Giant Planet Ionospheres and Thermospheres: The Importance of Ion-Neutral Coupling

Abstract

Planetary upper atmospheres -coexisting thermospheres and ionospheres -form an important boundary between the planet itself and interplanetary space. The solar wind and radiation from the Sun may react with the upper atmosphere directly, as in the case of Venus. If the planet has a magnetic field, however, such interactions are mediated by the magnetosphere, as in the case of the Earth. All of the Solar System's giant planets have magnetic fields of various strengths, and interactions with their space environments are thus mediated by their respective magnetospheres. This article concentrates on the consequences of magnetosphere-atmosphere interactions for the physical conditions of the thermosphere and ionosphere. In particular, we wish to highlight important new considerations concerning the energy balance in the upper atmosphere of Jupiter and Saturn, and the role that coupling between the ionosphere and thermosphere may play in establishing and regulating energy flows and temperatures there. This article also compares the auroral activity of Earth, Jupiter, Saturn and Uranus. The Earth's behaviour is controlled, externally, by the solar wind. But Jupiter's is determined by the co-rotation or otherwise of the equatorial plasmasheet, which is internal to the planet's magnetosphere. Despite being rapid rotators, like Jupiter, Saturn and Uranus appear to have auroral emissions that are mainly under solar (wind) control. For Jupiter and Saturn, it is shown that Joule heating and "frictional" effects, due to ion-neutral coupling can produce large amounts of energy that may account for their high exospheric temperatures.

Solar System Ionospheres

“…Recent progress in our understanding of these ionospheres has been reviewed by Nagy and Cravens (2002), Majeed et al (2004), Mueller-Wodarg (2004), Yelle and Miller (2004), Miller et al (2005) and Schunk and Nagy (2008). The main focus in this subsection is on Saturn and Titan as representative of current research topics.…”

“…Such fully coupled models will be a major focus of future efforts. Given the abundance of good review papers currently available regarding ionospheric modeling of Saturn (e.g., Nagy and Cravens 2002;Majeed et al 2004;Miller et al 2005), this article will discuss only the most recent theoretical results. Moore et al (2004) find that H + 3 can become the dominant ion in the afternoon on Saturn at altitudes near the peak, especially in regions where the daily average flux of ultraviolet radiation is lowest (e.g., in winter, at middle and high latitudes, at latitudes that experience ring shadowing for some portion of the day, and at solar minimum); the H + / H + 3 ratio therefore depends on latitude, local time, and solar flux.…”

The Outer Planets and their Moons

“…There are nonetheless variations in the radio emission (Desch and Kaiser, 1981;Zarka and Kurth, 2005, in this volume) and vortical perturbations of the field (Espinosa and Dougherty, 2000) with a period that corresponds to Saturn's rotation period. For a discussion of the auroral emissions, we refer to the articles by Zarka and Kurth (2005) and Miller et al (2005) in this volume. The latter authors also present recent theoretical work on convection patterns and auroral currents by .…”

“…In several articles in this volume the composition (Encrenaz, 2005), cloud structure (Atreya and Wong, 2005), dynamics (Beebe, 2005), photochemistry , ion-neutral coupling (Miller et al, 2005), and auroral emissions (Zarka and Kurth, 2005) of Saturn's atmosphere and ionosphere are discussed. Here, we only give a brief overview on observations of Saturn's atmosphere and ionosphere.…”

Electron spin resonance investigation of H2+, HD+, and D2+ isolated in neon matrices at 2 K

“…[10][11][12] Early calculations of the H 2 + hyperfine structure were focused on supporting astronomical observations of the molecular ion, [13][14][15] and theoretical studies continued to improve the accuracy of the calculated hyperfine values (A iso = 922.992 MHz and A dip = 42.83 MHz). [16][17][18][19][20][21] The H 2 + radical cation has not been directly observed in an astronomical setting, 22,23 but it remains important in proposed astrophysical processes, [24][25][26][27][28] including the formation of small polyatomic molecules with biological significance.…”