Second-Generation Optimized Fabry-Perot Doppler Imager (SOFDI)

Program Directive
To design and build a portable, remote controlled instrument that can map the dayglow and nightglow thermospheric horizontal and vertical winds and temperatures.

Program Manager Charlie Richey charlesrichey@michiganaerospace.com (734) 975-8777 x112

Program Affiliates

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SOFDI Links Abstract Introduction Science Objectives Future Plans Calibration/Testing Process Forward Model Pictures of the Instrument

Abstract
Daytime measurements of winds and temperatures based on the 630 nm emission from thermospheric atomic oxygen (OI) have been difficult to obtain because of the large solar background continuum which overpowers the comparatively weak emission. Though measurements of this dayglow transition have been attempted by a number of different groups in the past, the scientific significance of these measurements has been somewhat limited in scope. However, the Second generation Optimized Fabry-Perot Doppler Imager (SOFDI), based on the design of the UARS HRDI instrument, makes use of a number of innovative optical techniques which will allow for day and night measurements of the 630 nm line, as well as the 557 nm transition of OI, the 732 nm transition of O+, and the 840 nm 6-2 P1(2) rotational line of OH, and others. These results are expected to yield a wealth of information for the aeronomy community, including the mapping of the dayglow and nightglow thermospheric horizontal and vertical winds, study of the 630 nm dayglow brightness distribution as applied to the Appleton anomaly formation, 732 nm observations of ion temperatures and ion drifts during morning or evening twilight, measurements of the variability of thermospheric atomic oxygen concentrations, the investigation of possible “hot oxygen” at low latitudes, the determination of OH rotational temperature and winds, observations of the full diurnal variation of thermospheric temperature and winds, etc. As such, as part of the SOFDI initiative, it is the purpose of this paper to present 1) an up-to-date summary of the construction of the SOFDI instrument and 2) an overview of the instrument forward model and associated inversion algorithms based on theoretical calculations. It is shown that the construction of the SOFDI instrument is preceding as scheduled, and will be ready for test runs in early July 2003 in upstate New York. In addition, we show that the data analysis infrastructure is currently being optimized based on results from the forward model. We expect that by late August/early September, the SOFDI instrument will be ready for scientific data collection.

Introduction
Advances in Fabry-Perot technology over the past decade with regard to etalon assembly design, transmission of fiber optics cables, optical conversion of the interferogram from circle to line combined with major improvements in detector sensitivity and low noise characteristics have made possible the development of a new generation of the triple etalon Fabry-Perot interferometer that can successfully observe the 630 nm dayglow to measure thermospheric winds and temperatures with excellent stability and sensitivity. The SOFDI (Second-generation Optimized Fabry-Perot Doppler Imager) instrument is modeled after the HRDI (High Resolution Doppler Imager) interferometer that flew on the Upper Atmosphere Research Satellite to observe upper atmosphere winds and temperatures in the daytime. Numerous refinements in the design that greatly enhances the potential scientific yields from the measurements make SOFDI truly a remarkable instrument of great versatility with a broad range of applications pertaining to the observations of the mesosphere and thermosphere airglow emissions for both day and night. These refinements include:

• Automatic transformation of the Fabry-Perot triple etalon optics configured for dayglow measurements into a single high resolution etalon configuration for nightglow measurements
• Separate fields of view for simultaneous measurements of the line-of sight Doppler shifts in
  four directions
• Increase in instrumental sensitivity for nighttime measurements by field-widening over 12 orders
  at 630 nm
• Excellent instrumental stability of less than 1 m/s drift over ten hours
• Doppler calibration against the laboratory cerium emission line at 630.0202 nm
• Multiple wavelengths covering OI emissions at 557.7 and 630 nm (mesospheric and thermospheric winds and temperatures) + OH emissions at 840nm and 846.5 nm (daytime and nighttime OH rotational temperatures) + 732.0 emissions of O+ (ion drifts and ion temperatures
• High spectral resolution observations of the dayglow continuum with 2.4 Angstrom spectral range (covers Fraunhofer region of 630 nm dayglow and Fraunhofer region adjacent so an independent measurement of the Ring Continuum can be made separate from the 630 nm dayglow emission)
• Observations automatic with Internet access for remote control of operations.