International Reference Ionosphere (IRI) (II)
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Global modeling of hot O distribution in the upper thermosphere
Earth, Planets and Space volume 63, pages 391–396 (2011)
The existence of hot oxygen (hot O, Oh) in the upper thermosphere is mainly confirmed by optical observations of high-altitude airglow. In the experiments described here, a peak of Oh population was found at an altitude of approximately 550 km with a temperature of about 4000 K. Although it was shown that Oh concentration could reach a value of 1–2% with respect to ambient (cold) O, a realistic global distribution of Oh concentration and temperature has not been established. The presence of non-thermal atoms in the thermosphere leads to variations in the thermo-dynamical regime in the upper atmosphere. The major chemical processes involved in Oh production were taken into account in the time-dependent, Global Self-consistent Model of Thermosphere, Ionosphere and Protonosphere (GSM TIP) of the Earth in order to simulate global distribution of Oh concentration and temperature (Th). Calculations were carried out in the geomagnetic coordinate system for moderate solar, quiet geomagnetic conditions, and winter season. It was shown that the maximum Oh is located at −60° latitude, 300° longitude, and 24 UT. The Th maximum is about 2050 K. This temperature and Oh concentration cause an increase in neutral gas temperature at high thermosphere by ∼100 K during daytime and by ∼70 K during nighttime. Variations in the neutral gas velocity circulation were calculated. The maximum increase in neutral velocity was about 36 m/s, corresponding to Φ = 50°, Λ = 180° in the northern and Φ = −50°, Λ = 270° in the southern hemisphere.
Experimental evidence for the existence of hot oxygen (hot O, Oh) in thermosphere was first presented by Yee et al. (1980) who inferred a population of Oh atoms from twilight measurements of O+ emission at 732 nm. At an altitude of 550 km and temperature of ≥4000 K, the number density reaches 105–106 cm−3. Further experimental evidence was supplied by Hedin (1989), who estimated Oh densities to be equal to (1–3) · 105 cm−3 at an altitude of 550 km based on differences between MSIS model and satellite drag models. The high-resolution rocket data on atomic oxygen dayglow emissions at altitudes ranging from 150 to 960 km was obtained by Cotton et al. (1993a). These researchers reported that the modeled 130.4-nm and 98.9-nm emission lines underestimated the measured intensities of oxygen. Analyses of radar data have shown that a heat source is needed to explain the detected ion temperature (Oliver, 1997): the calculated Oh concentration was insufficient to account for the observed excess EUV emission.
Cotton et al. (1993b) subsequently reanalyzed the data incorporating the effects of Oh. A comparison of the new results with those obtained from the standard model of a neutral atmosphere shows that the Oh geocorona has an intensity peak as high as 106 cm−3 at 550 km, with the temperature of Oh being 4000 K.
A large amount of Oh is created as a result of photo-chemical processes. The importance of chemical sources for Oh geocorona has been discussed in detail by Gerard et al. (1995). The most complete list of the chemical reactions for Oh is given in papers by Richards et al. (1994) and Hickey et al. (1995).
Modeling of the Oh concentration is rather complicated procedure, and researchers very often adopt a diffusive equilibrium for the altitude profile of the Oh concentration. Various procedures have been adopted in attempts to obtain an altitude profile of Oh concentration. In the stochastic calculations of the oxygen energy distribution function (EDF) described by Shematovich et al. (1994), a non-thermal oxygen concentration of ∼1% with respect to cold O content was obtained with a characteristic hot temperature of ∼4000 K. In the study of Shoendorf et al. (2000), Oh profiles were obtained using mass and energy equations methods, and the ionosphere parameters (Ti, Te, Ne) were taken from International Reference Ionosphere (IRI) (Bilitza, 2001). However, these authors were used a simplified method of deriving Oh profiles and the variation in Oh temperature with changing altitude.
The loss processes of hot atoms are governed by thermalization. These processes include translational energy or velocity relaxation through collisions with molecules and atoms of ambient gases and ions. Inelastic collisions may also occur in which the kinetic energy of atoms is transferred to the rotational and vibrational motion of the ambient gas molecules. Obviously, the presence of oxygen atoms with energies exceeding the thermal energy of ambient oxygen in the exospheric altitudes would lead to a change in the thermodynamic regime of the upper thermosphere. It should be noted that this problem has as yet not been properly considered.
In the study reported here, we used the Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) of the Earth to carry out numerical calculations of Oh density and temperature on the global scale. We also studied the effect of Oh on temperature and circulation of the neutral gas.
Model calculations were executed for the winter solstice, moderate solar activity, and quiet geomagnetic conditions. Global distributions of Oh concentration and temperature were obtained by solving corresponding equations numerically using the appropriate technique.
2. Brief GSM TIP Description
The Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere was developed in the West Department of IZMIRAN of the Russian Academy of Sciences and simulates the time-dependent global structure of the near-Earth space environment from 80 km to 15 Earth radii.
In the thermospheric block of the model, global distribution of the neutral gas temperature (Tn) and of N2, O2, O, NO, N(4S), and N(2D) concentration, as well as the three-dimensional circulation of the neutral gas and , and NO+, and also their temperature (Ti) and velocities (Vi), are calculated in the range from 80 to 520 km in the spherical geomagnetic coordinate system. In the vertical dimension, the thermospheric code uses 30 layers, with each layer approximately equal to a thickness of one scale height.
In the ionospheric section of the model, global time-dependent distributions of ions, electron temperatures (Ti, Te), vector velocity (Vi), and O+ and H+ ion concentrations are calculated in the magnetic dipole coordinate system from 175 km in the northern hemisphere to 175 km in the southern hemisphere. In this case, the ionosphere code for atomic ions does not require the upper boundary condition. The convection patterns imposed by the magnetospheric electric field on plasma movements in the polar region are referenced to a fixed Sun-Earth frame, assuming pure E × B drifts. The electric field is derived from calculation of the two-dimensional distribution of the electric field potential of dynamo and magnetospheric origin. The solution of the full system of equations of the model is performed numerically on a global grid with resolutions of 5° in latitude and 15° in longitude as specified in the spherical geomagnetic coordinate system; the time step is 2 min.
Model inputs are the solar EUV and UV spectra (10–1760Å). The precipitating electron fluxes and distribution of the field-aligned currents in the first- and second high-latitude region are specified in the solar-magnetic frame. The transformations between all coordinate systems in the model are given by standard formulas.
In our model, the first region of the field-aligned currents (FACs) is located at ±75° magnetic latitude, while the location of the second region of the FACs is at ±65°. The values of FACs were adjusted so that the polar cup potential difference was in agreement with the statistical model of Oliver et al. (1983) for the quiet geomagnetic conditions (Kp ∼ 2). For the solar EUV flux, we have used the technique of Nusinov (1984) to construct the flux spectra for the period under study. For the electron precipitations, we mark out several precipitation zones. At both hemispheres, we have an auroral oval electron flux precipitation with a characteristic energy of 3 keV and a maximal flux of ∼4 erg/(cm2 s) at 00 MLT (Magnetic Local Time). Further, we have the soft electron flux in the cusp region of 0.2 keV, an energy flux of 0.2 erg/(cm2 s) and the diffusive precipitation in the nighttime sector with 0.1 keV electrons, an energy flux of ∼0.1 erg/(cm2 s). The spectral characteristic of the soft and energetic precipitating electrons was chosen according to the Maxwellian energy distribution. The spatial distribution has Gaussian form in both the longitudinal and latitudinal directions. The adopted auroral precipitating energy flux and spatial distribution are typical for lower geomagnetic conditions and in accordance with the statistical model of Hardy et al. (1985).
3. Hot Oxygen Model Equations
Two three-dimensional equations for Oh describe mass and energy conservation:
Where Th, nh are the temperature and density of the Oh, V is a vector of average mass velocity, qh, lh represent the sources and losses of Oh, Φ is a vertical flux due to diffusion, Ph is a pressure, λ is a conductivity for Oh, Q and L are Oh thermal energy sources and losses, and k is Boltzmann’s constant. The molecular diffusion coefficient for Oh differs from that of ambient oxygen and is calculated in accordance with the concepts of diffusion for a two-temperature mixture of gases (Ivanovsky et al., 1967).
From the 27 possible sources of Oh in the thermosphere that are presently known (Hickey et al., 1995), we take into account only main reactions. The list of the adopted reaction is presented in Table 1.
All components of the chemical reactions listed in Table 1, except for the concentration of vibrationally excited nitrogen (), are modeled in the GSM TIP. The concentration and temperature were adopted as reported in the paper by Korenkov et al. (1996):
where Tv is the temperature of , and i is the vibrational levels from 1 to 2. The temperature of the vibrationally excited nitrogen was approximated by a ·Te, where a is a coefficient ranging from 1.1 to 1.3 in our calculations.
The losses of Oh occurs in collisions with cold atoms and molecular and ions of O+:
where are the collision frequencies between Oh and O+, cold O, and N2, respectively.
Represent collisions are accompanied with heat transfer from Oh to cold neutral species and ions:
where Ti, Tn are the temperatures of the ion and cold neutral gas, respectively.
The boundary conditions for the Oh mass equation are chemical equilibrium at lower boundary and diffusion equilibrium at upper boundary. For the energy equation at lower boundary we put Th = Tn (cold), while at upper boundary we adopt dTh/dr = 0. The time step was chosen to be 2 min. The model equations were integrated until the solution reaches the quasi-stationary regime.
Input parameters are based on accumulated experimental data and represent average values for the geophysical condition under study. Input data cannot vary within wide limits for the given conditions.
Results of model calculations are presented in Figs. 1–3 in the geomagnetic Cartesian coordinate system for 24 UT for various values of Tv and, accordingly, . Several variants of the calculations were carried out, which are different in approximations for Tv. The main results were obtained at Tv = 1.1 · Te, where Te is electron temperature. Figures 1–3 correspond to that very case, unless specifically indicated otherwise.
Figure 1 shows the logarithm of the global distribution of Oh concentrations (cm−3) (Fig. 1(a)) and of temperature (K) of Oh (Fig. 1(b)) at 500 km. From the global distribution of Oh concentration, it can be seen that the maximum of lg(nh) is about 3.5 and located at −60° latitude and 285° longitude at these altitudes. The maximum Oh temperature is about 2050 K at the same altitude and located at −40° latitude and 270° longitude. Both maxima are detected in the southern-summer hemisphere. Global minima of the Oh concentration and temperature are located in the northern-winter hemisphere. Note here that maximum of Oh density does not coincide in space with the temperature maximum.
The altitude profiles of the Oh temperature and density corresponding to three model runs for various Tv values at the points of global temperature maximum and minimum are presented in Fig. 2(a) and (b), respectively. Solid lines correspond to the main version of calculations (Tv = 1.1 · Te), while dashed and dash-dotted lines correspond to Tv = 1.2 · Te and Tv = 1.3 · Te, respectively.
As can be seen from Fig. 2, the temperature profiles of Oh are similar to the standard temperature profile of neutral gas in, for example, the MSIS or IRI models because the thermal conductivity of Oh concentration is large enough to smooth any significant concentration profile variations. The temperature of Oh at daytime conditions is about twofold that of the standard temperature of neutral gas and may be as high as 2000 K (the main version of calculations), possibly even reaching a value of 3900 K for a model run with Tv = 1.3 · Te.
The altitude profiles of nh have pronounced maxima at altitudes of <200 km. A daytime maximum for the main version of calculations is located at about 180 km and reaches a value of 4 · 105 cm−3. A nighttime maximum is located at about 160 km and is as high as 4 · 104 cm−3. These maxima are due to chemical reactions in the lower thermosphere and diffusive equilibrium at the higher altitudes.
This temperature and Oh concentration increase the neutral gas temperature in the high thermosphere and modify the neutral gas circulation. Th spatial distribution of gain at the neutral temperature due to Oh heating is presented in Fig. 3(a) where Tn can be seen to increase by ∼100 K during the daytime and by ∼65 K during the nighttime.
Figure 3(b) illustrates the variability of the dynamical regime at ionosphere altitudes on a global scale. The distribution of neutral gas velocity gain due to taking into account Oh and normal circulation (without Oh) is shown in Fig. 3(b). The maximum increase in neutral gas velocity is about 36 m/s and corresponds to Φ = 50°, λ = 180° in the northern and Φ = −50°, Λ = 270° in the southern hemisphere at an altitude of 500 km.
As was mentioned above, Oh attracts attention because of discrepancies between experimental data (mostly from observations of O emissions) and theoretical estimates of ambient O near the exobase and above.
Most estimates give values for the effective Oh temperature that fall in a range of 4000–6000 K. In terms of its concentration, Oh is estimated to be present at between about 0.1% and 1% relative to the cold O content at an altitude of 400 km (Shematovich et al., 1994; Oliver, 1997; Litvin and Oliver, 2000).
The major part of the experimental data present here was obtained under the assumption that Ti and UV airglow intensity calculated from theory were significantly lower than those measured. This discrepancy could be reduced by including an additional heat source, for example, Oh.
Although qualitative estimates of Oh absolute density as a function of altitude do exist, a definitive density profile shape has not yet been established. At present, it is assumed that Oh is more likely to form a layer (Cotton et al., 1993a; Schoendorf et al., 2000) than a concentration profile which decreases with altitude (Shematovich et al., 1994).
However, in practical calculations, a diffusive equilibrium density profile is used with a reference density of 0.1–1% at an altitude of 400 km (Alcayde et al., 2001; Zettergren et al., 2006) with Th 4000–5000 K. These altitude profiles of the Oh population are taken in order to fit the experimental data. However, they do not have much physical basis.
Figure 4 illustrates some samples of the Oh density profile that have been presented by other authors
As can be seen from Fig. 4, the dispersion of experimental estimates of Oh concentration at 400–500 km is very large.
The calculation results within our model are also presented Fig. 4. However, one can directly compare model values and experimental estimates only at altitudes within the indicated range, where measurements of the appropriate parameters are carried out.
We obtained a density profile of Oh with a peak located at about 200 km, reaching the value of about 4 · 105 cm−3. These results do not agree with experimental data. Therefore, the model calculations show that the density of Oh at 400 km is about 104 cm−3 or 0.001% of the cold O concentration, while the experimental estimates give 0.1% with respect to cold O content. The temperature of Oh is about 2000 K in the GSM TIP model calculations, which is somewhat less than the universally adopted value. There could be several explanations for these discrepancies.
Firstly, our model does not take into consideration some of the chemical reactions mentioned in the paper by Hickey et al. (1995). Most of these reactions involve metastable species, for example, O(1D), O(1S), and electron exited ions, such as O+(2D) and O+(2P). The importance of these reactions lies in that they allow the electronic energy transfer to another species at the higher altitudes. However, Hickey et al. (1995) and Richards et al. (1994) studies these reactions, although they did not present the altitude profiles of the density and temperature of Oh. Another reason for the low Oh concentration and temperature in the model calculations could be the choice of inappropriate boundary conditions, in which the flux of Oh and energy are not included. These fluxes arise from ion-neutral reactions in the plasmasphere. Bisikalo et al. (1995) proposed energetic O+ ions precipitation to be a possible source of high-latitude Oh concentration, and hence the boundary conditions must be based on Oh temperature and density values found in the experimental data and literature. Another important point in terms of the model calculations is the choice of the temperature of the vibrationally excited N2 because this component plays an important role in the chemistry and energetics of the ionosphere (Richards and Torr, 1986; Korenkov et al., 1996). As can be seen from Fig. 2, a large Tv value produces a significant variation in Oh density and temperature. We note here that the vibrational temperature is substantially higher than the neutral temperature and a little less than the electron temperature (Richards and Torr, 1986). However, the exact value of the Tv has not yet been reliably established.
The GSM TIP model can simulate the global distributions of all thermosphere/ionosphere parameters. In this study, we present global distributions for the temperature and concentration of Oh and their effect on the thermospheric temperature and global circulation of neutral gas. As can be seen from Figs. 1 and 2, both maxima of Th and Oh are formed during the daytime, although Oliver and Schoendorf (1999) pointed out that Oh density at an altitude of 400 km is the lowest during the day and highest at nighttime. However, this assumption is scarcely relevant since all of the sources of Oh are regulated by the sun.
Increases in Tn (Fig. 3(a)) cause pressure disturbances in the upper thermosphere and, consequently, variations in the neutral gas velocity (Fig. 3(b)). It is noted here that the largest corrections to Tn took place in the evening sector, 90 K at Φ = −60° and Λ = 360° and 36 m/s in the latitudinal component of a vector neutral gas in the night sector.
Thus, our simulations of global distribution of Oh density and temperature within the GSM TIP model show that even a small fraction of Oh has a significant impact on the thermo-dynamical structure of the upper atmosphere. On the other hand, the issue of the self-consistent modeling of Oh remains unsolved and requires further study.
We have presented calculation results on the spatiotemporal distribution of Oh concentration and temperature on the global scale. These results were obtained using GSM TIP model.
The results of this study indicate that the Oh density profile has layer shape, with a peak located at about 200 km altitude.
Our calculations show that a small fraction of Oh (< 0.5% with respect to cold oxygen content) had a significant impact on the heat budget and dynamical regime of the neutral gas. Hot O causes an increase in neutral gas temperature up to 100 K at daytime and up to 65 K at nighttime. The vector velocity value rises to 36 m/s.
Our calculations also show that reactions with vibrationally exited N2 are very important for the production of Oh.
A number of problems, such as chemical processes, boundary conditions, and others, need to be solved.
These results may be used in the analysis of the energy and dynamical regimes of the Earth’s upper atmosphere.
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The authors thank the referees for critical reading manuscript and helpful comments.
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Bessarab, F.S., Korenkov, Y.N. Global modeling of hot O distribution in the upper thermosphere. Earth Planet Sp 63, 391–396 (2011). https://doi.org/10.5047/eps.2011.01.009
- Global modeling of the upper atmosphere
- photochemical processes
- hot oxygen