What do flare loops and ribbons tell us about reconnection energy release?
The global energy release in a solar flare is comprised of a large number of "elementary bursts", represented by an arcade of flare loops formed and heated by reconnection during the impulsive phase. Decades of observations have shown that the total emission of these flare loops may persist for several hours after the impulsive phase. How much energy is used to heat these loops? Why do they stay bright for hours after reconnection? To answer these questions, we explore observation-constrained modeling approaches. Flare loops are anchored to chromospheric ribbons, which respond prominently and almost instantaneously to reconnection energy release. Therefore, we can infer heating rates in hundreds of flare loops from spatially resolved light curves of flare ribbons. With these rates, we model the evolution of individual loops using the EBTEL code (Klimchuk et al. 2008) and compute synthetic total flare emissions in the corona. This method has been applied to model flare observations on the AIA scale and quantify heating energies in AIA loops. Most recently, we modeled a flare observed by the Goode Solar Telescope (GST), and derived heating rates in 77,000 GST-scale threads. Our studies suggest that the heating rate in a flare loop (of size 0.6") or thread (0.1") consists of an initial impulsive pulse lasting about a minute followed by a prolonged tail heating for approximately 10 minutes. The prolonged tail heating maintains loop temperature and density, producing the persistent flare SXR and EUV emissions. Comparing the derived heating rates and loop properties across the AIA and GST scales provides interesting insight into the impact of the filling factor on flare modeling. Furthermore, we have developed an approach to reconstruct post-reconnection magnetic field, by fitting thousands of observed EUV flare loops to extrapolated magnetic field lines anchored in the flare ribbons. Our first experiment on a two-ribbon flare demonstrates that, as the flare progresses, newly formed loops anchored to newly brightened ribbons are described by field lines with decreasing twist parameter $\alpha$. This indicates that the post-reconnection magnetic field is not linear force-free. This study demonstrates, for the first time, the physical nature of the long observed strong-to-weak shear evolution in two-ribbon flares, and implies incomplete relaxation by fast reconnection. It also verifies a long time delay (20 - 30 min) between reconnection forming a flare loop and its appearance in EUV passbands, suggesting that the prolonged heating of post-reconnection flare loops is likely driven by continuous dissipation of current-carrying fields after fast reconnection.
Dr. Qiu is a Professor of Physics at Montana State University. She earned her PhD in astrophysics from Nanjing University in China and subsequently conducted her postdoctoral work as a research scientist at the Big Bear Solar Observatory and the New Jersey Institute of Technology. Her early research spans solar physics as well as studying Earth's global atmospheric properties through Earthshine measurements. Dr. Qiu joined the faculty at Montana State University in 2005. She teaches physics and astronomy classes, advises graduate student research, and has managed NSF's Research Experience for Undergraduate (REU) program since 2008. Her research primarily focuses on solar flares and coronal mass ejections (CMEs). She has developed innovative methods to derive the properties of magnetic reconnection and investigate its role in CME evolution and structure. In the recent decade, Dr. Qiu has been interested in developing observation-constrained modeling approaches to infer the kinematic and magnetic properties of magnetic flux ropes prior to eruption, to quantify the heating rates of flare loops, and to construct post-reconnection magnetic fields, with the aim to bridge observations and theoretical understanding.