Ultraviolet and X-ray Precursors of Solar Flares

We examined pre-flare brightenings in ultraviolet (UV) and X-ray emissions during flares on September 23, 2014, and January 11, 2013. We analyzed UV maps at nine wavelengths based on SDO/AIA data and RHESSI X-ray images over a one-hour interval before the onset of the flare. We obtained the temperature and emission measure dependencies of the quasi-thermal X-ray emission during the precursor periods. The plasma temperature in the precursor regions exceeded 1 MK. The emission in the extreme UV (EUV) lines was used to identify the filamentary structure. The SDO/HMI vector magnetic field data allows the reconstruction of the magnetic field pattern in the coronal part of the filaments using the NLFFF method. We found magnetic field structures containing flux ropes during the precursor phase. We discuss the role of magnetic flux ropes, their temporal evolution in the dynamics of the pre-flare stage, and the transition from the precursor to the explosive phase of the flares. We conclude that there is a causal relationship between the precursors of the flares on September 23, 2014, and January 11, 2013, and the events themselves.