[Report published in Astronomy & Geophysics 39, 1.27-1.28 (February 1998)]
by M J Laird (King's College, London) and N F Arnold (University of Leicester)
Published in Astronomy & Geophysics 39, 1.27-1.28 (February 1998)
The annual one-day meeting of the MIST (Magnetosphere, Ionosphere and Solar-Terrestrial) community, held on 28th November 1997 at the Geological Society, Burlington House, set a new record for the number of presentations, thirty two, and still managed to end within five minutes of its scheduled close. The morning session, chaired by Stan Cowley, began with four talks about a series of new space mission proposals to be considered in the next few years.
Andre Balogh (IC) gave a short overview of the concept of small, technology testing missions, which would give the UK scientific and industrial communities a lead in miniaturised space instrumentation and science. Naturally, there are a number of challenges to be faced, not least being able to do good science on small satellites. Steve Schwartz (QMWC) gave the first example of such a scenario when he described SWARM, a collection of no less than thirty `micro-satellites' to study the magnetosphere in three dimensions. The main scientific objectives would include monitoring the response of the magnetosphere to a wide range of solar conditions on many different spatial and temporal scales. Alan Johnstone (MSSL) proposed a pair of satellites (Magnetospheric Plasma Imaging Explorer Satellites - MAGPIES) to provide a stereoscopic view of singly ionised helium emissions at 30.4 nm from the plasmasphere and magnetosphere. With an angular resolution of around 1° and a temporal resolution of a minute, the spacecraft would provide unique observations of the dynamical evolution of the magnetosphere and yet weigh less than 50 kg. This section was completed by a presentation from Mike Lockwood (RAL) where he set out to refute the claim that he intended to put biscuit tins into space, rather that his satellites could fit into them. FARADAY is a proposal to insert two satellites into an orbit which would pass over Svalbard as often as possible. MOLNIYA would occupy a Molniya orbit, spending up to seven hours near Svalbard. Svalbard is within the dayside cusp and is already heavily instrumented.
Moving on to experiments that have been successfully implemented, Patrick Chaizy (RAL) made use of WIND and GEOTAIL spacecraft data to investigate energetic electrons (> 20 keV). Having two measurements separated by over a million kilometres allows a distinction to be made between spatial and temporal effects. Richard Kilmurray (IC) examined over fifty Coronal Mass Ejections using the Ulysses spacecraft. He investigated the interaction of the magnetic boundary of these CMEs with the solar wind and observed discontinuities and planar magnetic structures. Some old HEOS high-resolution magnetic field data was dusted off by Malcolm Dunlop (IC) so that he could examine the Earth's cusp field geometry. Low-resolution global data in geocentric magnetospheric equatorial co-ordinates provided a good baseline for examining cusp traversal in the HEOS data during variations in solar wind conditions.
Around fifty 20-hour intervals of data from a series of satellites had been studied by Nick Flowers (MSSL) to establish a relationship between energy input into the magnetosphere and the size and growth/decay rates of the resulting magnetospheric substorms. The rate of change tended to be a more accurate indicator than its absolute size. Mervyn Freeman (BAS) found that the loss of energy from the magnetospheric cavity through leakage outside the magnetopause boundary was comparable to other loss mechanisms, such as compressional-transverse wave coupling and compressional wave propagation. The magnetopause transition parameter, derived from measurements of electron number density and temperature in the low latitude boundary layer, was employed by Mike Hapgood (RAL) to select ion velocity and magnetic field data for the `Whelan test'. This is a necessary condition for reconnection and the method works well when the magnetosheath is stable.
Mark Dieckmann (Warwick) presented results from a plasma sounder, which, as the name suggests, measures plasma density. Using the dispersive behaviour of wave packets, he demonstrated that there is a response peak doublet close to the upper hybrid Bernstein mode maximum. This is a more likely candidate for explaining the measurements obtained by the instrument than that of low group velocity resonances applied to date. A sophisticated chemical model of the D region was constructed to obtain the effective recombination coefficient for electrons below 95 km, where the E-region square law breaks down. John Hargreaves (Lancaster) explained that a three-halves power law dominates due to the relative increase in cluster ions rather than the simple ones found at greater heights.
The morning session was brought to a close by Henry Rishbeth (Southampton) informing the meeting about the current and proposed situation concerning copyright - both for printed matter and data. There are proposals to end the policy of `fair use' where non-commercial institutions can make limited numbers of reproductions without incurring royalty charges.
The ionospheric theme continued after lunch as, with Mike Hapgood in the chair, Henry Rishbeth (Southampton) spoke on annual and semi-annual variations in the F-layer. At Slough, for example, maximum electron density occurs in winter, whereas at Port Stanley maximum values occur at the equinoxes. Model studies show that such differences can be broadly explained by composition changes arising from the global thermospheric circulation.
Shifting earthwards, Gary Beard (Aberystwyth) described the application of bispectral analysis to non-linear interactions between atmospheric tides and planetary waves. Meteor-radar observations had shown modulation of tidal velocities with periods matching those of planetary waves. Non-linear theory predicts secondary waves with frequencies that are the sum and difference of the two primary wave frequencies, and the analysis confirmed their presence.
Vertical winds offer an important insight into the physics of gravity waves. The predicted spectral index of the vertical-wavenumber spectrum is -3 for the linear instability theory of wave breaking, whereas it is +1 for the diffusive filtering and saturated cascade theories. However, according to Nick Mitchell (Aberystwyth), measurements by the EISCAT VHF radar at mesospheric heights revealed a mean spectral index of -1.36±0.2, between the predictions of the rival theories! This suggested none of them could account for the observations, and it was proposed that the effect of ducted waves should be considered.
Further work on gravity waves, based on CUTLASS HF radar observations, was reported by Neil Arnold (Leicester). The radar system readily detects quasi-periodic disturbances in the ground back-scattered power, which could be interpreted as the superposition of a background gravity wave field and a source of additional waves, possibly from the troposphere. Analysis showed good agreement with the Hines dispersion relation. Barbara Jenkins (BAS) then presented features of mesospheric winds derived from observations made by the HF radar at Halley, Antarctica. Echoes within a range of 400 km are believed to be due to scattering from meteor trails, having the diurnal variation expected of meteors together with an annual variation possibly due to changes in the neutral atmosphere. The trails move with the neutral winds, and can be used as tracers. Evidence is that the semi-diurnal tide is dominant for most of the year, but with large planetary wave activity during the austral winter.
With a change of topic, William Wilkinson (Aberystwyth) showed how EISCAT observations of interplanetary scintillation over extended periods of time had enabled the determination of the meridional component in the velocity of the mid- and high- latitude solar wind. Measured velocities are between 10 and 20 km s-1 and are consistently equatorwards in both hemispheres, and imply that the inner high-latitude heliospheric magnetic field will likewise have a small non-radial component.
Y. Su (Sheffield) then described an investigation of annual and seasonal variations in the low latitude topside ionosphere using observations from the Hinotori satellite together with the Sheffield University plasmasphere-ionosphere model. Observed electron densities at 600 km showed a strong annual anomaly; model values reproduced the general behaviour but indicated that energy sources in addition to variations in the solar EUV flux may contribute. Seasonal behaviour is consistent with a transequatorial component of the neutral wind directed from the summer to the winter hemisphere.
The session concluded with two talks from the Aberystwyth group featuring ionospheric tomography, which is now well established as a valuable technique. First, Simon Berry compared structures seen in tomographic reconstructions of ionospheric electron density with optical observations of auroral red- and green-line activity. Signatures of discrete auroral arcs could be related to latitudinally narrow field-aligned density enhancements. Association could be made between satellite data representing hard precipitation to E-region altitudes and E-region enhancements. Second, Ian Walker discussed observed ionospheric effects of magnetopause reconnection. Tomographic imaging showed an F-region signature of ion dispersion and also a field-aligned current system consistent with earlier ideas. Features seen in the density structure also enabled identification in the ionosphere of the open/closed field line boundary.
Bill Wright (Vdstervik) opened the last session of the meeting, chaired by Alan Rodger, by reporting on progress toward definition and validation of an ionospheric sunrise index of neutral thermospheric composition. Some of its properties were exhibited using a fifty-year database of foF2 from Slough, and slightly shorter sequences from other ionosonde sites. Clear solar cycle, seasonal, latitudinal, longitudinal and geomagnetic activity variations were evident.
The geomagnetic disturbance - a `minor storm' - of 1997 January 10/11 was well observed by a number of spacecraft together with a widespread network of ground-based instruments. As reported by David Rees (Kent), it was thus possible to compute the evolution of the high-latitude electrostatic potential. Simulations using independent hemisphere patterns for time-dependent potential showed considerably larger increases of local and global thermospheric temperature, and substantially bigger high- and mid-latitude thermospheric winds compared with simulations using the Foster polar electric field model.
Attention then moved to the cusp region, with the presentation by Peter Cargill (IC) of a new model (for northward interplanetary magnetic field) in which the cusp is treated as an indentation in the magnetic field. When the magnetosheath flow is supersonic, it can `turn the corner' into the indentation by means of a Prandtl-Meyer fan; it must then be turned out again by an oblique shock. Difficulties associated with forming a standing shock lead to the possibility of turbulent rather than steady-state flow.
The TIMAS experiment on the Polar spacecraft can give full energy/pitch angle distributions for four ion species simultaneously. From data obtained while Polar was within the cusp, Andrew Fazakerley (MSSL) gave examples of overlapping ion signatures that could be compared with Lockwood's 1995 model of cusp ion injection. Some were consistent with the model for a single reconnection X line; however others could not be explained by a straightforward interpretation. One possibility would be further reconnection on an already open field line. Four more examples of overlapping velocity dispersion signatures, three of high energy and one of low energy, were then presented by Ian Krauklis (MSSL). The low energy dispersion could be explained by existing models; the high energy dispersion might be due to interaction - perhaps multiple interactions - with a rotational discontinuity.
Gareth Lawrence (Sussex) described a model of magnetohydrodynamic flow past the magnetopause in which a plasma depletion layer was observed. This was followed by a paper presented by S. Matsoukis (Warwick) on whistler mode wave coupling effects on electron dynamics in the near-earth magnetosphere. She showed that an electron interacting with two oppositely directed parallel propagating whistler mode waves exhibits stochastic behaviour in addition to resonance. This stochasticity can contribute to pitch angle diffusion into the loss cone, leading to auroral precipitation.
More results from Polar came from Jonathan Storey (Leicester), who reported on an investigation of substorm-associated features in the plasma sheet observed with the on-board CAMMICE instrument. Two intervals had been studied. In a substorm interval, field-aligned ion beams were seen, suggesting an active reconnection event. In addition, in comparison with a quiet interval, there was a count rate increase seen in all ions, and an energy increase seen in ions of solar wind origin. Reconnection was also the theme of the next speaker, M. Buchan (Sussex). She described a technique involving observations of the aurora which allows indirect measurements to be made of the reconnection rate. Using a reconnection model, one can infer the behaviour of plasma conductivity within the diffusion region; nightside optical data from Tromsø indicated that the onset of reconnection is characterised by an abrupt decrease in conductivity, which later increases.
To conclude the day, Ingo Mueller-Wodarg (UCL) discussed the effects of the forthcoming total solar eclipse on 1999 August 11 on the thermosphere and the ionosphere. Two simulations had been run using the Coupled Thermosphere-Ionosphere-Plasmasphere model (CTIP), one with the eclipse included, the other without, to predict the effects on winds, temperature and composition in the thermosphere and the ionosphere. Features included a substantial fall in thermospheric temperature in the totality region, and a downwelling of the F2 layer.
The meeting concluded with thanks to all contributors, chairmen and the organizer, A.J. Smith.