Initiation of solar flares: reconnection, heating and particle acceleration

Solar flares and coronal mass ejections (CMEs) are the most energetic phenomena in our solar system. The high radiation, fast particles and magnetic field brought by flare-CME events are the major sources of disturbances of our space weather at Earth. A fundamental process in magnetized plasmas, magnetic reconnection, is believed to drive these energetic events. However, how magnetic reconnection is initiated in flare-CME events, how and in which form the energy is originally released and distributed is still not sufficiently understood. The pre-flare and early rise phases contain important information on the reconnection onset and the process initiating the energy release. With the launch of NASAs Solar Dynamics Observatory (SDO) in the year 2010, a powerful suite of instruments is continuously observing the Suns magnetic field and its highly dynamic atmosphere with unprecedented spatial and temporal resolution over a broad temperature range from some 103 to 107 K. At the same time, NASAs Ramaty High Energy Solar Spectroscopic Imager (RHESSI) continues to provide high-quality X-ray imaging and spectroscopy to diagnose particle acceleration and plasma heating in solar flares. Based on these unique data sources and on innovative methods to discriminate the thermal and nonthermal flare radiation, the proposed project addresses the following scientific questions, which are important for a better understanding of flare-CME events: (1) In which form is the flare energy originally released (plasma heating vs. particle acceleration)? (2) How and where does magnetic reconnection set in and how does it evolve? (3) How is the flare energy release related to the initiation and evolution of an associated CME eruption?

Flares and coronal mass ejections (CMEs) from the Sun are the most energetic processes in our solar system. The high radiation, fast particles and magnetic field carried by these eruptions are the major sources of severe disturbances of our space weather at Earth. Magnetic reconnection is a fundamental process in magnetized plasmas, which drives these energetic events. The overall goal of the research project was to perform systematic studies of the pre- and early phase of solar flares, in order to better understand how magnetic reconnection is initiated in flares, how the released energy is converted into different forms, and how these processes are related to CME eruptions. Our main results are the following: - We provide the first comprehensive and comparative description of the properties of flares with/without associated CME, with all the main factors involved: the properties of the source active region, the flare location within the active region, and the evolution and properties of the flare ribbons. - We derived an estimate of the largest flare that can be produced by the present days Sun, which is of the order of GOES X100, i.e. about a factor of 3 bigger than then largest flares observed in the satellite era but on the lower limit of superflares recently detected on solar-like stars. - For the first time, the micro-processes that build up CME eruptions have been revealed in observations, thus bridging the gap between the CME microscale and macroscale dynamics. We demonstrate how a CME evolves from the detachment and merging of multiple plasmoids, i.e. mini flux ropes that are barely resolved. - Our studies challenge the traditional interpretation of the CME three-part morphology where the core corresponds to the erupting prominence. We demonstrate that in some strong events the core is a manifestation of the magnetic flux and frozen-in plasma added to the rising flux rope by flare-related magnetic reconnection and serving for its acceleration. - We derived a method to better separate the thermal and non-thermal emission in solar flares, which is important for the characterization of particle acceleration in solar flares and the atmospheric response to the impulsive energy input.