Volume Emission Rate Research Articles

Simultaneous measurements of the O2(¹Δ) and O2(¹Σ) Airglows and ozone in the daytime mesosphere

We report simultaneous measurements of the O2(¹Δ) and O2(¹Σ) airglow volume emission rate profiles in the daytime mesosphere. We use these measurements to derive ozone concentrations separately from each airglow. The measurements were made as part of the Mesosphere‐Thermosphere Emissions for Ozone Remote Sensing (METEORS) sounding rocket project launched from White Sands Missile Range, New Mexico. These data offer the opportunity to assess the consistency of ozone profiles derived from measurements of the oxygen airglows and thus provide a fundamental test of our understanding of dayglow physics and solar energy deposition. In both airglow‐derived profiles the ozone decreases with altitude throughout the middle mesosphere and a pronounced secondary maximum exists near 90 km. The derived ozone profiles are in agreement to within measurement uncertainty. This agreement confirms that the major processes responsible for the generation of mesospheric oxygen dayglow are well understood via measurement.

Temporal variation of the volume emission rates between OI5577 and O2(0,1)

Abstract The intensities of the upper mesospheric airglow emissions, OI 557.7 nm and O 2 (b) atmospheric (0,1) band at 864.5 nm, have been measured since October 1998 using a ground-based multichannel airglow photometer located near the equator at Sao Joao do Cariri (7S, 35W). The intensity ratio between the two emissions, OI5577/O 2 b(0,1), was obtained as a function of time. The annual average of the ratio R in 1998 was 0.41 ± 0.14, and showed considerable day to day variations, ranging from 0.2 to 0.6. Nocturnal variations of the ratio also demonstrate a large amplitude of oscillation, varying between 0.3 to 0.6. These results were compared with a model calculation.

Airglow intensity variations induced by gravity waves. Part 1: generalization of the Hines–Tarasick's theory

This study generalizes the gravity wave–airglow interaction theory of Hines and Tarasick (Planetary Space Science 35 (1987) 851). A set of equations describing the wave-induced fluctuations of temperature, line-of-sight (LOS) wind, and volume emission rate (VER) are obtained for limb observations with a photometer or interferometer. The results can also be applied to ground-based measurements. Approximations at small and large zenith angles θ are considered. As a central part of the treatment, the VER variation is expressed in terms of the atmospheric density (or temperature) perturbation and the displacements parallel and perpendicular to the LOS. The relative importance of these effects is characterized by an effective scale λ c of the photochemical–dynamical interactions between a wave and the airglow emission. This effective scale is shown to be ∼33 km in the mesosphere and lower thermosphere (MLT) for emission processes of three-body reactions, and to slowly increase with increasing wave vertical scales. At small zenith angle observations, the VER variation is dominated by the effect of the vertical displacement (in geographic frame) or of the density (or temperature) perturbation for waves with vertical scales smaller or larger than λ c, respectively. The small-scale height H ε of the emission layer may play an important role only for limb measurements. When λ c>2π H ε , the VER variation is primarily controlled by vertical wave motions for a broad spectrum of vertical scales. Comparisons with observations are presented in an accompanying paper.

Double‐peaked hydroxyl airglow profiles observed from WINDII/UARS

The WINDII instrument onboard the UARS satellite made many thousands of measurements of the apparent brightness of the hydroxyl airglow as a function of tangent height during the night. The measurements were mainly at latitudes between 42°S and 42°N. Most of the profiles of volume emission rate indicate a simple layer with a peak between 85 and 90 km. However, from 5 to 25% of the measured profiles have a more complex structure for the layer profile, most frequently with two peaks separated by 4 to 6 km. The horizontal extent varies from 200 to several thousand kilometers, and the duration may be as long as 1½ hours. Here the WINDII data from November 1991 to April 1995 are used to study the frequency of occurrence of the double‐peaked layers as a function of latitude, season, and local time and to explore possible causes of the unusual profiles. Early in the night the double‐peaked layers occur preferentially at low to medium latitudes, migrating toward the equator by midnight where they remain for the rest of the night. There is a pronounced asymmetry between the hemispheres which reverses between the March/April equinox and the September/October equinox. The solstice behavior is similar, though not so pronounced. Through simulation three possible causes to the observed OH* double‐peaked layers were identified. Gravity waves are a likely source of isolated small‐scale events but the large horizontal scale, long‐duration structures are more likely to result from temperature inversion layers or mixing events or a combination of the two.

Latitudinal and longitudinal dispersion of energetic auroral protons

Abstract. Using a collision by collision model from Lorentzen et al., the latitudinal and longitudinal dispersion of single auroral protons are calculated. The proton energies varies from 1 to 50 keV, and are released into the atmosphere at 700 km altitude. The dipole magnetic field has a dip-angle of 8 degrees. Results show that the main dispersion region is at high altitudes (300-350 km) and occurs during the first few charge exchange collisions. As the proton travels further down the atmosphere the mean free path becomes smaller, and as a result the spreading effect will not be as pronounced. This means that the first few charge exchange collisions fully determines the width of both the latitudinal and longitudinal dispersion. The volume emission rate was calculated for energies between 1 and 50 keV, and it was found that dayside auroral hydrogen emissions rates were approximately 10 times weaker than nightside emission rates. Simulations were also performed to obtain the dependence of the particle dispersion as a function of initial pitch-angle. It was found that the dispersion varies greatly with initial pitch-angle, and the results are summarized in two tables; a main and an extreme dispersion region.Key words. Ionosphere (auroral ionosphere; · particle precipitation) · Space plasma physics · (transport processes)

An anomalous subauroral red arc on 4 August, 1972: comparison of ISIS-2 satellite data with numerical calculations

Abstract. This study compares the Isis II satellite measurements of the electron density and temperature, the integral airglow intensity and volume emission rate at 630 nm in the SAR arc region, observed at dusk on 4 August, 1972, in the Southern Hemisphere, during the main phase of the geomagnetic storm. The model results were obtained using the time dependent one-dimensional mathematical model of the Earth's ionosphere and plasmasphere (the IZMIRAN model). The major enhancement to the IZMIRAN model developed in this study to explain the two component 630 nm emission observed is the analytical yield spectrum approach to calculate the fluxes of precipitating electrons and the additional production rates of N+2, O+2, O+(4S), O+(2D), O–(2P), and O+(2P) ions, and O(1D) in the SAR arc regions in the Northern and Southern Hemispheres. In order to bring the measured and modelled electron temperatures into agreement, the additional heating electron rate of 1.05 eV cm–3 s–1 was added in the energy balance equation of electrons at altitudes above 5000 km during the main phase of the geomagnetic storm. This additional heating electron rate determines the thermally excited 630 nm emission observed. The IZMIRAN model calculates a 630 nm integral intensity above 350 km of 4.1 kR and a total 630 nm integral intensity of 8.1 kR, values which are slightly lower compared to the observed 4.7 kR and 10.6 kR. We conclude that the 630 nm emission observed can be explained considering both the soft energy electron excited component and the thermally excited component. It is found that the inclusion of N2(v > 0) and O2(v > 0) in the calculations of the O+(4S) loss rate improves the agreement between the calculated Ne and the data on 4 August, 1972. The N2(v > 0) and O2(v > 0) effects are enough to explain the electron density depression in the SAR arc F-region and above F2 peak altitude. Our calculations show that the increase in the O++N2 rate factor due to the vibrationally excited nitrogen produces the 5–19% reductions in the calculated quiet daytime peak density and the 16–24% decrease in NmF2 in the SAR arc region. The increase in the O++N2 loss rate due to vibrationally excited O2 produces the 7–26% decrease in the calculated quiet daytime peak density and the 12–26% decrease in NmF2 in the SAR arc region. We evaluated the role of the electron cooling rates by low-lying electronic excitation of O2(a1Δg) and O2(b1Σg+), and rotational excitation of O2, and found that the effect of these cooling rates on Te can be considered negligible during the quiet and geomagnetic storm period 3-4 August, 1972. The energy exchange between electron and ion gases, the cooling rate in collisions of O(3P) with thermal electrons with excitation of O(1D), and the electron cooling rates by vibrational excitation of O2 and N2 are the largest cooling rates above 200 km in the SAR arc region on 4 August, 1972. The enhanced IZMIRAN model calculates also number densities of N2(B3Πg+),N2(C3Πu), and N2(A3Σu+) at several vibrational levels, O(1S), and the volume emission rate and integral intensity at 557.7 nm in the region between 120 and 1000 km. We found from the model that the integral intensity at 557.7 nm is much less than the integral intensity at 630 nm.Key words. Atmospheric composition and structure (airglow and aurora; thermosphere – composition and chemistry) · Ionosphere (ionosphere – magnetosphere interactions)

The nocturnal behavior of the hydroxyl airglow at the equatorial and low latitudes as observed by WINDII: Comparison with ground‐based measurements

Measurements of the hydroxyl nightglow from the Wind Imaging Interferometer (WINDII) satellite‐borne experiment and from ground‐based photometers are compared and combined in order to characterize the hydroxyl airglow nocturnal behavior at 23°S and the equatorial region. Ground‐based observations at Cachoeira Paulista (23°S, 45°W) and at Fortaleza (4°S, 38°W) from 1987 to 1994 and WINDII data from March 1992 to January 1994 are selected, and two periods are studied: the March/April equinox and the December/January solstice. The WINDII results for the equinox/solstice difference and for the nocturnal behavior of the vertically integrated emission rate of the hydroxyl nightglow are found to be in very good agreement with the ground‐based observations. The WINDII measurements revealed two general characteristics of the hydroxyl nightglow altitude variation at the latitudes above. The hydroxyl nocturnal variations are predominantly occurring below 90 km, and the vertically integrated emission rate and the peak height of the volume emission rate show an inverse relationship with seasonal, latitudinal, and inter annual effects. The interpretation of these results in terms of atmospheric tides is then discussed.

A new perspective on the molecular oxygen and hydroxyl airglow emissions

The mesospheric molecular oxygen and hydroxyl airglow emissions have traditionally been measured in order to derive minor species abundances or to diagnose dynamical phenomena. We present a new interpretation of these airglow emissions and show them to be fundamental measures of energy deposition from which rates of atmospheric heating are readily derived. The heating rate due to absorption of ultraviolet radiation in the Hartley band of ozone may be derived from simultaneous measurements of the oxygen atmospheric band and infrared atmospheric band volume emission rates independent of knowledge of the ozone density, the solar irradiance, and the ozone absorption cross sections. The heating rates due to key exothermic reactions may be derived directly from appropriate airglow observations independent of the reactant concentrations and the temperature‐dependent reaction rates. The accuracy of heating rates derived directly from airglow measurements is also inherently higher than that obtained in standard approaches. We suggest that heating rates derived in this manner be treated as data products and that they be compared with numerical model computations to enhance understanding of atmospheric thermodynamics. An initial comparison of airglow‐derived energy deposition rates with deposition rates traditionally computed from numerical models shows agreement to within 20% for the Hartley band of ozone in the lower and upper mesosphere.

WINDII measurements of nightglow enhancements in the South Atlantic Magnetic Anomaly Zone

Enhancements of the O(¹D) 630 nm and O(¹S) 558 nm volume emission rates were detected by WINDII, the Wind Imaging Interferometer on board the Upper Atmosphere Research Satellite, UARS, as its limb‐locked fields of view crossed the South Atlantic magnetic anomaly zone during periods of high geomagnetic activity. The enhancement attributed to electron precipitation is 20 R for the 630 nm red line and 3 R for the 558 nm green line, less than the natural variability of these nightglow emissions as seen from below. Nevertheless, statistical tests indicate with 95% confidence that the enhancements are significantly distinguishable from the natural variability. The red enhancement lies west of the green because eastward drifting trapped electrons move toward lower mirror altitudes in this region. These enhancements are energetically consistent with earlier measurements of electron fluxes in this region; we estimate the total precipitation energy deposition rate at a few megawatts.

Atmospheric Ultraviolet Radiance Integrated Code (AURIC): theory, software architecture, inputs, and selected results

The Atmospheric Ultraviolet Radiance Integrated Code (AURIC) is a software package developed under the sponsorship of the Air Force Phillips Laboratory/Geophysics Directorate (PL/GP) for upper atmospheric radiance modeling from the far ultraviolet to the near infrared. It effectively extends the PL/GP code MODTRAN for calculating atmospheric transmittance and radiance (IR and Rayleigh+aerosol scattering of sunlight) to altitudes above 100 km and wavelengths down to 1050 Å. The science content of the airglow and upper atmospheric composition codes is presented in this paper. The development has focused on the re-engineering of research codes according to modern software standards and on critical reviews of input parameters (cross sections, rate coefficients, molecular band constants, etc.). In addition to describing the structure and operation of AURIC, detailed, quantitative descriptions are given of the calculation of photoelectron fluxes, the chemistry associated with dayglow and nightglow, and the generation of spectral radiances. Numerous tables are presented of electron impact cross sections and rate coefficients along with associated bibliographies. Results include photoelectron fluxes, volume emission rates, limb profiles of selected emission features, spectral radiances, and densities of ionic and neutral species. To document validation activities, many of the results are compared with published rocket and satellite data showing good overall agreement with the measurements. The airglow modeling capabilities of AURIC make it a powerful tool for characterizing optical backgrounds at thermospheric heights, for developing remote sensing algorithms, for simulating data from rocket and satellite optical instrumentation, and for conducting science investigations (sensitivity studies and data analyses).

A note on gravity wave‐driven volume emission rate weighted temperature perturbations inferred from O2 atmospheric and O I 5577 airglow observations

A full‐wave dynamical model and chemistry models that simulate ground‐based observations of gravity wave‐driven O2 atmospheric and O I 5577 airglow fluctuations in the mesopause region are used to demonstrate that for many observable gravity waves modeling is required to infer temperature perturbation amplitudes from airglow observations. We demonstrate that the amplitude of the altitude‐integrated volume emission rate weighted temperature perturbation differs by at least about 30% from the amplitude of the temperature perturbation of the major gas in the vicinity of the peak of the airglow volume emission rate for gravity waves with horizontal phase speeds less than about 150 m s−1 and vertical wavelengths less than about 50 km and that the amplitude of the altitude‐integrated volume emission rate weighted temperature perturbation differs considerably from the amplitude of the temperature perturbation averaged over the vertical extent of the emission layer for waves with horizontal phase speeds less than about 65 m s−1 and vertical wavelengths less than about 20 km. For waves with phase speeds less than about 100 m s−1 and vertical wavelengths less than about 30 km the amplitude of the altitude‐integrated volume emission rate weighted temperature perturbation differs by at least about 30% from the altitude‐integrated mean volume emission rate weighted temperature perturbation, demonstrating that the nonthermal fluctuation contribution to the former (involving volume emission rate perturbations) needs to be included in such modeling. We conjecture that the observed brightness perturbation is a simpler and better quantity to simulate using detailed modeling than the observed airglow temperature perturbation for the determination of wave amplitude in cases where nonthermal effects or cancellation effects (for short vertical wavelengths) are strong.

An anomalous subauroral red arc on 4 August, 1972: comparison of ISIS-2 satellite data with numerical calculations

This study compares the Isis II satellite measurements of the electron density and temperature, the integral airglow intensity and volume emission rate at 630 nm in the SAR arc region, observed at dusk on 4 August, 1972, in the Southern Hemisphere, during the main phase of the geomagnetic storm. The model results were obtained using the time dependent one-dimensional mathematical model of the Earth's ionosphere and plasmasphere (the IZMIRAN model). The major enhancement to the IZMIRAN model developed in this study to explain the two component 630 nm emission observed is the analytical yield spectrum approach to calculate the fluxes of precipitating electrons and the additional production rates of N+2, O+2, O+(4S), O+(2D), O–(2P), and O+(2P) ions, and O(1D) in the SAR arc regions in the Northern and Southern Hemispheres. In order to bring the measured and modelled electron temperatures into agreement, the additional heating electron rate of 1.05 eV cm–3 s–1 was added in the energy balance equation of electrons at altitudes above 5000 km during the main phase of the geomagnetic storm. This additional heating electron rate determines the thermally excited 630 nm emission observed. The IZMIRAN model calculates a 630 nm integral intensity above 350 km of 4.1 kR and a total 630 nm integral intensity of 8.1 kR, values which are slightly lower compared to the observed 4.7 kR and 10.6 kR. We conclude that the 630 nm emission observed can be explained considering both the soft energy electron excited component and the thermally excited component. It is found that the inclusion of N2(v > 0) and O2(v > 0) in the calculations of the O+(4S) loss rate improves the agreement between the calculated Ne and the data on 4 August, 1972. The N2(v > 0) and O2(v > 0) effects are enough to explain the electron density depression in the SAR arc F-region and above F2 peak altitude. Our calculations show that the increase in the O++N2 rate factor due to the vibrationally excited nitrogen produces the 5–19% reductions in the calculated quiet daytime peak density and the 16–24% decrease in NmF2 in the SAR arc region. The increase in the O++N2 loss rate due to vibrationally excited O2 produces the 7–26% decrease in the calculated quiet daytime peak density and the 12–26% decrease in NmF2 in the SAR arc region. We evaluated the role of the electron cooling rates by low-lying electronic excitation of O2(a1Δg) and O2(b1Σg+), and rotational excitation of O2, and found that the effect of these cooling rates on Te can be considered negligible during the quiet and geomagnetic storm period 3-4 August, 1972. The energy exchange between electron and ion gases, the cooling rate in collisions of O(3P) with thermal electrons with excitation of O(1D), and the electron cooling rates by vibrational excitation of O2 and N2 are the largest cooling rates above 200 km in the SAR arc region on 4 August, 1972. The enhanced IZMIRAN model calculates also number densities of N2(B3Πg+),N2(C3Πu), and N2(A3Σu+) at several vibrational levels, O(1S), and the volume emission rate and integral intensity at 557.7 nm in the region between 120 and 1000 km. We found from the model that the integral intensity at 557.7 nm is much less than the integral intensity at 630 nm.Key words. Atmospheric composition and structure (airglow and aurora; thermosphere – composition and chemistry) · Ionosphere (ionosphere – magnetosphere interactions)

Coupled longitudinal and tidal variations of equatorial nightglow [formula omitted] zenith emission rates from WINDII and Christmas Island data

Vertically integrated volume emission rates of the O( 1S) nightglow measured by WINDII, the WIND Imaging Interferometer on UARS, NASA's Upper Atmosphere Research Satellite, over the equator were compared with zenith measurements of the O( 1S) nightglow over Christmas Island measured by a ground-based all-sky imager during the September, 1995, equinox period. The average variation with respect to local time, the migrating tidal signature, was almost identical for the two data sets. On one night, however, the Christmas Island nightglow showed virtually no tidal signature while WINDII showed that just 40 deg to the west the evening maximum was especially pronounced. This longitudinal variability is shown to be the result of the superposition of a planetary scale gravity wave with the diurnal tidal oscillation. The WINDII data also demonstrate modulation by a 5-day wave, with details to be presented in a later study.

The morphology of oxygen greenline dayglow emission

Abstract. In this study, the morphology of the oxygen greenline dayglow emission is presented. The volume emission rate profiles are obtained by using Solomon's glow model. The glow model is updated in terms of recent cross sections, reaction rate coefficients and quantum yield of greenline emission. Throughout most of the thermosphere the modelled and observed emission rates are in reasonably good agreement. In the region between 98 and 120 km, the modelled emission rates are substantially higher (about a factor of 1.7) than the observed emission rates. This discrepancy is discussed in terms of scaling of solar fluxes which accounts the variation of solar activity for the day on which calculations are made. The modelled morphology of greenline emission is compared with those cases where WINDII data is available. The modelled and observed morphology is in reasonably good agreement at most of the latitudes above 120 km. In the mesosphere the qualitative nature of morphology is very similar to those of WINDII observation except the modelled emission rates are about a factor of 1.7 higher than the observed emission rates.Keywords. Ionosphere (ion chemistry and composition; modeling and forecasting; solar radiation and cosmic ray effects).

Analytical models for the responses of the mesospheric OH* and Na layers to atmospheric gravity waves

Analytic models are developed to describe gravity wave induced perturbations in the high ν OH* Meinel Band emissions and in atomic Na density. The results are used to predict the fluctuations in OH* intensity and rotational temperature, Na abundance, and the centroid heights of the OH* and Na layers. The OH* model depends critically on the assumed form for the atomic oxygen profile. In this study the O profile is modeled as a Chapman layer, which is in excellent agreement with MSIS‐90. We also neglect the wave‐induced redistribution of O3 because the chemical lifetime of ozone in the mesopause region is short compared to most gravity wave periods. Under these conditions the OH* response is ΔV/Vu ≃ −3[1 ‐ (z ‐ zOH)hOH + (z − zOH)2/σ12]Δρ/ρu, where ΔV/Vu are the relative emission rate fluctuations, Δρ/ρu are the relative atmospheric density fluctuations, zOH ≃ 89 km is the layer centroid height, hOH ≃ 3.6 km, and σ1 ≃ 8.0 km. By using these results, we show that cancellation of the induced perturbations in emission intensity and rotational temperature is significant for short vertical wavelengths. The amplitude attenuation in both parameters is proportional to exp(−m2σ2OH/2), where m = 2π/λz and σOH ≃ 4.4 km is the rms thickness of the OH* layer. For example, at λz = 15 km, the predicted rotational temperature perturbation is only 20% of the atmospheric temperature perturbation. Because the most sensitive instruments are only capable of accuracies approaching ±2 K, there are few reported observations of waves with λz ≤ 15 km. The cancellation effects are not as limiting in OH intensity observations because the relative intensity perturbations are larger than the relative temperature perturbations, and intensities can be measured more accurately than temperature, especially with broadband instruments. Fluctuations in the emission rate are largest on the bottomside of the OH* layer, ∼ 3.75 km below the layer peak (∼89 km), where the effects due to the redistribution of atomic oxygen dominate. Fluctuations in rotational temperature are largest near the peak of the OH layer, where the volume emission rate is largest. The ∼3.75 km separation between the maxima of the intensity and rotational temperature perturbations is largely responsible for the phase differences observed in the fluctuations of these parameters. Rotational temperature and Krassovsky's ratio are found to be very sensitive to the form of the background temperature profile. Wave‐induced OH* layer centroid height fluctuations coupled with the mean lapse rate of the background temperature profile can contribute significantly to the observed rotational temperature fluctuations, especially for the shorter wavelength waves λz ≤ 15 km. The OH* intensity fluctuations are relatively insensitive to the temperature profile as well as variations in atomic oxygen density and therefore appear to be excellent tracers of gravity wave dynamics. OH temperature observations are best suited for studying long‐period waves, including tides, with λz ≥ 15 km.

Nightglow zenith emission rate variations in O(1S) at low latitudes from wind imaging interferometer (WINDII) observations

Volume emission rate data of the O(1S) nightglow layer measured by the wind imaging interferometer (WINDII) aboard the Upper Atmosphere Research Satellite (UARS) are vertically integrated to give equivalent zenith column emission rates. The data are restricted to tropical latitudes and categorized according to season. The migrating tidal signature is extracted by plotting zonally averaged data as a function of local time. The main features are a premidnight maximum at the equator during equinox intervals and a midnight maximum at 20°N and S. The two equinoxes are not equivalent, nor is the behavior identical in the two hemispheres. Winter periods show little or no well‐defined tidal signature. Longitudinal variations at fixed local time were also studied. These variations are larger than the tidal signature and are geographically random, showing no consistent identification with land masses. Some of these variations could be of planetary scale, while others are probably long‐wavelength gravity waves, but in any case they were seen to extend over 10° of latitude at least. Zenithal emission rate measurements exceeding 500 R occurred at the coincidence of these longitudinal wave structures and the tidal maxima. The suggestion is put forward that much of the perceived tidal variability in the tropical lower thermosphere may be explained by such wave‐tide superpositions.

Excitation of X rays in comet Hyakutake (C/1996 B2)

There is great disagreement between current evaluations of the role of bremsstrahlung and line excitation by electron impact and recombination in cometary X rays. To study the problem in detail, we use the exact values of the Roentgen satellite (ROSAT) high‐resolution imager and wide‐field camera instrument response functions and apply them to all nine exposure intervals in the ROSAT observation of comet Hyakutake to determine X ray two‐parameter power law and thermal bremsstrahlung spectra. Thermal bremsstrahlung is a convenient form for two‐parameter spectrum and not a physical reality in comets. Then spectral index is α = 1.97 ± 0.18 ≈ 2, spectral temperature is kT = 360 eV, and the exposure‐mean photon emission is 3 × 1025 ph s−1 and 1.3 × 1016 ergs s−1 in a range of 90–2000 eV and in an aperture radius of 1.2 × 105 km. The exposure‐mean ROSAT spectrum is in excellent agreement with the Extreme Ultarviolet Explorer (EUVE) observation of 7.5 × 1024 ph s−1 in a range of 100–165 eV from comet Hyakutake. Then we follow the same methods as Northrop et al. [1997] to calculate a volume emission rate at the comet brightness maximum and to calculate that from bremsstrahlung with the Vega 2 electron flux. Yet we obtain a much lower emission than Northrop et al., at a level of 0.5% of the observed emission in accordance with results of Krasnopolsky [1996, 1997a] and Bingham et al. [1997]. Electron impacts of molecular, atomic, and ion species in comets produce mostly excitation of the X ray lines C 277 eV and O 525 eV at a level of 2% of the total X ray emission from comet Hyakutake. This value is calculated for the Vega 2 electron flux. The value for the Giotto electron flux is 3 × 10−6 of the measured emission. Processes which are responsible for impurity radiation in fusion plasma either are inefficient or produce ultraviolet photons in cometary plasma, and the method of Bingham et al. is inapplicable to excitation of X ray lines in comets. The calculations show that the expected effects of bremsstrahlung and electron impact are very low in cometary X rays. The obtained spectral index α = 2 differs significantly from α ∼ 5 for scattering of solar X rays by attogram dust. These facts favor charge transfer of heavy solar wind ions as the dominant process of X ray excitation in comet Hyakutake. Charge transfer is not efficient in production of X rays on the Moon and other solid bodies.

Remote sensing of mesospheric temperature and O2(1Σ) band volume emission rates with the high‐resolution Doppler imager

An algorithm for obtaining temperatures and volume emission rate profiles in the mesosphere and lower thermosphere from HRDI measurements is described. Temperature and volume emission rates are derived via perturbation theory by comparing measured brightness to brightness computed from the forward model described here. Error analysis shows that the method recovers the temperatures with an error of 7° K and volume emission rates with an error of 3% from 80 to 100 km, providing global measurements of these fields in this altitude range for the first time. Results show the structure of the cold summer mesopause near the poles, the semiannual oscillation, and the diurnal tidal oscillations at the equator.

The morphology of oxygen greenline dayglow emission

In this study, the morphology of the oxygen greenline dayglow emission is presented. The volume emission rate profiles are obtained by using Solomon's glow model. The glow model is updated in terms of recent cross sections, reaction rate coefficients and quantum yield of greenline emission. Throughout most of the thermosphere the modelled and observed emission rates are in reasonably good agreement. In the region between 98 and 120 km, the modelled emission rates are substantially higher (about a factor of 1.7) than the observed emission rates. This discrepancy is discussed in terms of scaling of solar fluxes which accounts the variation of solar activity for the day on which calculations are made. The modelled morphology of greenline emission is compared with those cases where WINDII data is available. The modelled and observed morphology is in reasonably good agreement at most of the latitudes above 120 km. In the mesosphere the qualitative nature of morphology is very similar to those of WINDII observation except the modelled emission rates are about a factor of 1.7 higher than the observed emission rates.Keywords. Ionosphere (ion chemistry and composition; modeling and forecasting; solar radiation and cosmic ray effects).

Seasonal and local time variations in the O( 1S) green line, O 2 atmospheric band, and OH Meinel band emissions as measured by the High Resolution Doppler Imager

The High Resolution Doppler Imager (HRDI) instrument on the Upper Atmosphere Research Satellite (UARS) has been collecting nearly simultaneous altitude profiles of the O( 1S), O 2( 1Σ) , and OH (9-4) Meinel band emissions in the mesosphere and lower thermosphere (MLT) region during the nighttime portion of the satellite orbit since March of 1994. The length of the data record permits seasonal variations of the emissions to be examined and correlated with seasonal changes in the tidal influence of the wind field. The UARS spacecraft precesses in local time so that all nighttime hours are covered every 36 days. The volume emission rate profile can therefore be mapped as a function of local time and latitude approximately every month. This paper discusses the retrievel method, presents the height and brightness variations of the three emissions for one year of data and shows the local time variation of the OH (9-4) emission during March–April, 1994.