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by M J Laird (King's College, London)
Published in Astronomy & Geophysics (1997) 38(1), 19-20
The annual one-day meeting, held on 1996 November 22 at the Geological Society, Burlington House, again provided an opportunity to hear of the latest observational, experimental and theoretical results from the MIST (Magnetosphere, Ionosphere, and Solar-Terrestrial) community. With Mervyn Freeman in the chair, the first session began with four papers from the Imperial College group all related to the Ulysses spacecraft which, after its Jupiter fly-by, had proceeded to explore the interplanetary medium at high heliospheric latitudes. First, Coreena Lofting described two effects of the rotation of the Jovian magnetosphere on the dispersion relation of transverse Alfvén waves. She showed that the Coriolis force can impose circular polarization even in the presence of linearly polarized sources, and that this could lead to the production of doublet structures in the power spectrum resulting from the frequency shift between left- and right-handed components. Elizabeth Lucek then presented a wavelet analysis of an isolated wavepacket observed by the Ulysses magnetometer upstream of an interplanetary shock. The wavepacket, whose appearance was similar to the high frequency signatures associated with shocklets seen at planetary bow shocks, showed a very distinctive dispersive signature where the frequency decreased throughout the wavepacket.
Very few observations of slow mode shocks have been made by spacecraft in heliospheric space, but Ulysses' polar trajectory provides data from previously unexplored regions, and from regimes, such as stream interfaces, current sheets and boundaries of expanding coronal mass ejections (CMEs) where such shocks are most likely to form. Richard Kilmurray gave the results of a systematic scan, which confirmed that slow shocks are rare in the heliosphere, but not entirely absent. Lastly in this group, John Glover described an investigation into the magnetic signatures of 57 ICMEs (the interplanetary counterpart of CMEs) as observed by Ulysses, and the comparison made with the spacecraft's solar wind observations. Of the 31 ICMEs with some magnetic signature, the majority had coincident magnetic and plasma start and stop times. However, some magnetic signatures preceded the plasma start time, and some extended beyond the plasma stop time.
To follow, Paul Gough (Sussex) presented observations made on Space Shuttle flights. On STS-75, long periods of the mission were dedicated to further study of the MHz electron modulations seen when the DC electron beam was at 90° pitch angle. Modulations fell into two classes: narrow band dispersionless modulations at harmonics of the electron gyrofrequency Fce; and relatively broad band dispersive (n+1/2)Fce emissions. These were interpreted in terms of electrostatic cyclotron harmonic waves, and the results gave a direct experimental derivation of the dispersion relation for the (n+1/2)Fce waves. Sean Oughton (UCL) then showed how spectral anisotropy may be measured using single-spacecraft data, and was followed by Richard Horne (BAS), who presented a study of quasi-linear diffusion using a new Vlasov simulation code. The code had been tested for the case of Landau damping, and applied to beam plasma electrostatic instabilities of the type encountered in the auroral region. Initial wave growth was in accordance with linear theory, but then diffusion of particles in velocity space acted to limit amplitudes, the spatially averaged distribution developing a plateau in accordance with quasi-linear theory. Non-resonant wave-particle interactions heated the background thermal distribution.
In studies of magnetopause crossings, Hapgood and Bryant had introduced a transition parameter, based on electron characteristics, which ordered other independent data, such as ion flow, density and temperature, and the orientation and strength of the magnetic field, though it was not clear why this should work. Mike Lockwood (RAL) provided an explanation by showing, from AMPTE-UKS satellite data, that when magnetic reconnection generates newly-opened field lines, the transition parameter has a direct relationship with the time elapsed since the boundary-layer field line was opened. A simple model reproduced this behaviour. The next speaker, Anasuya Aruliah (UCL), presented evidence for a solar cycle dependence of the high latitude electric field. It had long been assumed that this field only depends on interplanetary magnetic field components By and Bz. However, the solar cycle dependence of plasma velocities observed by the EISCAT radar together with neutral winds at Kiruna measured with an FPI, and IMF measurements by satellites, indicated there was also a solar cycle dependence of the electric field. Hence, to derive this field, one cannot just rely on Kp.
Two-station interplanetary scintillation observations of the solar wind using EISCAT have enabled accurate measurement of the solar wind velocity for a number of years. Maximum cross-correlation between the antennae generally occurs when the baseline is parallel to the radial vector from the sun. Phil Moran (Aberystwyth) discussed long duration observations in which the baseline perpendicular to this radial vector varied significantly. On occasion, there was some evidence of an offset in the cross-correlation peak, indicating a possible off-radial component of the solar wind. The session ended with the presentation of a new theory of the development of solitary irregularities in the auroral E-region, with application to auroral backscatter, by Terry Robinson (Leicester). The theory includes coupling between drift and ion acoustic waves, and leads to the possibility of reverse diffusion, in which small broad irregularities may develop into large narrow structures.
After lunch, Sandra Chapman took the chair, and William Wilkinson (Aberystwyth) discussed the structure of low Mach number quasi-perpendicular bow shocks. This structure depends on the angle between the upstream magnetic field and the shock normal; dispersion is supposed to limit the steepening when this angle is less than about 80°, and resistivity at greater angles. However, a series of one-dimensional hybrid simulations showed that the "dispersive" description is valid for angles as large as 88°, the shock getting thinner as the angle increases. Yuanzhi Su (Sheffield) presented MU radar observations of altitude dependencies of the solar activity variations of ionospheric densities and temperatures and this was followed by a case study of a polar cap arc and HF radar backscatter given by Steven Milan (Leicester). The backscatter appeared as a region approximately 150 km wide to the east of the optical arc (observed by a meridian-scanning photometer located at Ny Ålesund). The luminosity of the optical feature appeared to be related to the plasma flow speed.
There followed three more contributions from Aberystwyth, the first two on ionospheric tomography, a technique in which observation of signals from navigational satellites provides a number of total electron content measurements. Inversion then gives a two-dimensional image of electron density. The technique is ideal for use at remote locations like the polar regions. Eleri Pryse reviewed recent developments, including a new algorithm, and also presented first results from a new tomographic chain with receivers at Ny Ålesund, Longyearbyen, Bjørnøya and Tromsø. Because the satellite does not pass directly overhead of the receivers, a correction is necessary for the satellite-receiver longitude difference. Cathryn Mitchell showed comparisons of tomographic measurements with independent measurements from the EISCAT radar, which indicated that accurate corrections can be made provided that a good estimate of the ionospheric height is obtained. A preliminary result from the campaign, using two chains and showing the early-evening east-west electron density gradient, demonstrated that ionospheric tomography has the potential to produce three-dimensional images. Francis Sedgemore then presented latest results from the EISCAT dynasonde, and illustrated how a dynasonde can help separate spatial and temporal variations and measure the drift of small-scale features associated with auroral arcs.
The POLAR satellite can observe the polar regions for extended time periods, and the ultraviolet imager on board yields high time resolution images of the aurora. Fred Rees (Southampton) described how the energy flux of precipitating electrons may be derived from the brightness of the images. Values of the auroral energy flux and of the characteristic energy will be available to the scientific community.
The session concluded with two papers from Sussex with the common theme of reconnection. Richard Rijnbeek presented, on behalf of Gareth Lawrence, a paper on reconstructing flux transfer events (FTEs) from remote data. The theory of flow around a structure gives the perturbation at a distant encounter. Also included is a model for reconnection based on the Petschek theory to give perturbations seen by a spacecraft. From observed perturbations, the FTE can then be reconstructed. Then Maria Buchan described a technique involving observations of the aurora, a visual manifestation of reconnection, which allows indirect measurements to be made of the reconnection rate. Again using the Petschek model, she also showed that details of the diffusion region, in which reconnection is initiated, may be inferred from observations of auroral arcs.
After tea, the final session, chaired by Paul Gough, opened with Sandra Chapman (Warwick) posing the question: Are particles detrapped by constant By in static magnetic field reversals? In simple models, the reversing field Bx(z) varies, the linking field Bz is constant, and the cross-tail field By is zero. She had introduced a constant By and looked for the equivalent of the well-known invariant of the cross-sheet motion in the By=0 case. Numerically integrated trajectories indicated that for certain values of energy and By/Bz the invariant is destroyed and particles are detrapped, which corresponds to an increase in phase space volume available to particles that transit the sheet. The question asked by Mervyn Freeman (BAS) was: How much solar wind energy and magnetic flux is transported into the magnetotail between successive substorm onsets? Data from Halley, Antarctica had been used to identify a sequence of eight onsets, and solar wind data from the Wind spacecraft used to estimate the solar wind power input and the magnetopause reconnection rate. It turned out that of nine different formulations of the measured solar wind parameters, two input measures were relatively invariant over the inter-substorm intervals, namely the accumulated time for which the IMF Bz was negative, and the time-integrated magnetopause reconnection rate predicted from the theory of Hill.
The Sheffield/UCL/SEL thermosphere/ionosphere/plasmasphere model is a non-linear, global three-dimensional model developed to increase our understanding of the processes occurring in the upper atmosphere. Richard Balthazor (Sheffield) introduced a video presentation illustrating dynamic event simulations. The first sequences showed some effects on polar cap morphology. The second sequence showed the application of a localized enhanced electric field at conjugate auroral latitudes, resulting in families of atmospheric gravity waves and travelling ionospheric disturbances. David Nunn (Southampton) then presented numerical simulations of VLF chorus. High quality VLF data has come from the Geotail satellite, with precise measurements of field amplitudes, propagation direction, electron distribution and ambient plasma density and magnetic field. Specific events, using data direct from the satellite, had been simulated using a one-dimensional Vlasov code; falling tones and rising frequency chorus had been reproduced very closely. The simulations provided strong confirmation that nonlinear trapping is the root cause of triggered VLF emissions and chorus.
There followed three contributions from Leicester. Brian Shand discussed interhemispheric contrasts in the ionospheric convection response (observed by the PACE radar system) to changes in the interplanetary magnetic field. Continuing with the topic of ionospheric convection, John Taylor gave results from CUTLASS-Finland radar data. During a 2 h interval, the IMF was variable but mainly contained a southward component. Analysis of the data showed bursts of high speed antisunward flow appearing in patches which moved poleward at latitudes greater than 70° and equatorward at lower latitudes. These features were interpreted in terms of a change in curvature of ionospheric flow following a pulse of reconnection. More CUTLASS observations were presented by Paul Eglitis, this time of artificial field-aligned irregularities generated by the RF heater facility at Ramfjordmoen, Tromsø. The phase speed of these irregularities was found to be equal to the component of the plasma drift velocity (measured independently by the EISCAT radar) in the direction of propagation of the plasma wave, thus demonstrating a way of detecting small drift velocities. It also appeared that the heater can modify the bulk plasma drift velocity.
Planetary waves have long been recognized as providing a coupling between different levels of the neutral atmosphere, and recently their influence has been shown to extend to F-region heights. In the last paper of the day, Nick Mitchell (Aberystwyth) discussed observations of horizontal lower-thermospheric winds made at 53.4°N using a meteor radar located at Sheffield, which revealed the presence of planetary waves. Analysis showed that, apart from the well-known two-day wave, the motion field was dominated by irregularly occurring bursts of activity with periods in the range 5-10 days. The meeting then ended with thanks to speakers, chairmen, other contributors, and the organizer, Andy Smith.