J. A. Fennelly

EUVAC: A solar EUV Flux Model for aeronomic calculations

This paper presents a new solar EUV flux model for aeronomic calculations (EUVAC), which is based on the measured F74113 solar EUV reference spectrum. The model provides fluxes in the 37 wavelength bins that are in widespread use. This paper also presents cross sections to be used with the EUVAC flux model to calculate photoionization rates. The flux scaling for solar activity is accomplished using a proxy based on the F10.7 index and its 81-day average together with the measured solar flux variation from the EUVS instrument on the Atmosphere Explorer E satellite. This new model produces 50-575 A integrated EUV fluxes in good agreement with rocket observations. The solar cycle variation of the chromospheric fluxes agrees well with the measured variation of the Lyman α flux between 1982 and 1988. In addition, the theoretical photoelectron fluxes, calculated using the new EUV flux model, are in good agreement with the solar minimum photoelectron fluxes from the Atmosphere Explorer E satellite and also with the solar maximum photoelectron fluxes from the Dynamics Explorer satellite. Its relative simplicity coupled with its ability to reproduce the 50-575 A solar EUV flux as well as the measured photoelectron spectrum makes the model well suited for aeronomic applications. However, EUVAC is not designed to accurately predict the solar flux variability for numerous individual lines.

Ionospheric electron densities calculated using different EUV flux models and cross sections: Comparison with radar data

The recent availability of the new EUVAC (Richards et al., 1994) and EUV94X (Tobiska, 1993b, 1994) solar flux models and new wavelength bin averaged photoionization and photoabsorption cross section sets led us to investigate how these new flux models and cross sections compare with each other and how well electron densities (Ne) calculated using them compare with actual measurements collected by the incoherent scatter radar at Millstone Hill (42.6°N, 288.5°E). In this study we use the Millstone Hill semiempirical ionospheric model, which has been developed from the photochemical model of Buonsanto et al. (1992). For the F2 region, this model uses determinations of the motion term in the Ne continuity equation obtained from nine-position radar data. We also include two simulations from the field line interhemispheric plasma (FLIP) model. All the model results underestimate the measured Ne in the E region, except that the EUV94X model produces reasonable agreement with the data at the E region peak because of a large Lyman β (1026 A) flux, but gives an unrealistically deep E-F1 valley. The ionospheric models predict that the O2+ density is larger than the NO+ density in the E region, while numerous rocket measurements show a larger NO+ density. Thus the discrepancy between the ionospheric models and the radar data in the E region is most likely due to an incomplete understanding of the NO+ chemistry. In the F2 region, the photoionization rate given by EUV94X is significantly larger than that given by the EUVAC and earlier models. This is due to larger EUV fluxes in EUV94X compared to EUVAC over the entire 300-1050 A wavelength range, apart from some individual spectral lines. In the case of EUVAC, this is partly compensated for by larger photoelectron impact ionization due to the larger EUV fluxes below 250 A. The differences between ionospheric model results for the different cross-section sets are generally much smaller than the differences with the data.

A method for the retrieval of atomic oxygen density and temperature profiles from ground-based measurements of the O+(²D - ²P) 7320-Å twilight airglow

This paper describes a technique for the retrieval of altitude profiles of the atomic oxygen concentration (n = [O]) and temperature (T) from ground-based measurements of the O+(²D - ²P) doublet at 7320 and 7330 A in the twilight airglow. The technique is based on previously demonstrated knowledge that at solar zenith angles (SZA) characteristic of twilight conditions, the upper state of the 7320-A doublet transition is produced by photoionization and photoelectron impact ionization of atomic oxygen and lost mainly by radiative decay, thereby providing a sensitive dependence on [O]. We apply inverse problem theory to retrieve the exospheric temperature (T∞), the atomic oxygen concentration at 120 km (n120), the temperature at 120 km (T120) and the temperature profile shape factor (S) using a Bates-Walker representation of n given approximately by n = (n120T120/T) exp[−z] where T = T∞ - (T∞ - T120)exp[−S(h - h120)], z is the reduced height, and h is the altitude. The algorithm is tested and theoretically verified using synthetic data sets where random errors of measurements are characterized by Poisson noise due primarily to sky background. In the tests that we report here the solar EUV flux is specified. In a separate paper we will report how the solar EUV ionization rate can be independently derived from various twilight emissions. By comparing retrieved with known input values, it is demonstrated that for the altitude range 200 to 500 km the atomic oxygen concentration [O] can be retrieved with relative errors ≃15% and systematic errors of about 25% if the solar EUV is given. Sensitivity of the results to noise, sample size (degrees of freedom), and absolute calibration are quantitatively evaluated. In addition, to demonstrate the validity of the technique experimentally, we utilized the Atmosphere Explorer E (AE-E) in situ measurements of the solar EUV flux and [O], with the latter taken when perigee was over Arecibo on an occasion when the observatory airglow spectrometer was simultaneously measuring the 7320-A emission from the ground during twilight. The results show excellent agreement with the measured [O] values which were ∼ 50% lower than the mass spectrometer incoherent scatter (MSIS-86) model values at ∼ 300 km on that day, thereby demonstrating the value of the method for monitoring day-to-day variations in [O] and the temperature.

Mesospheric nightglow spectral survey taken by the ISO Spectral Spatial Imager on ATLAS 1

This paper reports the first comprehensive spectral survey of the mesospheric airglow between 260 and 832 nm taken by the Imaging Spectrometric Observatory (ISO) on the ATLAS 1 mission. We select data taken in the spectral window between 275 and 300 nm to determine the variation with altitude of the Herzberg I bands originating from the vibrational levels v′ = 3 to 8. These data provide the first spatially resolved spectral measurements of the system. The data are used to demonstrate that to within an uncertainty of ± 10%, the vibrational distribution remains invariant with altitude. The deficit reported previously for the v′ = 5 level is not observed although there is a suggestion of depletion in v′ = 6. The data could be used to place tight constraints on the vibrational dependence of quenching rate coefficients, and on the abundance of atomic oxygen.

Retrieval of thermospheric atomic oxygen, nitrogen and temperature from the 732 NM emission measured by the ISO on ATLAS 1

The Imaging Spectrometric Observatory (ISO) was a part of the ATLAS 1 Mission flown on the shuttle Atlantis from March 24 to April 2, 1992. During limb scanning operations, the ISO measured the O+(2P) ion emission at 732 nm. We have used a numerical inversion technique to retrieve thermospheric atomic oxygen, molecular nitrogen and temperature profiles. These preliminary results indicate a lower thermospheric temperature cooler than that predicted by MSIS for the solar conditions during the mission. Although the densities agree at low altitudes, the reduced scale height produces O and N2 densities 25% lower than the MSIS at 300 km.

Simultaneous retrieval of the solar EUV flux and neutral thermospheric O, O2, N2, and temperature from twilight airglow

We present a method to retrieve neutral thermospheric composition and the solar EUV flux from ground-based twilight optical measurements of the O+(²P) 7320 A and O(¹D) 6300 A airglow emissions. The parameters retrieved are the neutral temperature, the O, O2, N2 density profiles, and a scaling factor for the solar EUV flux spectrum. The temperature, solar EUV flux scaling factor, and atomic oxygen density are first retrieved from the 7320-A emission, which are then used with the 6300-A emission to retrieve the O2 and N2 densities. The retrieval techniques have been verified by computer simulations. We have shown that the retrieval technique is able to statistically retrieve values, between 200 and 400 km, within an average error of 3.1 ± 0.6% for thermospheric temperature, 3.3 ± 2.0% for atomic oxygen, 2.3 ± 1.3% for molecular oxygen, and 2.4 ± 1.3% for molecular nitrogen. The solar EUV flux scaling factor was found to have a retrieval error of 5.1 ± 2.3%. All the above errors have a confidence level of 95%. The purpose of this paper is to prove the viability and usefulness of the retrieval technique by demonstrating the ability to retrieve known quantities under a realistic simulation of the measurement process, excluding systematic effects.

Technique to retrieve solar EUV flux and neutral thermospheric O, O2, N2, and temperature from airglow measurements

We describe a method for retrieving neutral thermospheric composition and solar EUV flux from optical measurements of the O+(2P) 732 nm and O(1D) 630 nm airglow emissions. The parameters retrieved are the neutral temperature, the O, O2 and N2 density profiles, and a scaling factor for the solar EUV flux spectrum. The temperature, solar EUV flux scaling factor, and atomic oxygen density are first retrieved from the 732 nm emission, which are then used with the 630 nm emission to retrieve the O2 and N2 densities. Between the altitudes of 200 and 400 km the retrieval technique is able to statistically retrieve values to within 3.1% for thermospheric temperature, 3.3% for atomic oxygen, 2.3% for molecular oxygen, and 2.4% for molecular nitrogen. The solar EUV flux scaling factor has a retrieval error of 5.1%. We also present the results of retrievals using existing data taken from both groundbased and spacebased instruments. These include airglow data taken by the Visible Airglow Experiment on the Atmosphere Explorer spacecraft and the Imaging Spectrometric Observatory flown on the ATLAS 1 shuttle mission in 1992.

Global observations and modeling of the ionosphere, thermosphere and mesosphere

The Imaging Spectrometric Observatory (ISO) flown on the Atmospheric Laboratory for Applications and Science (ATLAS) 1 mission between 24 Mar 1992 and 2 Apr 1992, acquired a database designed to study several outstanding problems in the ionosphere, thermosphere and mesosphere. In this paper we discuss the goals and preliminary results from three of these studies. To support these studies, the ISO acquired a database of: (1) emissions for the retrieval of neutral and ion densities to test global models of the ionosphere and thermosphere; (2) emissions for the retrieval of mesospheric composition of major and minor constituents needed to test models of the oxygen-hydrogen photochemistry, (3) emissions of the bands of the metastable states of O2, and O(S-1) produced by three-body recombination of O in the mesosphere.