Ultra-narrow Negative Flare Front Observed in Helium-10830~\AA\ using   the 1.6 m New Solar Telescope
  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. 
Yan Xu, Wenda Cao, Mingde Ding, Lucia Kleint, Jiangtao Su, Chang Liu,   Haisheng Ji, Jongchul Chae, Ju Jing, Kyuhyoun Cho, K...
astro-ph.SR