Magnetic flux emergence in the Sun

Space weather research is closely connected with the study of the solar magnetic activity. In past years, many solar missions ( e. g., YOHKOH, SOHO, TRACE, and RHESSI) have provided outstanding observations, which have been used to improve our understanding of the structure and the dynamical evolution of solar magnetic fields. In addition, the newly launched solar missions ( e. g., Hinode and STEREO) will study the interaction between the emerging magnetic field and the preexisting field in the corona ( increasing our understanding of the causes of solar variability), and they will also observe the three-dimensional evolution of solar eruptions as they leave the Sun and move into the interplanetary space. One of the most important processes, responsible for many dynamical phenomena observed in the Sun, is the emergence of magnetic flux from the solar interior in active regions and the modification of the coronal magnetic field in response to the emergence. In fact, magnetic flux emergence might be responsible for the appearance of small-scale events ( e. g., compact flares, plasmoids, and active-region-associated X-ray brightenings) and large-scale events ( e. g., X-class flares and CMEs), which are major drivers of space weather. However, it is clear that the question of how exactly the magnetic fields rise through the convection zone of the Sun and emerge through the photosphere and chromosphere into the corona has still not been solved. It is believed that understanding the process of flux emergence is an important step toward the understanding of the initiation mechanism of eruptive events in the Sun, which is another topic of great debate. This paper provides a brief review of the theory and the numerical models, which have been used to study the process of magnetic flux emergence into the outer atmosphere of the Sun. We underline the similarities and differences between these models, and we compare the basic features of the numerical results with observations. Finally, we review the recent progress and discuss what further developments are required in the models to best describe the essential physics in the process of flux emergence.