Flux emergence
Flux emergence is a highly dynamic process in which the solar magnetic field emerges from a dense solar interior into a highly stratified solar atmosphere (hereafter, emerging magnetic field). Various active phenomena observed on the Sun depend on physical properties of the emerging magnetic field. The emerging field is suggested to have the shape of a thin flux tube before it emerges to a solar surface, and when emerging there, it starts to expand rapidly because the gas pressure of a plasma surrounding the flux tube decreases sharply across the surface.
animation version
References
Fan, Y. 2001, ApJ, 554, L111
Magara, T. 2001, ApJ, 549, 608
Magara, T. & Longcope, D. W. 2003, ApJ, 586, 630
Pariat, E. et al. 2004, ApJ, 614, 1099
Shibata, K., Tajima, T., & Matsumoto, R. 1990, Phys. Fluids B, 2, 1989