An Airborne Infrared Spectrometer (AIR-Spec) for Solar Eclipse Observations
On August 21, 2017, the Airborne Infrared Spectrometer (AIR-Spec) observed the total solar eclipse at an altitude of 14 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 (1.43µm), S XI (1.92µm), Fe IX (2.85µm), Mg VIII (3.03µm), and Si IX (3.94µm). The linewidths, peak intensities, and center wavelengths of all five lines were measured radially outward from the limb at four positions in the corona. Characterizing these magnetically sensitive emission lines is an important first step in designing future instruments to monitor the coronal magnetic field, which drives space weather events as well as coronal heating, structure, and dynamics.
The AIR-Spec instrument includes an image stabilization system, feed telescope, grating spectrometer, and slit-jaw imager. The image stabilization system uses a fast steering mirror to correct the line-of-sight for platform perturbations, which are measured by a set of fiber-optic gyroscopes. The telescope collects light over a 0.4 degree (1.5 solar radius) field of view and feeds it into the spectrometer, which is based on a planar diffraction grating operating near the Littrow condition. The five wavelengths of interest are divided into two channels and imaged onto the cryogen-cooled InSb detector. The spectrometer optics are housed in a vacuum chamber and cooled with liquid nitrogen.
The instrument development and mission planning faced three major challenges. First, the line-of-sight stabilization system was required to operate without knowledge of the image, so that it could be tested and proven before the eclipse. Second, the thermal instrument background had to be dramatically reduced to allow the spectrometer to detect the faint coronal signal. Third, the flight plan needed to maximize the time spent in totality while optimizing the orientation of the aircraft before and during totality. The resolution of these issues had implications for both science and operations.