Modelling perturbations propagating through the mesopause into the earth's upper atmosphere

Global oscillations formed in the terrestrial troposphere, stratosphere and mesosphere propagate into the thermosphere and ionosphere where they change the dynamics, energy and composition. This thesis presents a series of studies which examine in detail the nature and influence of solar tides and the planetary 2-day wave above 80 km altitude. The Coupled Thermosphere-Ionosphere Model (CTIM) calculates self-consistently the dynamics, energy and composition of the terrestrial thermosphere and ionosphere in three dimensions and is used as the main tool in these studies. In order to simulate the upwardly propagating perturbations which are formed outside the height range of the model, the lower boundary of the CTIM at 80 km height was modified to allow the global profiles of pressure-, wind- and temperature oscillations to be specified. In principle, following the modification, any such profile can be used for the external forcing as long the parameters at the lower boundary' are self-consistent. One effective method of achieving this is to specify global perturbations of geopotential height, using Hough functions for the latitudinal structure, and calculating the simultaneous wind- and temperature oscillations at the lower boundary analytically with expressions from Classical Tidal Theory. The necessary formalism for this has been fully implemented. For validation of the new code a series of comparisons with other numerical models and Incoherent Scatter Radar measurements at equinox and solstice are presented and show that CTIM is capable of reproducing many tidal features found in the "real" thermosphere. A further study is presented which investigates processes causing planetary' wave signatures in the ionosphere. It is found not only that CTIM reproduces some key properties of upwards propagating planetary waves found in other theoretical and modelling studies, but also that upwards propagating tides may, through modulation of their amplitudes, carry planetary wave signatures into the 200 km height regime where they are transferred into the ionosphere by chemical processes. The new CTIM thus offers the possibility of carrying out many unprecedented studies exploring the nature of the Earth's upper atmosphere

Phosphine gas in the cloud decks of Venus

Measurements of trace gases in planetary atmospheres help us explore chemical conditions different to those on Earth. Our nearest neighbour, Venus, has cloud decks that are temperate but hyperacidic. Here we report the apparent presence of phosphine (PH3) gas in Venus’s atmosphere, where any phosphorus should be in oxidized forms. Single-line millimetre-waveband spectral detections (quality up to ~15σ) from the JCMT and ALMA telescopes have no other plausible identification. Atmospheric PH3 at ~20 ppb abundance is inferred. The presence of PH3 is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently known abiotic production routes in Venus’s atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery. PH3 could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of PH3 on Earth, from the presence of life. Other PH3 spectral features should be sought, while in situ cloud and surface sampling could examine sources of this gas

Plasma temperatures in Saturn’s ionosphere

Abstract Using a series of coupled models developed to help interpret Cassini observations, we have calculated self-consistent electron and ion temperatures in Saturn's ionosphere under solar maximum conditions. Electron temperatures in the topside ionosphere are calculated to range between 500 -560 K during the Saturn day --approximately 80 -140 K above the neutral temperature. Ion temperatures, calculated for only the major ions H + and H 3 + , are nearly equal to the neutral temperature at altitudes near and below the height of peak electron density, while they can reach nearly 480 K during the day at the topside. Plasma scale heights of the dusk electron density profile from radio occultation measurements of the Voyager 2 fly-by of Saturn have been used to estimate plasma temperature. Such an estimate agrees well with the temperatures calculated here, although there is a topside enhancement in electron density that remains unexplained by ionospheric calculations that include photochemistry and plasma diffusion. Finally, parameterizations of the heating rate from photoelectrons and secondary electrons to thermal, ambient electrons have been developed. They may apply for other conditions at Saturn, and possibly at other giant planets as well

Simulation of the Magnetosphere- Ionosphere Connection at Saturn

The giant planets in our solar system such as Saturn and Jupiter represent fascinating worlds which exhibit a range of electro-magnetic, collisional and chemical processes coupling the upper atmospheres with the magnetospheres and some of their moons. Observationally, they are explored either in-situ through magnetic and electric field as well as plasma observations, or remotely by observing auroral emissions or atmospheric occultations. Magnetosphere-ionosphere coupling has over the past decades been studied in depth on Earth and matured as a field, but for the giant planets our understanding is still in its early stages. A key aid for our understanding of the underlying physics are numerical models which simulate the relevant neutral-ion and ion-magnetosphere coupling processes. Some of the key currently unresolved science questions for Saturn include the origin of its high thermosphere temperatures ( energy crisis ), of its highly variable and structured ionosphere as well as the observed variations of Saturn\u27s apparent rotation rate. Work over the past years has shown that these all in one way or another rely on understanding magnetosphere-atmosphere coupling. Comparisons of Saturn and Earth are particularly interesting as well, as similar physical processes - well studied for Earth - act on both, but under different boundary conditions. Using our Saturn Thermosphere-Ionosphere model (STIM) with inputs from the University of Michigan Block Adaptive Tree Solar wind Roe-type Upwind Scheme (BATSRUS) MHD model, we calculate the coupling of Saturn\u27s magnetosphere with the planet\u27s upper atmosphere. At high latitudes STIM relies on electric fields and incident energetic particle fluxes which in turn ionise the upper atmosphere and generate ionospheric currents. These, in turn, lead to westward (anti-corotational) acceleration of ions and thereby neutral winds, whereby angular momentum is transferred from atmosphere to magnetospheric plasma. Within the atmosphere, strong auroral heating occurs which drives a complex system of global circulation and energy redistribution. For the first time we present calculations made at high spatial resolution and illustrate the relevance of that. By examining the time-dependent response of Saturn\u27s atmosphere to variations in solar wind pressure (via its magnetosphere), we infer the relevant physical processes and intrinsic atmospheric time scales. Our Saturn calculations are constrained by and compared with key observations, and parallels are drawn to any terrestrial equivalents in behaviour. We address the energy crisis and discuss possible solutions. Our simulations and tools, in tandem with Cassini and ground based observations form an important step towards understanding \u27\u27space weather\u27\u27 on Saturn

Neutral Upper Atmosphere and Ionosphere Modeling

Numerical modeling tools can be used for a number of reasons yielding many benefits in their application to planetary upper atmosphere and ionosphere environments. These tools are commonly used to predict upper atmosphere and ionosphere characteristics and to interpret measurements once they are obtained. Additional applications of these tools include conducting diagnostic balance studies, converting raw measurements into useful physical parameters, and comparing features and processes of different planetary atmospheres. This chapter focuses upon various classes of upper atmosphere and ionosphere numerical modeling tools, the equations solved and key assumptions made, specified inputs and tunable parameters, their common applications, and finally their notable strengths and weaknesses. Examples of these model classes and their specific applications to individual planetary environments will be described

Neutral Upper Atmosphere and Ionosphere Modeling

Numerical modeling tools can be used for a number of reasons yielding many benefits in their application to planetary upper atmosphere and ionosphere environments. These tools are commonly used to predict upper atmosphere and ionosphere characteristics and to interpret measurements once they are obtained. Additional applications of these tools include conducting diagnostic balance studies, converting raw measurements into useful physical parameters, and comparing features and processes of different planetary atmospheres. This chapter focuses upon various classes of upper atmosphere and ionosphere numerical modeling tools, the equations solved and key assumptions made, specified inputs and tunable parameters, their common applications, and finally their notable strengths and weaknesses. Examples of these model classes and their specific applications to individual planetary environments will be described

The formation of benzene and complex hydrocarbons in the auroral and non-auroral regions of Saturn

International audienceRecent infrared and ultraviolet observations from Cassini reveal an enhancement in the abundance of benzene at high latitudes on Saturn and indicate the presence of high-altitude hazes in Saturn's upper stratosphere (Guerlet et al. 2015, A&A 580, A89; Koskinen et al. 2016, GRL 43, 7895; Kim et al. 2012, PSS 65, 122). In contrast, photochemical models that include neutral photochemistry initiated by solar ultraviolet radiation alone (i.e., no auroral chemistry and no ion chemistry) predict a maximum in the benzene abundance at low latitudes, with related haze production deeper in the stratosphere (e.g., Moses & Greathouse 2005, JGR 110, E09007). This model-data mismatch supports the hypothesis that refractory high-molecular-weight organics are synthesized in Saturn's auroral regions as a result of ion chemistry driven by the precipitation of energetic magnetospheric particles into the upper atmosphere. Using a coupled ion-neutral photochemical model, we investigate the production of benzene, polycyclic aromatic hydrocarbons (PAHs), and other complex organic molecules in Saturn's high-latitude stratosphere and thermosphere and compare predictions with Cassini CIRS and UVIS observations. We identify the key chemical mechanisms involved, discuss the relative roles of auroral chemistry and solar photochemistry in producing refractory organics across the planet, better define the vertical profiles of benzene and other hydrocarbons as a function of latitude, and describe the differences between the hydrocarbon chemical pathways on Titan and Saturn. This work was supported by the NASA Solar System Workings program, grant number NNX16AG10G

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