The autumn one-day meeting of the MIST

(Magnetosphere, Ionosphere and Solar–

Terrestrial) community was held on 28

November 2003 at the Geological Society,

Burlington House. The first session of the meet-

ing was chaired by Anasuya Aruliah (University

College London) and started with four talks on

the outer environments of the outer planets.

Nick Achilleos (Imperial College) reported

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 fluctua-

tions. She found that the relationship between

distance from the planet, solar wind Mach

number and solar wind Alfven number agreed

well with magnetohydrodynamic (MHD) mod-

els out to 600 Jupiter radii. The electron heat-

ing depended on the solar wind bulk flow and

magnetosheath speed.

Correcting model temperatures

A three-dimensional model of the thermosphere

of the gas giants was introduced by Chris Smith

(UCL). Previous models underestimated the

observed temperatures by up to 400 K, so it was

necessary to consider the contribution from

auroral plasma current heating and the condi-

tions that would be required to ensure that this

extra energy was transported effectively to the

rest of the planet. Modelling of the martian

upper atmosphere was undertaken by Tracy

Moffet (also UCL). The introduction of an

ionosphere made it possible to carry out inter-

comparisons with observations from the Radio

Science experiment on board Mars Global

Surveyor of the free electron density profile. The

agreement was good above 150 km, but addi-

tional 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 investi-

gate time-varying phenomena it was necessary

Meeting report

1.27February 2004 Vol 45

Interplanetary MISTThe London MIST meeting in November covered current research using satellites to investigate planetary

environments across the solar system, including our own. Earth-bound instruments, GPS and modelling also

came under discussion, as did a series of results from the Cluster mission. Neil Arnold reports.

1: An importantenergy source inSaturn’sthermosphere isheatingassociated withthe aurorae inthe north andsouth polarregions. Thisheating sets up aglobal circulationthat redistributessome of theenergy to low-latitude regions.Colouredcontours indicatetemperature;arrows indicatewind direction(Chris Smith).2: Saturn’saurorae (HSTNASA/STSCI).

temperature (K)

–90 –45 0 45 90 latitude

log 1

0P (P

a)

–5

–4

–3

–2

–1

1000

800

600

400

200

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 observa-

tions 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 into the Central

Plasma Sheet, but the spacecraft geometry and

time histories suggested that this was not pos-

sible. Alternative explanations appear to be

required to account for this behaviour.

The IMF and plasma flows

Two more talks from Leicester examined

the impact of the interplanetary magnetic

field (IMF) on the resulting plasma flows in

the Earth’s ionosphere. Adrian Grocott

examined an event where the vertical com-

ponent 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 tan-

gential 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 ionos-

pheric 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 iono-

sphere. She made use of three chains of global

positioning satellite receivers in Scandanavia,

Alaska and Greenland to reconstruct tomo-

graphically 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

meteo-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

three-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 variabil-

ity 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 representa-

tions 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 signifi-

cant contribution to the heating rate.

Hien Vo (Aberystwyth) described some obser-

vations 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 equator-

ward of the normal two-cell ionospheric convec-

tion pattern and this feature corresponded well

to the plasmaspheric tail seen by the extreme

ultraviolet images from the IMAGE spacecraft.

Chorus waves

The session was brought to a close by Nigel

Meredith (MSSL). He described a statistical

search for the more favoured regions for cho-

rus-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 par-

ticular magnetic local times were the most con-

ducive conditions.

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 magneto-

spheric cusp crossing during a moderate geo-

magnetic storm. Three distinct anti-Sunward

ion flow events aligned roughly parallel to the

magnetopause surface were detected in the pres-

ence of strong field-aligned currents. Katarina

Nykyri (IC) examined ion cyclotron waves in

the high altitude cusp and took advantage of the

variable separations of the four spacecraft to

attempt to observe correlations between them.

The absence of any positive correlations indi-

cated that the waves appeared to be in highly

filamented shear flows with spatial scales much

less than 100 km.

Magnetopause alignment

Robert Fear (MSSL) looked at data from the

PEACE particle detectors when the four space-

craft were aligned in a line along the magneto-

pause 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 contrib-

utors 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) elec-

tron beams, presumably from magnetopause

reconnection.

Silvia Dalla (UMIST) examined the propaga-

tion times of solar energetic particles at a range

of heliospheric latitudes. It was found that coro-

nal mass ejections (CMEs) 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 east-

ward, due to the distortions of the Sun’s rotation.

Jason Dewhurst (MSSL) concluded the meet-

ing with a paper on Cluster PEACE observa-

tions 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. �

Neil Arnold, University of Leicester. Note: much of the work reported on above is col-laborative. Full programme details, including titles,authors, abstracts and affiliations, may be found onthe MIST web pages at www.mist.ac.uk.

Meeting report

1.28 February 2004 Vol 45

“The thermosphere isconsiderably more dynamic thanhad been previously assumed.”