Excitation of O(1S) and emission of 5577 Å radiation in aurora

The results of recent laboratory experiments suggest that the reaction N⁺ + O₂ → NO⁺ + O(¹S) is the principal source of O(¹S) in aurora. A negligible time delay between auroral ionization and O(¹S) production is associated with this indirect process, which is a necessary condition for a viable mechanism. The $\text{5577}\text{A}\text{?}\text{3914}\text{A}\text{?}$ volume emission rate ratio associated with this production source remains constant with altitude. The problems encountered by the currently accepted source of O(¹S), the reaction of N₂(A³Σ) molecules with atomic oxygen are explored, and the contributions of this and other reactions to the auroral green line emission are reevaluated.     

Energy transfer from excited N2 and O2 as a source of O(1S) in the Aurora

It is proposed that energy transfer from excited O₂ contributes to the production of O(¹S) in aurora. An analysis is presented of the OI5577 Å emission in an IBC II⁺ aurora between 90 and 130 km. The volume emission rate of the emission at these altitudes is consistent with the production rate of O(¹S) by energy transfer to O(³P) from N₂ in the A³Σ₂⁺ state and O₂ in the A³Σ_u⁺, C³Δc¹Σ_u^? states, the N₂A state being populated by direct electron impact excitation and B → A cascade and the excited O₂ states by direct excitation. Above the peak emission altitude (~105 km), energy transfer from N₂A is the predominant production mechanism for O(¹S). Below it, the contribution from quenching of the O₂ states becomes significant.     

The height,spectrum and mechanism of type-B red aurora and its bearing on the excitation of O(1S) in aurora

The height of the lower red border of type-B aurora has been determined by triangulation using TV cameras at two ground stations. A mean height of 91.4 ± 1.1 km was determined from a set of 12 measurements made under ideal conditions. A TV spectrograph was used simultaneously to seek possible spectral changes between 6400 and 6900 Å which would be indicative of changes in the vibrational distribution in the N₂ First Positive bands. No significant difference was found in this distribution between the spectra from 93 and 122 km. The height distribution of contributions to the OI 5577 Å emission relative to the N⁺₂ First Negative emission was modelled from 80 to 160 km. Contributions from electron impact on atomic O, O⁺₂ dissociative recombination and N₂(A)O energy transfer were included. Account was taken of recent laboratory data on O(¹S) quenching. It was concluded that these processes could explain the excitation of O(¹S) in normal aurora and the height distribution of OI 5577 Å in type-B red aurora. It was confirmed that the lifetime ofO(¹S) in type-B red auroral rapid time variations is about 0.5 s and it was found from the model that the observed time variation can be reproduced by the mechanisms considered, provided the concentration of NO in the auroral atmosphere is about 1 × 10⁹ at 95 km. Before reasonable certainty can be attained in the correctness of the interpretation it will however be necessary to have reliable simultaneous observations of neutral atmospheric composition particularly for O and NO as well as unchallengeable measurements of the yields of O(¹S) for the processes considered and for several other processes which have been suggested recently.     

Ground-based observations of O2 (b1Σ+g−X3Σ−g) atmospheric bands in high latitude auroras

Analysis of observed spectrograms is based on comparison with synthetic spectra. The O₂(b¹Σ⁺_g?X³Σ^?_g Atm. (1,1) band in high latitude auroras observed from the ground is found to be the strongest in the Δv = 0 sequence. It is enhanced with altitude relative to the N₂ 1P(2, 0)and N⁺₂ M(2,0) bands, but the O₂ Atm. (2, 2) band has an unexpected low intensity. The range of rotational temperatures of the O₂ Atm. bands varies from approx. 200 to above 500 K which indicates that the altitude of the centroid of the emission region varies from about 100 km to the F-region. The highest temperature is found in the midday aurora associated with the magnetospheric cusp. Conspicuous relative variations between the intensities of N₂ and O₂ spectra are documented, but a satisfactory explanation of the variety is not given. Deviations of the observed O₂ Atm. band intensities from the vibrational intensity distribution predicted by Franck-Condor factors indicate that the excitation of the O₂ Atm. bands in aurora is not mainly due to particle impact on O₂, and the contribution due to energy transfer from hot O(¹D) atoms has to be found in future research.     

Neutral temperatures and emission height changes in an E-region aurora

In this study a method is outlined which is capable of giving neutral temperatures and height changes in the aurora when the molecular emissions originate from the E-region.Absolute spectrometric measurements of N₂⁺ 1NG and O₂⁺ 1NG bands and the auroral green line are performed in a nightside aurora. Rotational temperatures and band intensities are deduced by a least-squares fit of synthetic spectra to observations. There is a close correlation between the variations in rotational temperatures and the relative intensity ratio of N₂⁺ 1NG(0,3) and O₂⁺ 1NG(1,0) bands. The change in the relative intensity ratio is similar to the intensity variation predicted by the changing N₂ and O₂ densities from 120 to 150 km, obtained from the MSIS 83 model atmosphere, and the derived neutral temperature variations are consistent with a similar change in emission height of the aurora. Therefore the changing temperature is most likely due to a changing emission height of the aurora, and no local heating can be inferred.     

Energy distribution function of translationally hot O(3P) atoms in the atmosphere of Earth

Translationally hot O(³P) atoms are produced in the atmosphere of Earth by photolysis of O₂ and O₃ and quenching of O(¹D). A rigorous kinetic theory analysis of this problem is developed and compared with the approach previously employed by Logan and McElroy [Planet. Space Sci.25, 117 (1977)]. It is shown that the kinetic theory employed by the previous workers is somewhat deficient. With the line-of-centers cross-section, the rates of reactions of the translationally hot O(³P) atoms with other atmospheric gases are calculated and found to be in some instances many orders of magnitude larger than the equilibrium rates. Though the non-equilibrium reaction rates with O(³P) are substantially increased, they are still not competitive with the corresponding reaction rates with O(¹D).     

Rocket measurements of oxygen and nitrogen emissions in the aurora

Altitude distributions of electronically excited atoms and molecules of oxygen and nitrogen in the aurora have been obtained by means of rocket-borne wavelength scanning interference filter photometers launched from Fort Churchill, Manitoba (58.4°N, 94.1°W) on January 23, 1974. Atomic oxygen densities derived from mass spectrometer measurements obtained during the flight are used in conjunction with the volume emission rate ratio of the N₂(C³Π_u?B³Π_g) (0-0) second positive and N₂(A³Σ_u⁺, v = 1?X¹Σ_g⁺) Vegard-Kaplan bands to derive a rate constant for quenching of the N₂(A³Σ_u⁺, v = 1) level with O(³P) of 1.7(±0.8) × 10^?11 cm³ s^?1 These data, together with O den derived from the O₂(b¹Σ_g⁺) state nightglow emission observed during the rocket ascent, suggest that quenching of the N₂(A³Σ_u⁺, v = 1) level by O₂ has a significant positive temperature dependence. The processes involved in the production and loss of the N₂(A³Σ_u⁺) state are considered and energy transfer from the N₂(A³Σ_u⁺) state to O(³P) is found to be a significant source of the OI 5577 Å green line in this aurora at altitudes below 130 km. Emission from the NO(A²Σ⁺?X²Π) gamma bands was not detected, an observation which is consistent with the mass spectrometer data obtained during the flight indicating that the NO density was <10⁸ cm³ at 110 km. On the basis of previous rocket and satellite measurements of the NO gamma bands, energy transfer from the N₂(A³Σ_u⁺) state to NO(X²Π) is shown to be an insignificant source of the gamma bands in aurora. Altitude profiles of the N₂(a¹Π_g?X¹Σ_g⁺) Lyman-Birge-Hopfield band system are presented.     

194.1 keV resonance contribution on 17O(p,α)14N reaction rate using R matrix code

Recently, a new resonance at E _Rlab =194.1 keV corresponding to a level in ¹⁸F at E _x =(5789.8±0.3) keV has been observed for the first time. In view of these new parameters, we propose here to examine this reaction using a numerical code based on the R-matrix method to show its contribution to the ¹⁷O(p,α)¹⁴N reaction rates. We also study the cross section as well as the differential cross section of the ¹⁷O(p,α)¹⁴N reaction. It is found that this resonance predominates the reaction rates in all the range of the stellar temperature for classical novae explosions. Our results are in good agreement with those of a recent experimental study.     

Solar Wind Control of Jupiter's H3Auroras

A program of observations of Jupiter's H⁺₃auroras was conducted during the first 4 months of 1992, a period that spanned the Ulysses encounter. Daily variations of auroral intensity are small in magnitude (∼20%) and well correlated with variations in the solar wind ram pressure arriving at Jupiter. A much larger (factor of 3) time variation is observed to occur on a time scale of approximately 2 months. During the first 2 months of observation, the inferred total intrinsic intensity of the northern aurora exceeded that of the southern aurora by a factor of 2. Throughout the latter 2 months, characterized by less intense auroral activity, the intensity of the northern aurora was comparable to that of the southern aurora.     

On the temperature dependence of the reaction O + NO → NO12

The published data on the temperature dependence of the radiative combination of atomic oxygen with nitric oxide at pressures near 1 torr is examined. Arguments are advanced to suggest that radiation near the cut-off wavelength (～ 3875Å) is coming from the unstabilized activated complex, No¹₂. At 4000Å a positive activation energy of 1 kcal mole^?1 is deduced. Application of this temperature dependence with the rate coefficient at 5200Å is made to airglow measurements in aurora. The deduced NO concentration is about 10⁹ cm^?3, in general agreement with that deduced from the measured NO⁺/O⁺₂ ratio as well as an auroral model prediction.     

The excitation of atomic oxygen to the O(1S) level by energy transfer from N2(A3∑u+) molecules in aurora

The part that the energy transfer reaction $\text{N}{}_2\text{(A}{}^3\text{∑}{}_{\mathrm{u}}{}^{\mathrm{+}}\text{) + O(}{}^3\text{P) → N}{}_2\text{(X}{}^1\text{∑}{}_{\mathrm{g}}{}^{\mathrm{+}}\text{) + O(}{}^1\text{S)}$ plays in the excitation of the auroral green line has been investigated. The contribution is estimated to be 40 per cent in this case, containing pulsating aurora in class IBC 1. Due to greater quenching of the A³∑_u⁺ state, the centroid of the VK emission is displaced 10 km upwards of the green line height, which is centred at 110 km.     

Evolution of CO on Titan

The early evolution of Titan's atmosphere is expected to produce enrichment in the heavy isotopomers of CO, ¹³CO and C¹⁸O, relative to ¹²C¹⁶O. However, the original isotopic signatures may be altered by photochemical reactions. This paper explains why there is no isotopic enrichment in C in Titan's atmosphere, despite significant enrichment of heavy H, N, and O isotopes. We show that there is a rapid exchange of C atoms between the CH₄ and CO reservoirs, mediated by the reaction ¹CH₂+*CO→¹*CH₂+CO, where *C is ¹³C. Based on recent laboratory measurements, we estimate the rate coefficient for this reaction to be 3.2×10⁻¹² cm³ s⁻¹ at the temperature appropriate for the upper atmosphere of Titan. We investigate the isotopic dilution of CO using the Caltech/JPL one-dimensional photochemical model of Titan. Our model suggests that the time constant for isotopic exchange through the above reaction is about 800 Myr, which is significantly shorter than the age of Titan, and therefore any original isotopic enhancement of ¹³C in CO may have been diluted by the exchange process. In addition, a plausible model for the evolution history of CO on Titan after the initial escape is proposed.     

1302Å absorption measurement of atomic oxygen concentrations in the high latitude winter upper atmosphere

The first measurement of atomic oxygen concentration in the upper atmosphere using the resonance absorption lamp method was made 23 January 1974 in a flight from the Churchill Research Range (58.4°N, 94.1°W, geographic) in northern Canada. Earlier difficulties with the data analysis have been resolved to a limited extent, and a concentration profile over the range 90–155 km is presented here. In the interim period, the same technique has been employed by a number of investigators, but the importance of the present data is that the atomic oxygen concentration was measured by three independent methods on the same flight, aeronomical and mass spectrometric methods being used as well as resonance absorption. The results are in good agreement when current aeronomical understanding is employed. Comparison is made with other measurements and it is concluded that the high degree of variability observed may well be real. A simple argument based on these data shows that O₂(c¹Σ) cannot be the sole source of O(¹S) and O₂(b¹Σ) in the aurora.     

Neutral particle release from Europa’s surface

In this paper, we look at space weathering processes on the icy surface of Jupiter’s moon Europa. The heavy energetic ions of the jovian plasma (H⁺, O⁺, S⁺, C⁺) can erode the surface of Europa via ion sputtering (IS), ejecting up to 1000 H₂O molecules per ion. UV photons impinging the Europa’s surface can also result in neutral atom release via photon-stimulated desorption (PSD) and chemical change (photolysis). In this work, we study the efficiency of the IS and PSD processes for ejecting water molecules, simulating the resulting neutral H₂O density. We also estimate the contribution to the total neutral atom release by the Ion Backscattering (IBS) process. Moreover, we estimate the possibility of detecting the sputtered high energy atoms, in order to distinguish the action of the IS process from other surface release mechanisms. Our main results are: (1) The most significant sputtered-particle flux and the largest contribution to the neutral H₂O density come from the incident S⁺ ions; (2) the H₂O density produced via PSD is lower than that due to sputtering by ∼1.5 orders of magnitude; (3) in the energy range below 1 keV, the IBS can be considered negligible for the production of neutrals, whereas in the higher energy range it becomes the dominant neutral emission mechanism; (4) the total sputtering rate for Europa is 2.0 × 10²⁷ H₂O s⁻¹; and (5) the fraction of escaping H₂O via IS is 22% of the total sputtered population, while the escape fraction for H₂O produced by PSD is 30% of the total PSD population. Since the PSD exosphere is lower than the IS one, the major agent for Europa’s surface total net erosion is IS on both the non-illuminated and illuminated side. Lastly, the exospheric neutral density, estimated from the Galileo electron density measurements appears to be higher than that calculated for H₂O alone; this favors the scenario of the presence of O₂ produced by radiolysis and photolysis.     

A uniform belt of diffuse auroral emission seen by the ISIS-2 scanning photometer

One of the most striking and persistent features in high latitude regions as seen by the ISIS-2 scanning auroral photometer is a fairly uniform belt of diffuse auroral emission extending along the auroral oval. Indications are that this region follows, contributes to, and may in a sense actually define the auroral oval during quiet times.The diffuse belt is sharply defined at its equatorward edge, which is located at an invariant latitude of about 65° in the midnight sector during relatively low magnetic activity (K_p = 1?3). The poleward edge of the region is not as sharply defined but is typically at about 68°. Discrete auroras (arcs and bands) are located, in general, near the poleward boundary of the diffuse aurora. The position of the belt appears to be relatively unaffected by the occurrence of individual substorms, even when discrete forms have moved well poleward. Representative intensities at 5577 Å are 1–2 kR (corrected for albedo) at quiet times and may reach 5 kR during an auroral substorm.It appears that the mantle aurora and proton aurora constitute this diffuse aurora in the midnight sector. Precipitating protons and electrons both contribute to the emissions in this region.     

Contribution of 19F resonances on 18O(p,α)15N reaction rate

The ¹⁸O(p,α)¹⁵N reaction influences the isotopes production such as ¹⁹F, ¹⁸O, and ¹⁵N which can be used to test the models of stellar evolution. ¹⁹F is synthesized in both asymptotic giant branch (AGB) and metal-rich Wolf-Rayet (WR) stars. Using R-matrix theory we allow new values of resonances parameters in ¹⁹F. We show that the most important contribution to the differential and total cross section at low energies, comes from the levels in ¹⁹F situated at resonances energies E _R =151, 680 and 840 keV with spin and parity 1/2⁺. The total width of the 680 keV resonance is badly known. So, we have focused on this broad resonance corresponding to the 8.65 MeV level in ¹⁹F. We delimit the temperature range in which each resonance contribution to the total reaction rate occurs by analyzing the ratio (N _A 〈σν〉 _i /N _A 〈σν〉). This allowed us to show that the 680 and 840 keV broad resonances strongly dominate the reaction rate over the stellar temperature range T ₉=0.02–0.06 and T ₉=0.5–5. Finally, these results were compared to NACRE and Iliadis astrophysical compilations.     

Modeling of the N2+ First negative bands in the sunlit aurora

The N_2⁺ First Negative band profiles in the high-altitude sunlit aurora are modeled by solving a set of simultaneous coupled equations for the population and depopulation of the N_2⁺ vibrational and rotational energy levels. Approximations due to computer processing time and the use of1-→A averaged solar flux resulted in the loss of the Swings effect, but otherwise the modeled spectra simulate the observations and the characteristic high rotational development quite closely. Comparisons with the well-known spectrum of the 3914 Å band in the sunlit auroral ray published by Hunten et al. (1959, Nature 183, 453) give the N_2⁺ ion lifetime of ∼10³ s and N₂X rotational temperature of ∼600 K, and are consistent with the fluorescent excitation mechanism; this contradicts conclusions made some two decades ago. A sample auroral cusp spectrum is included to illustrate the effects of Rayleigh scattering of solar flux.     

N2 vibrational distribution in aurorae

The N₂ vibrational distribution is calculated for a specific IBC Class II aurora using as input, data obtained from coordinated rocket and satellite observations and currently accepted excitation and quenching rates. The results of the calculations indicate no significant vibrational enhancements for this specific aurora nor for “upper limit” estimates for more intense aurorae. We conclude from this study that if significantly larger concentrations of vibrationally excited N₂ molecules exist in the aurora, as recent rocket e.u.v. measurements suggest, current concepts of the sources and sinks of N₂ vibrational excitation will require significant revision.