Discovery of New Coronal Lines at 2.843 & 2.853 Microns

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Tuesday, February 20, 2018

Jenna Samra and Philip Judge identify two new emission features observed during the 21st August 2017 total solar eclipse by a novel spectrometer, AIR-Spec, flown at 14.3 km altitude aboard the NCAR Gulfstream V aircraft. We derive wavelengths in air of 2.8427 ± 0.00009 μm and 2.8529 ± 0.00008 μm. One of these lines belongs to a transition in Ar-like Fe IX. This appears to be the first detection of this transition from any source.

 Summary of AIR-Spec observations during totality image
Summary of AIR-Spec observations during totality. (a) The four AIR-Spec slit positions (white lines) superimposed on a composite of images from the slit-jaw camera. (b) Average AIR-Spec spectrum at each slit position (blue). Si X, Mg VIII and a coronal line at 2.843 μm are seen in all four slit positions, while a 2.853 μm line appears only in position 3 (east limb). H I appears in the prominence in position 2. The black dashed line indicates the theoretical wavelength of Fe IX, with a ±2σ error bar.

Minimization of residual wavelength differences using both measured wavelengths, together with NIST extreme ultraviolet wavelengths, does not clearly favor assignment to Fe IX. However, the shorter wavelength line appears more consistent with other observed features formed at similar temperatures to Fe IX. The transition occurs between two levels within the excited 3p53d configuration, 429,000 cm−1 above the ground level. The line is therefore absent in photo- ionized coronal-line astrophysical sources such as the Circinus Galaxy. Data from an FTS instrument deployed from Wyoming show that both lines are significantly attenuated by telluric H2O, even at dry sites. We have been unable to identify the longer wavelength transition.

Publication Name: ApJL