  Recent observations in extreme-ultraviolet (EUV) wavelengths reveal an EUV
late phase in some solar flares, which is characterized by a second peak in the
warm coronal emissions (about 3 MK) occurring several tens of minutes to a few
hours after the corresponding main flare peak. We aim 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. We first
characterize the plateau-like late phase using EUV Variability Experiment (EVE)
full-disk integrated irradiance observations and Atmospheric Imaging Assembly
(AIA) spatially-resolved imaging observations on board the Solar Dynamics
Observatory (SDO). Then we perform a nonlinear force-free-field (NLFFF)
extrapolation, 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 carry out differential emission measure
(DEM) analysis to explore the thermodynamics of the late-phase loops. The MFR
shows a non-radial eruption from a fan-spine magnetic structure. The eruption
of the MFR and its interaction with overlying arcades invoke multiple magnetic
reconnections, which are responsible for the production of different groups of
late-phase loops. Afterwards, 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 cooling rates, which makes their warm
coronal emission peaks temporally separated from each other. Combing the
emissions from all late-phase loops together, an elongated plateau-like late
phase is formed.
