The MIGHTI Wind Retrieval Algorithm: Description and Verification

We present an algorithm to retrieve thermospheric wind profiles from measurements by the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) instrument on NASA's Ionospheric Connection Explorer (ICON) mission. MIGHTI measures interferometric limb images of the green and red atomic oxygen emissions at 557.7 nm and 630.0 nm, spanning 90-300 km. The Doppler shift of these emissions represents a remote measurement of the wind at the tangent point of the line of sight. Here we describe the algorithm which uses these images to retrieve altitude profiles of the line-of-sight wind. By combining the measurements from two MIGHTI sensors with perpendicular lines of sight, both components of the vector horizontal wind are retrieved. A comprehensive truth model simulation that is based on TIME-GCM winds and various airglow models is used to determine the accuracy and precision of the MIGHTI data product. Accuracy is limited primarily by spherical asymmetry of the atmosphere over the spatial scale of the limb observation, a fundamental limitation of space-based wind measurements. For 80% of the retrieved wind samples, the accuracy is found to be better than 5.8 m/s (green) and 3.5 m/s (red). As expected, significant errors are found near the day/night boundary and occasionally near the equatorial ionization anomaly, due to significant variations of wind and emission rate along the line of sight. The precision calculation includes pointing uncertainty and shot, read, and dark noise. For average solar minimum conditions, the expected precision meets requirements, ranging from 1.2 to 4.7 m/s.

Spectral airglow temperature imager (SATI): a ground-based instrument for the monitoring of mesosphere temperature

The spectral airglow temperature imager is a two-channel, Fabry-Perot spectrometer with an annular field of view and a cooled CCD detector. The detected fringe pattern contains spectral information in the radial direction and azimuthal spatial information from the annular field of view. The instrument measures the rotational temperature from the O2 atmospheric (0,1) nightglow emission layer at 94 km and from the Q branch of the OH Meinel (6,2) band emission layer at 87 km. The method for temperature derivation is based on the temperature dependence of the line-emission rates. This dependence allows a determination of the temperature by a least-squares fit of the measured spectrum to a set of synthetic spectra, an approach that minimizes the effect of noise from the sky background and the detector. The spectral airglow temperature imager was developed to meet a need for monitoring the role of the mesosphere in climate variability through long-term observation of the mean temperature and the gravity waves from a single station, as well as large-scale wave perturbations through the use of multiple stations.

Inversion of Fabry-Perot CCD images: use in Doppler shift measurements

We consider how to use a charge-coupled device (CCD) as an imaging device for a Fabry-Perot interferometer. When the CCD is used in a conventional manner (pixel by pixel readout), the read noise limits its usefulness. Instead, we propose using the CCD in an operational mode so that the 2-D circular symmetric Fabry-Perot interference pattern is read as a 1-D array. The recovery of the signal is effected by carrying out an Abel inversion of the 1-D array. We show how by carrying out this operation it is possible to reduce greatly the effect of read noise. Finally, we consider how this image technique reflects on the accuracy of a Doppler shift measurement.

Image plane detector for the Dynamics Explorer Fabry-Perot interferometer

A compact twelve-channel photon-counting device based on existing Generation II imaging technology has been developed for use as the image-plane detector of the Dynamics Explorer Fabry-Perot interferometer. The device has an S-20 photocathode, three-microchannel plate electron multiplication stages, and an equal-area concentric-ring segmented anode whose geometry mimics that of the interference ring pattern produced by a plane etalon. The twelve channels sample equal and contiguous intervals in the spectrum. The purpose of the development has been to utilize the signal multiplex advantage of a multichannel detector in the measurement of Doppler shifts and line-broadening effects for naturally occurring atmospheric emission features of low intensity. The design, testing, calibration, and flight performance of the novel detector system are presented. In addition, measured emission line profiles at high resolution from the satellite instrument are presented to illustrate the operation of the device.

Stable and rugged etalon for the Dynamics Explorer Fabry-Perot interferometer. 1: Design and construction

This is one of two papers which describe the development and performance of a very stable and rugged etalon designed for use in the Fabry-Perot interferometer, one of the instruments of the NASA Dynamics Explorer satellite mission, and which will obtain global measurements of the thermospheric and mesospheric wind and temperature with an accuracy of aporoximately 10 m/sec. The etalon consists of two flat plates of fused silica, with spacers constructed of Zerodur (a polycrystalline glass ceramic of extremely low expansion coefficient) which are cemented together using cyanoacrylic adhesives. This provides adequate mechanical integrity and stability for any space flight application and has a thermal expansion coefficient of the etalon cavity of <10(-7)/ degrees C.

High-resolution optical measurements of atmospheric winds from space. 1: Lower atmosphere molecular absorption

The brightness spectrum of light scattered by the earth's atmosphere in the vicinity of molecular rotational lines is investigated. These absorption features are shown to be Doppler shifted by horizontal atmospheric winds. It is demonstrated that a modest aperture multiple etalon Febry-Perot interferometer can be used to monitor the global wind system from space by observing these Doppler shifts.