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.     

Modeling of global variations and ring shadowing in Saturn's ionosphere

A time-dependent one-dimensional model of Saturn's ionosphere has been developed as an intermediate step towards a fully coupled Saturn Thermosphere-Ionosphere Model (STIM). A global circulation model (GCM) of the thermosphere provides the latitude and local time dependent neutral atmosphere, from which a globally varying ionosphere is calculated. Four ion species are used (H⁺, H⁺₂, H⁺₃, and He⁺) with current cross-sections and reaction rates, and the SOLAR2000 model for the Sun's irradiance. Occultation data from the Voyager photopolarimeter system (PPS) are adapted to model the radial profile of the ultraviolet (UV) optical depth of the rings. Diurnal electron density peak values and heights are generated for all latitudes and two seasons under solar minimum and solar maximum conditions, both with and without shadowing from the rings. Saturn's lower ionosphere is shown to be in photochemical equilibrium, whereas diffusive processes are important in the topside. In agreement with previous 1-D models, the ionosphere is dominated by H⁺ and H⁺₃, with a peak electron density of ∼¹⁰⁴ electrons cm⁻³. At low- and mid-latitudes, H⁺ is the dominant ion, and the electron density exhibits a diurnal maximum during the mid-afternoon. At higher latitudes and shadowed latitudes (smaller ionizing fluxes), the diurnal maximum retreats towards noon, and the ratio of [H⁺]/[H⁺₃] decreases, with H⁺₃ becoming the dominant ion at altitudes near the peak (∼1200-1600 km) for noon-time hours. Shadowing from the rings leads to attenuation of solar flux, the magnitude and latitudinal structure of which is seasonal. During solstice, the season for the Cassini spacecraft's encounter with Saturn, attenuation has a maximum of two orders of magnitude, causing a reduction in modeled peak electron densities and total electron column contents by as much as a factor of three. Calculations are performed that explore the parameter space for charge-exchange reactions of H⁺ with vibrationally excited H₂, and for different influxes of H₂O, resulting in a maximum diurnal variation in electron density much weaker than the diurnal variations inferred from Voyager's Saturn Electrostatic Discharge (SED) measurements. Peak values of height-integrated Pedersen conductivities at high latitudes during solar maximum are modeled to be ∼42 mho in the summer hemisphere during solstice and ∼18 mho during equinox, indicating that even without ionization produced by auroral processes, magnetosphere-ionosphere coupling can be highly variable.     

Measurements of positive ion conversion and removal reactions relating to the Jovian ionosphere

Measured rates are presented for the reaction of He⁺ ions with H₂ (and D₂) molecules to form H⁺, H₂⁺, and HeH⁺ ions, as well as for the subsequent reactions of H⁺ and HeH⁺ ions with H₂ to form H₃⁺. The neutralization of H₃⁺ (and H₅⁺) ions by dissociative recombination with electrons is shown to be fast. The reaction He⁺ + H₂ is slow (k = 1.1 × 10^?13 cm³/sec at300°K) and produces principally H⁺ by the dissociative charge transfer branch. It is concluded that there may be a serious bottleneck in the conversion of two of the primary ions of the upper Jovian ionosphere, H⁺ and He⁺ (which recombine slowly), to the rapidly recombining H₃⁺ ion (α[H₃⁺]?3.4 × 10^?7 cm³/sec at 150°K).     

The role of hydrocarbons in the lonospheres of the outer planets

The role of hydrocarbons as a possible sink for H⁺ and H₃⁺ ions in the lower ionosphere of the outer planets is examined. Calculations indicate that H⁺ and H₃⁺ are efficiently converted to hydrocarbon ions on reaction with methane. The terminal ions, CH₅⁺ and C₂H₅⁺ are rapidly neutralized in dissociative recombination with electrons. Extreme ultraviolet photolysis of hydrocarbons as a potential additional source of lower elevation ions in investigated.     

Thermal proton flow in the plasmasphere: The morning sector

Vertical profiles of electron density obtained in the vicinity of the plasmapause using the Alouette-II topside sounder have been analyzed to assess the presence of H⁺ flow in the topside ionosphere. The observations in the midnight sector show clearly the presence of the plasmapause; i.e. there is a sharp boundary separating the poleward regions of polar wind H⁺ flow and the more gentle conditions of the plasmasphere where light ions are present in abundance. In contrast, in the sunlit morning sector upwards H⁺ flow is deduced to be present to invariant latitudes as low as 48° (L = 2·2) in the regions normally known to be well inside the plasmasphere. The upwards H⁺ flux is sufficiently large (3 × 10⁸ ions cm^?2 sec^?1) that the plasmapause cannot be seen in the latitudinal electron density contours of the topside ionosphere. The cause for this flow remains unknown but it may be a result of a diurnal refilling process.     

The upper ionosphere of Titan

Photoionization of the upper atmosphere of Titan by sunlight is expected to produce a substantial ionospheric layer. We have solved one-dimensional forms of the mass, momentum, and energy conservation equations for ions and electrons and have obtained electron number densities of about 10³ cm^?3, using various model atmospheres. The significant ions in a CH₄H₂ atmosphere are H⁺, H₃⁺, CH₅⁺, CH₅⁺, CH₃⁺, and C₂H₅⁺. Electron temperatures may be as high as 1000°K, depending on the abundance of hydrogen in the high atmosphere. Interaction of the solar wind with the ionosphere is also discussed.     

Diffusion coefficients for three major ions in the topside ionosphere

Published experimental data on ion composition in the topside ionosphere are examined. For certain features (the light ion trough, the high-latitude trough, the high-latitude hole and the mid-latitude total ion concentration trough) it is pointed out that the number of major ions present may be 3 or more. Transport equations derived by Schunk and co-workers are extended to include the case of three major ions in the topside ionosphere. Specific calculations are made for the O⁺, H⁺ and He⁺ ions and the behaviour of the diffusion coefficients is discussed. From a model of the high-latitude ionization hole, described by Heelis et al., representative concentration and temperature profiles are obtained. These profiles are used to demonstrate further the behaviour of the ion diffusion coefficients.     

Ionospheric composition in SAR-arcs

The high electron temperatures existing within SAR-arcs can result in enhanced vibrational excitation of atmospheric N₂ molecules and, as a consequence, increase the rate coefficient of the reaction, O⁺ + N₂ → NO⁺ + N. This results in a change in the relative abundance of O⁺ and NO⁺+ in the SAR-arc region compared with that in the undisturbed ionosphere. Theoretical ion density profiles were computed by a triple ion analysis solving the mass, momentum and energy equations for O⁺, NO⁺ and O⁺₂ ions self-consistently. Although the electron temperature dependence of the recombination rate of NO⁺ is not well known, the results show that for a range of expected recombination rates NO⁺ still remains the dominant ion up to ca. 320 km at night within a bright SAR-arc. Studies were also made of the relative importance of a downward O⁺ flux and an upward ion drift in maintaining the F-region under SAR-arc conditions. It was found that the upward drift caused a marked increase in the NO⁺/O⁺ transition altitude as high as 460 km at night. However, for typical drift speeds up to 50 m sec^?1 the peak electron density was lower than experimental observations. The effect of a large, short-duration perpendicular electric field on the SAR-arc ion and electron density profiles was found to be small. In all cases considered the magnitude of the enhanced NO⁺ density as a result of vibrationally excited N₂ molecules was sufficient to prevent the electron density within the night-time SAR-arc from becoming vanishingly small.     

The topside equatorial ionosphere during daytime: Elevated plasma temperatures,the transequatorial O+ breeze and the dynamics of minor ions

Steady-state calculations are performed for the daytime equatorial F2-region and topside ionosphere. Values are calculated of the electron and ion temperatures and the concentrations and field-aligned velocities of the ions O⁺, H⁺ and He⁺. Account is taken of upward $\text{E × B}$ drift, a summer-winter horizontal neutral air wind and heating of the electron gas by thermalization of fast photoelectrons.The calculated plasma temperatures are in accord with experiment: at the equator there is an isothermal region from about 400–550 km altitude, with temperatures of about 2400 K around 800 km altitude. The transequatorial O⁺ breeze flux from summer to winter in the topside ionosphere is not greatly affected by the elevated plasma temperatures. The field-aligned velocities of H⁺ and He⁺ depend strongly on the O⁺ field-aligned velocity and on the presence of large temperature gradients. For the minor ions, ion-ion drag with O⁺ cannot be neglected for the topside ionosphere.     

A model of the terrestrial ionosphere in the altitude interval 50–4000 km II. Molecular ions (N2 +, NO+, O2 +) and electron density

Empirical models of molecular ion densities (N₂ ⁺, NO⁺, O₂ ⁺) and the electron density (N _e ) are presented in the altitude interval 50–4000 km as functions of time (diurnal, annual), space (position, altitude) and solar flux (F _10.7). Using observations of 6 satellites (AE-C, AE-D, AE-E, ALOUETTE-2, ISIS-1, ISIS-2), 4 incoherent scatter stations (Arecibo, Jicamarca, Millstone Hill, St Santin) and more than 700 D-region profiles, this model describes the global gross features of the ionosphere for quiet geophysical conditions (K _p 3).The molecular ion densities and the electron density increase with increasing altitude up to a maximum (or several maxima) - and decrease from thereon with increasing height. Between ~80 and 200 km, the main ionic constituents are NO⁺ and O₂ ⁺; below ~80 km cluster ions are predominating. During local summer conditions the molecular ions and N _e increase around polar latitudes and decrease correspondingly during local winter. The diurnal variations are intrinsically coupled to the individual plasma layers; in general, the molecular ion and electron densities are enhanced during daytime and depleted during nighttime (for details and exceptions, see text).     

The lower ionosphere of Titan

Ionization of the atmosphere of Titan by galactic cosmic rays is a very significant process throughout the altitude range of 100 to 400 km. An approximate form of the Boltzmann equation for cosmic ray transport has been used to obtain local ionization rates. Models of both ion and neutral chemistry have been employed to compute electron and ion density profiles for three different values of the H₂/CH₄ abundance ratio. The peak electron density is of the order 10³ cm^?3. The most abundant positive ions are C₂H₉⁺ and C₃H₉⁺, while the predicted densities of the negative ions H^? and CH₃^? are very small (<10^?4 that of the positive ions). It is suggested that inclusion of the ion chemistry is important in the computation of the H and CH₃ density profiles in the lower ionosphere.     

Predictions of the electrical conductivity and charging of the aerosols in Titan's atmosphere

The electrical conductivity and electrical charge on the aerosols in atmosphere of Titan are computed for altitudes between 0 and 400 km. Ionization of methane and nitrogen due to galactic cosmic rays (GCR) is important at night where these ions are converted to ion clusters such as CH⁺₅CH₄, C₇H⁺₇, C₄H⁺₇, and H₄C₇N⁺. The ubiquitous aerosols observed also play an important role in determining the charge distribution in the atmosphere. Because polycyclic aromatic hydrocarbons (PAHs) are expected in Titan's atmosphere and have been observed in the laboratory and found to be electrophilic, we consider the formation of negative ions. During the night, the very smallest molecular complexes accept free electrons to form negative ions. This results in a large reduction of the electron abundance both in the region between 150 and 350 km over that predicted when such aerosols are not considered. During the day time, ionization by photoemission from aerosols irradiated by solar ultraviolet (UV) radiation overwhelms the GCR-produced ionization. The presence of hydrocarbon and nitrile minor constituents substantially reduces the UV flux in the wavelength band from the cutoff of CH₄ at 155 to 200 nm. These aerosols have such a low ionization potential that the bulk of the solar radiation at longer wavelengths is energetic enough to produce a photoionization rate sufficient to create an ionosphere even without galactic cosmic ray (GCR) bombardment. At altitudes below 60 km, the electron and positive ion abundances are influenced by the three-body recombination of ions and electrons. The addition of this reaction significantly reduces the predicted electron abundance over that previously predicted. Our calculations for the dayside show that the peaks of the charge distributions move to larger values as the altitude increases. This variation is the result of the increased UV flux present at the highest altitudes. Clearly, the situation is quite different than that for the night where the peak of the distribution for a particular size is nearly constant with altitude when negative ions are not present. The presence of very small aerosol particles (embryos) may cause the peak of the distribution to decrease from about 8 negative charges to as little as one negative charge or even zero charge. This dependence on altitude will require models of the aerosol formation to change their algorithms to better represent the effect of charged aerosols as a function of altitude. In particular, the charge state will be much higher than previously predicted and it will not be constant with altitude during the day time. Charging of aerosol particles, whether on the dayside or nightside, has a major influence on both the electron abundance and electrical conductivity. The predicted conductivities are within the measurement range of the HASI PWA instrument over most but not all, of the altitude range sampled.     

Predictions of the electrical conductivity and charging of the aerosols in Titan's atmosphere

The electrical conductivity and electrical charge on the aerosols in atmosphere of Titan are computed for altitudes from 0 to 400 km. Ionization due to both galactic cosmic rays and electron precipitation from the Saturnian magnetosphere is considered. This ionization results in free electrons and the primary ions N₂⁺ and N⁺ which are then rapidly converted into secondary ions such as H₂CN⁺ and NH₄⁺ which in turn form ion clusters such as H₂CN⁺(HCN)_n and NH₄⁺(NH₃)_m. In contrast to the atmospheres of Venus and Earth, we find no species in the Titan atmosphere that lead to the formation of appreciable concentrations of negative ions. Consequently, the predicted conductivity is quite different in that a substantial concentration of electrons exists all the way to the surface of Titan. The ubiquitous aerosols observed in the Titan atmosphere also play an important role in determining the charge distribution in the atmosphere. At altitudes above 100 km and for aerosol concentrations above approximately 10/cc, the recombination of electrons and positive ions is controlled by the recombination on the surface of the aerosols rather than by the gas-kinetic recombination rate. For small aerosol concentrations, the ratio of the number of charges per particle to the radius of the particle is approximately 30, for radii in microns. This value is similar to that obtained by previous investigators for terrestrial noctilucent clouds. Because the aerosol particles are highly charged, coagulation is inhibited, particle sizes are smaller, and their settling rates are reduced. As a consequence, the optical depth of the atmosphere is much higher than it would be if the particles were uncharged.     

Aeronomy of extra-solar giant planets at small orbital distances

One-dimensional aeronomical calculations of the atmospheric structure of extra-solar giant planets in orbits with semi-major axes from 0.01 to 0.1 AU show that the thermospheres are heated to over 10,000 K by the EUV flux from the central star. The high temperatures cause the atmosphere to escape rapidly, implying that the upper thermosphere is cooled primarily by adiabatic expansion. The lower thermosphere is cooled primarily by radiative emissions from H⁺₃, created by photoionization of H₂ and subsequent ion chemistry. Thermal decomposition of H₂ causes an abrupt change in the composition, from molecular to atomic, near the base of the thermosphere. The composition of the upper thermosphere is determined by the balance between photoionization, advection, and H⁺ recombination. Molecular diffusion and thermal conduction are of minor importance, in part because of large atmospheric scale heights. The energy-limited atmospheric escape rate is approximately proportional to the stellar EUV flux. Although escape rates are large, the atmospheres are stable over time scales of billions of years.     

Ionospheric models of Saturn,Uranus, and Neptune

Model ionospheres are calculated for Saturn, Uranus, and Neptune. Protons are the major ions above 150 km altitude measured from a reference level where the hydrogen density is 1 × 10¹⁶ molecules cm^?3, while below 150 km quick conversion of protons to H₃⁺ ions by a three-body association mechanism leads to a rapid removal of ionization in dissociative recombination of H₃⁺. Electron density maxima are found at about 260 km for Saturn and Uranus and 200 km for Neptune. Present knowledge of the physical and chemical processes in the atmospheres of these planets suggests that their ionospheres probably will not be Jupiter-like.     

Ion irradiation experiments and nitrogen bearing species on Jovian and Saturnian icy surfaces

Implantation of reactive ions into targets of planetary interest is a relevant subject to be studied in the laboratory. It could in fact produce new molecular species that are not native to those surfaces. Presented here are new laboratory results obtained by nitrogen implantation (15–30 keV N⁺) on frozen mixtures of H₂O:CH₄ (2:1). These species have been chosen in view of their possible presence on the surface of Jovian and Saturnian satellites and rings. In fact these surfaces are exposed to intense irradiation by magnetospheric and/or solar energetic particles. The laboratory investigation utilizes IR spectroscopy. The main objectives of the present study are to identify newly produced species and to verify if these (or at least if the profile of their IR bands) are different from those produced by unreactive ions impinging on targets in which nitrogen is already present, occurring in the form of frozen NH₃ (Strazzulla and Palumbo, 1998) or N₂ (Palumbo et al., 1999). I find that CN-bearing group is in fact formed and its IR feature has a profile (peak position and band profile) that differs from that obtained after irradiation or frozen gases containing nitrogen. The relevance the results might have to elucidate the origin of some species observed on Jovian icy moons or predicted to be observed on Saturnian satellites are outlined.     

Silicon ion chemistry in the ionosphere

Loss processes which remove Si⁺ ions selectively relative to other meteor-derived atomic ions in the E- and D-regions of the ionosphere have been identified and measured in the laboratory. The major Si⁺ loss in the E-region is the reaction Si⁺ + H₂O → HSiO⁺ + H (1) with a rate constant 2.3 ± 0.9 × 10^?10 cm³s^?1 at 300 K. The corresponding reactions with Fe⁺, Mg⁺ and other metallic meteor ions are endothermic. Presumably (1) is followed by a fast dissociative-recombination with electrons to produce neutral SiO or Si. At lower altitudes Si⁺ ions associate in a three-body reaction with O₂ with a much larger rate constant than the corresponding associations of Fe⁺ and Mg⁺ with O₂.     

Ion chemistry in the cometary atmosphere

Dirty ice of a second kind (major components, H₂O, CO, and N₂; minor components less than several percents, NH₃, CH₄, and other organic substances such as HCN, CH₃CN etc.) is assumed for the composition of volatiles in the cometary nucleus. The consistency with the observations of molecular ions and daughter molecules in the cometary atmosphere is argued by taking into account various ion-molecular reactions and dissociative recombinations. There is a satisfactory agreement for the second kind of dirty-ice model, but the presence of large amounts of CH₄ and NH₃ is found to be rather in contradiction with observational evidence. A velocity of 8 km s^?1 for the hydrogen atoms, derived from analysis of the hydrogen Lyman-alpha corona around comets, is found from the dissociative recombination of H₃O⁺, the dominant constituent of cometary ionosphere, in accordance with H₃O⁺+e ^?→OH+H+H.     

Decrease of ozone and atomic oxygen in the lower mesosphere during a PCA event

It is argued that ozone measurements made by Weeks et al. (1972) can be interpreted in terms of the enhanced ionization present. The conversion of O₂⁺ ions to oxonium, H₃O⁺ · (H₂O)_n, ions plus the dissociative recombination of these ions provides for an increased OH and/or H formation rate. The resulting enhanced OH and HO₂ concentrations reduce the ambient atomic oxygen and hence ozone populations. The net excess H + OH formation rate is found to lie between one and two times the ionization production rate at altitudes where oxonium ions are the dominant positive ion species.