End of Meeting
Aberystwyth: University of Wales, Aberystwyth
Aerospace: Aerospace Corporation, Los Angeles; USA
Augsburg: Augsburg College, Minneapolis; USA
BAS: British Antarctic Survey
BGS: British Geological Survey
Boston: Boston University; USA
Calgary: Calgary University, Canada
Chicago: University of Chicago; USA
DCRS: Danish Centre for Remote Sensing; Denmark
DRA: Defence Research Agency
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FMI: Finnish Meteorological Institute; Finland
Garching: Max Planck Institute for Extraterrestrial Physics, Garching, Germany
GSCanada: Geological Survey of Canada; Canada
GSFC: NASA Goddard Space Flight Centre; USA
IAP: Institute of Applied Physics, Nizhny Novgorod; Russia
IC: Imperial College, London
ISAS: Institute of Space & Astronautical Sciences; Japan
JHU/APL: Johns Hopkins University/Applied Physics Laboratory; USA
Lancaster: University of Lancaster
LANL: Los Alamos National Laboratory; USA
Leicester: University of Leicester
Lindau: Max Planck Institute for Aeronomy, Lindau; Germany
Lockheed: Lockheed Martin Laboratory, Palo Alto; USA
LPCE/CNRS: Laboratory of the Physics and Chemistry of the Environment; France
Meudon: Observatory of Paris; France
Minnesota: University of Minnesota; USA
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New Hampshire: University of New Hampshire, USA
Newcastle: University of Newcastle, NSW, Australia
NIPR: National Institute of Polar Research; Japan
NIWA: National Institute of Water and Atmospheric Research; New Zealand
PGI: Polar Geophysical Institute; Russia
Phillips Lab: Phillips Laboratory; USA
QMWC: Queen Mary and Westfield College, London
RAL: Rutherford Appleton Laboratory
Sheffield: University of Sheffield
Sodankyla: Sodankyla Geophysical Observatory, Finland
Soton: University of Southampton
St Petersburg: St Petersburg State University; Russia
St Andrews: University of St Andrews
Stanford: Stanford University; USA
STEL: Solar Terrestrial Environment Laboratory; Japan
Sussex: University of Sussex
Texas: University of Texas at Dallas; USA
Tohoku: Tohoku University; Japan
UCL: University College, London
UNIS: UNIS, Longyearbyen, Svalbard
Warwick: University of Warwick
Waseda: Waseda University, Japan
York: University of York
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by M J Laird (King's College, London)
Published in Astronomy & Geophysics (1997) 38(3), 25-27
The spring meeting provided rich fare for the MIST community. To begin the session on aurora and the cusp, David Willis reported that a catalogue of ancient auroral observations from China, Japan and Korea had been used to generate a database that is being used to investigate both seasonal and secular variations, as well as the evidence for long-term variations in the occurrence of major geomagnetic storms. Peter Collis then presented coordinated observations using the Kilpisjärvi imaging riometer, the Porojärvi all-sky TV and the EISCAT UHF radar. Three auroral intensifications had been observed in the magnetic zenith, each separated by about 40 minutes. A novel comparison of latitudinal profiles of auroral light intensity and radio absorption was introduced, and the results showed that the pre-break up auroral arcs were collocated with arc-like features in absorption. Next, Steve Milan described CUTLASS Iceland radar observations of auroral electrojet irregularities at E and F region altitudes. At the higher altitude, a broad spectrum of line-of-sight velocities was consistent with zonal flow. In the E region, these velocities saturated at the ion acoustic speed cs, the spectra being broad (v < cs) or narrow (v ~ cs).
The magnetic cusps are of particular interest, as here magnetosheath plasma has more or less direct access to the inner magnetosphere. Karlheinz Trattner showed three dimensional plasma measurements from the toroidal imaging mass-angle spectrograph (TIMAS) on the Polar satellite. Observations of H+, O+, He+ and He++ in the energy range 16 eV - 33 keV revealed a velocity filter effect at the cusp, with magnetosheath ions arriving first at the observing satellite, while slower ions were seen later at higher latitudes. This is in agreement with dayside reconnection, and seemed to be a constant feature for all the cusp crossings. Simultaneous observations of the cusp by the Ny Ålesund meridian scanning photometer, the CUTLASS Finland HF radar and the DMSP F13 satellite were presented by Tim Yeoman. An interval of very strong radar returns coincided with optical and particle signatures of the cusp. The source of the backscatter was F region irregularities produced by soft ion precipitation. Peter Cargill had carried out MHD numerical simulations of the cusps for east-west and northward interplanetary magnetic field (IMF). In the east-west case, the magnetopause moved inward, its position being determined by balance between the magnetic pressure of the IMF and the compressed magnetosphere. For northward IMF, the simulations demonstrated the existence of lobe reconnection in the presence of a realistic cusp field geometry.
To conclude the session, Carlos del Pozo presented two case studies of multi-instrument observations of the dynamics of auroral arcs. For a substorm event, the Kilpisjärvi imaging riometer tracked several patches of intense ionisation drifting south-east until the appearance of a magnetic bay. Subsequently, an enhanced electrojet crossed Tromsø, associated with north-south moving arcs. A quiet time case showed evidence of a localised field-aligned current system producing a dim arc.
The MIST community has always taken much interest in techniques and instrumentation. Ian McCrae said that Ian Gordon, a sandwich student, had been working at Rutherford Appleton Laboratory for a year to develop a computer program for removing anomalous contamination (mainly from satellites) from EISCAT raw data. Early tests showed that it can reduce the number of fit failures from uncleaned topside data by large factors. Owen Jones demonstrated a new use of the Halley dynasonde, in which it doubles as an imaging doppler interferometer, to measure mesospheric winds. Soundings made every five minutes are processed in near-real time, and a database is being built up from 1996 December. Brian Shand introduced another use of CUTLASS by showing observations of the Odden ice tongue, to the east of Greenland. Sea ice had been seen in the radar data in the same region observed from the DMSP satellite, and its evolution could be tracked. Sea waves in the ice regions are damped, and this shows up in the radar scatter.
To conclude the day, Henry Rishbeth introduced a special session on current issues concerning STP data. He outlined the traditional principles of "free and open access", and stressed the need to maintain these principles despite new national and international measures on copyright of data bases. Toby Clark (British Geological Survey), Matthew Wild (RAL) and Martin Jarvis (British Antarctic Survey) described the principles and practice of supplying data in their institutions. In some cases access is prescribed by the EC directive on environmental data and the Antarctic Treaty. Scientists should take care to acknowledge the sources of the data they use.
Wednesday morning was devoted to the study of waves. David Orr discussed cavity resonances in the outer magnetosphere. A detailed study of pulsation activity using eleven magnetic observatories had allowed an estimate to be made of plasma density which was then used to model the characteristic periods of cavity modes and their amplitude structure. Agreement between the model and the experimental measurements supported the existence of these modes. Rod Mathie presented a statistical survey of high latitude Pc5 frequency, amplitude and phase characteristics observed on the IMAGE magnetometer array. From work in Canada, the possibility had been mooted of stable discrete frequencies 1.3, 1.9, 2.6 and 3.4 mHz, but though these frequencies were present in the IMAGE data, they were no more dominant than any other set of frequencies. Using data from an array of HF doppler sounders and magnetometers in Australia, Fred Menk had investigated the coupling of Pc2-4 waves to the low latitude ionosphere. The dominant spectral contribution was due to pulsations below the resonance frequency, and these magnetic and ionospheric oscillations showed a constant phase relationship not predicted by models, which accordingly were in need of generalisation to accommodate both Alfvén and fast mode waves. Theoretical work on these waves was presented by Jason Smith, who had studied coupling in a two-dimensional, non-uniform cavity driven by a random motion of the boundary. Numerical results showed that the system excited the fast modes which lay inside the broadband spectrum of the driver. Alfvén resonances were also excited when the fast mode eigenfrequency lay within the Alfvén continuum. Recently, ULF waves with large azimuthal wavenumber (large m) have been observed on similar L-shells and with the same or similar frequencies as small m field line resonances. In explanation, A.N.Wright proposed an extension to an earlier waveguide model, suggesting that tailward-propagating modes drive the resonances. Phase-mixing allows the development of the nonlinear Kelvin-Helmholtz instability, eventually leading to observable small m drift waves. In similar vein, Ian Mann presented MHD wave solutions for high m poloidal Alfvén waves with equatorwards phase propagation. He had used a box model, and introduced a driving term to represent energy input from a magnetospheric source, such as bounce resonance with energetic protons. His solutions provided a good explanation for latitudinal amplitude variations for giant pulsations observed by the EISCAT magnetometer cross.
Observations of NNSS signals have enabled total electron content (TEC) to be found for the ionosphere. Now the deployment of the GPS satellite constellation has made possible observations out to three earth radii anywhere at any time. GPS TEC measurements comprise both ionospheric and protonospheric contributions, and Nicholas Lunt showed that differencing between GPS and NNSS vertical TEC data has the potential to monitor the protonosphere. Nigel P. Meredith reported `pancake' electron distributions in the outer radiation belts observed by the low energy plasma analyser (LEPA) on the CRRES satellite, where it is possible to get a full pitch angle distribution over the whole energy range 10 eV - 30 keV. The higher the characteristic energy, determined from magnetometer data and the Iowa plasma data, the more sharply peaked were the distributions, which were interpreted as the remains of pitch angle diffusion by whistler mode waves. Further results from CRRES LEPA were given by Sarah Szita. Fitting diffusion curves to contours of constant electron counts in energy-pitch angle space gave a much lower value for the characteristic energy than that obtained from the Iowa wave data. More work was necessary, taking into account the gradient in counts down to the loss cone.
Gavin Wheeler discussed the growth and propagation of magnetosonic waves in the outer plasmasphere, having employed a model of the proton ring distribution with the inclusion of suprathermal electrons. Results from the HOTRAY ray tracing simulation showed strong growth possible for wave normal angles close to 90°. The waves can be guided by the density gradient at the plasmapause. Modulation of ELF/VLF emissions has been observed by an array of Antarctic stations, the modulations falling in two bands: 10-100 mHz, termed quasi-periodic (QP), and 200-300 mHz, termed periodic (PE). A.J.Smith said that the QPs could be correlated or uncorrelated with magnetic pulsations. The latter case never occurred without PEs; the proposed mechanism was that the PEs induced precipitation which gave rise to field-aligned currents which in turn generated field line resonant ULF waves driving the PEs. Andy Buckley presented measurements of pulsed plasma wave emissions observed upstream of the earth's bow shock by the AMPTE UKS} and IRM satellites. Waves near the electron plasma frequency were found to be highly time structured, giving rise to the observed emission pulses. David Nunn had successfully simulated risers, fallers, hooks and hiss triggered emissions corresponding to VLF data from Halley, Antarctica. Identification of generation regions for risers and fallers, and understanding of hook formation had been obtained. He was strongly of the opinion that nonlinear electron cyclotron resonant trapping is the root mechanism behind VLF chorus and emissions. To conclude the session, A.G.Demekhov discussed modelling of nonstationary electron precipitation by the whistler cyclotron instability. Comparison with case-study data from EISCAT and the CRRES and LANL satellites showed reasonable agreement.
To begin the ionosphere session, Alan Aylward reviewed polar mesospheric summer echoes, very high power echoes seen in radar returns, which had hitherto remained unexplained since their discovery in 1981. More recent experimental work has shown associations with electron density "bite-outs", tidal oscillations and water vapour. The explanation now put forward for the scattering mechanism involves the presence of negatively charged ice particles. This causes the electrons to have two diffusion modes: a fast one which is the usual diffusion, and a slow mode. The latter is picked out by the radar as it allows structures of half the radar wavelength to develop long enough for coherent scattering to take place. Michael Williams presented a study of the influence of the solar wind on the electron density structure at high latitudes using results from an ionospheric tomography campaign together with additional EISCAT data and solar wind information from the WIND satellite. In a case study, during active IMF conditions the trough was found to form a long way equatorward - ~5° of latitude - from its quiet time position. Attempts at ionospheric prediction have not always been too successful, predictions often being inferior to those of the `persistence' (i.e. no change) model. Nick Francis presented a novel technique using radial basis function neural networks, and prediction of foF2 values showed significant improvement over persistence. Terry Robinson described his solution of the equations associated with the Farley-Buneman instability, treating it as an initial value problem. The solution exhibits several stages, which include soliton-like collapse and finally true instability. Application to the E region plasma leads to final states ranging from arc-like single field aligned structures to small-scale turbulence.
Finding values for the ion-neutral collision frequency directly from EISCAT data is not easy, and requires a number of assumptions. Ken Freeman put forward an alternative technique, establishing for each neutral species a height profile of neutral density. From this, a profile of the collision frequency is easily found, which can then be fed back into the program used to process the EISCAT data, a procedure which can be repeated until convergence. Another study of ion-neutral interaction, involving data from the EISCAT radar and a Fabry-Perot interferometer run by UCL was reported by Chris Davis. The debate surrounding the calculation of the O-O+ collision cross-section by comparison of optical and radar data is now centred on the statistical methods used. A maximum likelihood method appeared to give the lowest error.
Why is energy release not steady during reconnection? This was the question Richard Rijnbeek set out to answer. An analysis of Petschek-type reconnection had shown that time-varying reconnection is a more efficient energy release mechanism than a steady-state one, in accordance with observations of impulsive phenomena such as solar flares. Reconnection is a feedback process, which is intrinsically time varying. Continuing on this theme, Gareth Lawrence showed how a reconnection electric field could be inferred from observations of flux transfer events (FTEs). He had considered ideal MHD flow past an obstacle, and shown, using a time-dependent reconnection model, that the shape of the obstacle could be inferred from the ambient field configuration, and furthermore that the reconnection rate could be estimated. Manuel Grande presented Polar observations of the magnetosheath during a persistent northward IMF condition. These observations had been combined with those from HEO to examine the spatial extent and characteristics of this region.
With a change of scene to Jupiter, Stan Cowley discussed Ulysses observations of field-perpendicular plasma flows in the Jovian magnetosphere. Independent estimates for the E x B velocity had been derived from energetic ion data from the ATs instrument and thermal electron data from SWOOPS. On the inbound pass these generally agreed, but on the outbound pass this was not so. Another Jovian puzzle is the heating of ions from Io to tens of keV, and Andrew Fazakerley suggested a possible mechanism in his discussion of J-violating acceleration in a rapidly rotating magnetosphere. He showed that typical bounce times were of the order of hours, so that, as Jupiter's rotation period is about ten hours, J will not be conserved. The perpendicular energy of the particles can then, via pitch-angle scattering, be increased irreversibly.
To finish the day, John Taylor presented CUTLASS observations during the magnetic storm of 1997 January 10-11. This storm was driven both by the shocked solar wind impact and a prolonged and enhanced southward IMF. Observations during the recovery phase showed the reconfiguration of the magnetosphere.
Thursday began with two papers on the substorm current wedge. Rob Lewis demonstrated how Goose Bay HF radar had been used to describe the behaviour of the electric field within the wedge for a small substorm, with magnetometer data from Canada and Greenland locating the radar backscatter with respect to the substorm currents. The main substorm onset was preceded by a pseudo-breakup, and it was suggested that near-Earth current disruption may be the favoured onset mechanism. Paul O'Pray then discussed modelling of the wedge location for an isolated substorm using the UK-SAMNET and the Canadian EMR magnetometer arrays. Previously, using SAMNET data and the York wedge location model it had been possible to estimate the location within 1-2 h of local time beyond the spatial range of the magnetometer array. Use of the Canadian data as well extended the local time range of wedge location.
Chris Owen presented a study of three plasmoid events observed by Geotail, located between 170 and 200 earth radii downtail, and also seen by IMP-8 at 40 earth radii. All three plasmoids showed earthward plasma flow at the leading edge. The results indicated that an active reconnection site must have been located tailward of Geotail despite the distance downtail and the fact that the plasmoids appeared to have been disconnected. Further observations from Geotail, together with data from Polar were shown by Manuel Grande in coordinated studies of plasmasheet dynamics during a substorm growth phase. At the time of the observations the satellites were located on similar L-shells. Both saw the same drop-out, indicating an observation of large scale magnetospheric behaviour. Continuing with the substorm theme, Mark Lester presented EISCAT observations of ionospheric convection in the dusk local time sector. DMSP passes were used to determine the location of the auroral precipitation, and indicated a reduction in polar cap size from expansion to recovery phase. It appeared that reconnection of open flux in the tail took place well into the expansion phase.
Jim Dungey emphasised the value of simulating neutral sheets. Several groups had demonstrated the feasibility of particle-in-cell simulations, and found that reconnection occurs. The latter implies that the particles come from outside the simulation box and the boundary conditions can be simple; discovering how they affect the rate of reconnection is an obvious objective. M.Tsalas discussed single particle dynamics in a simple model geotail field. Without a shear component By in the magnetic field, orbits may be classified as integrable, which make no contribution to the cross-tail current, transient, which make the main contribution, and stochastic, which contribute, but on a longer timescale. The introduction of By destabilizes the trapped particle orbits, and thus may influence the currents. Anders Ynnerman continued by showing a video displaying three coordinates in three-dimensional space. Such a display permits exploration of the characteristic geometrical properties of the orbits for different values of By, revealing key properties of the bifurcation sequence that are not accessible to two-dimensional Poincaré surface-of-section plots.
Nick Watkins presented first results from the application of modern time series analysis to WIND data in the deep solar wind. Wavelet analysis of observed ion acoustic turbulence showed a characteristic pattern in the nonlinear development which appeared to be more generally present than either visual or Fourier analysis would suggest. In the first of three presentations on comets, Geraint Jones discussed the interaction of the solar wind with comets Hyakutake and Hale-Bopp as revealed in imaging of ion features. Ion rays, first reported by Eddington in 1910, could be seen in observations of H2O+ in Hyakutake. They could also be seen in Hale-Bopp, though the effects of dust had first to be removed. It is generally accepted that the rays trace decelerated contaminated solar wind. Hale-Bopp had also been observed by the Japanese X-ray satellite ASDA. Barry J. Kellett reported that no X-ray emissions in the 0.5-10.0 keV range had been detected. However, the derived upper limits to the X-ray continuum and line emission can place strong constraints on the possible X-ray emission models of the comet. Another comet encounter was that between Grigg-Skjellerup and the Giotto spacecraft. According to Ian Krauklis, the implanted ion sensor on board revealed asymmetric cometary ion distributions in velocity space. Numerical simulation showed that the observed one-sided velocity distributions were the result of ion implantation in the upstream diffusion gap, with the Alfvénic Mach number near unity, and ion lifetimes less than the isotropization time.
Peter Cargill discussed the interaction of interplanetary coronal mass ejections with the solar wind. Observations came from in situ measurements at 1 AU, and also from Ulysses at several AU. MHD numerical simulation of the interaction of a magnetic flux rope with the solar wind plasma showed strong coupling with reconnection at the trailing edge. Simon Walker presented results from a study of the spatial scales of the ramp region of supercritical, quasi-perpendicular bow shocks. The study used data from the AMPTE UKS and IRM satellites during crossings of the Earth's bow shock. It showed that the scale size of the ramp was usually of the order of a few electron inertial lengths, consistent with theoretical predictions based on nonlinear particle dynamics.
Opening the final session, Toby Clark reported on the effects of thermospheric winds on spacecraft such as the ERS series of satellites controlled by ESOC (the European Space Operations Centre). Use of the UCL thermosphere-ionosphere model to compute winds showed that the drag coefficient could vary by ±25%, and it was suggested that study of this variation could improve estimates of atmospheric density from the MSIS-86 model.
Vikki Howells discussed EISCAT observations of gravity waves at mesopause heights. Measurements made by the Tromsø radar showed that the vertical velocity field at the mesopause was dominated by short period gravity waves. Thermal and doppler ducting give plausible explanations of the many cases of no exponential growth with height, and long-lasting variance profiles suggest that the wave fields are shaped by the background structure. Results of a study of large-scale atmospheric gravity waves (AGWs) and associated travelling ionospheric disturbances (TIDs) were presented by Richard Balthazor. In the Sheffield coupled thermosphere-ionosphere-plasmasphere model, families of AGWs propagated equatorwards and interfered at the equator. The TIDs became decoupled and there were persisting disturbances in the ionosphere, believed to be due to the initial large thermospheric wind, and the neutral winds associated with the slower AGWs. More work on gravity waves was reported by Neil Arnold. Quasi-periodic fluctuations in the returned ground-scatter power from the CUTLASS radars have been linked to the passage of medium-scale AGWs. Ray-tracing calculations indicate that the range of the first radar returns from the F layer are closely related to a small number of basic ionospheric parameters which vary with local time, season and geomagnetic activity. Using bandpass filtering, the AGW component of these returns can be extracted. Results show a sensitive response to the AGWs.
According to Anasuya Aruliah, the prevailing assumption that the neutral atmosphere moves in a slow and steady manner needs to be revised. In support, she presented evidence of small-scale structure in the high-latitude thermosphere derived from Fabry-Perot interferometer observations. Comparison of vertical winds showed highly localized upwelling, due to Joule heating, seen at Kiruna but not 150 km away at Kilpisjärvi. Such small-scale structure will have important consequences for the calculation of momentum transfer and Joule heating. Is there long-term change in the temperature and wind in the Antarctic thermosphere? In the final contribution Martin Jarvis sought to answer this question with the aid of 39 years of monthly mean ionospheric data from Argentine Islands, Antarctica and Port Stanley, Falklands. Analysis showed a downward trend in the height of the F region peak of about 0.15 km/y and 0.5 km/y at Argentine Islands and Port Stanley respectively plus a reduction in the effect of the thermospheric neutral wind of about 8%. A fall in the geomagnetic field strength of 10% over the last 40 years in the region could be significant. The meeting concluded with thanks to all who contributed, and especially to Sandra Chapman, the local organizer.