by
Published in
Astronomy & Geophysics 45, 1.27-1.28 (February 2004)
The annual one-day meeting of the MIST (Magnetosphere, Ionosphere and
Solar-Terrestrial) community was held on the 28th November, 2003 at the
Geological Society, Burlington House. The first session of the meeting was
chaired by Anasuya Aruliah (UCL) and started with four talks on the outer
environments of the outer planets. Nick Achilleos (Imperial College)
reported on magnetic field observations at Jupiter's bow shock as the
Cassini spacecraft flew by on the way to its rendezvous with Saturn. The
solar wind pressure varied by an order of magnitude in the space of a few
days, making it possible to observe a wide range of angles between the wind
and the bow shock normals over a range of distances from the planet,
greater than were possible during the Voyager and Pioneer encounters.
During this passage, Abigail Rymer (Mullard Space Science Laboratory)
examined the electron temperature and density variations that accompanied
the magnetic field fluctuations. She found that the relationship between
distance from the planet, solar wind Mach number and solar wind Alfven
number agreed well with magnetohydrodynamic (MHD) models out to 600 Jupiter
radii. The electron heating depended on the solar wind bulk flow and
magnetosheath speed.
A three-dimensional model of the thermosphere of the gas giants was
introduced by Chris Smith (University College London). Previous models
underestimated the observed temperatures by up to 400 Kelvin, so it was
necessary to consider the contribution from auroral plasma current heating
and the conditions that would be required to ensure that this extra energy
was transported effectively to the rest of the planet. A similar approach
was taken by Tracy Moffet (also UCL), but this time for Mars. The
introduction of an ionosphere made it possible to carry out
inter-comparisons with observations from the Radio Science experiment on
board the Mars Global Surveyor spacecraft of the free electron density
profile. The agreement was good above 150 km, but additional processes
will need to be included to improve the representation below this height.
Ilya Alexeev (MSSL) described his research into magnetic
reconnection. In order to investigate time varying phenomena, it was
necessary to develop a self-consistent numerical model that allowed a
non-steady state solution. This allowed feedback structures to evolve. It
was found that pulsation rates with a period that was proportional to the
diffusion region size were obtained, thereby possibly accounting for the
bursty nature of reconnection. Natalie Draper (Leicester) examined Cluster
observations of a magnetic field cavity in the Earth's plasma sheet. She
took advantage of the unique four spacecraft configuration to isolate
unusual events where field and particle data indicated levels consistent
with entry of the Central Plasma Sheet, but the spacecraft geometry and
time histories suggested that this was not possible. Alternative
explanations appear to be required to account for this behaviour.
Two more talks from Leicester examined the impact of the
Interplanetary Magnetic Field on the resulting plasma flows in the Earth's
ionosphere. Adrian Grocott examined an event where the vertical component
of the IMF was northward. Normally, reconnection does not occur under
these conditions, but open flux production was maintained by the existence
of a strong tangential component. It was believed that the reconnection
was linked to a reconfiguration of the asymmetric tail associated with
convective transport in the distant tail, rather than being directly
related to substorms. Gabrielle Provan had identified a second period
where signatures of reconnection were evident in the ionosphere under
similar IMF conditions. She described how the changing nature of the IMF
resulted in a major reconfiguration of the global ionospheric convection
pattern, from the classical twin cell to a multi-cell circulation.
The second session of the meeting was chaired by Jason Dewhurst
(MSSL). The first talk by Helen Middleton (Aberystwyth) continued the
theme of IMF influences on the Earth's ionosphere. She made use of three
chains of Global Positioning Satellite receivers in Scandanavia, Alaska and
Greenland to reconstruct tomographically the large scale plasma
distributions in near real time. She provided an example of a secondary
peak in the electron density that was isolated from the auroral oval.
There then followed two presentations from Bath University on
observations from a new meteor radar that has recently been deployed on
Ascension Island in the Equatorial Atlantic Ocean where it can measure
winds in the upper mesosphere and lower thermosphere. Nick Mitchell
reported on several years' worth of data which showed a significant signal
from 3-day waves, that were thought to be Kelvin waves which appeared to be
depositing large amounts of momentum and energy into the background flow at
these altitudes. Peter Younger then discussed the tides that were observed
by this radar. A strong 24-hour tide was present in the data with a
clearly defined seasonal cycle. However, considerable variability on
timescales of a month or less indicated that coupling with planetary scale
waves was an important factor.
Scott England (Leicester) described recent efforts to improve the ability
of the UCL Coupled Middle Atmosphere Thermosphere general circulation model
to reproduce the equatorial winds that have been observed by satellites and
radars such as the one described above. It was found that accurate
representations of the impact of sub-grid scale waves, such as gravity
waves brought the model much closer to the observed upper mesosphere and
lower thermosphere. The first direct evidence of meso-scale variability on
ion-neutral dynamics was provided by Anasuya Aruliah (UCL) using co-located
tristatic Fabry-Perot Interferometers and the three receiver configuration
of the European Incoherent Scatter radar in Northern Scandinavia. Winds
and temperatures on a scale of tens of kilometres were recorded, indicating
that the thermosphere is considerably more dynamic than had been previously
assumed and that the neutral wind dynamo made a significant contribution to
the heating rate.
Hien Vo (Aberystwyth) described some observations of the sub-auroral
electric field as seen by the Millstone Hill incoherent scatter radar over
two complete solar cycles. A broad region of ring current induced electric
field was seen equatorward of the normal two-cell ionospheric convection
pattern and this feature corresponded well to the plasmaspheric tail seen
by the extreme ultraviolet images from the IMAGE spacecraft. The session
was brought to a close by Nigel Meredith (MSSL). He described a
statistical search for the more favoured regions for chorus-driven electron
acceleration to relativistic energies in the Earth's outer radiation belt
and the conditions most likely to induce these forces. It was found that
enhanced chorus waves, active geomagnetic conditions and particular
magnetic local times were the most conducive.
The third and final session of the day, chaired by Nick Mitchell, was
dominated by Cluster science presentations. Nanan Balan (Sheffield)
presented observations of a structured magnetospheric cusp crossing during
a moderate geomagnetic storm. Three distinct anti-sunward ion flow events
aligned roughly parallel to the magnetopause surface were detected in the
presence of strong field-aligned currents. Katarina Nykyri (IC) examined
ion cyclotron waves in the high altitude cusp and took advantage of the
four spacecraft with varying separations to attempt to observe correlations
between them. The absence of any positive correlations indicated that the
waves appeared to be in highly filamented shear flows with spatial scales
much less than 100 km.
Robert Fear (MSSL) looked at data from the PEACE particle detectors when
the four spacecraft were aligned in a line along the magnetopause boundary
near the cusp. They observed a series of transient bursts of
magnetosheath-like plasma. A flux transfer like event was found to be more
consistent with motion around a moving boundary caused by a pressure pulse.
Yulia Bogdanova (MSSL) examined the ion outflow from the cleft-cusp. This
flow is one of the main contributors of ionospheric ions in the
magnetosphere. Three such crossings were identified and the location of
the region with the most intense ion heating was well correlated with uni-
and bi-directional soft (< 500 eV) electron beams, presumably from
magnetopause reconnection.
Silvia Dalla (UMIST) examined the propagation times of solar energetic
particles at a range of heliospheric latitudes. Is was found that Coronal
Mass Ejections were observed near the Earth by the WIND spacecraft much
earlier than at the high latitudes observed by Ulysses. One possible
interpretation is that the particles were delayed by scattering in the
interplanetary medium. Matthew Owens (IC) described attempts to predict
the time of arrival at the Earth of these CMEs. Up to half of the events
were found to exhibit smooth magnetic field rotations and that twice as
many propagated in the westward direction, rather than the eastward, due to
the distortions of the Sun's rotation.
Jason Dewhurst (MSSL) concluded the meeting with a paper on Cluster PEACE
observations of flow shears in the plasma sheet. He derived electron
velocity and magnetic field moments using high resolution data from the
four spacecraft. The effect of flow shears on field-aligned flows within
the plasma sheet was discussed. There was some evidence of twisting of the
flux tubes related to field-aligned current generation with reconnection
occurring at the Near Earth Neutral Line.
Return to MIST home page
Meeting Report
Neil Arnold (University of Leicester)