  On the Sun, Doppler shifts of bidirectional outflows from the
magnetic-reconnection site have been found only in confined regions through
spectroscopic observations. Without spatially resolved spectroscopic
observations across an extended region, the distribution of reconnection and
its outflows in the solar atmosphere cannot be made clear. Magnetic
reconnection is thought to cause the splitting of filament structures, but
unambiguous evidence has been elusive. Here we report spectroscopic and imaging
analysis of a magnetic-reconnection event on the Sun, using high-resolution
data from the Interface Region Imaging Spectrograph and the Solar Dynamics
Observatory. Our findings reveal that the reconnection region extends to an
unprecedented length of no less than 14,000 km. The reconnection splits a
filament structure into two branches, and the upper branch erupts eventually.
Doppler shifts indicate clear bidirectional outflows of ~100 km/s, which
decelerate beyond the reconnection site. Differential-emission-measure analysis
reveals that in the reconnection region the temperature reaches over 10 MK and
the thermal energy is much larger than the kinetic energy. This Letter provides
definite spectroscopic evidence for the splitting of a solar filament by
magnetic reconnection in an extended region.
