appear as a loop structure, i.e., the green lines linking points A, B, C, D, and E, since the optical thickness is much larger here. When the CME is observed face-on as a halo event, however, the optical thickness is large only near the torus linking points B and D, i.e., the blue lines in Figure 45, which surrounds the solar disk to the observer. If so, the nature of halo CMEs would be the same as the normal CMEs. For example, Krall et al. (2006) extended the flux rope model, which was demonstrated to be applicable to limb CMEs (Chen, 1996; Krall et al., 2001), to halo CMEs, and found that the model can reproduce both quantitative near-Sun properties of the 2003 October 28 CME and the timing, strength, and orientation of the fields measured in situ near the Earth orbit. Of course, the nature of CME frontal loops is still under debate, and there are other possibilities, such as that the CME frontal loop is due to the compression of magnetic field lines which are stretched successively, as discussed in Section 4.4.
If the nature of halo CME fronts is the same as normal CMEs, there is a serious problem: why halo CMEs are on average twice faster than normal CMEs? Noticing that the Thomson scattering is significantly reduced for halo CMEs, Andrews (2002) proposed that many dim and slow halo CMEs are missed by coronagraphs so that the average velocity of the observed halo CMEs is high. Following this line of thought, Zhang et al. (2010) performed Monte Carlo simulations to investigate how the white-light brightness of CMEs with an average velocity of 523 km s–1 is reduced when they are observed as halo events. They found that the brightness of many narrow and slow CMEs, when they are observed as full halo CMEs, is reduced to a level comparable to the solar wind fluctuations, and therefore, these events would be missed to be identified in the coronagraph images. The remaining observable halo CMEs have an average velocity of 922 km s–1, quite similar to the value in observations.
An alternative view is that the halo CME fronts are completely different from the frontal loops of limb CMEs in physics. For example, Lara et al. (2006) proposed that the halo CME fronts might be the combination of the CME-driven shock wave and the CME material itself. Based on MHD numerical simulations, Manchester IV et al. (2008) synthesized the white-light images of a halo CME event, and found that the halo CME front can be identified as the CME-driven shock wave.
Since the fast-mode wave in the corona is of the order of 1000 km s–1, it easily explains why the average velocity of halo CMEs is as high as 957 km s–1. One may argue that the piston-driven shock wave is much weaker in white light than the CME frontal loop, and therefore can be barely visible in the coronal images. This is true for the limb events (e.g., Vourlidas et al., 2003). However, for halo CMEs, as illustrated by Figure 45, the situation may change. If we consider the Thomson scattering, which remarkably favors the plasma moving in a direction closer to the plane of the sky, the scattered white-light emission of the shock wave front at point F in Figure 45 could be stronger than that of the CME frontal loop near point B when both points are observed at the same projected heliocentric distance in the plane of the sky, although the plasma density is higher at point B than point F. Such a possibility needs to be studied quantitatively. Another question in this theoretical framework is how to interpret a couple of slow halo CME events, whose velocities are 100 – 200 km s–1. Maybe weak magnetic field?
Living Rev. Solar Phys. 8, (2011), 1
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