[Report published in Astronomy & Geophysics 40, 1.29-1.30 (February 1999)]
by N F Arnold (University of Leicester)
Published in Astronomy & Geophysics 40, 1.29-1.30 (February 1999)
The annual one-day meeting of the MIST (Magnetosphere, Ionosphere and Solar-Terrestrial) community, held on 27th November at the Geological Society, Burlington House, continued to thrive with 31 presentations over a wide range of topics. The morning session was chaired by Francis Sedgemore-Schulthess (Southampton). Panagiota Petkaki (Imperial College) started the proceedings with a presentation on how the magnetometer on the Ulysses spacecraft measured ion-cyclotron waves in the middle magnetosphere current sheet during a number of magnetodisk crossings. The ion gyrofrequencies were examined at one-second time resolution to provide a diagnostic of the species present.
A new three-dimensional model of the Martian thermosphere and ionosphere has developed by Bevis Peters (University College London). In addition to solving the time dependent momentum, energy and continuity equations, a comprehensive radiation scheme was included. The model simulations were in reasonably good agreement with observations from the Viking and Pathfinder spacecraft and further improvements are envisaged when the effects of ion chemistry will be considered.
The next three talks returned to the results from the Ulysses spacecraft. Geraint Jones (IC) examined evidence for Planar Magnetic Structures in the solar wind in the low latitude heliosphere. These structures occur when the magnetic field vector is confined to a plane for periods of several hours or more. Suggested formation mechanisms included solar re-entrant loops and draping and/or compression of existing features of the ambient wind in the coronal mass ejection (CME) sheath. Observations of large, energetic particle events from the Anisotropy Telescope on board the spacecraft were described by Silvia Dalla (IC). At low heliographic latitudes, around 5 AU from the sun, both sunward and anti-sunward flows were observed as well as the bidirectional motions typical of closed magnetic field structures. She then proceeded to discuss the implications for the large-scale structure of the heliosphere. Markus Fraenz (Queen Mary and Westfield College) concluded the Ulysses talks with a study of large magnetic depressions in the solar wind similar to those observed in the Earth's bow shock. It appears that these depressions are a ubiquitous feature of the solar wind. He suggested that they may be related to solar wind magnetic holes and discontinuities and explored some of the possible formation mechanisms.
The focus then shifted to the complexities that arise as the plasma from the solar wind propagates towards the Earth. Firstly, Patrick Chaizy (Rutherford Appleton Laboratory) discussed the effects of cross field gradients in the profiles of solar energetic particle events. Using three years of satellite data, he indicated that spatial variations between WIND and GEOTAIL spacecraft observations were important in around 30% of cases over a three year period. He then compared these results with a model where there is a variation in the source intensity at the sun. Hina Khan (Leicester) then proceeded to make use of IMP-8 spacecraft and ground-based EISCAT incoherent scatter radar data to study the response time delay of the ionosphere following an Interplanetary Magnetic Field polarity reversal. During the day, the time difference was only 2-4 minutes but increased to around 10 minutes at midnight. From these timings it was possible to estimate the velocity of the convection propagation of the flow from a typical cusp latitude of around 7km/s.
Joachim Schmidt (IC) presented a magnetohydrodynamic simulation of interplanetary magnetic clouds in a sheared solar wind flow. He found good agreement with Ulysses data at solar minimum provided magnetic tension forces were present. With no magnetic tension forces present, the CME is torn apart. Thus, these clouds are more stable in inhomogeneous media than previously anticipated. Huiyu Tao (Lancaster) compared a model of the absorption of High Frequency electromagnetic radiation that used inputs from EISCAT measurements with the IRIS riometer observations of a Coronal Mass Ejection in January 1997.
The direction of the meeting took an unexpected turn when Sandra Chapman (Warwick) suggested that the behaviour of the magnetosphere had a lot in common with a pile of sand. The coupled solar wind-magnetosphere system exhibits a broken power law dependence over certain regimes in a number of geomagnetic indices. To complicate this picture, geomagnetic substorms occur with well-defined probabilities for their intensity and duration. One interpretation of these observations suggests a scale-free "self-organised criticality" that can be modelled by an avalanche model. The one-dimensional 'sand-pile' model is a simple example of this. Mervyn Freeman (Rutherford Appleton Laboratory) developed this theme by generating power spectra from time series of geomagnetic indices. The magnetosphere acts as a low-pass filter for periods below 30 minutes. The power spectrum of the substorm 'unloading' of the magnetosphere may be interpreted being derived from a time series with similarly shaped pulses with variable amplitude, duration and recurrence intervals.
The final talk of the morning was presented by Rosalind Mist (QMW). She examined an unusual GEOTAIL interval, where the spacecraft remained in the central plasma sheet for a prolonged period deep in the tail of the magnetosphere. The magnetic field and plasma motions indicated that the sheet was formed by reconnection processes. An analysis of the ion distributions made it possible to quantify the degree of heating undergone by the inflowing plasma.
The first session of the afternoon was chaired by Tim Yeoman (Leicester) with Maximos Tsalas (Warwick) the first speaker. He presented an adiabatic model of the motion of a charged particle in a magnetic reversal, analogous to a simplified quiet-time geotail. Small changes in the dawn-dusk component of the geomagnetic field can dramatically change the velocity phase space topology, such that transient phenomena can become periodic or chaotic and vice versa. Thus the cross-tail current becomes a rather sensitive region of the magnetosphere. Several papers then made use of observations from the short-lived Equator-S satellite mission. Elizabeth Lucek (IC) found examples of mirror mode magnetosheath waves in the dawn sector Equator-S data. Analysis of a large number of these waves indicated that they occurred when the field orientation was consistent with draping around the magnetopause. No intervals of electromagnetic ion cyclotron waves were observed, nor was there any evidence for a plasma boundary layer. Malcolm Dunlop (IC) continued this study with an examination of a large number of Equator-S crossings of the magnetopause. Most events showed a well ordered tangential magnetosheath field with a significant number of mirror-like signatures. However, a few events exhibited unusually high pressure states and a high number of multiple crossings per event.
Two speakers from the University of Wales at Aberystwyth made use of the tomographic reconstruction technique to obtain electron density distributions from satellite/ground total electron content measurements. Simon Berry investigated ionospheric signatures of magnetospheric boundaries in the post-noon sector using electron densities deduced by tomography and particle fluxes from a DMSP satellite. Discrete electron density structures linked to the boundary plasma sheet precipitation were clearly identifiable. Adam Smith (Aberystwyth) carried out a multi-instrument study of the cusp/cleft ionosphere under steady northward IMF conditions to observe lobe reconnection. It was then possible to identify the adiaroic boundary, regions of particle precipitation and the convection flow in the polar cap.
In the first of a group of five talks from Leicester, Terry Robinson, presented evidence from SOUSY radar observations that field aligned currents can generate plasma irregularities in the lower ionosphere that propagate parallel to the geomagnetic field. A new instability that depends on electron inertial effects was proposed to explain these observations. Jonny Rae showed evidence for low energy helium ion pulsations in the cusp using the Polar spacecraft with a period of around nine minutes. The populations on closed field lines were dominated by dispersed high energy protons. He then proceeded to discuss these observations in the context of large-scale changes in the interplanetary magnetic field.
The Scandinavian CUTLASS HF radars figured prominently in the next three talks. Mark Lester discussed a night-time interval of coincident northward IMF and auroral activity using CUTLASS and a meridian scanning photometer. He indicated that it may be possible to identify the locations of the boundary between the central plasma sheet and the boundary plasma sheet under certain conditions. High time resolution CUTLASS and magnetometer data was employed by Jim Wild to investigate the properties of auroral omega bands. It was possible to deduce the ionospheric current signatures and their conductivities as they propagated eastward over the field of view of the Finland radar.
Refreshed by a welcome tea break, Nick Watkins (British Antarctic Survey) took the chair to allow Tim Yeoman to take his turn on the floor. He examined the extent of conjugacy, between the Northern and Southern hemispheres, in the response of the ionosphere to a substorm. Coincident HF radars, magnetometers and satellite observations formed the basis for this study. The agreement was good in the growth phase during the due to the similarities in the substorm electrojets, but was less pronounced during the later expansion phase that followed. Anasuya Aruliah (UCL) compared the relative strength of the high latitude neutral wind dynamo with the magnetospheric dynamo using a numerical model and Fabry-Perot interferometer data. During the recovery phase of the ionosphere to a weak substorm, the neutral winds decay more slowly than the ionosphere and these winds may generate a significant contribution to the electric field.
Gary Abel (Mullard Space Science Laboratory) used the low energy plasma analyser on the CRRES satellite to demonstrate that field aligned electrons may be an integral part of the substorm process. Earlier studies from the AMPTE satellite suffered from a restricted sampling capability that seriously underestimated the probability of detecting these electrons. Dave Neudegg (Leicester) outlined work on propagation in an ionospheric waveguide of plasma waves originating in the magnetosphere. The results of the first high latitude waveguide observations taken in Antarctica by the Antarctic Division and University of Newcastle (Australia) were compared with existing theory and mid-latitude results.
Taking advantage of a late withdrawal from the original programme, the new Particle Physics and Astronomy Research Council strategy for astronomical computing was discussed by Chris Thomas (Leicester). The MIST community falls squarely within its remit so everyone was encouraged to take an active interest in the proposals and their likely impact on future research activities. Changes are being planned to improve the continuity of provision, especially for computer management/administration as well as for software and hardware. Normal scientific proceedings were resumed by Olivier Moullard (QMW). He examined WIND spacecraft observations of low frequency electrostatic waves during solar type III radio emission events. The resultant Doppler shifted ion-acoustic waves were examined for sensitivity to the proximity to the Earth's bow shock region. Mark Dieckmann (Warwick) modelled the non-linear ionospheric response induced by a plasma sounder. In the solar wind, the response is characteristic of electron trapping, whilst in the ionosphere wave decay instability appears to be the dominant process.
Over thirty years of ionosonde data from Port Stanley in the Southern Hemisphere and Slough in the Northern Hemisphere were examined by Henry Rishbeth (Southampton). He discussed the anomalous behaviour of the annual and semi-annual variations in the heights of the respective ionospheric F2-layers. Some progress using a computational model was made when the contribution from solar activity was taken into account but it appears that upwelling waves from the mesosphere may also be significant. Eoghan Griffin (UCL) compared the thermospheric neutral winds derived empirically from the EISCAT radar and digisondes with those obtained from a Fabry Perot Interferometer. The model-derived values tended to overestimate the magnitude of the day time winds compared with the FPI and the agreement also broke down during periods of high geomagnetic activity as the assumptions made in the model no longer applied. Phil Williams (Aberystwyth) gave the final talk of the meeting with the first thermospheric tidal observations from the EISCAT Svalbard Radar. The data suggested that planetary waves are modulating the amplitude of the semi-diurnal tide and the non-linear interaction may be responsible for a 10-hour oscillation similar to one found in the Antarctic.
The meeting concluded with thanks to all contributors, chairmen and the organiser, Andy Smith (BAS).