  Solar flares are sudden flashes of brightness on the Sun and are often
associated with coronal mass ejections and solar energetic particles which have
adverse effects in the near Earth environment. By definition, flares are
usually referred to bright features resulting from excess emission. Using the
newly commissioned 1.6~m New Solar Telescope at Big Bear Solar Observatory,
here we show a striking "negative" flare with a narrow, but unambiguous "dark"
moving front observed in He I 10830 \AA, which is as narrow as 340 km and is
associated with distinct spectral characteristics in H-alpha and Mg II lines.
Theoretically, such negative contrast in He I 10830 \AA\ can be produced under
special circumstances, by nonthermal-electron collisions, or photoionization
followed by recombination. Our discovery, made possible due to unprecedented
spatial resolution, confirms the presence of the required plasma conditions and
provides unique information in understanding the energy release and radiative
transfer in astronomical objects.
