Ejection of nitrogen from Titan's atmosphere by magnetospheric ions and pick-up ions

A 3-D Monte Carlo model is used to describe the ejection of N and N₂ from Titan due to the interaction of Saturn's magnetospheric N⁺ ions and molecular pick-up ions with its N₂ atmosphere. Based on estimates of the ion flux into Titan's corona, atmospheric sputtering is an important source of both atomic and molecular nitrogen for the neutral torus and plasma in Saturn's outer magnetosphere, a region now being studied by the Cassini spacecraft.     

On the structure of Mars' atmosphere at 120–220 km

Altitude dependences of [CO₂] and [CO₂⁺] are deduced from Mariner 6 and 7 CO₂⁺ airglow measurements. CO₂ densities are also obtained from n_e radio occultation measurements. Both [CO₂] profiles are similar and correspond to the model atmosphere of Barth et al. (1972) at 120 km, but at higher altitudes they diverge and at 200–220 km the obtained [CO₂] values are three times less the model. Both the airglow and radio occultation observations show that a correction factor of 2.5 should be included into the values for solar ionization flux given by Hinteregger (1970). The ratio of [CO₂⁺]/n_e is 0.15–0.2 and, hence, [O]/[CO₂] is ～3% at 135 km. An atmospheric and ionospheric model is developed for 120–220 km. The calculated temperature profile is characterized by a value of T ≈ 370°K at h ? 220 km, a steep gradient (～2°/km) at 200-160 km, a bend in the profile at 160 km, a small gradient (～0.7°/km) below and a value of T ≈ 250°K at 120 km. The upper point agrees well with the results of the Lyman-α measurements; the steep gradient may be explained by molecular viscosity dissipation of gravity and acoustical waves (the corresponding energy flux is 4 × 10^?2 erg cm^?2sec^?1 at 180 km). The bend at 160 km may be caused by a sharp decrease of the eddy diffusion coefficient and defines K ≈ 2 × 10⁸cm²sec^?1; and the low gradient gives an estimate of the efficiency of the atmosphere heating by the solar radiation as ? ≈ 0.1.     

The upper atmosphere as a regulator of subauroral red arcs

The mechanisms for producing a subauroral red arc (SARARC) are studied by solving a system of basic ionospheric and atmospheric equations. It is shown that many of the observed features of a SARARC can be explained within the framework of the two processes generally responsible for the ionospheric behavior during a magnetic storm: these are (1) energy conduction from the magnetosphere to the ionosphere and (2) the changes in neutral composition of the lower atmosphere caused by the increase in turbulent mixing. Both the processes trigger a complex chain of events which ultimately results in the redistribution of both the charged and neutral particles, an increase in the electron, ion, and neutral temperatures, and a decrease in the electron density in the altitude region near the F2 peak. It is shown that both the occurrence and the emission intensity of a SARARC are regulated by the neutral atmosphere, even though conduction of the thermal energy from the magnetosphere to the ionosphere provides the excitation energy fo the optical remission. Recent satellite measurements of the ionospheric parameters have confirmed the validity of these findings and have provided grounds for rejecting several other theories which have been proposed in the literature.     

Effects of solar XUV variations on the thermosphere and the variability of the photoionization efficiency parameter during the solar cycle 21

Making use of the latest available semi-empirical atmospheric models, solar XUV radiations rates of photoionization and absorbed energy profiles have been graphically presented showing the latitudinal, seasonal and solar cycle variations. The photoionization limits of the major neutral constitutents of the terrestrial atmosphere O₂, O, and N₂ that occur at wavelengths 102.7, 91.2, and 79.6 nm, respectively have been quantified by showing the photoionization rates of O ₂ ⁺ , O⁺, and N ₂ ⁺ for different spectral groups both under quiet and different solar flare conditions. The variability of the photoionization efficiency parameter which is height-dependent, from winter to summer, for solar minimum to solar maximum for four significantly different latitudes under local noon conditions have been investigated during the solar cycle 21. More energy is required to produce an electron-ion pair in a denser atmosphere than in a thinner atmosphere and hence more energy is being deposited in the height range between 100–120 km which itself manifests in raising the electron gas temperatures higher than the neutral gas temperatures.     

Ion temperature troughs induced by a meridional neutral air wind in the night-time equatorial topside ionosphere

A mathematical model has been developed to calculate consistent values for the O⁺ and H⁺ concentrations and field-aligned velocities and for the O⁺, H⁺ and electron temperatures in the night-time equatorial topside ionosphere. Using the results of the model calculations a study is made to establish the ability of F-region neutral air winds to produce observed ion temperature distributions and to investigate the characteristics of ion temperature troughs as functions of altitude, latitude and ionospheric composition. Solar activity conditions that give exospheric neutral gas temperatures 600 K, 800 K and 1000 K are considered.It is shown that the O⁺-H⁺ transition height represents an altitude limit above which ion cooling due to adiabatic expansion of the plasma is extremely small. The neutral atmosphere imposes a lower altitude limit since the neutral atmosphere quenches any ion cooling which field-aligned transport tends to produce. The northern and southern edges of the ion temperature troughs are shown to be restricted to a range of dip latitudes, the limiting dip latitudes being determined by the magnetic field line geometry and by the functional form of the F-region neutral air wind velocity. Both these parameters considerably influence the interaction between the neutral air and the plasma within magnetic flux tubes.     

Geomagnetic storm effects on the thermosphere and the ionosphere revealed by in situ measurements from OGO 6

The temporal response of ion and neutral densities to a geomagnetic storm has been investigated on a global scale with data from consecutive orbits of OGO-6 (>400km) for 4 days covering both magnetically quiet and disturbed conditions. The first response of the neutral atmosphere to the storm takes place in the H and He densities which start to decrease near the time of the storm sudden commencement. The maximum decreases in H and He were more than 40% of the normal density at high latitudes. A subsequent increase in O and N₂ densities occurs about 8 hours later than the change in H and He densities, while the relative O and N₂ density changes indicate a depletion of atomic oxygen in the lower thermosphere by more than a factor of two. The overall features of the change in the neutral atmosphere, especially the patterns of change for individual species, strongly support the physical picture that energy is deposited primarily at high latitudes during the storm, and the thermosphere structure changes through (1) heating of the lower thermosphere and (2) generation of large scale circulation in the atmosphere with upwelling at high latitudes and subsidence at the equator. The storm-time response of H⁺ occurs in two distinct regions separated by the low latitude boundary of the light ion trough. While on the poleward side of the boundary the H⁺ density decreases in a similar manner to the decrease in H density, on the equatorward side of the boundary the H⁺ decrease occurs about half a day later. It is shown that the decrease of H⁺ density is principally caused by the decrease in H density for both regions. The difference in H⁺ response between the two regions is interpreted as the difference in H⁺ dynamics outside and inside the plasmasphere. The O⁺ density shows an increase, the pattern of which is rather similar to that for O. Two possibilities for explaining the observed change in O⁺ density are suggested. One attributes the observed increase in O⁺ density to an increase in the plasma temperature during the storm. The other possibility is that the increase in the production rate of O⁺ due to an increase in O density exceeds the increase in the loss rate of O⁺ due to an increase in N₂ density, especially around the time of sunrise. Hence the change in O⁺ density in the F-region may actually be controlled by the change in O density.     

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.     

The evolution and variability of atmospheric ozone over geological time

The rise of atmospheric O₃ as a function of the evolution of O₂ has been investigated using a one-dimensional steady-state photochemical model based on the chemistry and photochemistry of O_x(O₃, O, O(¹D)), N₂O, NO_x(NO, NO₂, HNO₃), H₂O, and HO_x(H, OH, HO₂, H₂O₂) including the effect of vertical eddy transport on the species distribution. The total O₃ column density was found to maximize for an O₂ level of 10^?1 present atmospheric level (PAL) and exceeded the present total O₃ column by about 40%. For that level of O₂, surface and tropospheric O₃ densities exceeded those of the present atmosphere by about an order of magnitude. Surface and tropospheric OH densities of the paleoatmosphere exceeded those of the present atmosphere by orders of magnitude. We also found that in the O₂-deficient paleoatmosphere, N₂O (even at present atmospheric levels) produces much less NO_x than it does in the present atmosphere.     

Electron content modelling: The significance of protonospheric contents

The principal advance of the ATS-6 satellite beacon experiment was the ability to deduce continuously the electron content along the entire slant path from ground-based measurements of the signal group delay. This feature has been exploited in conjunction with the more usual Faraday rotation technique to separate the total electron content into ionospheric and protonospheric components. The physical validity of the deduced quantities is investigated using a mathematical model of the plasmasphere in which integration of the time-dependent continuity and momentum equations for oxygen and hydrogen ions along selected L shells yields the ion concentrations and field-aligned fluxes. The ion concentrations are then integrated along the propagation path to various ground stations from ATS-6 to give computed values for comparison with observations. The mathematical model is used with different sets of atmospheric parameters to investigate the significance of ionospheric and protonospheric contents as derived from beacon data.The calculated electron concentrations are able to reproduce mid-latitude equinoctial electron content observations. The shape parameters τ and F can also be simulated by day, but night-time values do not match the observations well, a greater protonospheric content being required. The calculations show that the quantity N_p, which is readily derived from ATS-6 observations, may be interpreted as the slant H⁺ content above some fixed height in the case of some stations (but not others) if the plasmasphere is reasonably full. The total slant content of H⁺ is approx. twice the value of N_p, though it appears that for the Lancaster raypath a closer relationship exists between N_p and the H⁺ tube content at L = 1.8. In general,N_p is most closely related to the tube content for an L value slightly greater than the minimum L intersected along the raypath.     

Structure and time variations of the Jovian ionosphere

The problem of the ionospheric formation in the Jovian upper atmosphere is examined. By adopting two plausible atmospheric models, we solve coupled time-dependent continuity equations for ions H₂⁺, H₅⁺, H⁺, H₃⁺ and H_eH⁺ simultaneously. It is shown that both radiative and three body association of H⁺ to H₂ are important for the determination of the structure of the Jovian ionosphere. The maximum electron density in the daytime is found to be about 10⁵ cm^?3. It is also shown that diurnal variation with large-amplitude can exist in the Jovian ionosphere.     

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.     

Ionospheric warming by neutral winds

The effect of frictional heating by means of neutral winds on the ion and electron temperature in the undisturbed ionosphere is studied theoretically by solving a system of basic ionospheric and atmospheric equations. The study shows that both the electron and ion temperatures are increased in the night-time ionosphere through friction. In the region between 150 and 200 km T_i may exceed T₆ by as much as 130°. The increase of T_i due to friction amounts to about 100–200°, depending on the atmospheric model employed in calculating the neutral wind velocity. It is illustrated that frictional heating may be very important for the determination of the neutral temperature from measured ion temperature values.     

Possible new atmospheric sources and sinks of odd nitrogen: 2. A revisit with O2(B) and discussions of other possibilities

Recently, modelers have expressed a concern that the currently known chemistry of atmospheric NO_y is deficient. It is therefore necessary to explore possible sources and sinks of atmospheric NO_x that may have been overlooked. In this context, it is noteworthy that the experimentally observed, four-center, Woodward-Hoffman forbidden, reaction 0₂(B ³Σ) + N₂ → NO(X) + NO (X) is atmospherically significant. In the 20 to 30 km region NO_x production from this reaction may potentially exceed the production from the “classical” N₂0 + O(¹D) reaction, and may provide a new mechanism to couple the solar UV variability and stratospheric ozone. The avoidance of the non-conservation of the orbital symmetry via the production of one NO in the excited electronic state being endothermic, it appears that the interaction of 0₂(B ³Σ) with the adjoining ¹Λ, ³Λ and ³Σ_u⁺ states might be responsible for the reaction. Experimental studies of the reaction as a function of the vibrational levels of the B-state, temperature and pressure are needed for reliable atmospheric applications of this reaction. At altitudes greater than about 50 km the production of NO from 0₂(B) begins to decrease rapidly. The NO production from 0₂ (A ³Σ₊⁺) + N₂ → NO + NO reaction may become important here, if the reaction is confirmed by experiments. These new sources of NOx call for new sinks of this species. In the upper stratosphere and mesosphere the chemical acceleration of NO dissociation via the reactions of electronically and vibrationally excited NO with 0₂ may help. In the lower atmosphere there is a possibility of the annihilation of NO, N0₂pair leading to the recreation of a stable NN bond. This might happen if N₂0₃ from NO and N0₂ recombination may photodissociate as N₂0 + 0₂. Unfortunately the requirements are stringent, and only experiments can tell whether or not this mechanism operates in the atmosphere.     

Heavy ion formation in Titan's ionosphere: Magnetospheric introduction of free oxygen and a source of Titan's aerosols?

Discovery by Cassini's plasma instrument of heavy positive and negative ions within Titan's upper atmosphere and ionosphere has advanced our understanding of ion neutral chemistry within Titan's upper atmosphere, primarily composed of molecular nitrogen, with ~2.5% methane. The external energy flux transforms Titan's upper atmosphere and ionosphere into a medium rich in complex hydrocarbons, nitriles and haze particles extending from the surface to 1200 km altitudes. The energy sources are solar UV, solar X-rays, Saturn's magnetospheric ions and electrons, solar wind and shocked magnetosheath ions and electrons, galactic cosmic rays (GCR) and the ablation of incident meteoritic dust from Enceladus’ E-ring and interplanetary medium. Here it is proposed that the heavy atmospheric ions detected in situ by Cassini for heights >950 km, are the likely seed particles for aerosols detected by the Huygens probe for altitudes <100 km. These seed particles may be in the form of polycyclic aromatic hydrocarbons (PAH) containing both carbon and hydrogen atoms C_nH_x. There could also be hollow shells of carbon atoms, such as C₆₀, called fullerenes which contain no hydrogen. The fullerenes may compose a significant fraction of the seed particles with PAHs contributing the rest. As shown by Cassini, the upper atmosphere is bombarded by magnetospheric plasma composed of protons, H₂⁺ and water group ions. The latter provide keV oxygen, hydroxyl and water ions to Titan's upper atmosphere and can become trapped within the fullerene molecules and ions. Pickup keV N₂⁺, N⁺ and CH₄⁺ can also be implanted inside of fullerenes. Attachment of oxygen ions to PAH molecules is uncertain, but following thermalization O+ can interact with abundant CH₄ contributing to the CO and CO₂ observed in Titan's atmosphere. If an exogenic keV O+ ion is implanted into the haze particles, it could become free oxygen within those aerosols that eventually fall onto Titan's surface. The process of freeing oxygen within aerosols could be driven by cosmic ray interactions with aerosols at all heights. This process could drive pre-biotic chemistry within the descending aerosols. Cosmic ray interactions with grains at the surface, including water frost depositing on grains from cryovolcanism, would further add to abundance of trapped free oxygen. Pre-biotic chemistry could arise within surface microcosms of the composite organic-ice grains, in part driven by free oxygen in the presence of organics and any heat sources, thereby raising the astrobiological potential for microscopic equivalents of Darwin's “warm ponds” on Titan.     

Observations of outflowing ion beams on auroral field lines at altitudes of many earth radii

The composition, energy and angular characteristics of upward flowing ionospheric ions at altitudes greater than ～ 20,000 km have been studied by means of the PROGNOZ-7 ion composition experiment. Very narrow beams, having widths corresponding to a mirroring altitude of the order a few thousand kilometers or less, may be found up to altitudes exceeding 30,000 km on the nightside. At much higher altitudes and in regions connected to the dayside/flank boundary layer and plasma mantle, the beams are much broader than expected from adiabatic particle motions from an ionospheric source/acceleration region, suggesting that pitch angle scattering or transverse acceleration processes are present there. Considerable mass dispersion effects have also been observed in some upward flowing ionospheric ion beams. The peak energy for the O⁺ ions may differ by several keV compared to that for the H⁺ ions in one and the same ion beam at altitudes above ～ 20,000 km. The O⁺ ions in these beams have gained considerably more energy than H⁺ in the acceleration process. Many examples with a much higher O⁺ than H⁺ content in the beam have been observed. Possible mechanisms giving rise to the observed effects are discussed, one being several kV of potential drop below the neutral H, O-crossover altitude (500–1500 km). At altitudes where the upflowing ionospheric ions are intermixed with magnetosheath ions, mass dispersion effects are also observed. This dispersion often appears to be the result of a velocity filtering effect caused by the dawn-dusk electric field (earthward convection).     

Mutual neutralization rates of ionospherically important ions

Following our recently published measurements of the rate coefficients for mutual neutralization, α, of the ionospherically important reactions NO⁺ + NO₂^?(α₁) and NO⁺ + NO₃^?(α₂) carried out in ion-ion flowing afterglow plasmas at 300 K, we have determined the mutual neutralization rates for the water cluster ion H₃O⁺ · (H₂O)₃ with a mixture of several negative ions which are known to exist in the D region. The α coefficients for these cluster ion reactions do not differ significantly from alpha;₁ and α₂, all of these reactions having α ?6 × 10^?8 cm³/sec which is significantly smaller than values usually adopted in ionospheric calculations. Current information on the ionic composition of the D region and the implications of the present results to de-ionization rate calculations are discussed.     

Mid-latitude electron content modelling: The role of interhemispheric coupling

A modelling study of the electron content of the mid-latitude ionosphere and protonosphere has been carried out for solstice conditions using the mathematical model of Bailey (1983). In the model calculations coupled time-dependent O⁺, H⁺ continuity and momentum equations and O⁺, H⁺ and electron heat balance equations are solved for a magnetic shell extending over both hemispheres. The inclusion of interhemispheric flow of plasma and of heat balance has enabled us to investigate the role of interhemispheric coupling on the electron content and related shape parameters. The computed results are compared with results from slant path observations of the ATS-6 radio beacon made at Lancaster (U.K.) and Boulder, Colorado (U.S.A.).It has been found that the conjugate photoelectron heating has a major effect on the shape of the daily variation of slant slab thickness (τ) and also on the magnitude of the protonospheric content (N_p). Some of the main features of τ are closely related to the sunrise and sunset times in the conjugate ionosphere. Also it is found that night-time increases in total electron content (N_T) and F2 region peak electron density (N_max) in winter are natural consequences of ionization loss at low altitudes causing an enhanced downward flow of plasma from the protonosphere which is coupled to the summer hemisphere. One other important consequence of the coupled protonosphere is that the effects on N_T of the neutral air wind are not much different in winter from those in summer.     

Effects of ion excitation on charge transfer reactions of the Mars, Venus, and Earth ionospheres

The absolute reaction cross sections and reaction rate coefficients as a function of photoionisation energy for 25 ion-molecule reactions (charge transfer reactions except for one) have been measured between the most abundant species present as ions or neutral in the Mars, Venus and Earth ionospheres: O₂, N₂, NO, CO, Ar and CO₂.This study shows the strong influence of electronic as well as vibrational internal energy on most ion-molecule reactions. In particular endothermic charge transfer reactions are driven by electronic excitation of O₂⁺ and NO⁺ ions in their a⁴Π_u and a³Σ⁺ metastable states, respectively. Moreover, it is shown that lifetimes of these metastable states are sufficient to survive the mean free path in the lowest part of ionospheres and therefore express their enhanced reactivity. The reactions of O₂⁺ with NO as well as the reactions of CO₂⁺ with NO, O₂, CO and to a less extent N₂ are driven by vibrational excitation. N₂⁺ and CO⁺ reactions vary much less with photon energy than the other ones, except for the case of reactions with Ar. The effects of the molecular ion internal energy content on their reactivity must be included in the ionospheric models for most of the reactions investigated in the present work. It is also the case for the effect of collision energy on the CO⁺+M reactions as we expect that a significant proportion of these CO⁺ could be produced with translational energy by dissociation of doubly charged CO₂²⁺, in particular in the Mars ionosphere. Recommended effective rate constant values are given as a function of VUV photon energy.     

The calculated and observed diurnal variation of the ionosphere over Millstone Hill on 23–24 March 1970

A numerical model of current F-region theory is use to calculate the diurnal variation of the mid-latitude ionospheric F-region over Millstone Hill on 23–24 March 1970, during quiet geomagnetic conditions. From the solar EUV flux, the model calculates at each altitude and time step primary photoelectron spectra and ionization rates of various ion species. The photoelectron transport equation is solved for the secondary ionization rates, photoelectron spectra, and various airglow excitation rates. Five ion continuity equations that include the effects of transport by diffusion, magnetospheric-ionospheric plasma transport, electric fields, and neutral winds are solved for the ion composition and electron density. The electron and ion temperatures are also calculated using the heating rates determined from chemical reactions, photoelectron collisions, and magnetospheric-ionospheric energy transport. The calculations are performed for a diurnal cycle considering a stationary field tube co-rotating with the Earth; only the vertical plasma drift caused by electric fields perpendicular to the geomagnetic field line is allowed but not the horizontal drift. The boundary conditions used in the model are determined from the incoherent scatter radar measurements of T_e, T_i and O⁺ flux at 800km over Millstone Hill (Evans, 1971a). The component of the neutral thermospheric winds along the geomagnetic field has an important influence on the overall ionospheric structure. It is determined from a separate dynamic model of the neutral thermosphere, using incoherent scatter radar measurements.The calculated diurnal variation of the ionospheric structure agrees well with the values measured by the incoherent scatter radar when certain restrictions are placed on the solar EUV flux and model neutral atmospheric compositions. Namely, the solar EUV fluxes of Hinteregger (1970) are doubled and an atomic oxygen concentration of at least $\text{10}{}^{11}\text{cm}{}^3$ at 120 km is required for the neutral model atmosphere. Calculations also show that the topside thermal structure of the ionosphere is primarily maintained by a flow of heat from the magnetosphere and the night-time F2-region is maintained in part by neutral winds, diffusion, electric fields, and plasma flow from the magnetosphere. The problem of maintaining the calculated night-time ionosphere at the observed values is also discussed.     

Intensity variation of atomic oxygen red line in the day airglow with solar activity

Rates of production of O(¹ D) atoms in the upper atmosphere by photodissociation of O₂, dissociative recombination of O₂ ⁺, NO⁺ and electron impact excitation of O(³ P) have been calculated for low, medium and high levels of solar activity. Variations with solar activity, of neutral and ionic composition, electron and neutral temperatures of the upper atmosphere and solar extreme ultraviolet fluxes incident on it have been taken into consideration.Emission rates ofOi red line (6300Å) have been computed taking into account the deactivation both by molecular oxygen and nitrogen. It has been shown that the integrated intensity from low to high activity period varies by approximately an order of magnitude in agreement with the results of experimental observations.