An Airborne Infrared Spectrometer for Coronal Observations: Development, Characterization, and First Science Results from the 2017 Solar Eclipse

Abstract

The solar magnetic field enables the heating of the corona and provides its underlying structure. Energy stored in coronal magnetic fields is released in flares and coronal mass ejections and ultimately drives space weather. Therefore, direct measurements of the coronal magnetic field have the potential to significantly enhance understanding of coronal dynamics and improve solar forecasting models. High-precision measurements are difficult to make due to the low field strengths that characterize most of the corona, but previous work suggests that emission lines in the shortwave and mid-infrared are a promising target for future magnetometers. Characterizing these magnetically sensitive emission lines is an important first step in developing the next generation of instrumentation. A new imaging spectrometer has just taken a step toward the direct observation of coronal magnetic fields by measuring infrared emission in the corona at high spectral resolution. On August 21, 2017, the Airborne Infrared Spectrometer (AIR-Spec) observed the total solar eclipse at an altitude of 14.3 km from aboard the NSF/NCAR Gulfstream V research aircraft. The instrument successfully observed the five coronal emission lines that it was designed to measure: Si X/1.43 μm, S XI/1.92 μm, Fe IX/2.84 μm, Mg VIII/3.03 μm, and Si IX/3.94 μm. This thesis describes the instrument design and development, the data processing and calibration, and the first science results from the 2017 eclipse observation. AIR-Spec is a slit spectrometer that measures light over a 1.55 solar radius field of view in four spectral passbands between 1.4 and 4 μm. The package includes an image stabilization system, feed telescope, grating spectrometer, and slit-jaw imager. The instrument development encountered a number of challenges, centered around maintaining adequate resolution and signal-to-noise ratio in a compact and inexpensive package on a moving platform. Meeting all of the engineering challenges resulted in a successful mission. During the eclipse observation, AIR-Spec measured the average linewidths, peak intensities, and center wavelengths of all five lines radially outward from the limb at four positions in the corona. The observation of Fe IX at 2.84 μm was the first of that line. The radial intensity gradient of Si X was measured with high sensitivity, providing information on the dominant excitation processes for that line. The relative Doppler velocity of Si X was measured with a resolution of 5 km/s, revealing variations across different coronal structures and an interesting case of bimodal velocities near the solar prominence. Several follow-on experiments are being proposed to expand on the results from the 2017 eclipse. These include a re-flight of AIR-Spec during the 2019 total eclipse, development of a spectropolarimeter operating at AIR-Spec wavelengths, and a laboratory study of infrared coronal emission lines.