RAS/G-MIST meeting
10 January 2003

Society of Antiquaries, Burlington House, London

[Report published in Astronomy & Geophysics 44, 2.33 (April 2003)]

A. Programme B. Report C. Abstracts

RAS/G-MIST Meeting: Comparative Aeronomy in the Solar System

10 January 2003
Society of Antiquaries, Burlington House, Piccadilly, London

here for a map
Speakers: click here for further information
Click here for a pdf version of this schedule, including abstracts

Organizers:     Ingo Mueller-Wodarg (i.mueller-wodarg@ucl.ac.uk), Emma Bunce (emma.bunce@ion.le.ac.uk)

Morning Session
Chair: Ingo Mueller-Wodarg (UCL)
10:00-10:30       Coffee
10:30-10:35       Introduction
10:35-11:05       M. Mendillo (BU), Thermospheres and Ionospheres in the Solar System
11:05-11:25       G. Millward  (UCL), Comparative modelling of planetary ionosphere-thermosphere systems
11:25-11:45       N. Achilleos (IC), Thermosphere-Ionosphere Coupling at Jupiter
11:45-12:05       T. Stallard, G. Millward, S. Miller (UCL), Jupiter's auroral/polar winds: observations and implications
12:05-12:25       J. Bailey (AAO), Observations of the Venus Night Airglow
12:25-12:55       M. Blanc (OAMP), Magnetospheres in the Solar System
Lunch (a light lunch will be available for purchase)
Afternoon Session
Chair: Emma Bunce (Leicester)
14:00-14:20       J. D. Nichols, S.W.H. Cowley, and E.J. Bunce (Leicester) Magnetosphere-Ionosphere Coupling Currents in Jupiter's Middle Magnetosphere

14:20-14:40       S. W. H. Cowley, E J Bunce, and J D Nichols (Leicester), Magnetosphere-Ionosphere Coupling Currents in Jupiter's and Saturn's Magnetospheres

14:40-15:10       S. E. Milan (Leicester), Convection and auroral signatures of solar wind-magnetosphere coupling at Earth - What might we expect at Saturn?
15:10-15:30       A. Coates (MSSL/UCL) Ion pickup in the solar system
15:30-16:00       Tea at the Geological  Society
16:00-18:00       RAS Monthly A&G (Ordinary) Meeting
18:00-19:00       Drinks Party (in RAS' Burlington House apartments)

RAS/G-MIST meeting, 10 January 2003
Meeting Report

by Ingo Mueller-Wodarg (University College London) and Emma Bunce (University of Leicester)

Published in Astronomy & Geophysics 44, 2.33 (April 2003)

Comparative Aeronomy in the Solar System: An RAS-G/MIST Discussion Meeting

The coupled upper atmospheres, ionospheres and magnetospheres of the planets were the subject of a joint RAS-G/MIST discussion meeting hosted by the Royal Astronomical Society on Jan 10, 2003, organised by Dr. Ingo Mueller-Wodarg (University College London) and Dr. Emma Bunce (University of Leicester). The scientific programme consisted of 10 presentations by speakers from the UK, France, USA and Australia.

The subject of this meeting, aeronomy, is the study of the upper atmospheres (thermospheres, ionospheres) of planets and their interaction with the solar wind and magnetosphere (where applicable) as well as their coupling to the lower regions of an atmosphere. The aim in this meeting was not to discuss a planet in isolation, but rather to look at them collectively, contrasting their features and finding similarities. By doing so, we obtain a far deeper insight into the underlying physical processes which control their behaviour, and we are thus able to place specific features into broader context by realizing how unique or common they are in the solar system. Furthermore, the aim of this meeting was to bring together two scientific communities which interact too rarely, the terrestrial and planetary aeronomers. While studying different planets, they often investigate the same physical processes and use similar observational techniques and models. A more intense dialogue between them is therefore of obvious mutual benefit.

The morning session concentrated on discussing the atmospheres of planets and was started with a comprehensive review of thermospheres and ionospheres in our solar system given by Prof. Michael Mendillo (Boston University). He demonstrated how peak electron densities generally decreased with distance from Sun, due to the drop in ionising EUV radiation. Particularly the ionospheres of Venus, Earth and Mars are controlled by solar radiation, due to their proximity to the Sun, with terrestrial E-region variability and Martian peak density changes correlating well with variations in solar flux. This review talk was followed by two presentations demonstrating the capabilities of modern numerical models to study planetary atmospheres. Numerical models calculate the global response of an atmosphere to solar heating and magnetospheric driving, using basic physical laws, and form an essential aeronomy tool to complement equally vital observations. Dr. George Millward of University College London (UCL) introduced suites of General Circulation Models developed in the UK and the USA to study the coupled thermospheres and ionospheres of most planets in the solar system. He illustrated with a few sample studies the high capabilities of these codes. For both the Earth and Jupiter, they allow us to study the atmosphere's response to the time-varying magnetospheric forcing. Dr. Nick Achilleos (Imperial College London) presented more calculations of Jupiter's coupled thermosphere-ionosphere system from the Jovian Ionosphere Model (JIM). Ions are accelerated by electric fields from the magnetosphere, collide with neutral gas particles at high latitudes and accelerate them to supersonic velocities. The calculations agreed well with ground-based observations of strong ion drifts in Jupiter's auroral zone which were presented by Dr. Alan Aylward (UCL). He outlined the capability and importance of ground-based observations to study planetary atmospheres, presenting studies of the H3+ emissions from Jupiter's polar regions carried out by Drs. Tom Stallard and Steve Miller using the UK Infrared Telescope (UKIRT) facility on Hawaii. Another example of ground-based observations was given by Dr. Jeremy Bailey (Anglo-Australian Observatory), who presented recent observations of a strong 1.27 mm O2 airglow emission from the night side of Venus. These observations indicated large day-to-day variability of this emission, which originates from around 100 km altitude as a result of atomic oxygen recombination and appears brightest at the antisolar point.

To introduce the subject of the second half of the meeting, Dr. Michel Blanc (Observatoire Astronomique de Marseille-Provence) presented a comprehensive overview of magnetospheres in the solar system, distinguishing between intrinsic magnetospheres (Giant planets, Earth, Mercury) and those which are induced (Venus, Comets). While the Earth and the Gas Giants are shielded from the solar wind by their magnetic fields, one of Mercury's poles is connected to the solar wind leading to non-uniform precipitation of particles from the solar wind onto one hemisphere. Surface material can be accelerated into Mercury's magnetosphere and lead to additional low-latitude precipitation. The afternoon session continued with magnetospheres, and started with Mr. Jonathan Nichols (University of Leicester) reviewing the recent discovery that the main auroral oval at Jupiter is due to the breakdown of corotation of plasma within the magnetosphere. He told us of his recent work, specifically the dependence of the theoretical model of the oval on the ionospheric conductivity and the mass outflow rate from Io. He indicated that new results show a variable conductivity with latitude is necessary to be consistent with spacecraft observations. Next Dr. Emma Bunce (University of Leicester) spoke about the kronian auroral oval and recent enquiries into the origin of this emission. The group at Leicester showed that the same mechanism which produces an auroral oval at Jupiter, does not work at Saturn. The currents are of the wrong magnitude and do not flow at the observed latitude in the ionosphere. They therefore concluded that the kronian aurora was likely to be driven and modulated externally by the solar-wind, and hence probably more Earth-like. Dr. Steve Milan (University of Leicester) presented a review of the auroral processes in the Earth's magnetosphere, and gave an insight into the sort of dynamics we might expect to see at Saturn. In fact, he concluded that the only way to really understand the kronian aurora would be to study the dynamics and variability of the emissions, over reasonable time-scales. We hope that the Hubble Space Telescope campaign during the Cassini approach to Saturn may provide such an opportunity. Finally, Dr. Andrew Coates (Mullard Space Science Laboratory, UCL) gave an overview of the non-magnetised bodies and the process of ion-pick up in the solar system, with specific reference to comets.

We are very grateful to the RAS for their considerable financial and organising support, which made this meeting possible. Our hope is that it achieved some of its aims and inspired new ideas and collaborations within the community.


Meeting Summary: The planets and satellites in our solar system interact with the solar wind and radiation in ways which are determined by their atmospheres, magnetospheres, geometry and distance from the Sun. Comparing the upper atmospheres and magnetospheres of planets and satellites helps us better understand the underlying physical laws which control these interactions, putting the different planetary configurations into perspective. The aim of this discussion meeting is to address the diversity of coupling, energetics and dynamics found in planetary and satellite environments, through theory, modelling, and observations.
Michael Mendillo (Center for Space Physics, Boston University, mendillo@bu.edu), Thermospheres and Ionospheres in the Solar System
The neutral atmospheres and ionospheres of the planets offer a rich set of conditions to test our understanding of upper atmospheric physics and solar-planetary coupling.  The major constituents of the atmosphere (e.g., CO2 versus N2 versus H2) determine the processes of solar photon absorption and planetary radiation, and thus the resulting neutral temperature profiles and upper atmosphere global winds.  Atmospheric chemistry determines if plasmas produced by photo-ionization remain as ions related directly to their parent neutrals or to different ions via plasma-neutral transformations.  Subsequent plasma chemistry determines if the ionosphere is one dominated by atomic or molecular ions, and thus to the resulting diurnal patterns of global ionospheric behaviour.  The presence or absence of a planetary magnetic field governs the types of electrodynamical interactions between the solar wind and a planet's space environment.
George Millward (Atmospheric Physics Laboratory, University College London, george@apl.ucl.ac.uk), Comparative modelling of planetary ionosphere-thermosphere systems
On a fundamental level, a vertical slice through the atmosphere of a planet can be divided into three distinct regions. At the lowest level, the troposphere and stratosphere, an atmosphere consists of a dynamic mixture of neutral gases. In contrast, at the very outer extremity, an atmosphere consists entirely of charged particles. This plasma environment is known as the magnetosphere (for planets with a magnetic field), a region characterised by the interaction of the planets atmosphere with the high velocity stream of charged particles from the sun, known as the solar wind. Sandwiched in between is a transition region in which both the neutral gases of the lower atmosphere, and the charged particles of the outer atmosphere, co-exist. This region is the ionosphere-thermosphere. At the Atmospheric Physics Laboratory, University College London, sophisticated computational models of the global ionosphere-thermosphere system have been developed for the planets Earth, Mars, Jupiter, and Titan. In addition, recent developments have been concerned with the modelling of Saturn (in readiness for the arrival of the Cassini spacecraft in 2004) and also Jovian-like exoplanets. My talk will describe how three-dimensional, time dependent modelling of these systems provides invaluable insights into the underlying physical processes at work. It will focus on our comparative planetology approach in which the aim is to gain a better understanding by investigating the similarities and differences of the various systems.
Nick Achilleos (Space and Atmospheric Physics Group, Imperial College London, n.achilleos@ic.ac.uk), Thermosphere-Ionosphere Coupling at Jupiter
Observations of the motions of ions in Jupiter's auroral region have revealed that the dynamics of the ionosphere in this region are strongly coupled to the motions of the neutral thermospheric gas. Detailed global modelling using JIM (Jovian Ionospheric Model) has shown that this coupling is the result of: (i) The high ionospheric conductivity of Jupiter's auroral region; (ii) The strong electric field projected down onto the auroral ionosphere by the breakdown in corotation between the planet itself and the plasma in the distant magnetosphere. The results of this modelling study are presented in the wider context of the physical 'feedback' mechanisms, which link the dynamics and composition of the ionosphere, thermosphere and magnetosphere.
Tom Stallard, G. Millward, S. Miller (Atmospheric Physics Laboratory, University College London, tss@star.ucl.ac.uk), Jupiter's auroral/polar winds: observations and implications
Recent observations of the auroral / polar wind system on Jupiter show it to consist of several distinct regions. The main auroral oval is accompanied by a strong electrojet, with ion speeds reaching up to 2km/s. Poleward of the oval are two major regions - the Dark Polar Region (DPR) and the Bright Polar Region (BPR). The latter is seen to have rather flaccid winds. But the former has strong anti-sunward winds, as viewed in the planetary frame of reference. Transformed to the magnetic pole reference frame, this is shown to be a region of stagnation, linked to magnetotail field lines. Modelling of the electrojet shows that neutral winds are generated, with velocities reaching between 50% and 70% of the ions at the peak ionisation level. These winds may carry energy equatorward, helping to explain the high exospheric temperature of Jupiter.
Jeremy Bailey (Anglo-Australian Observatory, jab@aaoepp.aao.gov.au), Observations of the Venus Night Airglow
I will present the preliminary results of recent observations of the distribution and variability of the 1.27 micrometre O2 airglow emission from the night side of Venus. These and ot her recent results on the Venus airglow will be compared with the corresponding terrestrial emission. The implications for the photochemistry and dynamics in the Venus upper atmosphere will be discussed.
Michel Blanc (Observatoire Astronomique de Marseille-Provence, Michel.Blanc@oamp.fr), Magnetospheres in the Solar System
Steve Milan (Radio and Space Plasma Physics Group, University of Leicester, ets@ion.le.ac.uk), Convection and auroral signatures of solar wind-magnetosphere coupling at Earth - What might we expect at Saturn?
The circulation of plasma within planetary magnetospheres is controlled by the interplay between several competing processes.  For instance, in the case of colder plasma, for which the effects of magnetic curvature and gradient drifts can be ignored, the two main sources of impetus for the motion are angular momentum donated from planetary rotation (corotation) and the coupling of momentum from the solar wind across the magnetopause via magnetic rec onnection (the "Dungey cycle"). Observations and theoretical modelling show that magnetospheric convection driven by the latter dominates at Earth, whereas corotation is the dominant circulation at Jupiter.  At Saturn, however, it is thought that the two processes compete on a more or less equal footing, and hence solar wind-magnetosphere coupling should play a much more important role in modulating the Kronian aurora than their Jovian counterpart.  In this talk I will summarize the main features of the Earth's dayside aurora and convection which arise due to magnetic reconnection, to provide a guide for the possible identification of similar features during the Cassini mission at Saturn.
Stan Cowley, E. J. Bunce, and J. D. Nichols (Radio and Space Plasma Physics Group, University of Leicester, swhc1@ion.le.ac.uk), Magnetosphere-Ionosphere Coupling Currents in Jupiter's and Saturn's Magnetospheres
The dynamics of Jupiter's and Saturn's magnetospheres are both dominated by plasma pick-up and radial transport in corotation-dominated flow.  In this poster we compute and compare the magnitude and spatial distribution of the currents that couple angular momentum from the planetary atmosphere and ionosphe re to the magnetospheric plasma.  We also comment on the consequent relationship of the current systems to the patterns of observed aurora.
Jon Nichols, S. W. H. Cowley, and E. J. Bunce (Radio and Space Plasma Physics Group, University of Leicester, jdn@ion.le.ac.uk), Magnetosphere-Ionosphere Coupling Currents in Jupiter's Middle Magnetosphere

The dynamics of Jupiter's plasma environment is dominated by the outflow of material originating from the moon Io, which orbits deep within the magnetospheric cavity.  Breakdown of corotation associated with this radial transport results in the formation of a magnetosphere-ionosphere coupling current system which transfers angular momentum to the magnetospheric plasma and has been linked to the formation of the main jovian auroral oval.  In this poster we compute solutions for this current system based on steady plasma outflow from the Io torus, and consider how they depend on the values of the effective jovian ionospheric Pedersen conductivity and iogenic plasma mass outflow rate.  We go on to consider how the auroral precipitation associated with regions of upward field-aligned current flow modulates the effective Pedersen conductivity and thereby effect the solutions.
Andrew Coates (Mullard Space Science Laboratory/UCL, ajc@mssl.ucl.ac.uk), Ion pickup in the solar system
Ion pickup is an important process at work in several solar system contexts. It is central to the solar wind interaction with comets. In addition it plays an important role at Mars, Venus and probably Pluto. At Mars it has been responsible for atmospheric loss over the last 3.8 billion years. Ion pickup is also important at satellites such as Io and Titan and wherever a source of neutral particles occurs. The interstellar medium also provides a source of pickup ions. Here we review this process and its effects in the different contexts.

Last updated: 9 April 2003

Return to
MIST home page