An atypical plateau-like extreme-ultraviolet late-phase solar flare driven by the nonradial eruption of a magnetic flux rope

Abstract

Context. Recent observations in extreme-ultraviolet (EUV) wavelengths reveal an EUV late phase in some solar flares that is characterized by a second peak in the warm coronal emissions (∼3 MK) occurring several tens of minutes to a few hours after the corresponding main flare peak. Aims. Our aim is to clarify the physical origin of an atypical plateau-like EUV late phase in an X1.8-class solar flare occurring on 2011 September 7 from active region (AR) 11283. Methods. We mainly took advantage of observations with the three instruments on board the Solar Dynamics Observatory (SDO). We first characterized the plateau-like late phase using EUV Variability Experiment (EVE) full-disk integrated irradiance observations and Atmospheric Imaging Assembly (AIA) spatially resolved imaging observations. Then we performed a nonlinear force-free-field (NLFFF) extrapolation of the AR magnetic fields based on the photospheric vector magnetogram with the Helioseismic and Magnetic Imager (HMI), from which a filament-hosting magnetic flux rope (MFR) is revealed. The eruption of the MFR is tracked both in the plane of the sky (POS) and along the line of sight (LOS) through visual inspection and spectral fitting, respectively. Finally, we carried out differential emission measure (DEM) analysis to explore the thermodynamics of the late-phase loops. Results. The MFR shows a nonradial eruption from a fan-spine magnetic structure. The eruption of the MFR and its interaction with overlying arcades invoke multiple magnetic reconnections that are responsible for the production of different groups of late-phase loops. Afterward, the late-phase loops enter a long-lasting cooling stage, appearing sequentially in AIA passbands of decreasing response temperatures. Due to their different lengths, the different groups of late-phase loops cool down at different rates, which makes their warm coronal emission peaks temporally separated from each other. Combining the emissions from all late-phase loops together, an elongated plateau-like late phase is formed.