Reconnection in the lower solar atmosphere and coronal mass ejections Original Research Article

In 1985, a phenomenon in the solar photosphere, called magnetic flux cancellation, was first described in detail by Livi et al. (1985) [The cancellation of magnetic flux. I – On the quiet sun, Aust. J. Phys. 38, 855–873, 1985] and Martin et al. (1985) [The cancellation of magnetic flux. II – In a decaying active region, Aust. J. Phys. 38, 929–959, 1985]. Since then, it has been revealed that flux cancellation is intrinsically correlated to most, if not all, types of solar activity, such as flare, filament formation and eruption, and ubiquitous small-scale activity, e.g., X-ray bright point, explosive event, mini-filament eruption and so on. Only recently, it was discovered that flux cancellation appeared to be a key part of magnetic evolution leading to the initiation of coronal mass ejections (CMEs) [Zhang et al., Magnetic flux cancellation associated withthe major solar event on 2000 July 14. Astrophys. J. 548, L99–102, 2001; Zhang et al., 2001b. Filament-associated halo coronal mass ejection, Chin. J. Astron. Astrophys., 1, 85–98, 2001; Zhang and Wang, Filament eruptions and halo coronal mass ejections, Astrophys. J. 554, 474–487, 2001]. On the other hand, the nature of flux cancellation has been a topic of persistent interest and debate. We review the observational properties of magnetic flux cancellation and the relevant theoretical studies, describe the vector magnetic field changes in flux cancellation in CME-associated active regions (ARs), and demonstrate that the well-observed flux cancellations fit nicely the scenario of magnetic reconnection in the lower solar atmosphere. It is suggested that magnetic reconnection in the lower solar atmosphere is a ubiquitous process on the Sun. It is a key element in the magnetic evolution of CMEs.