Extremely Energetic Extreme-ultraviolet Late Phase of a Pair of C-class Flares Caused by a Noneruptive Sigmoid

Abstract The extreme-ultraviolet (EUV) late phase (ELP) is the second increase of the irradiance of warm coronal lines during solar flares, and has a crucial impact on the Earth’s ionosphere. In this paper, we report on the extremely energetic ELP of a pair of C-class flares (SOL2012-06-17T17:26:11) observed on 2012 June 17 in NOAA active region 11504 by the Atmospheric Imaging Assembly (AIA) instrument on board the Solar Dynamics Observatory. The light curves integrated over the flaring region show that a factor of 4.2 more energy is released in the “warm-temperature” (2–3 × 10 6 K) passbands (e.g., AIA 335 Å) during the late phase than during the main peaks. The origin of the emission in this extremely energetic ELP is a noneruptive sigmoid situated in a multipolar magnetic field configuration, which is rapidly energized by C-class flares. The sigmoid plasma appears to reach temperatures in excess of 10 7 K, before cooling to produce the ELP emission. This is seen in high-temperature passbands (e.g., AIA 131 Å) and by using differential emission measure analysis. Magnetic extrapolations indicate that the sigmoid is consistent with formation by magnetic reconnection between previously existing J-shaped loops. The sigmoid experienced fast and slow cooling stages, both of which were dominated by conductive cooling. The estimated total cooling time of the sigmoid is shorter than the observed value. So, we proposed that the noneruptive sigmoid, heated by continuous magnetic reconnection, leads to the extremely energetic ELP.