Ultra-narrow Negative Flare Front Observed in Helium-10830~\AA\ using the 1.6 m New Solar Telescope

TL;DR

This paper reports the first observation of a narrow, dark 'negative' flare front in Helium-10830 Å using the 1.6 m New Solar Telescope, revealing new insights into solar flare plasma conditions.

Contribution

It presents the first high-resolution observation of a negative flare front in Helium-10830 Å, confirming theoretical predictions and enhancing understanding of flare energy release mechanisms.

Findings

Detected a 340 km wide dark moving front in He I 10830 Å

Associated spectral features observed in H-alpha and Mg II lines

Supports models involving nonthermal electron collisions or photoionization

Abstract

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.