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.

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.

Analytical description of a Fabry-Perot spectrometer. 8: Optimum operation with equidistant equal-noise sampling

The optimum parameters of operation for a Fabry-Perot spectrometer used in the equidistant equal-noise sampling method have been calculated. The results, expressed in terms of normalized halfwidths at half-height, are: etalon (a*) = 0.078 (0.095), linewidth (dg*) = 0.13 (0.19), and aperture (f*) = 0.10 (0.19) for temperature (wind) determinations. The etalon widths correspond to rather low reflectivities, namely, 0.62 and 0.56. The critical number of samples, required for unambiguous determinations of a measured profile, are found to be equal to 8 and 12. The usefulness of the equal-noise method in absorption measurements is discussed.

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.

The Sun's Influence on the Earth's Atmosphere and Interplanetary Space

The bulk of the sun's radiation is in the visible and infrared. Solar radiation at these wavelengths controls the weather in the lowest levels of the earth's atmosphere. The rate at which this energy is emitted (the so-called solar constant) varies by a few tenths of 1 percent over a time scale of days. Longer period variations may exist, but have yet to be detected. Far more variable are the amounts of energy emitted as ultraviolet, extreme ultraviolet, and x-rays, and in the continuous outflow of ionized solar particles. The latter controls the properties of the space between the earth and the sun as well as those of the earth's magnetosphere. The ultraviolet and particle emissions control the properties of the earth's upper atmosphere, including the global wind circulation and changes therein associated with intense auroral storms. While considerable progress has been made in exploring the solar-terrestrial system since the advent of space research, many problems remain. These include the question of how magnetic energy is converted into ionized particle energy in the sun and in the earth's magnetosphere, the way in which solar and terrestrial magnetic fields join or merge, and how large electric fields are generated and sustained a few thousand kilometers above the earth's poles. Perhaps the most intriguing question concerns the possible relation between solar variability and the earth's weather and climate.

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.

Design of a multiple-zone aperture and application to a Fabry-Perot interferometer

A design for a multiple-zone aperture (MZA), which is applied to a Fabry-Perot interferometer for gaining greater throughput, is described. It is shown that there exists a definite value for the width of the aperture that gives, the minimum error in Doppler temperature measurements when instrumental parameters and the width of the source profiles are given. The effects caused by the imperfections of the MZA are analyzed. The successful use of an MZA with a 15-cm (6-in.) Fabry-Perot interferometer for measurement of an airglow emission line is described.

Thermospheric dynamics investigations with very high resolution spectrometers

Since 1972 high resolution Fabry-Perot spectrometers have been used at Fritz Peak Observatory (39.9 degrees N, 105.5 degrees W), Colorado to measure the nighttime variation of thermospheric temperatures and winds from the line profiles and Doppler shifts of the OI 15,867 K (630.0-nm) line emission in the nightglow. With the aid of these measurements we have defined the nighttime variation of winds and temperatures at F-layer heights for the various seasons of the year during geomagnetic quiet periods. During geomagnetic storm periods deviation in the nighttime variation of the winds and temperatures from those determined during geomagnetic quiet conditions have been shown to occur. In addition, measurements made during geomagnetic disturbed conditions have shown the existence of large-scale thermospheric waves generated at high latitudes by impulsive auroral heating events that are observed to propagate equatorward. The nighttime winds and temperatures measured from Fritz Peak Observatory have been used in various heoretical models of global thermospheric dynamics to infer the global circulation patterns, temperature structure, and thermospheric response to geomagnetic activity. By requiring agreement between the calculated and measured winds and temperatures over Fritz Peak Observatory, the over-all magnitude of the thermospheric high latitude heat source due to auroral processes has been inferred for both geomagnetic quiet and disturbed conditions. This energy source has been shown to be related to dissipation of the ring current energy in the high latitude ionosphere. The results of various geophysical studies using Fritz Peak Observatory data and theoretical model calculation are summarized.

Piezoelectric scanning of Fabry-Perot spectrometers: nonlinearities

Piezoelectric materials used in mechanical scanning of Fabry-Perot spectrometers are found to have measurable nonlinearities. Dynamic scanning has been used in the present investigation in order to decrease the effects of some of the variables, such as creep, of the piezoelectric material in static operation. The results show mainly a decrease in the distance between corresponding maxima, or apparent free spectral range, as the driving voltage increases to higher values. A relationship between this voltage and displacement (in terms of orders) has been derived, and it is given by v(i) = In {1 - (x(i) - x(0))/[a/(1 - b) - x(0)]}/In b, where v(i) is the displacement in orders from an arbitrary starting point, x(i) is a quantity directly proportional to the voltage, while x(0) is also a quantity proportional to the voltage at the arbitrary reference point, a is the distance between the arbitrary reference point and the next order, and b is the nonlinearity constant for a given piezoelectric material. Examples are given for emission and absorption measurements where the usually large effects of the piezoelectric material nonlinearities are derived, as well as schemes to minimize or altogether remove the effects by suitable handling of the data or by changing the experiment such that the scanning is forced to be linear.