  Triggering mechanisms of solar eruptions have long been a challenge. A few
previous case studies have indicated that preceding gentle filament merging via
magnetic reconnection may launch following intense eruption, according with the
tether-cutting (TC) model. However, detailed process of TC reconnection between
filaments has not been exhibited yet. In this work, we report the high
resolution observations from the Interface Region Imaging Spectrometer (IRIS)
of TC reconnection between two sheared filaments in NOAA active region 12146.
The TC reconnection commenced since 15:35 UT on 2014 August 29 and triggered an
eruptive GOES C4.3-class flare 8 minutes later. An associated coronal mass
ejection appeared in the field of view of SOHO/LASCO C2 about 40 minutes later.
Thanks to the high spatial resolution of IRIS data, bright plasma outflows
generated by the TC reconnection are clearly observed, which moved along the
subarcsecond fine-scale flux tube structures in the erupting filament. Based on
the imaging and spectral observations, the mean plane-of-sky and line-of-sight
velocities of the TC reconnection outflows are separately measured to be 79 and
86 km/s, which derives an average real speed of 120 km/s. In addition, it is
found that spectral features, such as peak intensities, Doppler shifts, and
line widths in the TC reconnection region evidently enhanced compared with
those in the nearby region just before the flare.
