Jie Zhang
Department of Physics and Astronomy, George Mason University
On the Initiation Processes of Solar Flares and Coronal Mass Ejections
Flares and coronal mass ejections (CMEs) are both energetic phenomena originated in the Sun’s corona. Flares are manifested as a sudden, in a time scale of minutes, generation of electromagnetic radiations in all wavelengths, while equally energetic CMEs are shown as a large-scale eruption of an organized magnetic structure into the interplanetary space. It is well recognized that they are the drivers of space weather that may have adverse effects on advanced technological systems in the space and on the ground. In this talk, I will first reflect on the debate in the 1990s on “solar flare myth”, which was about what is the true cause of space weather, i.e., whether flares or CMEs. A comprehensive overview on our improved understanding about the relations between flares and CMEs will be presented, thanks to the advancement in observations made by a series of modern space observatories, including SOHO, SDO and STEREO. Very often, it is found that the energetic process of a flare (e.g., increase of X-ray flux) and a CME (e.g., increase of CME velocity) is synchronized in time; thus, they can be collectively called a solar eruption. On the other hand, they can also occur independently, namely compact/confined flares and “stealthy” CMEs, respectively. I will further discuss how such diversity of observed phenomena poses challenges to theoretical and numerical models of solar eruptions, which are largely divided into two camps depending on whether the eruption is triggered by magnetic reconnection or instability of magnetic flux ropes.