Solar flares and coronal mass ejections (CMEs) are closely related phenomena (see Section 5.3.5), and it now seems very likely that they are simply different manifestations of a single, underlying physical process, namely, the release of magnetic energy stored in the magnetic field of the solar atmosphere. In the past there has been considerable controversy about the relation between CMEs and flares. Some authors have argued that flares cause CMEs by creating high enough temperatures to eject both plasma and magnetic field into the interplanetary medium. However, most CMEs are not associated with what is normally considered a flare (Gosling, 1993), and even in those cases that are, the thermal pressure is never enough to force the field open (Low, 2001).

As discussed in the previous chapter, flares occur over a span of energy scales that ranges from very small (microflares at the observable limit) to very large (>1032 ergs). Some time ago, Švestka and Cliver (1992) suggested that the main factor that determines whether a CME will be associated with a flare or not, is the strength of the magnetic field in the erupting region. If the ambient magnetic field strength is weak, then the emitted radiation, although still present, is just too faint to be considered a flare according to the traditional definition (Zirin 1988). According to some models, it is possible to have two CMEs with nearly the same trajectories and speeds but with an order of magnitude or more difference in the peak intensities of their light curves (Reeves and Forbes, 2005).