Marsha R. Torr

Chemistry of the thermosphere and ionosphere

The ionosphere offers an excellent laboratory for the study of atomic and molecular processes. Densities are low, permitting highly reactive species to accumulate in measurable quantities. Temporal and spatial scales are large, and the solar energy source causes substantial departures from thermodynamic equilibrium. This laboratory has been exploited by the Atmosphere Explorer Program of NASA. Simultaneous measurements of a large number of interrelated atmospheric parameters to altitudes as low as 150 km have provided data that can be analyzed in a quantitative manner to derive precise rates, as functions of temperature, for many important chemical reactions, including the reactions of electronically or vibrationally excited metastable species. Analysis of AE results has provided new information on the photochemical role of N in the thermosphere; the rate coefficients for dissociative recombination of NO+, O2+ and N2+; reactions of O+ with N2, N2+ with O, and O2+ with O; and reactions of metastable ions, atoms, and molecules. The progress in our understanding of ionospheric chemistry during the last few years shows the power of space science measurement programs carefully designed to provide rigorous tests of quantitative theoretical predictions.

Use of FUV auroral emissions as diagnostic indicators

In an earlier study we modeled selected FUV auroral emissions (O I (1356 A), N2 Lyman-Birge-Hopfield (LBH) (1464 A), and LBH (1838 A)) to examine the sensitivity of these emissions and their ratios to likely changes in the neutral atmosphere. In this paper we extend that study to examine the dependence of these same emissions and their ratios on the shape of the energy distribution of the auroral electrons. In particular, we wish to determine whether changes in energy spectra might interfere with our determination of the characteristic energy. Modeled column-integrated emissions show relatively small (<30%) dependences on the shape and width of the incident energy spectrum, provided the average energy and total energy flux of the energy distribution are held constant. Long-wavelength FUV emissions, which are relatively unaffected by O2 absorption losses, exhibit virtually no dependence on the shape of the incident energy distribution. Changes in ratios of FUV short- to long-wavelength emissions as a function of characteristic energy are much larger than those due to changes in energy distribution. As a result, the determination of characteristic energy using these emission ratios is relatively unambiguous. We also examine the relative intensities of the aurora and the dayglow for various conditions. The intensities of modeled FUV auroral emissions relative to the dayglow emissions are presented as a function of solar zenith angle and incident energy flux. Under certain conditions (energy flux ≤ 1 erg cm−2 s−1 and solar zenith angle ≤50°) the dayglow will be the limiting factor in the detection of weak auroras.

Vacuum ultraviolet thin films. 1: Optical constants of BaF 2 , CaF 2 , LaF 3 , MgF 2 , Al 2 O 3 , HfO 2 , and SiO 2 thin films

The optical constants of MgF(2) (bulk) and BaF(2), CaF(2), LaF(3), MgF(2), Al(2)O(3), HfO(2), and SiO(2) films deposited on MgF(2) substrates are determined from photometric measurements through an iteration process of matching calculated and measured values of the reflectance and transmittance in the 120-230-nm vacuum ultraviolet wavelength region. The potential use of the listed fluorides and oxides as vacuum ultraviolet coating materials is discussed in part 2 of this paper.

Intercalibration of airglow observatories with the atmosphere explorer satellite

Abstract The visible airglow photometer on the Atmosphere Explorer C Satellite has been used to compare the calibrations of a number of ground-based airglow observatories. Discrepancies between different ground stations as large as a factor of six have been revealed. Efforts to account for these discrepancies have resulted in the discovery of differences as large as a factor of 2 in the standard light sources in use at different observatories. The participation of additional observatories in the intercomparison of standard sources is solicited. The project has also led to the discovery of a source of error that can amount to another factor of 2 in the procedure used to calibrate many airglow instruments. In the course of the project detailed maps, based on satellite data, have been made of the galactic and zodiacal light background at a number of wavelengths, and a substantial source of contaminating emission has been discovered in the satellite data; the contamination appears to result from interaction of the spacecraft and the atmosphere at altitudes below 170 km.

The calculated and observed ionospheric properties during Atmospheric Explorer-C satellite crossings over Millstone Hill

The Atmospheric Explorer-C (AE-C) satellite passed almost directly over the Millstone Hill incoherent scatter radar station on 14 February 1974 and passed within the near vicinity of the station on 15 February 1974. Measurements of ionospheric and atmospheric properties were made simultaneously by the incoherent scatter radar and the AE-C satellite instruments. The incoherent scatter radar measured vertical profiles of the electron and ion temperatures and electron density and these data were used to derive a neutral gas temperature profile. The AE-C satellite measured the electron and ion densities and electron and ion temperatures, neutral gas composition, solar EUV flux, photoelectron spectra, the 6300 A volume emission rate profile and the distribution of NO along the satellite path. These simultaneous measurements provide a consistent set of data to examine current F-region theory in the daytime ionosphere. We used a time-dependent coupled model of the ionospheric E- and F-region to calculate the ionospheric properties over Millstone Hill at the times of the AE-C crossings and then compared the calculated structure to the observed structure. The results show good agreement between the incoherent scatter radar measurements and the model calculations. There is also good agreement among satellite and incoherent scatter radar measurements and model calculations for the altitude of the satellite crossing, 161 km. The satellite measurements along the orbital path, however, reveal considerable horizontal gradients in the measured ionospheric properties.

Further quantification of the sources and sinks of thermospheric O1D) atoms

Abstract In this paper we confirm an earlier finding that the reaction constitutes a major source of OI 6300 A dayglow. The rate coefficient for this reaction is found to be consistent with an auroral result, namely k1 ≈ 6 × 10−12 cm3s−1. We correct an error in an earlier publication and demonstrate that reaction (1) is consistent with the laboratory determined quenching rate for the reaction where k 2 = 2.3 × 10 −11 cm 3 s −1 . Dissociative recombination of O+2 with electrons is found to be a major daytime source in summer above ∼220 km.

Intensified charge coupled devices for use as a spaceborne spectrographic image-plane detector system

An instrument that has been developed for studies of atmospheric emissions from the Space Shuttle comprises an array of imaging spectrometers to observe features over the 300-12,000-A wavelength range. Each spectrometer has a 2-D image-plane detector system on which spectral information is dispersed in one direction and spatial information is resolved in the other. The detectors consist of CCD arrays to which are coupled to proximity focused image intensifiers. The intensifier in each case is selected for the wavelength range covered by the particular spectrometer. The result is a compact low-power spectrographic detector system whose properties are described in this paper. In the course of building the five flight detector systems, we had occasion to evaluate a larger number of the charge coupled devices and the proximity focused image intensifiers. We report here various characteristics, advantages, and shortcomings of these devices and the overall intensified CCD system.

Spectroscopic imaging of the thermosphere from the Space Shuttle

An important need for studies of the thermospheric chemistry and dynamics is multispectral information on the emissions over a broad wavelength range. An instrument that has been developed to meet this need is the Imaging Spectrometric Observatory which is currently undergoing integration for flight on the Shuttle Spacelab 1 mission (mid-1983). The instrument represents the coincidence of three major factors: the payload capacity of the Shuttle which permits the placing into orbit of a sufficiently large and comprehensive spectrometric system; the recent advances in solid state imaging detector components which have permitted use of very fast multiplexing detectors; and the maturing of knowledge of the thermosphere permitting clear identification of the desired measurements. The instrument consists of an array of five imaging spectrometers, each covering a portion of the 300-12000-A total wavelength range. The five spectrometers operate simultaneously with the spectrum being dispersed in one dimension on the focal plane detectors and spatial information along the slit length being resolved in the other. The detector system in each spectrometer is an intensified-charge coupled device, optimized for the wavelength region in question. The instrument has been designed in a modular fashion to permit variation in instrument parameters on subsequent flights. For the first flight the typical spectral resolution is 3 A (6 A below 1200 A). The sensitivity has been selected to permit coverage of a large dynamic range extending from weak nocturnal signals (approximately 1 R) to the bright earth (approximately10(7) R/A). The instrument has a minicomputer system which is located in the Spacelab module itself allowing payload crew interaction with the observation sequences. In addition, during the actual 7 days in orbit, the telemetry data stream from the instrument will be recorded directly on an instrument ground support minicomputer, permitting real-time and near-real-time evaluation of the data. The objectives on the initial flight are to obtain a survey atlas of the dayglow, nightglow, and twilightglow over the full wavelength range together with obtaining data necessary for the solution of several specific problems.

An imaging spectrometric observatory for spacelab

The capability to measure nearly simultaneously the entire spectrum of atmospheric emission from the extreme ultraviolet to the near infrared, with relatively high spectral resolution and high sensitivity, while also obtaining global and altitude coverage, would provide a database from which significant advances could be made in our current understanding of the atmosphere and its processes. The large payload capacity of the shuttle orbiter offers the first opportunity to put such instrumentation into space. The Imaging Spectrometric Observatory (ISO) comprises an array of five spectrometers designed to make full use of the shuttle as an observing platform for remote sensing of the atmosphere. ISO covers the wavelength range 300–12000 Å at 2–7 Å resolution. Use of area array detectors (intensified-CCDs) permits simultaneous measurements of ∼1000 Å at a time. The instrument is capable of scanning the entire wavelength range in less than 20 s, or dwelling on weaker features for longer periods of time. The detectors are two dimensional and permit spectral imaging in one direction and spatial imaging in the other. The spatial imaging and spatial scanning features permit measurement of altitude profiles, or mapping of strongly spatially varying features such as aurorae. The instrument is designed to allow versatility. The various functions are programmable and software controlled. The key subsystems are modular for convenient replacement or upgrading. It is anticipated that the instrument will have applications not only in the area of atmospheric science, but also in studies of the ionosphere and magnetosphere, and in support of active experiments to be performed in space.

The seasonal effect of nitric oxide cooling on the thermospheric U.V. heat budget

Abstract It has recently been shown that radiative cooling of vibrationally excited NO at 5.3 μ m could result in a significant loss of heat from the lower thermosphere. In contrast to this effect, recent rocket measurements of atomic oxygen fine structure radiation at 63 μ m suggests that radiation entrapment may greatly reduce the effectiveness of this process in cooling the lower thermosphere. In this paper we examine the effects of these processes on the u.v. heating efficiency ϵ . We point out that in the past the definition used for the heating efficiency runs counter to its logical application, and that strictly speaking, the above processes should be included as cooling processes in the energy equations. However, in order to compare out findings with past work we include the 0 and NO radiative cooling in the calculation of an effective heating efficiency, ϵ . We find that ϵ can vary from 45 to 60% depending on how the two radiative cooling mechanisms are included in the calculation. In addition it is found that the shape of the altitude profile of the heating efficiency varies significantly with season, while the peak value remains relatively invariant.