1I: Successful Spectral measurements in the mid-IR during the Solar Eclipse of 2017

“As part of the larger ground and airborne project we plan to measure, for the first time, the infrared spectrum of the solar corona from 2 to 12 μ. No such IR spectral survey of the corona has ever been performed, yet some of the most magnetically sensitive spectral lines are theoretically predicted in this region. All of the infrared coronal lines are forbidden lines of magnetic dipole character. We do not know how infrared emission line intensities are distributed in the corona, nor do we know wavelengths accurately enough to determine detrimental effects of telluric absorption on these particular lines.  The project is led by NCAR/HAO, the G-V is operated by NCAR/EOL, the primary airborne instrument is from SAO (Smithsonian Astronomical Observatory) and the ground-based IR survey is by NCAR/ACOM.”

The ACOM component of the SolarEclipse2017 mission was the deployment of the NCAR Airborne Interferometer (NAI) to Casper Mountain, WY for the mid-IR spectral observations.  The instrument had to be adapted from the designed task of airborne high resolution measurements of telluric atmospheric trace gases to the broadband coronal survey desired here.  Support and effort by both HAO and ACOM lead to the completion of the re-tasked spectrometer.  The fundamental optical design of the instrument allowed us to reposition the center-burst in the optical path and take an 10cm symmetric scan in 2.5 seconds.  This relieved our dependence on phase errors and corrections which are problematic in the emission spectra we expect during totality.  The solar disk is about 0.5º in diameter viewed from the earth.  To include > 90% of the coronal flux the instrumental field of view (FOV) was increased to 1.5º with an enlarged aperture of 11.6mm diameter.

Tracking the sun as it traverses the sky if usually performed with some dynamical feedback using the bright solar disk.  Not being an option during the eclipse we adopted an ephemeris only driven tracking system with direct injection of the parallel solar beam into the instrument. This has an advantage of less photon loss with fewer reflections and no vignetting of the beam but limited the duration of stability of the static tracker.  We were able to make continuous scans confidently locked on the eclipsed solar disk for 210 seconds. Ten scans were taken before totality, 60 during and the balance after.

Raw data from the totality scans are still being processed.  These spectra show some features but as calculated prior to the mission the coronal emission features in which we are interested are at the detection limit for this re-tasked instrument.  Below are first spectra of the prominence that is the last 20 seconds before totality.

Figure 1. The figure shows a broadband spectra from 2000-10000 (5-1µm)
Figure 1. The figure shows a broadband spectra from 2000-10000 (5-1µm) showing the terrestrial atmosphere absorption from gases such as water, CO2 and many trace species but also emission lines from the very limb of the photosphere.  These are mostly hydrogen and helium.


Figure 2. A zoom into the hydrogen emission line
Figure 2. A zoom into the hydrogen emission line at 2467cm-1 over a band of N2O.

Figure 3. Shows the same NAI spectrum at 6990cm-1
Figure 3. Shows the same NAI spectrum at 6990cm-1 and a calculated emission of Si X and is an example of the need for the IR survey instrument.

Next steps are continued analysis of totality spectra.  With the limits of the adapted instrument largely known but the utility shown with these initial spectra current plans are to design and construct a smaller HIAPER–worthy instrument that is evacuated and cooled providing a much lower noise threshold targeting coronal emission in future eclipses and at the DKIST observatory.