  This study presents a C3.0 flare observed by the BBSO/GST and IRIS, on
2018-May-28 around 17:10 UT. The Near Infrared Imaging Spectropolarimeter
(NIRIS) of GST was set to spectral imaging mode to scan five spectral positions
at $\pm$ 0.8 \AA, $\pm$ 0.4 \AA and line center of He I 10830. At the flare
ribbon's leading edge the line is observed to undergo enhanced absorption,
while the rest of the ribbon is observed to be in emission. When in emission,
the contrast compared to the pre-flare ranges from about $30~\%$ to nearly
$100~\%$ at different spectral positions. Two types of spectra, "convex" shape
with higher intensity at line core and "concave" shape with higher emission in
the line wings, are found at the trailing and peak flaring areas, respectively.
On the ribbon front, negative contrasts, or enhanced absorption, of about $\sim
10\% - 20\%$ appear in all five wavelengths. This observation strongly suggests
that the negative flares observed in He I 10830 with mono-filtergram previously
were not caused by pure Doppler shifts of this spectral line. Instead, the
enhanced absorption appears to be a consequence of flare energy injection,
namely non-thermal collisional ionization of helium caused by the precipitation
of high energy electrons, as found in our recent numerical modeling results. In
addition, though not strictly simultaneous, observations of Mg II from the IRIS
spacecraft, show an obvious central reversal pattern at the locations where
enhanced absorption of He I 10830 is seen, which is in consistent with previous
observations.
