The goal of this project is to demonstrate the functionality of a ground-based, high-resolution, high-throughput portable spectrograph for dedicated aeronomic investigations. The Compact Echelle Spectrograph for Aeronomical Research (CESAR) was constructed by SRI International with prior support from NSF to provide the finest-resolution wide-bandwidth spectra. Through this project, CESAR will collect simultaneous data over a broad spectral range and enable aeronomic investigations about relationships between spectral features that may be separated by hundreds of nanometers. The system is designed to be operated remotely during all dark nights. Initially sited at Poker Flat, CESAR will be co-located with Poker Flat Incoherent Scatter Radar (PFISR)--an NSF-funded, SRI-operated, phased-array incoherent scatter radar.
CESAR is an echelle spectrograph that produces broad-band, high-resolution optical spectra from the ultraviolet to the infrared and is specifically designed for auroral studies. It has been deployed at the Poker Flat Research Range in Alaska since November 2013. CESAR is an outgrowth of low-latitude investigations made over the last decade and a half with sky spectra obtained at the giant astronomical telescopes (Keck I, Keck II, VLT). These spectra, which show the terrestrial nightglow in amazing detail, led to the formulation of a plan to enable a similar capability for high-latitude studies. This was the genesis of the CESAR instrument, which was funded by the NSF Major Research Instrumentation (MRI) program. SRI International was responsible for designing and building CESAR. Many spectral studies of aurorae have been conducted over the years, but CESAR brings the capacity to simultaneously carry out such studies over a very broad spectral region, under conditions of normal nightglow and superimposed auroral excitation. The instrument has only been in operation for 6 months, but has already demonstrated its capabilities by investigating the oxygen component of aurorae, which is a little-studied area because of interference from the characteristic strong neutral and charged nitrogen species. This "oxygen aurora" is potentially very useful, because the shape of the individual spectral bands is related to the temperature of the atmosphere where the aurora originates. In particular, CESAR has demonstrated that there is a single oxygen band, with emission close to 697 nm, which can be used as a temperature probe. In the short period that CESAR has been deployed, the level of auroral activity has been low. With more intense aurorae, the gathering of data will be enhanced. The O2 Atmospheric band at 865 nm has been used countless times by aeronomers for nightglow studies, where it probes the atmosphere near an altitude of 95 km. In aurora, however, it originates at a much higher altitude, and this is reflected in the spectrum of the band shown by CESAR, which exhibits a two-temperature appearance. One portion corresponds to the normal nightglow fraction at its temperature of ~190 K, while an auroral portion is much hotter, and has a different mode of production than the same entity in the nightglow. The training of students has been a very important part of the CESAR enterprise. Deepali Saran, who was awarded an NSF CEDAR postdoctoral fellowship, played a critical role in preparing CESAR for deployment. She worked on the instrumental alignment, local deployment, and spectral calibration. Stefan Mellem, an NSF REU student, contributed to the project by assisting in the camera alignment via computer calculations. Oleg Kostko, a postdoctoral student, had an important role in getting the instrument moved to Alaska and re-establishing the alignment that it had at the construction site at SRI. This is of course a critical point for an instrument that is designed to be relocatable.
