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?  (Stephen Topliss, Chris Owen, Ian Krauklis, Mullard Space Science Laboratory, UCL; Craig Kletzing University of Iowa)

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?  (Stephen Topliss, Chris Owen, Ian Krauklis, Mullard Space Science Laboratory, UCL; Craig Kletzing University of Iowa)

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.

Thursday April 13

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.   

One-dimensional particle-in-cell simulations of the lunar wake (Paul Birch and Sandra Chapman, University of Warwick)

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 (K_p = 7^- - 5^-) and quiet (K_p = 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.

Posters

New method for monitoring the state of the ionosphere (Péter Bakki,  Technical University of Budapest, Hungary)

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 (69.59^o N, 19.23^o 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 (69.35^o N, 20.36^o 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 (12.8^o) and Spitze (6^o) 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.

VLF, Magnetic and Pi2 Substorm Signatures (A.J. Smith, M.P. Freeman and S. Hunter British Antarctic Survey)

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.