The Origins of Explosive Solar Activity*

The Sun’s atmosphere, the corona, is characterized by ubiquitous bursts of energy release ranging from giant coronal mass ejections (CMEs) and flares to the orders of magnitude smaller coronal jets and bright points that are responsible for much of the mass and energy of the solar wind. All these forms of solar activity share the common underlying origin that, as a result of stressing by the subsurface convective flows, magnetic free energy first builds up in the corona and then is released impulsively to the plasma in the form of heating, mass motions, and/or particle acceleration. We present recent high-resolution observations by the Solar Dynamics Observatory (SDO), the Interface Region Imaging Spectrograph (IRIS), and the Hinode mission, showing that the energy buildup process appears to be similar for solar activity ranging across orders of magnitude in scale and energy. Furthermore, the observations demonstrate conclusively that magnetic reconnection is the energy release process. We also present the latest MHD numerical simulations of solar activity that, for the first time, include self-consistently both the energy buildup and explosive release. The models show that current sheet formation leading to reconnection and significant energy release occurs almost continuously in the corona, but explosive energy release occurs only when there is strong feedback between the reconnection and the global ideal evolution. Capturing this multiscale feedback accurately remains as the greatest challenge to understanding and eventually predicting solar activity. The observations and models demonstrate that the corona is an amazing example of a magneto-plasma system that constantly self-organizes into an eruptive state.