MILLENIUM MIST: ABSTRACTS
Tuesday April 11
Long term trends in thermospheric neutral winds
derived from ionosonde measurements (Eoghan Griffin,
Anasuya Aruliah, Ingo Muller-Wodarg Atmospheric Physics Laboratory, University College London.)
A number of previous studies have reported on the variation of thermospheric neutral winds with solar activity and season, mostly for mid-latitude meridional winds derived from ionospheric measurements. These studies have typically used data from periods of around one solar cycle from individual sites. In this study data from ionosondes have been used to derive neutral winds over 2 solar cycles using 3 mid-latitude sites evenly spaced in longitude. Results are presented from the comparison of the sites, the different solar cycles and to results derived from ionospheric parameters using other techniques. In addition comparisons are presented between direct neutral wind measurements from FPI at Kiruna and from ionosonde measurements at the same site. CTIM numerical model output is used to attempt an explanation of the differences observed in the datasets.
A study of thermosphere-mesosphere
photochemical coupling due to medium
energy high latitude particle precipitation using the UCL Coupled Middle
Atmosphere and Thermosphere General Circulation model (CMAT) ( M.J.
Harris(1), A. Aylward(1), M.V. Codrescu(2), T.J. Fuller-Rowell(2), F.
Eparvier(3): (1) Atmospheric Physics Laboratory, University College London, (2)
Cooperative Institute for Reserach in Environmental Sciences, University of
Colorado and NOAA, Space Environment Center.)
The Coupled Mesosphere and Thermosphere (CMAT)
model is an extension of the UCL Coupled Thermosphere-Ionosphere-Plasmasphere
(CTIP) model. In addition to
thermosphere/ionosphere physics, it incorporates mesospheric dynamics,
energetics and chemistry down to a lower boundary of 10mb (~30km). Previously,
low energy (300eV-20keV) particle precipitation as measured by the TIROS/NOAA
polar orbiting satellites was included in the code. This has now been extended
to include medium energy (30keV-2.5MeV) precipitation. The effects upon minor
constituent chemistry, specifically with respect to ozone, shall be presented.
Modelling Helium Ions in the Earth's Upper
Atmosphere (C.R.Wilford, R.J.Moffett, J.M.Rees. Space and
Atmosphere Research Group Department of Applied Mathematics University of
Sheffield
The upper atmosphere is an extremely dynamic system
and is dependent on the Sun's influence on the Earth. This influence results in complex physical and chemical
processes, which, with the aid of mathematical models, can be analysed. The Sheffield coupled
thermosphere-ionosphere-plasmasphere model (SCTIP) is one such model. It is a three-dimensional, non-linear model
that solves equations of continuity, momentum and energy balance, to calculate
quantities such as ion and electron density, temperature and field-aligned
fluxes. Currently these equations are solved for the atomic ions O+ and
H+. However, observations of the upper
atmosphere have shown the occurrence of regions where He+ becomes the dominant
ion or dominant light ion. A
description of SCTIP is given, along with a description of a recent
modification by the inclusion of He+ in the model. First results of modelled He+ are presented.
Observations of planetary waves in the
stratosphere, mesosphere and ionosphere in antarctica, (Andrew
R.Lawrence (University of Cambridge) Martin J.Jarvis (British Antarctic
Survey))
Planetary wave signatures of about 16, 10 and 5 days have been observed at heights ranging from 30km to 250km at Halley, Antarctica. The analysis, conducted over a 130 day period in 1997, utilizes stratospheric ECMWF (European Centre of Medium Range Weather Forecasts) operational analyses, mesospheric IDI (Imaging Doppler Interferometer) data and ionospheric dynasonde data. By studying the periodicities of planetary waves at different altitudes, their vertical propagation through the lower, middle and upper atmosphere can be characterised. In addition, the interaction of these waves with atmospheric tides and mean winds is considered as well as their significance on the dynamics of the atmosphere.
Vertical wavenumber spectra associated with
gravity waves in the mesosphere/lower thermosphere (V.St.C.Howells(RAL)
& N.J.Mitchell(UW Aberystwyth)
The EISCAT VHF radar regularly measures vertical winds in the mesosphere/lower-thermosphere region. Vertical velocities measured on 74 days between 1990 and 1998 reveal the ubiquitous presence of gravity waves and are used to calculate the associated vertical-velocity vertical-wavenumber spectra. The daily-mean spectra are found to be highly variable on a day-to-day basis. However, calculation of the mean spectrum reveals two distinct spectral regimes in which the mean spectrum is found to have an index of about -1.3 at smaller wave numbers (wavelengths between about 4-10 km) and -0.3 at larger wave numbers (wavelengths between 2-4 km). The spectral index can be used as a diagnostic of current gravity-wave saturation theories and comparisons with a number of such theories are made.
Comparing the measurements from a new meteor
radar to the results from CUTLASS Finland, (Peter Anthony
Cook, Terry Robinson and Neil Arnold, University of Leicester, and Nick
Mitchell, University of Wales
Aberystwyth.)
A new meteor detecting radar has been set up at Esrange in Northern Sweden, providing continuous accurate measurements of meteor positions and altitudes. Since there is some overlap between the fields of view of this radar and the CUTLASS Finland radar, individual meteors can be detected by both radars and the results compared. This provides a means of examining the quality of CUTLASS measurements, picking out meteors from other features such as sporadic E, and calibrating the CUTLASS radar in terms of altitude. This calibration will give us the potential of improving the data quality from the whole SuperDARN network. Radial drift measurements from the two radars can also be compared, as can the relative strength of the radar echoes, and diffusion in the upper atmosphere from the CUTLASS spectral widths and the meteor trail decay times.
Geomagnetic influences on the winter
stratospheric circulation (N.F. Arnold and T.R. Robinson,
University of Leicester)
Whilst the impact of geomagnetic activity on the thermosphere has long been recognised, reported connections with the atmosphere lower down have remained something of a mystery. Clues have been found in the form of sensitivity to global scale atmospheric oscillations, suggesting that a possible coupling mechanism may exist via transport processes. Following on from our work with planetary wave coupling of the stratosphere to the thermosphere under a range of solar irradiance conditions, we present results from a three-dimensional model of the middle and upper atmosphere to demonstrate that a similar process is feasible under varying geomagnetic activity levels.
What happened to superrotation? (Henry
Rishbeth, Dept of Physics & Astronomy, University of Southampton)
Thirty years ago the "F-layer dynamo theory" was invented, largely to explain superrotation (i.e, the west-to-east prevailing wind of ~100 m/s in the low-latitude thermosphere), which was deduced from observations of satellite orbits. The F-layer dynamo seems to be real, and it accounts for the observed plasma drifts in the equatorial F-layer – but according to in-situ measurements by DE-2 and other satellites, the superrotation does not exist. What went wrong?
First results from an optical/radar campaign,
Svalbard, December 1999. (Stuart Robertson, University of
Southampton, Ian Furniss, University College London)
A new platform of optical instruments was installed on Svalbard in time for a winter campaign in December 1999 in conjunction with the EISCAT Svalbard Radar. The optical instruments included the new High Throughput Imaging Eschelle Spectrograph (Hi-TIES). The results from this instrument and from a narrow angle imager and photometers show a variety of events, many with bright and dynamic aurora at different Universal Times. The data from all instruments are centred on the field-aligned direction at Longyearbyen. These will be discussed in the context of the position of the cusp and auroral oval at this location.
Multi - instrument observations of processes in
the ionospheric cusp region (A. M. Smith and S. E. Pryse University
of Wales, Aberystwyth)
Radio tomography provides a powerful tool for
investigation of the footprints of boundary-layer processes coupling the solar
wind to the ionosphere in the dayside cusp. This paper combines tomographic
images of the spatial distribution of ionospheric electron density, with
temperature and velocity measurements by the EISCAT Svalbard radar and DMSP
particle fluxes and spectra. The relationship between features seen by the
different techniques is discussed in terms of an ionospheric current system.
Substorm Effect on HF radio Propagation from
Simultaneous IRIS Dynasonde and EISCAT Observations (H.
Tao, F. Honary (Lancaster) C. Davis (RAL))
Simultaneous observations of the substorm activities on 20 January 1998 by IRIS, Dynasonde and EISCAT are presented. At the onset of the substorm,indicated by Pi2 pulsation, IRIS observes a spike in absorption which lasts several minutes and reaches 1.6 dB. The absorption continues at a reduced level of about 0.5 dB for almost one hour. A Comparison of Dynaosonde and IRIS data indicates that during the periods when the absorption exceeds above 0.2 dB, the Dynaosonde received signal is completely blackout. In addition, when the electric field is above 20 mV/m, the Dynasonde received signal is reduced considerably and is accompanied by spread F. Also the period of enhanced electric field coincides with the time of reduced absorption. An ionospheric absorption model is produced to study this event. There exists an excellent agreement between IRIS observations and Model calculations.
Poleward-moving HF radar flow bursts in the
cusp: Transient changes in flow speed or direction? (G.
Chisham, M. Pinnock, and A.S. Rodger British Antarctic Survey, Cambridge,
U.K.)
Poleward-moving line-of-sight velocity "flow bursts" have been observed in the cusp by two southern hemisphere SuperDARN HF radars with overlapping fields-of-view. This has allowed the estimation of unambiguous two-dimensional velocity vectors in the vicinity of the "flow bursts". Rather than showing enhancements in the flow magnitude, the velocity vectors suggest that the line-of-sight velocity enhancements are a result of a change in the direction of the flow associated with latitudinal motion of the convection reversal boundary. These observations may have important implications for understanding the ionospheric footprint of flux transfer events, and also illustrate that caution is needed when interpreting line-of-sight velocity data from single radars.
A multi-instrument study of spectral width
boundary motion in data from the CUTLASS, Finland HF radar. (E.E.Woodfield,
J.A.Davies, Department of Physics and Astronomy, University of Leicester.
P.Eglitis, Swedish Institute of Space Physics, Uppsala and the Finnish
Meteorological Institute. M.Lester, Department of Physics and Astronomy,
University of Leicester.)
During the interval 02 - 05 UT on 20th December 1998 a distinct boundary in the spectral width parameter is observed in the CUTLASS HF coherent radar at Hankasalmi, Finland. Such a boundary has been suggested as the ionospheric footprint of the open_closed field line boundary. This interval is chosen for study because of the useful overlap of instrumentation. Both the EISCAT mainland and EISCAT Svalbard radar cover the area of interest during the period. These, and several other data sets commonly available, are used in an attempt to discern the reason for the presence of this boundary and its subsequent motion. Preliminary results are given.
Auroral evidence for avalanching dynamics in
the magnetosphere (S.C. Chapman, [Warwick], A. T.Y.Lui,
K. Liou, P.T. Newell, C.I. Meng [JHU/APL], M. Brittnacher and G.K. Parks [U.
Washington])
The
analogy between the dynamic magnetosphere and an avalanche system is examined
by using the global auroral energy deposition obtained by the Polar UVI as a
measure of the energy output of the magnetosphere. The analysis reveals two
types of energy dissipation: those internal to the magnetosphere occurring at
all activity levels with no intrinsic scale, and those associated with active
times corresponding to global energy dissipation with a characteristic scale.
The internal events exhibit the same power law index in both active and quiet
times. These features are consistent with a simple avalanche (`sandpile')
numerical model.
Joint SuperDARN and VIS observations of
ionospheric flows during magnetospheric substorms, (M.
Lester (1), N.J. Fox (2) and J.B. Sigwarth (3) (1) University of Leicester
(2) NASA/GSFC (3) University of Iowa)
The behaviour of ionospheric flows during substorms continues to be a topic of considerable importance. Observations at the time of expansion phase onset have suggested that there is limited evidence for ionospheric flow initiated by reconnection in the tail. It is clear, however, that the flow is severely perturbed by the enhanced ionospheric conductivity caused by particle precipitation into the E-region. In this paper we utilise the global nature of the SuperDARN radars and the Visible Imaging System (VIS) on board the Polar spacecraft to investigate the flow and conductivity relationship on a large scale basis. The study includes the behaviour of the flows at expansion phase onset as well as during the late expansion and recovery phases.
Ionospheric signatures of dayside antiparallel
merging, (Iain J Coleman, Mike Pinnock, Alan S
Rodger British Antarctic Survey)
The antiparallel merging hypothesis states
that reconnection takes place on the
dayside magnetopause where the solar and geomagnetic fields are oppositely
directed. With this criterion, we have mapped the predicted merging regions to
the ionosphere using the Tsyganenko 96 magnetic field model. The resulting
ionospheric footprint, and the associated potential patterns, are critically
dependent upon dipole tilt and IMF clock angle. We discuss the relevance of
these results to HF radar data, and their implications for dayside
reconnection.
Proton and electron heating by radially
propagating fast magnetosonic waves (Richard B. Horne British Antarctic Survey, Gavin V. Wheeler
and Hugo St.C. K. Alleyne Department of
Automatic, Control and Systems Engineering,
University of Sheffield)
We investigate the propagation, growth and
decay of fast magnetosonic waves in the
Earth's magnetosphere which are
believed to contribute to proton heating
up to energies of a few hundred eV near the magnetic equator. We construct a model of the proton and electron distribution functions from
spacecraft data and use the HOTRAY code to calculate the path integrated growth and decay of the waves
over a range of L shells from L = 2 to L = 7.
Instability calculations show that the waves are excited at very large
angles of propagation with respect to
the magnetic field, at the harmonics of
the proton gyrofrequency f_{cH+} up to the lower hybrid resonance frequency f_{LHR} by a
proton ring distribution at energies of the order of 10 keV. As a `rule of thumb', we find that growth is
possible for f > 30 f_{cH+} when the ring velocity exceeds the Alfv'en speed
v_{R} > v_{A}, and for f < 30
f_{cH+} when v_{R} > 2 v_{A}. For propagation in the meridian plane, waves
generated just outside the plasmapause grow with large amplification as they
propagate away from the Earth but
eventually lose energy to plasmasheet electrons at energies of a few keV by
Landau damping. The waves grow to large
amplification at frequencies just below f_{LHR}. For inward propagation, we find that waves generated just outside
the plasmapause can propagate to L = 2
with very little attenuation suggesting that waves observed well inside the plasmasphere could originate from
a source region just outside the plasmapause.
Strong wave growth only occurs for large angles of propagation and thus
the waves are confined to within a few
degrees of the magnetic equator. Waves generated near geostationary orbit and
which propagate towards the earth are absorbed by Doppler shifted cyclotron
resonance when they propagate into a
region where v_{R} < v_{A}.
Cyclotron resonant absorption causes pitch angle scattering and heating
transverse to the ambient magnetic field.
The amount of absorption, and hence transverse proton heating, increases
significantly as the thermal proton
temperature is increased up to 100 eV suggesting
a feedback process. Ray tracing shows
that transverse heating of the thermal proton distribution is most likely to
occur is just outside the plasmapause where v_{A} is large. Since proton ring distributions are formed
during magnetic storms at ring current energies, we suggest that fast
magnetosonic waves provide an additional energy loss process for ring current
decay.
Evidence of a connection between enhanced
relativistics MeV electron flux and storm-time Pc5 pulsation power in the
magnetosphere. (R.A. Mathie and I.R. Mann
Magnetospheric Physics Group, University of York, R.H.A. Iles and A.N.
Fazakerly Mullard Space Science Laboratory, UCL)
Some very recent publications have suggested
that the enhanced fluxes of relativistic MeV electrons observed during
geomagnetic storms may be driven by a drift-resonant interaction with the
enhanced ULF wave activity observed during storm times. We examine solar wind
characteristics, Pc5 ULF wave power and relativistic electron flux during the
first half of 1995 to examine this proposed connection. We identify a total of
eighteen geomagnetic storms during this interval and demonstrate that Pc5 wave
power and subsequently MeV electron flux are seen to increase dramatically at
storm time, especially during high solar wind speed intervals. Further, we
demonstrate that the largest relativistic MeV electron flux increases are seen
to occur in association with Pc5 wave power that is sustained over a number of
days, in response to long-lived, high speed solar wind streams. Given the
likely importance of long-lived MeV electron fluxes in producing satellite
anomalies, further work should examine whether a causal connection between the
Pc5 waves and the enhanced MeV electron flux exists.
Modelling the generation of geomagnetic giant
pulsations (L.G.Ozeke and I.R.Mann Department of Physics,
University of York, York, UK. G.Chisham British Antarctic Survey)
Geomagnetic giant pulsations (Pgs) are extremely monochromatic waves believed to be generated by unstable particle distributions in the magnetosphere. By modelling the trajectories of protons in the magnetosphere we have been able to predict the location of these unstable distributions. The bounce resonance equation has been solved to determine where these unstable distributions will cause wave growth. This model has been compared with a Pg observed by the SAMNET magnetometer array. The properties of the observed Pg closely match that predicted by the model. This is the first time a model has been able to explain why Pgs are extremely monochromatic, highly localized in latitude and only occur in the morning sector.
Burst lifetimes in AE indices and solar wind inputs (M.
P. Freeman, N. W. Watkins and D. J. Riley, British Antarctic Survey)
We examine the claim that the power law
distribution of burst lifetimes in the
AE index is evidence that the magnetosphere is a Self-Organized Critical (SOC)
system. To do this we compare the burst lifetime distributions of the AU and
|AL| indices with those of the vB{s} and e solar wind input
functions. We show for the first time
[Freeman et al, GRL, in press] that both the vB{s} and e
Large-scale corotating shock orientations as
observed by Ulysses (D. Clack and R. J. Forsyth Imperial
College),
Following its encounter with Jupiter in early
1992, Ulysses travelled south of the ecliptic plane for the first time. During
this passage to high southerly latitudes the spacecraft encountered a series of
corotating interaction regions (CIRs) between January 1992 and July 1993. We
have previously established the presence of large- and small-scale planar
structuring of the magnetic field within these events, aligned in accordance
with the tilted-dipole model of three-dimensional corotating stream
interactions. Here we examine the large-scale magnetic planarity in the
vicinity of the forward and reverse shocks and contrast this with previously
published analyses of shock front orientations carried out using velocity and
magnetic field coplanarity techniques. As an indicator of the large-scale
configuration of the magnetic field near the shock fronts our results provide
an important parameter for use in studies of charged particle acceleration
within CIRs.
Numerical models of magnetic reconnection
between a CME and the solar wind (J. M. Schmidt and
P. J. Cargill, Imperial College)
Coronal Mass Ejections (CMEs) are major
features of the solar wind which couple activity with the interplanetary medium. Their interactions
with the Sun's magnetic field, sheared solar wind streams and the heliospheric
current sheet are not well understood. In particular a strong external solar
wind magnetic field can reconnect with the CME field in a variety of ways. We
will present 2.5 dimensional MHD simulations of these processes. The initial
conditions will, for the first time, be based on an exact solution of the MHD
equations of a flux rope in a diverging magnetic field. It will be shown that
in a unidirectional field, the flux rope undergoes reconnection on one side
only. When the flux rope travels along
a current sheet, it is entirely destroyed if its field rotates with a sense
opposite to that of the field reversal. In the opposite case, the flux rope
travels outward without disruption.
Three spacecraft observations of solar wind
structures, (T. S. Horbury, D. Burgess, M. Fraenz (QMW), C. J. Owen (MSSL))
Close conjunctions between Wind, Geotail and IMP 8 allow multiple measurements of structures in the solar wind on scales of a few hundred thousand km. It is possible to identify the same magnetic field structures at the three spacecraft and, using trajectory information and plasma velocity measurements, deduce their orientation in the plasma frame. We present results of this work, including information on the alignment of these structures with the ambient magnetic field; differences between minimum variance and multi-spacecraft estimates of orientations; and new estimates of the relative abundances of tangential and rotational discontinuities in the solar wind.
Ulysses Observations Of MHD Waves And
Turbulence In A Co-rotating Interaction Region (M.Y.
Gulamali & P.J. Cargill, Imperial College)
During its mission around the poles of the Sun,
the Ulysses spacecraft recorded a number of well-developed co-rotating
interaction regions (CIRs) at mid-latitudes.
We discuss the observations of MHD wave phenomena and turbulent
behaviour within a CIR recorded between day 333 and day 362 of 1992, using data
from the magnetic field and plasma experiments. This case study is of particular interest because Ulysses
records a magnetic cloud embedded within the rarefaction region of the
CIR. We find that a proton density
enhancement before the magnetic cloud affects the propagation direction of
Alfvenic fluctuations in the CIR, as well as the mean power in the magnetic
field fluctuations. We attempt to
reconcile these observations with current plasma theory and give possible
explanations of the behaviour being exhibited.
Interplanetary scintillation observations using
EISCAT and MERLIN highlights of the 1999 campaign. (P.J.
Moran, A.R. Breen, R.A. Fallows, A. Canals, University of Wales, Aberystwyth)
Interplanetary scintillation observations allow measurements of the solar wind to be made at all heliocentric latitudes and at a range of distances from the Sun. In 1999 several hours of observations were made with EISCAT and MERLIN operating at different observing frequencies which enabled greater coverage of the heliosphere to be made. Collaborative work was carried out with the solar heliospheric observatory (SOHO) as well as two other IPS systems in India and Japan, the Ooty Radio Telescope (ORT) and the Nagoya array of telescopes. Results include:- The acceleration profile of the slow solar wind. Velocity profiles of Coronal Mass Ejections (CMEs). Measurements of non-radial flow. Comparison of IPS system observations. Variation with distance of alfven wave flux. Solar cycle variation of velocity.
An Unusual Feature in the Solar Wind detected
by Ulysses (Geraint H. Jones, Andre Balogh, Imperial
College; Timothy S. Horbury, Queen Mary and Westfield College)
The Ulysses spacecraft encountered a highly unusual feature in the high-speed solar wind. The
feature's presence was detected by several instruments aboard the spacecraft, including the magnetometer. We
have determined the cause of this unique event. Details of the structure's
identification and our analysis of the magnetic field data returned during its
traversal are presented.
CME Masses at Solar Minimum, (GR
Lawrence, GM Simnett - Univ. of Birmingham, RA Howard - Naval Research Laboratory)
Much has been learned from the LASCO data set about the spatial and temporal distributions of
CMEs, and also about some key parameters such as their typical speeds and -
sometimes - accelerations, and the plane-of-sky lengthscales and geometry. Less
is known about the masses involved - the excess mass carried away in these
explosive events. We present the results of calculations of masses of all
'significant' - i.e., ~1e13g or greater - CMEs observed by LASCO in a period
around the minimum period of solar cycle 23 spanning several months. The
question of the proportion of solar wind mass flux contained in CMEs at solar
minimum is addressed. We also analyse
in detail the masses of CMEs observed during the high-cadence (~3 min) LASCO/C3
programme run in Jan. 1996.
A new EISCAT for the new millennium (Ian
McCrea Space Science and Technology Department Rutherford Appleton
Laboratory)
A number of major upgrades to the EISCAT radar
systems are currently being carried out.
The result will be a significant change in the operating procedures for
the mainland radars and changes in some of the experiments with which current
EISCAT users are familiar. The benefit
should be the availability of a more flexible facility with higher quality
data. This presentation reviews the
work being undertaken and discusses what these changes will mean for the EISCAT
user.
POLAR particle observations of the cusp
boundary layer for northward IMF. Open or closed field lines?
The equatorward edge of 15 POLAR cusp crossings
during periods of N IMF, are examined for evidence of magnetosheath populations
trapped on closed field lines. This may indicate reconnection of solar wind
field lines at high latitudes in both hemispheres, forming closed field lines.
POLAR particle observations of the cusp
boundary layer for northward IMF. Open or closed field lines?
The equatorward edge of 15 POLAR cusp crossings
during periods of N IMF, are examined for evidence of magnetosheath populations
trapped on closed field lines. This may indicate reconnection of solar wind
field lines at high latitudes in both hemispheres, forming closed field lines.
A model of magnetic fluctuations in the cusp (M.G.G.T.
Taylor and P.J. Cargill, Imperial College)
The high altitude cusp is gradually being revealed as a region with magnetic field fluctuations on all scales. Using simple energy and characteristic scale considerations, we seek to create a model in which we can examine the effect of various energy sources in and around the cusp and investigate their influence on the overall dynamics of the system. We present a simple energy based model detailing the interaction of low frequency waves with a cusp-like field geometry. The waves are generated at the magnetopause, and are assumed to move into a region of changing field and plasma. We find the amplitudes of the velocity and magnetic field fluctuations decrease as they evolve through our idealised cusp geometry. Such behaviour implies that, for our model, fluctuations originating from magnetosheath generated disturbances cannot simply be convected into the cusp by a background flow and that fluctuations found within the cusp have a more local origin.
A survey of whistler mode chorus in the inner
magnetosphere using data from the CRRES Plasma Wave Experiment (Nigel
P. Meredith(1) and Richard B. Horne(2): (1) Mullard Space Science
Laboratory (2) British Antarctic Survey)
Intense interest currently exists in
determining the roles played by various wave particle interactions in the
production of the diffuse aurora and in the acceleration of electrons to
relativistic energies during/following geomagnetic storms. Here we present a
survey of data from the CRRES Plasma Wave Experiment for upper (0.5 -> 1.0
fce) and lower (0.1 -> 0.5 fce) band chorus. For quiet periods lower band
chorus exhibits two populations, one inside L = 3 and one for 4 < L < 7
confined to within approximately 15 degrees of the magnetic equator. Upper band
chorus exhibits an inner cut-off near L = 2.5, which appears to be related to
the magnetic field strength. Both upper and lower band chorus wave amplitudes
are significantly enhanced during active periods (AE > 300 nT) for 4 < L
< 7 in the region (23:00 -> 13:00 MLT) consistent with electron injection
and drift around dawn to the dayside. Lower band chorus wave amplitudes are
more enhanced than the upper band and are spread over a wider range of
latitudes. We conclude that the significant increase in wave amplitudes during
magnetically disturbed times supports the idea of electron acceleration in the
radiation belts by whistler mode waves as presented by Horne and Thorne (1998)
and Summers et al., (1998).
Ion distributions in the distant plasma sheet (R.
T. Mist and C. J. Owen, Mullard
Space Science Laboratory, University College London, T. Mukai, Institute of
Space and Astronautical Science, Japan)
Two dimensional distributions from the Geotail
data set are used to examine the structure of the deep tail plasma sheet and
plasma sheet boundary layer. As the spacecraft crosses these regions, the data
consistently shows the evolution of the distributions from a cold, lobe-like distribution into a
multi-component plasma sheet distribution. Examining several sequences of
plasma sheet crossings has enabled us to identify distinct features of these
distributions. These features are formed as a result of particle acceleration
in the hairpin field structure of the distant tail. The observed distributions
are compared to a theoretical model of this interaction which accounts for both
heating and acceleration of the incoming lobe particles and also the role of
the observed high energy plasma sheet boundary layer population.
Extent of the PDL in the near magnetosheath on the dawn-side magnetopause. (M. W. Dunlop, E. A. Lucek, A. Balogh and P. Cargill. Blackett Laboratory, Imperial College)
The Equator-S satellite often remained close to
the magnetopause for long periods on the morning side of the magnetosphere. We
have reported earlier that strong signatures consistent with mirror mode
activity were observed on a large minority (30%) of the orbits. For the majority of these passes, the signatures start close, or
adjacent, to the magnetopause, despite
a variety of upstream solar wind conditions. We suggest that in most
cases the plasma depletion layer (PDL) was either narrow or absent and have
extended the study to all magnetopause encounters. High resolution magnetic
field data, taken within the boundary region, can be used to identify the
presence or absence of electromagnetic ion cyclotron waves. In the absence of
suitable plasma data, we adopt these wave signatures as an indicator of the
presence of a PDL. This diagnostic is
most relevant when the waves lie between the magnetopause boundary crossing and
the onset of mirror activity.
Local Time Asymmetry of the Equatorial Current
Sheet in Jupiter's Magnetosphere (E. J. Bunce and S.
W. H. Cowley University of Leicester, U.K.)
We provide a first systematic comparison of the radial fields associated with the equatorial current sheet in the jovian magnetosphere which were observed during the flybys of the Pioneer-10 and -11, Voyager-1 and -2 and Ulysses spacecraft. We show that these fields are systematically weaker on the dayside than on the nightside at distances beyond ~20 Rj, and fall more rapidly with jovicentric distance in the former regime than in the latter. Fits to the current sheet radial field as a function of distance give rise to a simple model which predicts the radial field outside the current sheet as a function of distance and local time. We finally conclude that these results imply a significant divergence of the azimuthal equatorial current.
Further Evidence of Low Frequency Waves in
Jupiter's Middle Magnetosphere (R Wilson and M K
Dougherty, Imperial College, London).
Previous analysis of Voyager and Ulysses fly-by
data indicated ULF waves in Jupiter's magnetosphere. In this talk I investigate the data provided by Galileo for 10 -
20 minute period waves. I show that the
waves are prominent in the perpendicular magnetic field component and that they
are not dependent on the Local Time or radial distance of the spacecraft from
Jupiter.
Surprising Periodic Perturbations in Saturn's
Magnetic Field (S. A. Espinosa and M. K. Dougherty, Imperial
College)
Analysis of all the existing magnetic field data from Saturn's magnetosphere is carried out. This data arises from the three spacecraft encounters with Saturn (Pioneer 11, Voyager 1 and 2, in 1979, 1980 and 1981 respectively). In the cases of Pioneer 11 (whole encounter) and Voyager 2 (inbound) a bipolar perturbation is observed (with a period close to the planetary rotation period) in the radial and/or azimuthal components of the field. Existing models of Saturn's magnetic field imply a negligible tilt between the rotation and dipole axes and therefore such models cannot explain this periodic perturbation. A mechanism involving a ballooning of the field and plasma is described, together with some supporting evidence provided by a modulation of the magnetopause position.
A preliminary study of 30eV electrons observed
on entering Earth's plasma sheet - Cassini ELS (Abigail
Rymer, Gary Abel, Andrew Coates, Dave Linder & the CAPS team)
Cassini flew by Earth in August 1999 during a
particularly fast period of solar wind speeds and enhanced levels of storm
activity. Here we explore the nature of
a 30eV electron 'beam' observed for about 45 minutes as Cassini travelled
downtail from about 7 Re to 12 Re. In
particular we ask whether this beam is 'real' or has a source on the spacecraft
by looking at how it is related to Cassini actuator position and pitch angles.
Overview of Magnetic Field Measurements from
the Cassini Earth Swing-by (M. K. Dougherty, D. J. Southwood
(Imperial College) and the magnetometer team.)
The Cassini spacecraft Earth Swing-By (ESB)
took place on 18th August 1999. The
primary purpose of ESB magnetometer operations was deployment of the
magnetometer boom and flight test and proof of principle for dual scalar-vector
operations. However, plans were also
made for ancillary observations for solar terrestrial science as well as for
observations downstream of the Earth. Preliminary results are reported here.
Results from simulations of the lunar wake are presented
here. A simplified description of the lunar wake can be modelled via a
one-dimensional particle-in-cell simulation. Such a simulation represents both
ion and electron f(v) by a collection of superparticles. These superparticles,
along with the electric and magnetic fields, can exhibit both full ion and
electron kinetics. Since the velocities of the superparticles and the fields
have components in three dimensions, but their positions are only in
one-dimension, these simulations are referred to as one-and-a-half-dimensional
simulations. This simulation begins with a cross-section of the wake, along the
interplanetary magnetic field, immediately downstream of the moon, i.e. before
the solar wind plasma has infilled the wake region. As the simulation evolves
and the void infills, the cross-section effectively moves back along the wake
at the solar wind velocity. By using a sufficient number of particles per cell,
we are able, for the first time, to resolve the full dynamics of both electrons
and ions. The electrons immediately begin to form beams in the void, but a
significant charge imbalance causes them to deflect, generating vortices in
phase space on both sides of the wake. Ion beams are generated after the
lighter electrons have moved into the void, creating a two-stream distribution
at 10 lunar radii behind the moon. The resulting two-stream instability causes
the beams to mix in phase space at about 22 lunar radii. This is consistent
with both WIND observations and the results of earlier simulations which focus
only on the ion dynamics. It is this mechanism by which the wake is thought to
fill. Other structures are also evident including an electron rarefaction wave
and ion density fluctuations travelling away from the wake, coinciding with
ion-acoustic waves.
Artificial Field-Aligned Irregularities: Decay
Times and ACF Decorrelation Times, (Ranvir Dhillon,
Darren Wright and Terry Robinson University of Leicester)
Ionospheric modification (using high-power radio waves) is a well-known technique for generating and sustaining artificial field-aligned irregularities (FAI). There are a number of important differences between the coherent radar backscatter obtained from artificial and natural FAI, most notably the extremely low spectral widths associated with artificial FAI. Data, produced during experimental campaigns at the EISCAT Heating Facility, were collected using the CUTLASS Finland radar. CUTLASS consists of a pair of radars situated in Finland and Iceland and is part of the HF SuperDARN array. These data show the growth and decay of artificial FAI and characteristic decay times may be obtained from them. The most recent campaign (October 1999) was the first where long-lag ACFs (with lag separations longer than the standard 2400 microseconds) were obtained. These ACFs allowed decorrelation times to be calculated for backscatter from artificial irregularities. This cannot be done usually as the standard-lag ACF does not decorrelate for artificial FAI. It was found that the irregularity decay time after switch-off of the heater (corresponding to persistence of the irregularities) was of the order of five times the decorrelation time (corresponding to changes in the irregularity distribution) as derived from the long-lag ACFs.
Modelling Studies of the Equinoctial
Asymmetries at Low Latitudes (Y. Z.
Su and G.
J. Bailey Department of Applied Mathematics, University
of Sheffield)
The Sheffield Coupled Thermosphere-Ionosphere- Plasmasphere model (SCTIP) has been used to investigate the existence of equinoctial asymmetries in the low-latitude ionosphere and thermosphere. In accord with observations, the modelled electron density in the ionosphere is higher at the March equinox in the northern hemisphere and at the September equinox in the southern hemisphere. It is found that the thermosphere also shows an equinoctial asymmetry and the asymmetry in neutral wind plays an important role in the occurrence of the equinoctial asymmetries in the ionosphere. A mechanism for the equinoctial asymmetries in the thermosphere and ionosphere will be discussed.
A new method for the investigation of the
ionosphere via measuring the spectrum of average radio power density (Csaba Szombathy
Technical University of Budapest, Hungary)
Conventional investigation methods of the
Ionosphere include experiments with ionosondes, the measurement of Alfvén and
Schumann frequencies, etc. These methods are performed on the Earth and require
terrestrial equipment. Our long term
aim is to measure the power density spectrum radiated from the Earth to the
Space at frequencies between 500 kHz and 30 MHz, thus we could monitor the transparency
of the Ionosphere and study upper-layer waveguide properties. These
measurements are to be carried out by a programmable spectrum analyser
developed by the Space Research Group of the Technical University of Budapest.
The spectrum analyser is to be assembled on a satellite on Polar orbit 700 km
above the surface of the Earth. Though
the satellite carrying our instruments has not been launched yet, we have
already performed test measurements on the Earth with the aid of the equipment
mentioned above. We studied the variation of the waveguide properties of the
Ionosphere during the total eclipse of the Sun in 1999, and the changes in the
charge density profile of the Ionosphere could clearly be detected. My presentation includes the evaluation of
the test results measured during the total eclipse of the Sun in 1999 and the
principles of studying the state of the Ionosphere via the average radio power
density measured in the air.
The Tidal Thermospheric Spoon (Mueller-Wodarg,
I.C.F., A.D. Aylward Atmospheric
Physics Laboratory, University College London)
An idea originally presented by Fuller-Rowell
[1998] and labelled "Thermospheric Spoon" suggests that meridional
cross-hemispheric circulation at solstice causes the terrestrial thermosphere
to be more mixed, with an enhanced mean molecular mass, at solstice than at
equinox. Tides propagating upwards into the lower thermosphere dissipate and
release considerable amounts of energy and momentum into the background
atmosphere, altering the global mean circulation profile at low-to mid latitudes.
We investigate in how far the tides will affect thermospheric mean molecular
mass, similar to the spoon effect, and what implications that has on the effect
of tides on the globally averaged thermospheric temperature profile.
A mechanism for floating Q-resonances in the
context of topside sounding. (Mark Eric Dieckmann, ITN, University
of Linkoeping, Campus Norrkoeping, Sweden)
A plasma sounder is a combined radio wave emitter and receiver. Some peaks in the response intensity to sounding are attributed to waves propagating at the same (low) velocity as the satellite in the plasma frame of reference. One class of such resonances is the Q- resonances. The Q-resonances excited by topside sounders have a floating character, i.e. they are not detected immediately after the sounder pulse. In addition they can be excited even if the Q-resonance frequency is different from the emission frequency. Our particle in cell simulations show that the Q-wave is, in this case, generated by the switching off of the emitter. The floating nature of the resonance is apparently linked to a partial collapse of the Q-wave packet at the antenna location.
T hermospheric
ion-drag time constants using EISCAT and a fabry-perot interferometer (M. J. Kosch, K. Cierpka, M.
Rietveld, T. Hagfors and K. Schlegel, Max Planck Institut für Aeronomie)
It has been conclusively
demonstrated that thermospheric winds follow, but generally lag behind, the ion
drift pattern of magnetospheric convection. Analysis of the thermospheric
momentum equation shows that ion drag accounts for about two thirds of neutral
momentum forcing with minor contributions from coriolis, advection, pressure
and viscous forces. Neglecting the minor terms, an e-folding time constant is
defined which describes the time taken for the neutral gas velocity to approach
the ion velocity after a step change in convection. F-region ion drift and
neutral winds have been observed by the EISCAT incoherent scatter facility and
a ground-based Fabry-Perot interferometer, respectively, from northern
Scandinavia. Observations were made on 9 and 10 November 1998 (17-23 UT) for a
geomagnetically active (Kp = 7- - 5-) and
quiet (Kp = 0+ - 0) period, respectively. The e-folding
time is highly variable with lags up to 6 hours but compares well with
previously satellite measurements of 1 - 3 hours.
The Flywheel Effect: Consequences of neutral
wind inertia on the ionosphere-thermosphere system after a geomagnetic storm (A.L.Aruliah*,
I.C.F.Mueller-Wodarg*, A.D.Aylward*, J.Ruxton*, Mike Kosch#, Mark Lester%
*Atmospheric Physics Laboratory, University College London #Max Planck
Institute)
The ionosphere responds almost immediately to
changes in the high-latitude electric field. However, the neutral atmosphere,
which is the vast bulk of the upper atmospheric composition, takes a few hours
before such changes feed through to the large-scale motion via ion-neutral
interactions. This has major consequences for the ionosphere-thermosphere
system, in particular the currents and energy dissipation. This paper presents
a multi-instrument and model study of a storm that suddenly shuts off thus
allowing a clear observation of the flywheel effect.
Drift-bounce resonance interactions in ULF
waves observed in artificially -induced radar backscatter (T.
K. Yeoman, and D. M. Wright, Department of Physics and Astronomy,
University of Leicester)
HF radar backscatter which has been
artificially-induced by a high power RF facility such as the EISCAT heater at
Tromso has been demonstrated to provide ionospheric electric field data of
unprecedented temporal resolution and accuracy. Here such data are used to investigate ULF wave processes
observed by the CUTLASS HF radars.
Within a short period of time three distinct wave types are observed,
with differing periods, and latitudinal and longitudinal phase evolution. Combining information from the three waves
allows the drift-bounce resonance interactions which cause the waves to be
determined.
A new principle for the investigation of the Ionosphere has been introduced by the Space Research Group of the Technical University of Budapest. According to this the transparency of the Ionosphere could be monitored in the function of the frequency by a spectrum analyser. The spectrum analyser is to be assembled on a satellite on polar orbit, 700 km above the surface of the Earth. The satellite instruments for this purpose have already been developed by the Space Research Group of the Technical University of Budapest. The spectrum analyser developped by the Space Research Group is controlled by a microcontroller which is connected to the main CPU of the satellite. The programming of the system is performed from the Earth via microwave link. The measured data are transferred to the terrestrial control centre via the same microwave link. Data processing and the evaluation of the measurement results are to be carried out on the Earth. My presentation includes the principles of the satellite-based monitoring of the Ionosphere and the introduction of the spectrum analyser developed at our institute. Besides presenting the hardware, the several measurement modes and options offered by our controlling software will also be shown.
Thermospheric heating from the middle
atmosphere. (R. Balthazor and R Erdelyi, Space and
Atmosphere Research Group, University of Sheffield, Sheffield, S3 7RH)
Models of Joule heating and Lorentz forcing from observed auroral electric fields underestimate the thermospheric heating required to produce TIDs observed at mid latitudes. First results are shown of a thermospheric heating contribution from plasma waves propagating up from the turbulent middle atmosphere.
VR visualization as a tool for exploring
astroplasma systems (S. Chapman and B. Hnat, University of
Warwick)
A generic problem with astroplasma systems is
that the physics is multidimensional. Although it may ultimately be possible to
reduce the number of degrees of freedom of a system once it is understood,
initially one is faced with geometry
that cannot be represented fully by 2 dimensions plus time. Here we explore the
possibilities of semi immersive virtual reality as a tool for understanding
multidimensional systems. We discuss two case studies, i) single particle
dynamics in current sheets and ii) sandpile models for magnetospheric activity.
This will highlight the challenges of complex geometry, and selfsimilar
strucutres respectively.
The response of the HF radar spectral width
boundary to a switch in the IMF By direction: Ionospheric consequences of
transient dayside reconnection? (G.Chisham,
M.Pinnock, A.S.Rodger British Antarctic Survey, Cambridge, U.K.)
In the high-latitude dayside ionosphere, the
movement of the HF radar spectral width boundary (SWB) provides a good proxy
for the movement of the open-closed field line boundary around magnetic local
noon. By studying the dynamics of the spectral width boundary we can
investigate features of the dayside ionospheric response to changes in the
Interplanetary Magnetic Field (IMF). The high temporal and spatial resolution
of the SuperDARN HF radars make them good tools to study these features. In
this paper, we use the Halley HF radar in Antarctica to study the equatorward
motion of the SWB which appears to occur in response to a large change in the
direction of IMF By. The spectral width boundary initially moves equatorward in
the form of a U-shaped bulge close to magnetic local noon. This bulge then
expands longitudinally to earlier and later magnetic local times. Merged
velocity vectors from two Antarctic HF radars describe the flow velocity
variation in the boundary region. The flow equatorward of the boundary follows
the contours of the boundary as it
expands. The flow poleward of the boundary is directed at more oblique angles
to the boundary. This study represents the first clear two-dimensional
observation of the formation of an equatorward bulge on the polar cap boundary
which may be associated with changes in dayside reconnection and also presents
a unique observation of the variation of the ionospheric flow in the locality
of the boundary. We discuss the possible interpretations of this event and the
possible consequences to our present
understanding of the ionospheric response to changes in the IMF.
Monitoring Geomagnetically Induced Currents in
the Scottish Power Grid (TDG Clark, AWP Thomson, E Clarke, P
White, British Geological Survey)
We have previously reported on a study of the occurrence of problems related to Geomagnetically Induced Currents (GICs) in the UK power grid. A major drawback in this study was the lack of relevant data in the power industry to correlate against geomagnetic data, with only about 5 GIC events being known about in the last two decades. In January 2000 Scottish Power installed instruments to monitor GICs in their portion of the UK grid covering central Scotland. We present here some first results, comparing direct measurements of GICs with magnetic field variations recorded at the three UK magnetic observatories.
Long term changes in EUV and X-ray emissions
from the solar corona and chromosphere as measured by the response of the
Earth's ionosphere during total solar eclipses. (E.M.Clarke
[Sheffield Hallam University], C. J.
Davis, M. Lockwood, S. A. Bell [RAL])
During the August 1999 eclipse, ionosonde measurements were made in Helston, Cornwall. Using these data a method has been established which estimates the percentage of unobscured ionising solar radiation at any time during a solar eclipse. A study of ionospheric mearurements during eclipses since 1932 has now been carried out. Initial results show a long term increasing trend in the Sun's coronal intensity at EUV and soft X-ray wavelengths. This result concurs with other recent work, which has detected changes in the coronal magnetic field.
Further evidence of 154 day
periodicity in solar energetic particle fluxes at 5 AU. (S.
Dalla, A. Balogh (IC), B. Heber (MPAe))
A possible 154 day recurrence in fluxes and
anisotropies of 1.7 MeV solar protons was identified in data from the ATs
instrument, on board the Ulysses spacecraft. Fluxes from the KET instrument,
detecting protons in the 30-125 MeV range and electrons in the 2-10 MeV range,
are compared with ATs fluxes and are shown to support the hypothesis of
recurrent behaviour.
EISCAT VHF observations in the region of the
poleward expanding auroral bulge (J.A. Davies, S.W.H.
Cowley and M. Lester Department of Physics and Astronomy, University of
Leicester)
The growth phase of a substorm is characterised by the
presence of equatorward drifting arcs on the nightside. At expansion phase
onset, the most equatorward of these arcs suddenly brightens in a restricted
local time sector near magnetic midnight, forming an auroral bulge which
expands poleward as well as towards dusk and dawn. EISCAT VHF split beam
observations associated with the substorm bulge on two consecutive days in
December 1992 are presented and compared; in one case the bulge was observed
prior to magnetic midnight and in the other, during the post magnetic midnight
hours. In both cases the electron temperature boundary observed in the radar
data, which is taken to be a proxy of the poleward boundary of the bulge, is
straddled by a narrow band of enhanced ion temperature indicating the regime of
high electric field necessary for current continuity across the feature.
Conventional beam swinging to provide velocities in the beam plane by combining
the line-of-sight ion velocities from the same range gate along each beam is
inappropriate in the bulge region as, in both cases, the boundary is oriented
significantly away from the beam-bisector normal. A novel method of velocity
determination combines line-of-sight velocities estimates on each beam which are
equidistant from the electron temperature boundary, as it is surmised that in
situations such as these the velocities are more likely to be ordered with
respect to the feature itself.
THE JOVIAN CURRENT SHEET GEOMETRY (M.W.
Dunlop, Blackett Laboratory, Imperial College)
The magnetospheric field of jupiter, as seen by
both early and more recent spacecraft
is investigated using a global magnetic field model, modified to contain a
warped current sheet. The current sheet
model was originally developed to fit the Ulysses observations during its
dayside pass of Jupiter. Here, we investigate the required variation of the key
parameters defining current sheet shape to fit observations taken during other
spacecraft passes. We compare these parameters to previous analysis of the
older data sets and to the different regions of the magnetosphere. The magnetospheric field configuration is
compared to the model to imply particular features of global geometry,
particularly near the current sheet crossings.
Changes in magnetic field geometry while the spacecraft is traversing
the transition region between the outer and middle magnetosphere are also
considered. The analysis tests the robustness of the model for different
spacecraft trajectories.
MHD and kinetic aspects of magnetic depressions
in the solar wind (M.Fraenz, D. Burgess, T. S. Horbury
Astronomy Unit Queen Mary & Westfield College)
Depressions of the magnetic field magnitude are observed on a wide range of scales in the solar wind. Traditionally these have been explained by kinetic instabilities envolving anisotropic proton distributions. But observational evidence for unstable plasma environments around depressions in the solar wind has been poor. Recently it has been proposed that the stability of these depressions can be explained in pure MHD by soliton waves (K.Baumgaertel, JGR, A12, 1999). We investigate whether observations of the Ulysses, Helios and Ace spacecraft support the soliton or kinetic model of magnetic depressions.
Solar cycle variation of solar wind ram
pressure and its implications for mission planning (Mike
Hapgood, CLRC Rutherford Appleton Laboratory)
We have used the OMNI solar wind dataset to calculate the distribution of ram pressure values for each month from November 1963 to August 1999. The medians and sextiles of these distributions show a clear solar cycle variation. There is a rise in all three values between four and six years after solar minimum. The values then slowly decay until the equivalent rise in next cycle. This result conflicts with claims that there is no solar cycle variation in solar wind momentum flux. The implications of these statistics for the planning of magnetospheric missions will also be discussed.
O BSERVATIONS
OF HF INDUCED AIRGLOW BY DASI AND CUTLASS (M. J. Kosch (1), M. T.
Rietveld (1), F. Honary (2), T. B. Leyser (3) and T. Hagfors (1): (1) Max Planck Institut für Aeronomie, (2)
Swedish Institute of Space Physics, S-75591 Uppsala, Lancaster University)
HF induced airglow has been
observed on 21 February 1999 during a geomagnetically quiet period (Kp=1
preceeded by 9 hours of Kp=0+). The EISCAT Heating experiment (69.59o
N, 19.23o E) was operated at 4.04 MHz (O-mode) in the local zenith
from 16:40 to 18:32 UT using a 4-min. on, 4-min. off duty cycle.
Simultaneously, the Dynasonde measured the HF reflection height which increased
from ~200 to ~290 km during the experiment. After 18:16 UT, the ionospheric
critical frequency dropped below 4.04 MHz and no further airglow was observed.
The Digital All-Sky Imager (DASI) was recording at 630 nm from Skibotn (69.35o
N, 20.36o E), about 50 km east of the HF facility. The development
and fading of an airglow patch(es) corresponding to the Heater on, off times,
respectively, could be clearly observed with an intensity of up to ~100
Rayleighs. This observation is unique because the artificial airglow patch
appeared equatorward of the HF facility, generally in the vicinity of the
magnetic dip (12.8o) and Spitze (6o) zenith angles. A
similar equatorward shift in the region of backscatter is observed by the
CUTLASS radar. On one occasion two patches were sometimes generated
simultaneously. There is no evidence of ExB or neutral wind drifting of the
airglow.
Two Point observations of Lobe-reconnection (Ian
Krauklis, Andrew Coates, (Mullard Space Science Laboratory). Oleg Vaisberg
(Marshall Space Flight Centre, Huntsville Alabama). B Peterson (Lockheed-Martin Space Sciences Laboratory, Palo Alto,
California))
Observations of a Lobe-reconnection by the
Polar and Interball Tail spacecraft have been made when the two spacecraft were
seperated in Ygse by 2Re. The similarity of the two sets of observations
suggest that magnetopause and possibly reconnection site motion strongly effect
these observations.
Equator-S magnetic field observations of high
frequency waves at magnetopause crossings (E. A. Lucek,
P. Cargill, M. W. Dunlop and A. Balogh, Imperial College)
The magnetic field instrument on the Equator-S
satellite made high resolution magnetic field measurements, with a maximum of
132 vectors/seconds, covering about 130 magnetopause crossings between 06 00
and 10 40 LT. An analysis of the small scale features of some of these
magnetopause crossings has shown that a number of them contain small amplitude
wave packets within the magnetopause ramp. The waves are predominantly
compressive, and have a frequency close to the lower hybrid frequency. The
waves are of very low amplitude, and so far have only been identified at
exceptionally smooth magnetopause crossings, where the background power is low.
They do not persist throughout the whole of the magnetic field ramp. This might
be expected if the waves are generated by the lower hybrid drift instability,
where the growth rate is dependent on the density shear, which is often
observed to be located at the inner edge of the magnetic field ramp. We examine
the dependence of the wave characteristics on the level of magnetic shear, and
on the magnetic field orientation at the boundary.
DABS - A first look at the B.A.S. Data Access
and Browsing System (Nick Mattin and Rob Hibbins, BAS)
We will present a demonstration of a new data access and browsing tool (DABS) developed at the British Antarctic Survey. DABS provides a single web based interface to the last 5 years of data held in the upper atmospheric databases.
A CTIP Model
Investigation of the Influence of Tidal Forcing on Equatorial Vertical Ion
Drift. (G. H. Millward
Atmospheric Physics Laboratory, University College London)
A recent development of the CTIP (Coupled Thermosphere - Ionosphere - Plasmasphere) model has been the inclusion of the electrodynamic coupling between the equatorial ionosphere and thermosphere. The vertical ion drifts which result are shown to be largely in agreement with empirical data, based upon measurements made at the Jicamarca radar and other equatorial sites. Of particular importance, the CTIP model clearly reproduces the 'Pre-Reversal Enhancement' in vertical ion drift, a key feature of the observational data. Inacurnacies in the modelled daytime upward ion motion are investigated with regard to changing the magnitude and phase of components of the lower-thermospheric tidal forcing. The results show that daytime upwards ion motion is highly dependant upon both the magnitude and phase of the semi-diurnal [2,2] tidal component. The author speculates that the observed daily variation in daytime equatorial vertical ion drift could be a direct result of changes in tidal forcing, the latter of which are known to be highly variable.
The application of General Circulation Models
to Titan and Triton (Mueller-Wodarg, I.C.F.(1,2), A.D.
Aylward(1), R.V. Yelle(3), M. Mendillo(2)
(1) Atmospheric Physics Laboratory, University College London (2) Center
for Space Physics, Boston University, Boston, MA, USA (3) Northern Arizona
University, Flagstaff, AZ, USA)
We present 3-dimensional time-dependent calculations of thermospheric energetics and dynamics on the only two terrestrial-sized moons in our solar system with a nitrogen-rich non-transient atmosphere, Titan and Triton. Given their small sizes and, for the case of Titan, the extended nature of the atmosphere, the balances of momentum and energy terms are different on the two moons, and both in addition differ significantly from the terrestrial thermospheric dynamics.
Reconnection in an Accelerating Plasma (R.
P. Rijnbeek Space Science Centre, School of CPES, University of Sussex,
Brighton BN1 9QH V. S. Semenov Institute of Physics, State University, St.
Petersburg)
Using a time-dependent Petschek-type model, we have
analysed the effects of reconnection in an accelerating plasma. This problem
was solved analytically for a configuration in which a current sheet separates
two uniform plasmas with antiparallel magnetic fields. We considered two cases:
a stationary reconnection site, and a reconnection site which accelerates with
the plasma (so it remains stationary in the plasma rest frame). We present
results showing that in a superalfvenic plasma, reconnection can lead to the
initial destruction, and subsequent reformation of a current sheet. Such a
feature can be used to explain certain features reported in YOHKOH spacecraft
data.
A class of post-midnight ELF/VLF wave events
has been recognised as a signature of the substorm expansion phase onset. These
are known as substorm chorus events (SCEs) and have been observed near L=4 at
Halley, Antarctica (76S,27W) since 1992. From our multiyear catalogue of
>1000 SCEs we recently used a set of SCE epochs within 1h of MLT midnight at
Halley (02-04 UT) as a basis for a superposed epoch analysis of 3-component
Halley fluxgate magnetograms to determine the size, and time dependence
relative to the substorm expansion phase onset of a typical magnetic substorm
bay (Smith et al. J. Geophys. Res., 104, 12351, 1999). The results were
interpreted in terms of a substorm current wedge with an approximately westward
electrojet poleward of Halley. Pi2 pulsations are well-known ground-observable
signatures of the substorm, and in this paper we have applied the same
superposed epoch analysis to ULF power in the Pi2 period range, specifically
40-150s. This was obtained by filtering the Halley 3-component 1s fluxgate magnetometer
data, and taking the root mean square with 1 minute averaging. In all three
components, H, D, and Z, the power began to increase slowly 20-30 min before
the SCE epoch, but then rose rapidly at the epoch, reaching a peak at a value
corresponding to ~1nT rms above the reference curve for the H component (and
~0.5 nT and ~0.25 nT rms, respectively, for the D and Z components). The
decrease back to the pre-event level took place with the same time constant as
the magnetic bay, i.e. ~2 h, reflecting the decay of the electrojet current. A
similar analysis using data from the lower latitude Faraday (65S, 64W, L=2.5)
showed the peak Pi2 power, which again was reached at or just after the SCE
epoch, was about 100 times smaller.
Convection electric field effects creating
asymmetries in the occurrence of magnetosheath particles in the magnetosphere (T.
J. Stubbs, P. Cargill, Imperial College, M. Grande, B. Kellett, M.
Lockwood, C. Perry Rutherford Appleton Laboratory)
Solar wind/magnetosheath particles in the magnetosphere
are characterised as having high charge state, low density and, at initial
entry, low energy. In this study we have used He2+ observations from the
CAMMICE MICS instrument aboard POLAR taken over 3 years. To remove any bias in
the data set, occurrence probabilities were plotted. For all energies (1-35
keV) and solar wind conditions, the occurence probabilites peaked around the
cusp region and along the dawn flank. The solar wind conditions were filtered
to see if this dawnward asymmetry was controlled by the Svalgaard-Mansurov
Effect (IMF By) or by Fermi acceleration of He2+ at the bow shock (IMF Bx/By).
The asymmetry remained persistently on the dawn flank, suggesting the cause did
not occur directly at entry into the magnetosphere. Filtering the data by the
ratio of the magnetospheric particle energy to solar wind particle energy saw
the asymmetry flip to the dusk flank for higher energy ratios (> 7.5), this
was particularly noticeable for northward IMF Bz conditions. This behaviour can
be explained by convection electric field effects in the tail, where low energy
He2+ in the distribution see an electric field as they are being accelerated
earthward. For northward IMF, the convection electric field in the tail is
weaker than for southward IMF; therefore particles need less energy to drift to
the dusk flank so the asymmetry flips at lower energy ratios.