D. G. Torr

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.

Determination of ionospheric conductivities from FUV auroral emissions

The purpose of this paper is to examine the viability of using N{sub 2} Lyman-Birge-Hopfield (LBH) emission ratios to infer auroral conductances and to quantify the strengths and weaknesses of this technique. The authors show that column-integrated Hall and Pedersen conductances may be determined from a single remote measurement of a pair of auroral LBH emissions, one in the region of strong O{sub 2} absorption (1464 {angstrom}) and one lying outside of this region (1838 {angstrom}). The dependence of the determined conductivities on incident average energy, total energy flux, and changes in solar and magnetic activity levels is examined. They show that, for energies above 5 keV, auroral conductances may be scaled by the square root of the incident energy flux with errors less than about 20%. For lower energies, however, the scaling may deviate significantly from a power of 0.5. The authors provide appropriate scaling factors as a function of average energy. They also note that the choice of either a Gaussian or Maxwellian distribution can significantly affect low-energy (<1 keV) conductance calculations. Finally, they quantify the conditions under which the mean energy of a Gaussian energy distribution may differ significantly from the characteristic energy. 46 refs., 10 figs.,morexa0» 1 tab.«xa0less

Reevaluation of the O+(²P) reaction rate coefficients derived from Atmosphere Explorer C observations

O+(²P) is an important species for studies of the ionosphere and thermosphere: its emission at 7320 A can be used as a diagnostic of the thermospheric atomic oxygen density. Unfortunately, there are no laboratory measurements of the O and N2 reaction rates which are needed to determine the major sinks of O+(2P). The reaction rates that are generally used were determined from aeronomic data by Rusch et al. (1977) but there is evidence that several important inputs that they used should be changed. We have recalculated the O and N2 reaction rates for O+(2P) using recent improvements in the solar EUV flux, cross sections, and photoelectron fluxes. For the standard solar EUV flux, the new N2 reaction rate of 3.4 ± 1.5 × 10−10 cm3 s−1 is close to the value obtained by Rusch et al. (1977), but the new O reaction rate of 4.0±1.9 × 10−10 cm3s−1 is about 8 times larger. These new reaction rates are derived using neutral densities, electron density, and solar EUV fluxes measured by Atmosphere Explorer C in 1974 during solar minimum. The new theoretical emission rates are in good agreement with the data for the two orbits studied by Rusch et al. and they are in reasonable agreement with data from five additional orbits that are used in this study. We have also examined the effect of uncertainties in the solar EUV flux on the derived reaction rates and found that 15 % uncertainties in the solar flux could cause additional uncertainties of up to a factor of 1.5 in the O quenching rate.

F region climatology during the SUNDIAL/ATLAS 1 campaign of March 1992: Model‐measurement comparisons and cause‐effect relationships

We present the first joint comparison of global measurements of F region characteristics with three models used widely in the specification of the ionospheric-thermospheric system. The models, the International Reference Ionosphere (IRI), the field line interhemispheric plasma (FLIP) model, and the Thermospheric-Ionospheric General Circulation Model (TIGCM), represent a unique set of capabilities with major differences in approaches to the prevailing physics and different levels of computational complexity. The database was developed by a global network of 53 ionosonde stations operating around-the-clock for the period March 22 through April 4, 1992 in collaboration with the ATLAS 1 mission. The emphasis is on the F region characteristics of peak heights (h m F 2 ) and densities (N m F 2 ), their climatological (i.e., average) behavior during the ATLAS 1 period, and associated cause-effect relationships. We explore latitudinal and local time variations with attention to the influences of meridional winds and plasmaspheric fluxes in the maintenance of different domains in the ionospheric-thermospheric system. We find that all three models tend to underestimate the values of h m F 2 and N m F 2 with the largest discrepancies in N m F 2 resulting in the FLIP and TIGCM representations at night. These discrepancies can grow to levels as large as 110% near 0400 LT, a rediscovery of the old but unsettled issue of maintenance of the nighttime ionosphere. This nighttime discrepancy is traceable in first order to model underestimates of prevailing meridional winds. The contributions of plasmaspheric fluxes are also considered, with the conclusion that they are of secondary importance, but substantially more work is necessary to uniquely quantify their role. In contrast to their nighttime characteristics, the FLIP and TIGCM generally have excellent agreement (i.e., 6±6%) with daytime observations of N m F 2 , and the IRI tends to underestimate the observed values of N m F 2 by a nominally LT-insensitive level of 28±6%. Other campaign results are reviewed in this issue, with a focus on regional responses to the prevailing conditions and their characterization in terms of latitudinal distributions of F region heights and densities.

Atmospheric Physics and Earth Observations Atmospheric Spectral Imaging

An array of imaging spectrometers flown on the Spacelab 1 mission was capable of providing spectra of the atmospheric emissions over a broad wavelength range from 300 to 12,700 angstroms and acquiring each complete spectrum nearly simultaneously. The instrument was used to make observations on the day side and night side of the earth, looking down in the nadir direction, radially away from the earth, and in various limb-scanning modes. Observations were made looking at various angles to the vehicle velocity vector and during thruster firings and water dumps as well as at times when such events were inhibited. As a result of the mission a data base has been acquired that is valuable for studies of both the upper atmosphere and the shuttle-Spacelab vehicle environment.

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.

N2 Lyman-Birge-Hopfield dayglow from ATLAS 1

Spectral measurements of the far ultraviolet dayglow were made from the ATLAS 1 shuttle mission using an intensified charge coupled device (CCD) imaging spectrometer array. The instrument imaged relatively large (177 A) segments of the spectrum simultaneously and had the capability to image altitude in the second dimension of the two-dimensional focal plane detector when the entrance slit of the instrument was oriented perpendicular to the limb of the Earth. The far ultraviolet (FUV) channel of the instrument used a CsTe photocathode rather than the more conventionally used and more solar blind CsI photocathode material. As a result, the spectra were also measured with good sensitivity at FUV wavelengths longer than 1500 A. In this paper, data are selected of specific interest to a study of the N2 Lyman-Birge-Hopfield system using two shuttle attitudes: In the first the field of view was held at a fixed tangent ray height of approximately 140 km while scanning in wavelength, providing repeated FUV spectra over a range of latitudes and allowing comparison with our thermospheric airglow model. In the second observing sequence the line of sight of the instrument was scanned down through the atmosphere but covered about 75% of the full wavelength range. This sequence allows comparison of height dependence,including O2 absorption effects, with the thermospheric model. In all cases, relatively good agreement is obtained with the model (run strictly in a predictive mode) in comparisions in which the line of sight must be intergrated along a path that spans a considerable range of conditions (height, local time, latitude, solar zenith angle). In the case of the data sets examined in this study the vibrational population distributions show mixed results. In one case the distribution agrees well with a theoretical model based on direct electron impact excitation without a significant cascade contribution. A second case indicated a lower population in nu = 3 and a higher population in nu = 6 than would be expected from direct impact excitation alone. This supports previous indications that conditions resulting in the cascade mechanism may not always be operative.

Ionospheric Effects caused by Electrons precipitated from the Outer Radiation Belt

DURING the period October 1962 to March 1963, there was a disturbance in the ionosphere at the Antarctic station Sanae (70° 18′ S., 2° 22′ W.) whenever a high flux of precipitated electrons was observed in the magnetically conjugate area by the satellite Alouette1. Together with Gledhill, we2 have shown that similar disturbances are produced at other stations lying near the same magnetic shell (L = 4) in both the northern and southern hemispheres. We established that flux values above a certain level are observed only when the ionosphere is disturbed, and that the percentage of total time for which this level is exceeded at each station is almost exactly the same as the percentage time for which the ionosphere is disturbed. Our results showed that the frequency of occurrence of ionospheric disturbances on L = 4 increases markedly in the vicinity of the South Atlantic geomagnetic anomaly. This observation was attributed to the fact that the mirror points of particles trapped in the outer radiation belt are much lower there than anywhere else, being responsible for the high intensities of charged particles observed there3,4.

Sensitivity of the 6300 A twilight airglow to neutral composition

The field line interhemispheric plasma (FLIP) model is used to study the 6300 A line intensity measured during three morning twilights from the McDonald Observatory in Texas. The Imaging Spectrometric Observatory (ISO) measured the 6300 A intensity during the winter of 1987 and the spring and summer of 1988. The FLIP model reproduces the measured intensity and its variation through the twilight well on each day using neutral densities from the MSIS-86 empirical model. This is in spite of the fact that different component sources dominate the integrated volume emission rate on each of the days analyzed. The sensitivity of the intensity to neutral composition is computed by varying the N2, O2 and O densities in the FLIP model and comparing to the intensity computed with the unmodified MSIS-86 densities. The ion densities change self-consistently. Thus the change in neutral composition also changes the electron density. The F2 peak height is unchanged in the model runs for a given day. The intensity changes near 100° SZA are comparable to within 10% when either [O2], [N2] or [O] is changed, regardless of which component source is dominant. There is strong sensitivity to changes in [N2] when dissociative recombination is dominant, virtually no change in the nighttime (SZA ≥ 108°) intensity with [O2] doubled, and sensitivity of over 50% to doubling or halving [O] at night. When excitation by conjugate photoelectrons is the dominant nighttime component source, the relative intensity change with [O] doubled or halved is very small. This study shows the strong need for simultaneous measurements of electron density and of emissions proportional to photoelectron fluxes if the 6300 A twilight airglow is to be used to retrieve neutral densities.

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.