Response of Saturn's auroral ionosphere to electron precipitation: Electron density, electron temperature, and electrical conductivity

In the high-latitude regions of Saturn, the ionosphere is strongly coupled to the magnetosphere through the exchange of energy. The influx of energetic particles from Saturn's magnetosphere enhances the ionospheric densities and temperatures, affects the electrodynamical properties of the ionosphere, and contributes to the heating of the thermosphere. It is therefore critical to accurately model the energy deposition of these magnetospheric particles in the upper atmosphere in order to evaluate key ionospheric quantities of the coupled magnetosphere-ionosphere system. We present comprehensive results of ionospheric calculations in the auroral regions of Saturn using our Saturn Thermosphere-Ionosphere Model (STIM). We focus on solar minimum conditions during equinox. The atmospheric conditions are derived from the STIM 3-D General Circulation Model. The ionospheric component is self-consistently coupled to the solar and auroral energy deposition component. The precipitating electrons are assumed to have a Maxwellian distribution in energy with a mean energy E-m and an energy flux Q(0). In the presence of hard electron precipitation (1 < E-m <= 20 keV) with Q(0) > 0.04 mW m(-2), the ionospheric conductances are found to be proportional to the square root of the energy flux, but the response of the ionosphere is not instantaneous and a time delay needs to be applied to Q(0) when estimating the conductances. In the presence of soft electron precipitation (E-m < 500 eV) with Q(0) <= 0.2 mW m(-2), the ionospheric conductances at noon are found to be primarily driven by the Sun. However, soft auroral electrons are efficient at increasing the ionospheric total electron content and at heating the thermal electron population

Comparative ionospheres: terrestrial and giant planets

The study of planetary ionospheres within our solar system offers a variety of settings to probe mechanisms of photo-ionization, chemical loss, and plasma transport. Ionospheres are a minor component of upper atmospheres, and thus their mix of ions observed depends on the neutral gas composition of their parent atmospheres. The same solar irradiance (x-rays and extreme-ultra-violet vs. wavelength) impinges upon each of these atmospheres, with solar flux magnitudes changed only by the inverse square of distance from the Sun. If all planets had the same neutral atmosphere—with ionospheres governed by photochemical equilibrium (production = loss)—their peak electron densities would decrease as the inverse of distance from the Sun, and any changes in solar output would exhibit coherent effects throughout the solar system. Here we examine the outer planet with the most observations of its ionosphere (Saturn) and compare its patterns of electron density with those at Earth under the same-day solar conditions. We show that, while the average magnitudes of the major layers of molecular ions at Earth and Saturn are approximately in accord with distance effects, only minor correlations exist between solar effects and day-to-day electron densities. This is in marked contrast to the strong correlations found between the ionospheres of Earth and Mars. Moreover, the variability observed for Saturn's ionosphere (maximum electron density and total electron content) is much larger than found at Earth and Mars. With solar irradiance changes far too small to cause such effects, we use model results to explore the roles of other agents. We find that water sources from Enceladus at low latitudes, and ‘ring rain’ at middle latitudes, contribute substantially to variability via water ion chemistry. Thermospheric winds and electrodynamics generated at auroral latitudes are suggested causes of high latitude ionospheric variability, but remain inconclusive due to the lack of relevant observations

Composition of Titan's ionosphere

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94758/1/grl21212.pd

Surfaces, atmospheres and magnetospheres of the outer planets and their satellites and ring systems: Part X

International audienc

Preface to the special issue of PSS on “Surfaces, atmospheres and magnetospheres of the outer planets and their satellites and ring systems: Part XI”

International audienc

Neutral atmospheres

International audienceThis paper summarizes the understanding of aeronomy of neutral atmospheres in the solar system, discussing most planets as well as Saturn's moon Titan and comets. The thermal structure and energy balance is compared, highlighting the principal reasons for discrepancies amongst the atmospheres, a combination of atmospheric composition, heliocentric distance and other external energy sources not common to all. The composition of atmospheres is discussed in terms of vertical structure, chemistry and evolution. The final section compares dynamics in the upper atmospheres of most planets and highlights the importance of vertical dynamical coupling as well as magnetospheric forcing in auroral regions, where present. It is shown that a first order understanding of neutral atmospheres has emerged over the past decades, thanks to the combined effects of spacecraft and Earth-based observations as well as advances in theoretical modeling capabilities. Key gaps in our understanding are highlighted which ultimately call for a more comprehensive programme of observation and laboratory measurements

On the thermal electron balance in Titan’s sunlit upper atmosphere

International audienceThe Cassini mission has investigated Titan’s upper atmosphere in detail and found that, under solar irradiation, it has a well-developed ionosphere, which peaks between 1000 and 1200 km. In this paper we focus on the T40, T41, T42 and T48 Titan flybys by the Cassini spacecraft and use in situ measurements of N2 and CH4 densities by the Ion Neutral Mass Spectrometer (INMS) as input into a solar energy deposition model to determine electron production rates. We combine these electron production rates with estimates of the effective recombination coefficient based on available laboratory data for Titan ions’ dissociative recombination rates and electron temperatures derived from the Langmuir probe (LP) to predict electron number densities in Titan’s upper atmosphere, assuming photochemical equilibrium and loss of electrons exclusively through dissociative recombination with molecular ions. We then compare these predicted electron number densities with those observed in Titan’s upper atmosphere by the LP. The assumption of photochemical equilibrium is supported by a reasonable agreement between the altitudes where the electron densities are observed to peak and where the electron production rates are calculated to peak (roughly corresponding to the unit optical depth for HeII photons at 30.38 nm). We find, however, that the predicted electron number densities are nearly a factor of two higher than those observed throughout the altitude range between 1050 and 1200 km (where we have made estimates of the effective recombination coefficient). There are different possible reasons for this discrepancy; one possibility is that there may be important loss processes of free electrons other than dissociative recombination in Titan’s upper atmosphere