Richard L. Rairden

Imaging spectroscopy for two-dimensional characterization of auroral emissions

A large throughput transmission spectrometer, with a grating on a prism as the diffraction element, has been developed to study altitude distributions of auroral emissions. The imaging spectrometer disperses spectrally in one dimension while spatial information is preserved in the orthogonal direction. The image is projected onto a CCD array detector. Image processing methods have been developed to calibrate for wavelength, uniform field, spectral sensitivity, curvature of field, and spatial mapping. Single images are processed to represent a measured signal brightness in a unit of Rayleighs/pixel, from which area integrations can be made for desired spatial-spectral resolution. System performance is ~1.5-nm resolution over a 450-nm bandwidth (420-870 nm). Two spectrometer systems of this design were operated simultaneously, one with additional optical instruments and an incoherent scatter radar at Sondrestrom, Greenland, and the other at Godhavn, Greenland, which lies 290 km to the northwest and nearly in the magnetic meridian of Sondrestrom. The developed system, calibration method, and examples of performance results are presented.

New imaging spectrometer for auroral research

A Loral 1024 X 1024 CCD array with 15-micron pixels has been incorporated as the focal plane detector in a new imaging spectrometer for auroral research. The large format low-noise CCD provides excellent dynamic range and signal to noise characteristics with image integration times on the order of 60 seconds using f/1.4 camera optics. Further signal enhancement is achieved through on-CCD pixel binning. In the nominal binned mode the instrument wavelength resolution varies from 15 to 30 angstrom across the 5000 to 8600 angstrom spectral range. Images are acquired and stored digitally on a Macintosh computer. This instrument was operated at a field site in Godhavn, Greenland during the past two winters (1993, 1994) to measure the altitude distribution of the various spectral emissions within auroral arcs. The height resolution on an auroral feature 300 km distant is approximately 1 km. Examples of these measurements are presented here in snapshot and summary image formats illustrating the wealth of quantitative information provided by this new imaging spectrometer.

Tropospheric Infrared Mapping Spectrometers (TIMS) to Provide Measurements with much Improved Vertical, Temporal and Spatial Resolution in the Lower Troposphere

The Earth Science Decadal Survey Report of the U.S. National Research Council (NRC) describes requirements for improved atmospheric measurements to gain crucial understanding for air quality, climate change, and weather [1]. Improved vertical and horizontal resolution, temporal resolution and coverage are required. Our NASA Earth Science Technology Office (ESTO) Instrument Incubator Program (IIP) project is responsive as it is focused on demonstrating a Tropospheric Infrared Mapping Spectrometers (TIMS) technology that would provide considerably improved vertical and horizontal resolution, temporal resolution and coverage for measurements of Carbon Monoxide CO. It uses the CO solar reflective band near 2.3 mum and thermal emissive near 4.65 mum. It would also facilitate improved measurements of CH4, and H2O partial columns (vertical information), including considerable improvement in the boundary layer. The technology readily extrapolates to spectral regions that provide for retrieval of other important species (e.g., 9.6 and 3.6 mum for ozone O3 and formaldehyde HCOH) [2,3]. We describe the TIMS hardware and demonstration measurements, and a concept for application by the NRC mission GEO-CAPE.

A high spectral resolution solid state infrared spectrometer for atmospheric air quality measurement

Spectrometers, in which a grating is coupled with a two dimensional detector array to provide high resolution spectra without the need for spectral scan mechanisms can be designed in compact, rugged, configurations, making them well suited for spaceborne spectral mapping applications. We are pursuing the use of this technology for spaceborne tropospheric air quality monitoring, targeting high spectral resolution solar reflective and thermal emission spectroscopy in the wavelength range 2 to 5 μm. In this region key tropospheric pollutant and greenhouse gases such as O3, CO, CO2, CH4, HCHO, and H2O, have strong spectral features. The relatively short wavelengths allow for the use of well-developed detector technology and passive cooling. With sufficient resolving power, sensitivity, and judicious combination of spectra, good information on tropospheric vertical distributions, including boundary layer data, can be obtained. This paper describes the performance characteristics of a laboratory prototype of such a spectrometer, focused on the measurement of CO spectra in the range 4.56 to 4.73 μm. The design uses a cooled grating and optical train, coupled with a cooled 1024 x 1024 pixel HgCdTe array. It achieves a spectral resolution of ~0.32 cm-1 and NESR of 5.8x10-9 w/cm2/sr/cm-1. Both laboratory absorption spectra and zenith-looking air emission spectra of CO are presented. The spectrometer is the pre-cursor to a combined 4.6/2.33 μm instrument being developed under NASA funding and designed to demonstrate the unique vertical information capability of such a combination for tropospheric CO measurement. We give a brief discussion of a spaceborne concept focused on this technique.

High-resolution solid air gapped etalon in the 9500-nm region: application for nadir remote sounding of tropospheric ozone

We present test data for a solid ZnSe air gapped etalon with free spectral range 3 cm-1 and finesse >70 (i.e., spectral resolution <0.043 cm-1). We present an instrument concept, the Tropopsheric Ozone Sounding (TOS) Dual Etalon Cross Tilt Order Sorting Spectrometer (DECTOSS), that would use an etalon like this to acquire nadir data at resolution <0.06 cm-1 and signal to noise the order 1000 on a range from 1036 to 1071 cm-1 in footprints with crosstrack dimension selectable (e.g., the order tens to hundreds of km), and with along track dimension the order 17 km. Instrument accommodation is the order 25 kg, 110 W and 1 mbps. We present linear error analysis for retrieval of tropospheric ozone from the data acquired by the TOS-DECTOSS. Indication is that more than 2.5 vertical layers of information on tropospheric information are retrievable. An example of the deployment of the TOS-DECTOSS would be as an instrument of opportunity (IOO) add on to the US National Polar-orbiting Operational Environmental Satellite System (NPOESS). The huge advantage of the TOS-DECTOSS as compared with UV techniques for tropospheric ozone measurement is that it the can be used both day and night, the latter is not possible in the UV. The considerable advantage in signal to noise compared with a Fourier Transform Spectrometer (FTS) for tropospheric ozone measurement, on considering that for a given footprint the DECTOSS and FTS integration times are comparable, is that the DECTOSS noise per spectral sample is dominated by statistical fluctuations of signal photons that are passed through its narrow 0.06 cm-1 bandpass, while for a similar FTS spectral sample the noise is due to fluctuations of the signal photons through the FTS bandpass of tens of cm-1. The TOS-DECTOSS signal to noise advantage on the FTS is also enhanced in that the spectral sample density of the TOS-DECTOSS data is more than one hundred times larger than for the FTS.

Sublimb CO2 4200 nm measurements of small-scale internal gravity wave (GW) sources and their propagation and effects on the OH airglow

The Waves middle class Explorer mission (WE) is proposed to observe and quantify the effects of small-scale internal Gravity Waves (GW) in the Earths atmosphere from source regions in the troposphere and lower stratosphere to the mesosphere, lower thermosphere, and ionosphere (MLTI) where the GW have their most dramatic effects. These are now understood to be a key element in defining large-scale circulation, thermal and constituent structures, and variability of the stratosphere and MLTI. The WE instrumentation consists of 5 nadir and limb viewing sensors of the wave perturbed emission structure due to GW throughout the source and affected regions. The WE PI is Prof. G.R. Swenson. This paper addresses the measurement strategy and implementation for two of these instruments, the Source Wave And Propagation Imager (SWAPI), and the Hydroxyl Airglow Wave Imager (HAWI). The SWAPI uses multi-spectral sublimb imaging measurements in the CO2 (nu) 3 band near 4210 nm to identify GW sources, and their propagation through the stratosphere. Its measurement strategy is driven by data, particularly sublimb images in the CO2 (nu) 3 band that were obtained by instrumentation deployed on the Midcourse Space Experiment (MSX) satellite, and by the WE team members data analysis and models. Similarly team members ground based observational experience and data analysis drives the HAWI measurement strategy.

Performance of short wave cutoff MBE HgCdTe 2D arrays: spaceborne application for sensing OH airglow wave structure

Passive radiative cooling is desirable for space borne detectors because it is generally cheaper, less massive and power consumptive than cooling by a mechanical refrigerator or expendable cryogens. Our interest is space borne nadir imaging the OH airglow in Q-branch features of the 9->6 band at approximately 1382.3 nm, and the 2->0 band at approximately 1434.4 nm with sufficient signal to noise to quantitatively retrieve wave structure. Low noise 256 X 256- 40 micrometer pitch HgCdTe detector arrays are available for our application. E.g., the Rockwell Science Center standard 2.5 micrometers PACE product bonded on to the PICNIC read out MUL satisfies our high sensitive and low read noise requirements, but would require a mechanical refrigerator or expendable cryogen to cool sufficiently to satisfy our dark current requirement. To demonstrate an option that would provide our required performance at viable passive radiative cooling temperature, we have procured examples of the more recent RSC double layer planar heterogenous HgCdTe 2D arrays with shorter wavelength cutoff and produced by molecular beam epitaxy on a CdZnTe substrate, and bonded to the PICNIC MUL. Here we describe our test procedures and results that these at relatively warm temperature, the order 160 to 170K, satisfy the requirements for our OH airglow wave imaging application. We describe an instrument model and observational operations to observe the OH airglow wave structure with signal to noise > 100.

Test for mechanical-cooler-induced noise in a low-noise infrared 2D detector array: spaceborne application for sensing wave structure in thermal CO2 emission

For IR detectors that require cooling to temperatures lower than viable by passive radiative cooling, the mechanical refrigerator is an attractive alternative to expendable cryogen. It provides dramatic reduction in mass, and increased lifetime. For very low noise detectors, there may be some concern that mechanical cooler operation could provide an additional significant detector noise source. Here at LMAATC we have developed a mini-cooler for space borne application, a Stirling compressor driving a pulsetube, and have conducted test to determine if it would induce significant additional noise no cooling a low noise Mie HgCdTe 2D detector array with 3800 nm cutoff. We set up to cool the detector with our mini-cooler, and measure the noise with the cooler running, and with it turned off. We found that cooler operation increased noise barely perceptibly over the cooler off case. We will present implications for our planned space borne instrument, the Source Wave and Propagation Imager. It is an imaging spectrometer that will obtain measurements just below the limb in the 4180 to 4250 nm region of the CO2 band. Tropospheric production of atmospheric internal gravity waves, and their subsequent propagation through stratospheric will be retrieved from these data.