  Using the multi-wavelength data from Atmospheric Imaging Assembly on board
the Solar Dynamic Observatory, we investigated two successive solar flares, a
C5.1 confined flare and an X4.9 ejective flare with a halo coronal mass
ejection,in NOAA AR 11990 from 2014 Feb 24 to 25. Before the confined are
onset, EUV brightening beneath the filament was detected. As the are began, a
twisted helical flux rope (FR) wrapping around the filament moved upward and
then stopped, and in the meantime an obvious X-ray source below it was
observed. Prior to the ejective X4.9 flare, some pre-existing loop structures
in the active region interacted with each other, which produced a brightening
region beneath the filament. Meanwhile, a small flaring loop appeared below the
interaction region and some new helical lines connecting the far ends of the
loop structures was gradually formed and continually added into the former
twisted FR. Then, due to the resulting imbalance between the magnetic pressure
and tension, the new FR together with the filament erupted outward. Our
observations coincide well with tether-cutting model, suggesting that the two
flares probably have the same triggering mechanism, i.e., tether-cutting
reconnection. To our knowledge, this is the first direct observation of
tether-cutting reconnection occurring between the pre-existing loops in active
region. In the ejective flare case, the erupting filament exhibited an
omega-like kinked structure and underwent an exponential rise after a slow-rise
phase, indicating the kink instability might be also responsible for the
eruption initiation.
