Homologous Flares and Magnetic Field Topology in Active Region NOAA 10501 on 20 November 2003

We present and interpret observations of two morphologically homologous flares that occurred in active region (AR) NOAA 10501 on 20 November 2003. Both flares displayed four homologous H-alpha ribbons and were both accompanied by coronal mass ejections (CMEs). The central flare ribbons were located at the site of an emerging bipole in the center of the active region. The negative polarity of this bipole fragmented in two main pieces, one rotating around the positive polarity by ~ 110 deg within 32 hours. We model the coronal magnetic field and compute its topology, using as boundary condition the magnetogram closest in time to each flare. In particular, we calculate the location of quasiseparatrix layers (QSLs) in order to understand the connectivity between the flare ribbons. Though several polarities were present in AR 10501, the global magnetic field topology corresponds to a quadrupolar magnetic field distribution without magnetic null points. For both flares, the photospheric traces of QSLs are similar and match well the locations of the four H-alpha ribbons. This globally unchanged topology and the continuous shearing by the rotating bipole are two key factors responsible for the flare homology. However, our analyses also indicate that different magnetic connectivity domains of the quadrupolar configuration become unstable during each flare, so that magnetic reconnection proceeds differently in both events.Comment: 24 pages, 10 figures, Solar Physics (accepted

An Observational Overview of Solar Flares

We present an overview of solar flares and associated phenomena, drawing upon a wide range of observational data primarily from the RHESSI era. Following an introductory discussion and overview of the status of observational capabilities, the article is split into topical sections which deal with different areas of flare phenomena (footpoints and ribbons, coronal sources, relationship to coronal mass ejections) and their interconnections. We also discuss flare soft X-ray spectroscopy and the energetics of the process. The emphasis is to describe the observations from multiple points of view, while bearing in mind the models that link them to each other and to theory. The present theoretical and observational understanding of solar flares is far from complete, so we conclude with a brief discussion of models, and a list of missing but important observations.Comment: This is an article for a monograph on the physics of solar flares, inspired by RHESSI observations. The individual articles are to appear in Space Science Reviews (2011

The timing of non-thermal soft X-ray emission line broadenings in solar flares

We study 59 solar limb flares using the Bragg Crystal Spectrometer (BCS) on Yohkoh and the Burst and Transient Source Experiment (BATSE) to investigate the relative timings between the Hard X-Ray (HXR) emission and the observed non-thermal broadenings of X-ray emission lines (V\sb{\rm nt}). We show that the duration of the HXR flux rise to maximum emission affects the relative timing of the main V\sb{\rm nt} peak with respect to the main HXR peak. In ≈20% of the flares studied, secondary peaks in V\sb{\rm nt} are observed. These are always associated with a strong HXR pulse and usually occur after the associated HXR pulse. There are also flares that show a relationship between the decay times of V\sb{\rm nt} and HXR flux. These results are conducive to a causal relationship between the HXR flux and V\sb{\rm nt}. We divided the sample of flares into two classes, gradual rise and impulsive rise, depending on the shape of the HXR lightcurve up to maximum emission. We show that the V\sb{\rm nt} behaviour differs in the two classes. The implications are discussed with a view to understanding the mechanism of V\sb{\rm nt} generation