  Solar flares typically have an impulsive phase that followed by a gradual
phase as best seen in soft X-ray emissions. A recent discovery based on the EUV
Variability Experiment (EVE) observations onboard the Solar Dynamics
Observatory (SDO) reveals that some flares exhibit a second large peak
separated from the first main phase peak by tens of minutes to hours, which is
coined as the flare's EUV late phase. In this paper, we address the origin of
the EUV late phase by analyzing in detail two late phase flares, an M2.9 flare
on 2010 October 16 and an M1.4 flare on 2011 February 18, using multi-passband
imaging observations from the Atmospheric Imaing Assembly (AIA) onboard SDO. We
find that: (1) the late phase emission originates from a different magnetic
loop system, which is much larger and higher than the main phase loop system.
(2) The two loop systems have different thermal evolution. While the late phase
loop arcade reaches its peak brightness progressively at a later time spanning
for more than one hour from high to low temperatures, the main phase loop
arcade reaches its peak brightness at almost the same time (within several
minutes) in all temperatures. (3) Nevertheless, the two loop systems seem to be
connected magnetically, forming an asymmetric magnetic quadruple configuration.
(4) Further, the footpoint brightenings in UV wavelengths show a systematic
delay of about one minute from the main flare region to the remote footpoint of
the late phase arcade system. We argue that the EUV late phase is the result of
a long-lasting cooling process in the larger magnetic arcade system.
