Jovian proton Aurora

The planet Jupiter possesses a magnetic field and is surrounded by a magnetosphere. The occurrence of auroral and polar cap phenomena similar to those found on earth is very likely. In this work auroral and polar cap emissions in a model Jovian atmosphere are determined for proton precipitation. The incident protons, which are characterized by representative spectra, are degraded in energy by applying the continuous slowing down approximation. All secondary and higher generation electrons are assumed to be absorbed locally and their contributions to the total emissions are included. Volume emission rates are calculated from the total direct excitation rates with corrections for cascading applied. Results show that most molecular hydrogen and helium emissions for polar cap precipitation are below the ambient dayglow values. Charge capture by precipitating protons is an important source of Lyman α and Balmer α emissions and offers a key to the detection of large fluxes of low energy protons.     

A phase and amplitude study of auroral pulsations

To supplement a rocket investigation of the auroral green line, data from some auroral pulsations have been analyzed by direct integration of the time-dependent continuity equations for each of the sources now thought to contribute. It is shown that indirect processes are not incompatible with studies of auroral pulsations, and that a non-negligible contribution to the green-line intensity comes from dissociative recombination. Calculations using the theoretical O(¹S) lifetime show that the remaining green line must come from a direct or very fast process; however, if the O(¹S) lifetime can be reduced, significant portions of the green line can come from the transfer of energy from N₂(A³Σ_u⁺) to atomic oxygen.     

Antarctic substorm events observed by sounding rockets,ionization of the D- and E-regions by auroral electrons

The ionization structure of the auroral arc was measured on a sounding rocket which penetrated into a bright auroral arc. The E-region electron density becomes large (2 ～ 5 × 10⁵ el/cm³ only in the moving auroral arc, whose N₂⁺ 4278 Å brightness is 1 ～ 2·5 kR. The electron density in the D-region beneath the lower boundary of the arc (75 ～ 98 km in altitude) is also considerably enhanced to 2 ～ 5 × 10⁴ el/cm³.The observed E-region electron density can be interpreted theoretically as due to the direct ionization by precipitating electrons, whose energy spectrum is approximately represented by an exponential type having the characteristic energy of 2 keV. The correlation between the electron density and the N₂⁺ 4278 Å brightness can be reasonably explained by considering the simultaneous effects on the ionization and the optical excitation caused by the primary electrons having a flux of 9 × 10⁹ el/cm²/sec per 1 kR of the 4278 Å emission.Further analyses using the electron density data from four other sounding rockets have shown that the D-region ionization has good correlations to the cosmic noise absorption (CNA) and the magnetic substorm activities observed simultaneously at the ground station, whereas it has poor correlation to the same quantity of the E-region measured in the same experiment. It is found that the observed D-region ionization is much larger than that predicted by the theory which takes into account the Bremsstrahlung X-ray ionization along with the direct impact ionization when it is applied to the precipitating electron flux spectrum consistent to the E-region ionization and optical excitation.After all the present experimental results suggest a dual nature of the electron precipitation spectrum in the substorm, i.e. the softer part which is localized in the auroral arc and the harder part which is spatially wide-spread over the substorm area.     

Jovian auroral spectroscopy with FUSE: analysis of self-absorption and implications for electron precipitation

High-resolution (∼0.22 Å) spectra of the north jovian aurora were obtained in the 905-1180 Å window with the Far Ultraviolet Spectroscopic Explorer (FUSE) on October 28, 2000. The FUSE instrument resolves the rotational structure of the H₂ spectra and the spectral range allows the study of self-absorption. Below 1100 Å, transitions connecting to the v^″?2 levels of the H₂ ground state are partially or totally absorbed by the overlying H₂ molecules. The FUSE spectra provide information on the overlying H₂ column and on the vibrational distribution of H₂. Transitions from high-energy H₂ Rydberg states and treatment of self-absorption are considered in our synthetic spectral generator. We show comparisons between synthetic and observed spectra in the 920-970, 1030-1080, and 1090-1180 Å spectral windows. In a first approach (single-layer model ), the synthetic spectra are generated in a thin emitting layer and the emerging photons are absorbed by a layer located above the source. It is found that the parameters of the single-layer model best fitting the three spectral windows are 850, 800, and 800 K respectively for the H₂ gas temperature and 1.3×¹⁰¹⁸, 1.5×¹⁰²⁰, and 1.3×¹⁰²⁰ cm−2 for the H₂ self-absorbing vertical column respectively. Comparison between the H₂ column and a 1-D atmospheric model indicates that the short-wavelength FUV auroral emission originates from just above the homopause. This is confirmed by the high H₂ rovibrational temperatures, close to those deduced from spectral analyses of H⁺₃ auroral emission. In a second approach, the synthetic spectral generator is coupled with a vertically distributed energy degradation model, where the only input is the energy distribution of incoming electrons (multi-layer model ). The model that best fits globally the three FUSE spectra is a sum of Maxwellian functions, with characteristic energies ranging from 1 to 100 keV, giving rise to an emission peak located at 5 μbar, that is ∼100 km below the methane homopause. This multi-layer model is also applied to a re-analysis of the Hopkins Ultraviolet Telescope (HUT) auroral spectrum and accounts for the H₂ self-absorption as well as the methane absorption. It is found that no additional discrete soft electron precipitation is necessary to fit either the FUSE or the HUT observations.     

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.     

Spatially Resolved Far Ultraviolet Spectroscopy of the Jovian Aurora

Spatially resolved spectra in four 50-Å FUV spectral windows were obtained across the jovian aurora with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope. Nearly simultaneous ultraviolet imaging makes it possible to correlate the intensity variations along the STIS slit with those observed in the images and to characterize the global auroral context prevailing at the time of the observations. Spectra at ∼1-Å resolution taken in pairs included an unabsorbed window and a spectral region affected by hydrocarbon absorption. Both sets of spectra correspond to an aurora with a main oval brightness of about 130 kilorayleighs of H₂ emission. The far ultraviolet color ratios I(1550-1620 Å)/I(1230-1300 Å) are 2.3 and 5.9 for the noon and morning sectors of the main oval, respectively. We use an interactive model coupling the energy degradation of incoming energetic electrons, auroral temperature and composition, and synthetic H₂ spectra to fit the intensity distribution of the H₂ lines. It is found that the model best fitting globally the spectra has a soft energy component in addition to a 10 erg cm⁻² s⁻¹ flux of 80 keV electrons. It provides an effective H₂ temperature of 540 K. The relative intensity of temperature-sensitive H₂ lines indicates differences between the auroral main oval and polar cap emissions. The amount of methane absorption across the polar region is shown to vary in a way consistent with temperature. For the second spectral pair, the polar cap shows a higher attenuation by CH₄, indicating a harder precipitation along high-latitude magnetic field lines.     

On the envelope soliton below the H + gyrofrequency in low auroral region

Two envelope soliton events below the H ⁺ gyrofrequency with localized density depletion were discovered in low auroral region (∼ 1760 km)by Freja satellite. These events were correlated in time with the observations of the ratio of oxygen ion density to hydrogen ion density sharp increase and the electrons energization. These envelope solitons have a characteristic frequency at ∼ 180–190 Hz, which are obviously different from the electron-ion lower hybrid wave frequency and the helium ion gyrofrequency in low auroral plasma, but it is close to the resonancefrequency of hydrogen ion-oxygen ion hybrid wave. A modulational instability model of an ion-ion hybrid wave has been discussed here. It is found that the envelope soliton below the H ⁺ gyrofrequency in low auroral region may be generated by this modulational instability on condition that the local oxygen ion density is larger than the local hydrogen ion density. This revised version was published online in July 2006 with corrections to the Cover Date.     

A measurement of the O2(b1Σg+−X3Σg−) atmospheric band and the OI(1S) green line in the nightglow

Simultaneous measurements of the nightglow profiles of the O₂(b¹Σ_g⁺?X³Σ_g^?) A-band, the atomic oxygen green line and the OH (8?3) Meinel band are presented. The altitude profiles are used to determine both the excitation mechanisms for the oxygen emissions and the atomic oxygen altitude distribution. It is shown that the measurements are consistent with a green line excitation through the Barth mechanism and that the molecular oxygen emission is excited through oxygen recombination and the reaction between OH¹ and atomic oxygen. The derived atomic oxygen concentrations,6.2 × 10¹¹cm^?3at 98km, are consistent with the Jacchia (1971) model.     

An auroral enhancement of O2λ1.27-μm emission

The ground-level zenith radiance of the atmospheric emission at λ1.27 μm was radiometrically observed to increase by a factor of approximately two with the onset of an IBC III⁺ auroral breakup above Chatanika, Alaska, on 10 March 1975. Time-resolved optical spectra clearly show that the slow component of the enhancement is associated with the (0,0) band of the infrared atmospheric system of O₂. Photometric and incoherent scatter radar data are used to define the energy-deposition profile and the absolute energy flux for the event. The magnitude of the O₂λ1.27-μm enhancement compares favourably with the predictions of an auroral excitation model which includes only secondary-electron excitation of molecular oxygen in the O₂(a¹Δ_g) source term.     

The excitation of O2(b1Σg+) in the nightglow

It is proposed that the available measurements of the O₂(b¹Σ_g⁺ ?X³Σ_g^?) atmospheric bands both in the nightglow and in the laboratory indicate that the excitation mechanism is a two-step process rather than the direct three body recombination of atomic oxygen. It is shown that such a two-step mechanism can explain observations of the atmospheric bands both in altitude and intensity.     

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.     

The absorption of energetic electrons by molecular hydrogen gas

The processes by which energetic electrons lose energy in a weakly ionized gas of molecular hydrogen are analysed and calculations are carried out taking into account the discrete nature of the excitation processes. The excitation, ionization and heating efficiencies are computed for electrons with energies up to 100 eV absorbed in a gas with fractional ionizations up to 10^?2 and the mean energy per neutral hydrogen atom pair is calculated.     

Auroral recombination of N and O: A possible source for emission in the γ and δ bands of NO

Radiative recombination of N and O provides a significant source for auroral emission in the γ and δ bands of NO with selective population of vibrational levels in the A²Σ⁺ and C²Π states. This mechanism may account for emissions detected near 2150 Å. Models are derived for the auroral ionosphere and include estimates for the concentrations of N and NO. The concentration of NO is estimated to have a value of about 10⁸ cm^?1 near 140 km in an IBC III aurora. The corresponding density for N is about 5 × 10⁷cm^?3 and the concentration ratio $\text{NO}{}^{\mathrm{+}}\text{O}{}_2{}^{\mathrm{+}}$ has a value of about 5.5.     

Radial distribution of production rates, loss rates and densities corresponding to ion masses ?40 amu in the inner coma of Comet Halley: Composition and chemistry

In this paper we have studied the chemistry of C, H, N, O, and S compounds corresponding to ions of masses ?40 amu in the inner coma of the Comet 1P/Halley. The production rates, loss rates, and ion mass densities are calculated using the Analytical Yield Spectrum approach and solving coupled continuity equation controlled by the steady state photochemical equilibrium condition. The primary ionization sources in the model are solar EUV photons, photoelectrons, and auroral electrons of the solar wind origin. The chemical model couples ion-neutral, electron-neutral, photon-neutral and electron-ion reactions among ions, neutrals, electrons, and photons through over 600 chemical reactions. Of the 46 ions considered in the model the chemistry of 24 important ions (viz., CH₃OH⁺₂, H₃CO⁺, NH⁺₄, H₃S⁺, H₂CN⁺, H₂O⁺, NH⁺₃, CO⁺, C₃H⁺₃, OH⁺, H₃O⁺, CH₃OH⁺, C₃H⁺₄, C₂H⁺₂, C₂H⁺, HCO⁺, S⁺, CH⁺₃, H₂S⁺, O⁺, C⁺, CH⁺₄, C⁺₂, and O⁺₂) are discussed in this paper. At radial distances <1000 km, the electron density is mainly controlled by 6 ions, viz., NH⁺₄, H₃O⁺, CH₃OH⁺₂, H₃S⁺, H₂CN⁺, and H₂O⁺, in the decreasing order of their relative contribution. However, at distances >1000 km, the 6 major ions are H₃O⁺, CH₃OH⁺₂, H₂O⁺, H₃CO⁺, C₂H⁺₂, and NH⁺₄; along with ions CO⁺, OH⁺, and HCO⁺, whose importance increases with further increase in the radial distance. It is found that at radial distances greater than ∼1000 km (±500 km) the major chemical processes that govern the production and loss of several of the important ions in the inner coma are different from those that dominate at distances below this value. The importance of photoelectron impact ionization, and the relative contributions of solar EUV, and auroral and photoelectron ionization sources in the inner coma are clearly revealed by the present study. The calculated ion mass densities are compared with the Giotto Ion Mass Spectrometer (IMS) and Neutral Mass Spectrometer (NMS) data at radial distances 1500, 3500, and 6000 km. There is a reasonable agreement between the model calculation and the Giotto measurements. The nine major peaks in the IMS spectra between masses 10 and 40 amu are reproduced fairly well by the model within a factor of two inside the ionopause. We have presented simple formulae for calculating densities of the nine major ions, which contribute to the nine major peaks in the IMS spectra, throughout the inner coma that will be useful in estimating their densities without running the complex chemical models.     

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).     

Time dependent studies of the aurora—I. Ion density and composition

Ion densities and composition are investigated in a time varying model aurora. There is a time lag between turning on the source of ionization and the resulting increase in ion densities that depends on the species and the height level in the ionsophere, so that altitude profiles of auroral electron densities evolve with time. Characteristic buildup times for the ionization are a few seconds at the altitude of maximum energy deposition, increasing to tens of seconds above and below this level. A wide range of composition ratios, n(NO⁺/n(O₂⁺ and n(NO⁺/n(O⁺), can be expected, depending on the time an observation is made during buildup or decay of ionization. The concentrations of atomic nitrogen and nitric oxide increase as a result of auroral ionization, but the associated characteristic times are long compared to the average duration of ‘auroral event’. Thus, intermittent auroral bombardment could result in a gradual buildup of these minor neutral constituents in the auroral atmosphere. Variations in the electron density during pulsating, fluctuating or coruscating aurora lag the source function variations by a few seconds in a typical aurora.     

Changes in the Martian atmosphere induced by auroral electron precipitation

Typical auroral events in the Martian atmosphere, such as discrete and diffuse auroral emissions detected by UV spectrometers onboard ESA Mars Express and NASA MAVEN, are investigated. Auroral electron kinetic energy distribution functions and energy spectra of the upward and downward electron fluxes are obtained by electron transport calculations using the kinetic Monte Carlo model. These characteristics of auroral electron fluxes make it possible to calculate both the precipitation-induced changes in the atmosphere and the observed manifestations of auroral events on Mars. In particular, intensities of discrete and diffuse auroral emissions in the UV and visible wavelength ranges (Soret et al., 2016; Bisikalo et al., 2017; Gérard et al., 2017). For these conditions of auroral events, the analysis is carried out, and the contribution of the fluxes of precipitating electrons to the heating and ionization of the Martian atmosphere is estimated. Numerical calculations show that in the case of discrete auroral events the effect of the residual crustal magnetic field leads to a significant increase in the upward fluxes of electrons, which causes a decrease in the rates of heating and ionization of the atmospheric gas in comparison with the calculations without taking into account the residual magnetic field. It is shown that all the above-mentioned impact factors of auroral electron precipitation processes should be taken into account both in the photochemical models of the Martian atmosphere and in the interpretation of observations of the chemical composition and its variations using the ACS instrument onboard ExoMars.     

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.     

Excitation processes of infrared atmospheric emissions

Excitation rates of the infrared emissions which are likely to occur in the mesosphere and thermosphere are quantitatively evaluated. They include the 9.6 μm band of O₃, the 15 and 4.3 μm bands of CO₂ and the 5.3 and 2.8 μm bands of NO. These emissions may be excited through nonthermal processes such as chemiluminescent reactions and resonant fluorescence in the thermosphere, whereas they are of thermal origin in the stratosphere and mesosphere. Increase of the non-thermal excitation rate caused by precipitating electrons could be responsible for the enhancement of the 4.3 μm band of CO₂, and the 5.3 and 2.8 μm bands of NO observed in the auroral thermosphere.