Jovian ionospheric models

This paper reports results obtained on ionosphere formation in the Jovian upper atmosphere with special reference to some of the recently available reaction rates, and to recent models of the Jovian neutral atmosphere based on the possibility of a warmer mesopause. We find that the role of the hypothetical radiative association of H⁺ to H₂ to form H₃⁺, as brought to light in our earlier study, is still important, even with a reaction rate as low as 10^?15 cm³sec^?1. In the lower regions of the ionosphere three-body processes leading to the formation of H₃⁺ and H₅⁺ ions, which have very fast dissociative recombination rates, produce a dramatic reduction in the electron density. When no radiative association takes place, and the H⁺ ions are lost by radiative recombination alone, we confirm that the photochemical equilibrium profile is also the diffusive equilibrium profile. However, with collisional-radiative recombination, whose rate becomes altitude-dependent, diffusion tends to bring about some redistribution of the ionization. Inclusion of radiative association enhances the role of diffusion. In this case, diffusion brings about all the expected changes. In particular, the differences in the electron density profile, originated in the lower-middle ionosphere by radiative association, are propagated up to all higher altitudes by diffusion. The rate constant of radiative association is, however, unknown. It is hoped that the critical importance of this reaction for the Jovian ionosphere will be an incentive towards a careful laboratory determination of its rate coefficient. In the older models of the Jovian ionosphere the major ions were H⁺ which were lost only by pure radiative recombination. This led to high electron densities and practically no diurnal change. In contrast, our new models have relatively much smaller electron densities, especially in lower regions, and may be susceptible to significant diurnal variation.     

Impact of atmospheric gravity waves on the jovian ionosphere

We investigate the effects of atmospheric gravity waves on the vertical and horizontal structure of the ionosphere of Jupiter. The presented non-linear, two-dimensional model of the jovian ionosphere allows for spatially and temporally varying neutral wind and temperature fields and tracks the time evolution of six ionospheric species, , and . An analytical approach is used to validate the model results for linear, small-amplitude waves and to elucidate the mechanisms that leads to perturbations in the density of the main ion species, H⁺ and . We demonstrate that the long-lived H⁺ ions are perturbed directly by wave dynamics whereas short-lived ions such as are perturbed by chemical interactions with other perturbed ion species. The model is then applied using larger gravity wave amplitudes consistent with observations. Atmospheric gravity waves propagating at high altitudes create layers of enhanced electron density similar to the system of layers observed during the J0-ingress radio occultation of the Galileo spacecraft. Our best fit to the J0-ingress observation is achieved using an 82 min period forcing wave with horizontal and vertical wavelengths of 500 km and 60 km respectively, and peaks at 510 km above the 1 bar pressure level. We further investigate the effects of the wave-induced ion flux on the background ionospheric structure and demonstrate that in the presence of a gravity wave the background density profiles of the H⁺ and ions are significantly modified. We also find that the column density of has variations that can exceed 10% as the wave propagates.     

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.     

Oxygen ion escape at Mars in a hybrid model: High energy and low energy ions

We have studied the escape and energization of several O⁺ populations and an population at Mars by using a hybrid model. The quasi-neutral hybrid model, HYB-Mars model, included five oxygen ion populations making it possible to distinguish photoions from oxygen ions originating from charge exchange processes and from the ionosphere.We have identified two high-energy ion components and one low-energy ion component of oxygen. They have different spatial and energy distributions near Mars. The two high-energy oxygen ion components, consisting of a high-energy “beam” and a high-energy “halo”, have different origins. (1) The high-energy (>∼100 eV) “beam” of O⁺ and ions are originating from the ionosphere. These ions form a highly asymmetric spatial distribution of escaping oxygen ions with respect to the direction of the convective electric field in the solar wind. (2) The high-energy (>∼100 eV) “halo” component contains O⁺ ions which are formed from the oxygen neutral exosphere by extreme ultraviolet radiation (EUV) and by charge exchange processes. These energetic halo ions can be found all around Mars. (3) The low energy O⁺ and ions (<∼100 eV) form a relatively symmetric spatial distribution around the Mars-Sun line. They originate from the ionosphere and from charge exchange processes between protons and exospheric oxygen atoms.The existence of the low- and the high-energy oxygen components is in agreement with recent in situ plasma measurements made by the ASPERA-3 instrument on the Mars Express mission. The analysis of the escaping oxygen ions suggests that the global energization of escaping planetary ions in the martian tail is controlled by the convective electric field.     

The structure and time variability of the ring atmosphere and ionosphere

The saturnian system is subject to constant bombardment by interplanetary meteoroids and irradiation by solar UV photons. Both effects release neutral molecules from the icy ring particles either in the form of impact water vapor or gas emission in the form of H₂O, O₂ and H₂. The observations of the Cassini spacecraft during its orbit insertion have shown the existence of molecular and atomic oxygen ions. Subsequent modeling efforts have led to the picture that an exospheric population of neutral oxygen molecules is probably maintained in the vicinity of the rings via photolytic-decomposition of ice and surface reactions. At the same time, ionized products O⁺ and ions move along the magnetic field lines and, depending on the optical local thickness rings, can thread through the ring plane or impact a ring particle, the ion principal sink. In addition, collisional interactions between the ions and neutrals will change the scale height of the ions and produce a scattered component of O₂ molecules and O atoms which can be injected into Saturn’s upper atmosphere or the inner magnetosphere. The ring atmosphere, therefore, serves as a source of ions throughout Saturn’s magnetosphere. If photolysis of ice is the dominant source of O₂, then the complex structure of the ring atmosphere/ionosphere and the injection rate of neutral O₂ will be subject to modulation by the seasonal variation of Saturn along its orbit. In this work, we show how the physical properties of the ring oxygen atmosphere, the scattered component, and the magnetospheric ion source rate vary as the ring system goes through the cycle of solar insolation. In particular, it is shown that the magnetopheric ions should be nearly depleted at Saturn’s equinox if O₂ is produced mainly by photolysis of the ring material.     

Jupiter's hydrocarbon polar brightening: Discovery of 3-micron line emission from south polar CH4, C2H2, and C2H6

Jupiter exhibits bright H⁺₃ auroral arcs at 3-4 microns that cool the hot (>1000 K) ionosphere above the ∼¹⁰−7 bar level through the infrared bands of this trace constituent. Below the ¹⁰−7 bar level significant cooling proceeds through infrared active bands of CH₄, C₂H₂, and C₂H₆. We report the discovery of 3-micron line emission from these hydrocarbon species in spectra of the jovian south polar region obtained on April 18 and 20, 2006 (UT) with CGS4 on the United Kingdom Infrared Telescope. Estimated cooling rates through these molecules are 7.5×¹⁰−3, 1.4×¹⁰−3, and , respectively, for a total nearly half that of H⁺₃. We derive a temperature of 450 ± 50 K in the ¹⁰−7-¹⁰−5 bar region from the C₂H₂ lines.