[Report published in Astronomy & Geophysics 44, 2.33 (April 2003)]
by Ingo Mueller-Wodarg (University College London) and Emma Bunce (University of Leicester)
Published in
Astronomy & Geophysics 44, 2.33 (April 2003)
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 Report
Comparative Aeronomy in the Solar System: An RAS-G/MIST Discussion Meeting
Abstracts
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
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