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by R V Lewis and A S Rodger (British Antarctic Survey)
Published in Q. J. R. astr. Soc. (1996) 37, 307-313
The autumn MIST meeting was held on 24 November 1995 at the Geological Society, Burlington House. This was the first meeting organised by Dr Andy J Smith (British Antarctic Survey) who had recently taken over as MIST coordinator. Dr Smith paid warm tribute to Dr Mark Lester (University of Leicester) who had acted very effectively in that role for the preceding 6 years. The meeting showed their appreciation for Dr Lester's untiring efforts with resounding applause.
The first session, chaired by Dr Mark Lester, began with a presentation by J E Insley, V Moore (Imperial College) and R A Harrison (Rutherford Appleton Laboratory). Detailed analysis of coronal holes identified in magnesium, X ray and XUV images of the sun taken by the NRL XUV spectroheliograph flown on Skylab in the early 1970s were presented. In general, the analysis showed that there was excellent agreement between images at various wavelengths but careful choice of threshold was important. This work was essential to underpin the anticipated data from the Coronal Diagnostic Spectrometer (CDS) to be launched on SOHO in the next week. The CDS had much greater spatial, temporal and wavelength resolution than the Skylab instrument, and thus would likely provide considerable new insight into the structure, dynamics, cause and evolution of coronal holes, and could establish how these regions could be the source of the solar wind.
The next paper by S W H Cowley, A Balogh, M K Dougherty, M W Dunlop, T M Edwards, R J Forsyth, R J Hynds, N F Laxton and K Staines (Imperial College) discussed plasma flows in the dayside Jovian magnetosphere. These are poorly described and understood despite four fly-bys by Pioneer and Voyager spacecraft. New analysis of the anisotropy of the 1-2 MeV ion fluxes has allowed Cowley and co-workers to demonstrate that in the inner magnetosphere (10-15 Rj) the plasma corotates with the planet to a high degree of accuracy. Between ~20-45 Rj when the Jovian magnetosphere is compressed, and up to ~70 Rj when it is expanded, the azimuthal flow diminishes to values ~150-250 km s^-1. In the outer magnetosphere, plasma flow speeds in the dawn and dusk flanks are directed anti sunward, but in the pre-noon sector the plasma motion depends upon the state of the magnetosphere. These new results were compared with expectations based upon the transmission of angular momentum between the magnetosphere and ionosphere.
Solar wind velocity measurements made using the EISCAT radar between 1990-95 were presented by P J Moran, A R Breen, P J S Williams, B Tegid (University College of Wales, Aberystwyth), W A Coles, M Klingesmith and R Grall (UCLA, San Diego) and J Markkanen (EISCAT Scientific Association). They showed that the solar wind comprises two streams, one with a mean speed of ~750 km s^-1, and the other, with a mean speed of about 350 km s^-1. The observations also suggest that the solar wind must be accelerated to its full speed within 11 Rs and 30 Rs for the fast and slow speed stream respectively. Diurnal observations of the scintillation activity allowed the authors to demonstrate that the solar wind at 30-50 Rs was flowing radial away from the sun to a high degree of accuracy.
An improved technique for the analysis of interplanetary scintillation (IPS) measurements recorded by EISCAT was presented by A R Breen, P J Moran, P J S Williams (Aberystwyth), W A Coles, M Klinglesmith, R Grall (San Diego) and J Markannen (EISCAT). By combining the EISCAT data with white light coronagraph images, the IPS line of sight measurements are projected back to the solar corona. For occasions when the IPS data project back into a northern polar hole, the EISCAT data are interpreted as a `clean' fast stream, whereas when the IPS data map to the equatorial region, the EISCAT data are best interpreted as a slow speed solar wind stream. These data have been found to be consistent with recent ULYSSES observations of fast and slow streams. Examples from the revised analysis method also showed possible measurements of fast/slow interaction regions. The ensuing turbulence produces a very strong scintillation which previously might have been interpreted erroneously as a slow stream.
The relationship between the substorm onset and the pre-onset bay as observed by the imaging riometer operating at 38.2 MHz at Kilpisjärvi, Finland was described by J K Hargreaves (Lancaster). He showed that spiked absorption events lasting no more than 2 minutes were preceded by weak bay events which drift slowly equatorward at a few 100 m s^-1. The spike events are extended in the east-west direction. The absorption associated with the bay-like features can wax and wane, but the spatial structure can be remarkably constant. Interaction between the two features often occurs equatorward of the field of view, although in one example when the interaction did occur within the field, a fading of the pre-onset arc was observed prior to onset. In another example, an absorption feature was observed to propagate from the east.
The authors of a discontinuity analyser for CLUSTER were M W Dunlop, T I Woodward (Imperial College, London), R Elphic (Los Alamos) and K-H Glassmeier (Braunschweig) and the paper was presented by T I Woodward. The forthcoming ESA CLUSTER multi-spacecraft mission comprises four spacecraft. In one formation the spacecraft are to be situated at the four corners of a tetrahedron, which will enable the investigation of shocks and discontinuities in space plasmas with considerably more information than in previous missions. Simulations showed how such an arrangement of the spacecraft would observe both planar and curved discontinuities and highlighted several subtleties arising from the competing influences of the physical properties reflected in the data, such as curvature and acceleration.
R Hide provided a simple electrical equivalent circuit of the Earth's dynamo. It comprised an inductance, resistance and a motor, which can be considered to have the properties of a capacitance. He demonstrated that by varying both the values for the components and the starting conditions, the output of the system was highly variable ranging from DC, through perfect sine waves to a chaotic response. He drew a close parallel with the Earth's dynamo which at times is relatively stable for 50 M years, and yet through other epochs the field changes rapidly (~1 M year). However to achieve the range of variations seen in the historical record of the geomagnetic field, he emphasised that it was essential to include mechanical friction of the dynamo, a factor which is often ignored.
In order to understand waves in the vicinity of the Earth's bow shock, X Blanco-Cano and S J Schwartz (Queen Mary and Westfield College) have investigated the fluctuations of the particle population as well as considering the specific wave parameters derived using a kinetic, rather than a fluid model approach. Using data from the AMPTE spacecraft, the five parameters that they selected to describe the waves were the observed Alfvén ratio, parallel compressibility, cross-helicity, magnetic compression and the polarisation. This approach has allow them to identify separate regions where waves are Alfvénic from those where they are magnetosonic.
T K Yeoman and M Lester (Leicester) presented a preliminary account of the ionospheric convection observed by the CUTLASS HF radar during an interval of considerable substorm activity as ascertained by Pi2 wave activity and magnetic bays recorded by the SAMNET and IMAGE magnetometer arrays. CUTLASS was operating in a mode where the radar samples beam 9 every 14 seconds and produces full 16 beam scan information every 4 minutes. The observations characterized the ionospheric flow before, during and after a series of substorm onsets. Although some backscatter was lost at substorm onset, strong flows towards the radar were observed during the expansion phase. Prior to the onsets, the radar observed the return flow in the dusk convection cell, but after onset the flows were consistent with return flow in the dawn convection cell. These observations suggested that the substorm activity occurred within the vicinity of the Harang discontinuity.
In the final talk before lunch, A S Rodger and M Pinnock (British Antarctic Survey) utilised 10-s resolution measurements from the PACE HF radar at Halley, Antarctica to explore the evolution of the ionospheric response during the first few minutes after enhanced reconnection occurs at the magnetopause. They showed that the plasma velocity increases associated with flux transfer events (FTEs) occur first ~100-200 km equatorward of the region to which magnetosheath (cusp) precipitation maps in the ionosphere. They suggested that these velocity variations occur first near the ionospheric footprint of the boundary between open and closed magnetic field lines, and almost immediately after enhanced reconnection at the magnetopause starts. They proposed a hypothesis to explain these velocity variations involving the very rapid outflow of magnetospheric electrons into the magnetosheath along the most recently reconnected field lines.
Dr Michèle Dougherty (Imperial College) took the chair for the first session of the afternoon, which opened with a presentation by M Grande, C H Perry, M K Carter (Rutherford Appleton Laboratory), F Soraas, J Stadsness (Bergen), K Maseide (Oslo), M Smith (NASA), M J D Weber and T A Fritz (Boston) describing an investigation of pulsating aurora with the PULSAUR II rocket. The rocket was launched into the recovery phase of a large substorm on 9 February 1994. During the ascent, the rocket penetrated a structured part of the aurora, observing characteristic inverted-V kV electrons, but no pulsations. However, on the descent, pulsating aurora was penetrated. Strong pulsations of 6 s period were seen in the electron precipitation over a broad range of the higher energies, which could be directly correlated to bright pulses in bremsstrahlung X-rays and optical emissions imaged by the rocket. The wave measurements did not correlate well with the pulsating electron observations.
G O L Jones and R V Lewis (British Antarctic Survey) continued with the theme of active aurorae, with a talk based on ground-based data from Antarctica. Recent additions to the suite of instrumentation operated by BAS include an all sky camera (ASC) at Halley, and Automated Geophysical Observatories (AGOs) further polewards. Active auroral features, such as surges and arcs observed by the ASC, are confirmed by their signatures on magnetometers and riometers both at Halley and on the AGOs. The separation of the observing stations allowed some spatial as well as temporal development of the events to be obtained and in addition, for some of the events, further information on the spatial dynamics was provided by the Halley dynasonde.
M P Gough (University of Sussex), D A Hardy (Phillips Laboratory, USA) and H G James (Communications Research Center, Canada) described the first results from the rocket experiment Oedipus-C, which had been successfully launched only 18 days before the meeting. The rocket comprised of a father-son pair tethered together 1 km apart for the initial half of the flight before being separated near apogee. Both satellites were similarly instrumented with wave and particle diagnostics, but one satellite had a transmitter whilst the other a receiver operating in the frequency range 25 kHz to 8 MHz. There were many interesting features in the data such as that the wave amplitude appears to have reduced dramatically when the tether was cut. The experiment promised to provide a wealth of new knowledge on wave-particle interaction, wave generation and particle acceleration processes.
C N Mitchell, J A T Heaton, S E Pryse, L Kersley (University College of Wales, Aberystwyth) and G O L Jones (British Antarctic Survey) presented examples of tomographic images from three very different geomagnetic areas, and under widely different experimental geometries. They showed one example during a geomagnetically active period in April 1993 when the development and structure of the mid-latitude trough could be described in considerable detail. The inversion techniques were verified with ionosonde data from Lannion and Slough. Data from a different configuration of tomographic receivers has been used to demonstrate how auroral sporadic-E could be accurately imaged. This was an important development as on some occasions the sporadic-E could contribute a significant proportion of the total electron content. The final examples were taken from Antarctica from two stations separated by 10° in latitude and over 30° in longitude. The tomographic reconstructions were excellent despite the rather poor configuration of the stations, but the peak electron concentration was slightly over-estimated. The authors suggested that a better agreement could be achieved if a more realistic treatment of the E region were included in the reconstructions.
The F-region vertical E x B ion drift generates the plasma fountain that gives rise to the equatorial anomaly. Using the Sheffield University Plasmasphere Ionosphere Model (SUPIM) to investigate this phenomenon, B Jenkins, G J Bailey (Sheffield), N Balan (ISAS) and M A Abdu (INPE) showed that, under certain conditions, an additional layer, termed by them the F3 layer, develops initially at around 700 km altitude, i.e. significantly above the F2 layer peak concentration. This layer starts to form in the morning when the drift drives plasma upwards. At the same time, plasma outside the plasma fountain converges in the topside, a process which increases in the morning and gives rise to the F3 layer. It was shown that the F3 layer only forms within ±10° of the equator and is of greater magnitude than the F2 layer for a short time around local noon. Its production is also highly sensitive to the wind model used. Support for the formation of the F3 layer was provided by the ionogram records from Fortaleza, Brazil.
The first comparisons of the line-of-sight velocity measurements from the HF CUTLASS radar experiment and the EISCAT incoherent scatter radar were presented by J A Davies, M Lester, S E Milan, E C Thomas and T B Jones (University of Leicester). There were two intervals studied in detail from equinox and early summer 1995. If the data were sampled at similar temporal resolution, the observed velocities were remarkably similar for most occasions especially considering the very much larger volume sampled by the HF radar and the different integration times of the two instruments. On the few occasions when there was some disagreement, it was likely to have been caused by a mis-identification of ground-scatter in the HF radar data.
In the paper before afternoon tea, the ion-neutral collision frequency had been calculated by K S C Freeman and R C Cheater (Rutherford Appleton Laboratory) to determine the relative contributions of the two components, polarisation and change exchange. They used formulae for the collision frequency published in the open literature, combined with a neutral atmosphere model, MSIS. The approach assumed that the neutral and ion temperatures were the same. They showed that under most circumstances above ~140 km altitude, charge exchange dominated the collision frequency. However for O² below 140 km, polarisation could be an important contribution to the ion-neutral collision frequency.
The final session, with Dr Richard Horne (British Antarctic Survey) in the chair, opened with T R Robinson and J A Davies (University of Leicester) asking how systematic is auroral E-region electron heating? Large electric fields drive F-region ion drifts and E-region electron drifts which lead to F-region ion frictional heating and E-region electron turbulent heating respectively. A statistical analysis of 770 hours of field-aligned EISCAT UHF data has demonstrated the variation of the ion temperature and the electron temperature with the electric field. The ion temperature increases with the electric field, but the relative enhancement is less near 300 km than at altitudes around 150 km. The electron temperature also increases with the electric field, but requires a threshold of 15 mV m^-1 before heating occurs. Also the electron heating at 117 km is much less than at 110 km for the same level of applied electric field.
The Sheffield/UCL/SEL coupled thermosphere/ionosphere/plasmasphere (CTIP) model has been used by R L Balthazor and R J Moffett to study the effects of imposed short-duration local enhancements to the electric field at midnight in the auroral zone during winter solstice conditions at moderate solar activity. An electric field increase of 50 mV m^-1 was applied for 19 minutes caused both thermal expansion and the launching of atmospheric gravity waves both poleward and equatorward from the heated volume, but with little AGW activity generated zonally. At E-region altitudes the atmospheric responses were much smaller owing to the higher collision frequency compared with F-region altitudes.
C G Mouikis and S C Chapman (University of Warwick) have used two approaches to study the effects of dipolarisation of the geomagnetic tail on the energetic particle distribution. They used a self-consistent 1D hybrid particle simulation code and ran it in two limits, the ion kinetic limit where the ion moments and the electromagnetic fields are well resolved on a scale of the ion gyroradius, and the MHD limits where the ion moment and the electromagnetic field are considered on a larger scale than the kinetic simulation. It is found that the magnetic field, the ion pressure tensor and the energisation of the ions are markedly different in the two cases.
W Brown and S C Chapman (University of Warwick) have compared the output from the kinetic limit of the particle simulation models presented by Mouikis and Chapman with data from the Low Energy Plasma Analyser on the Combined Radiation and Release Effects Satellite (CRRES) when the satellite was near the equatorial plane at L=6 in February 1991. In particular, the plasma velocity and local AlfvŠn speed agree well with the theoretical calculations for two intervals. Even better agreement may be achieved if the data were transformed from the GSM co-ordinate system into a more appropriate system using minimum variance techniques. A more extensive statistical study of the CRRES data is now being undertaken.
Studies of the magnetic field wake of a conducting body within a moving plasma regime has been studied by P Thompson, D J Southwood and S Goodman (Imperial College). The analysis extends previous work, carried out using an MHD approach, by considering higher frequency variations in the range between the ion and electron gyro-frequencies. The results show the presence of a wake swept back in the flow, the front of which is a straight line determined by the group velocity of the waves. However the characteristic directions of constant phase of the field variables are found to lie along a series of parabolas rather than in a straight line, as given by the MHD approach. The results have relevance in understanding the electro-dynamic coupling between a conducting satellite and multi-ion magnetospheres such as Titan, Triton, and the asteroid Gaspra.
P J S Williams, N J Mitchell, T S Virdi and G Beard (University College of Wales, Aberystwyth) presented observations by the EISCAT VHF radar of atmospheric gravity waves near the mesopause. Earlier work had shown that the amplitude of gravity waves diminish at the mesopause (85 km), and that the same feature was apparent in recent observations. An example was shown where the amplitude of the wave increased with height, saturated and was then cut off sharply at 86 km. It was suggested that the sharp minimum in temperature at the mesopause could be responsible for changing the propagation of the gravity wave, and that gravity waves are also an essential component of the generation of PMSEs. In general, EISCAT data show a whole zoo of wave activity with periods between 20 and 40 min, as well as evidence for tidal activity and planetary waves, all of which shall be the subject of future experimental and modelling studies.
A data base of winds from 90-95 km altitude determined using the meteor radar system deployed near Sheffield between 1988-1994 has been exploited by N Mitchell, P J S Williams G Buesnel, G Beard (University College of Wales, Aberystwyth) and H Müller (University of Sheffield) to investigate the variability of the tides. The experimental technique integrates the wind over a relative large volume and thus is ideal for studying large scale features, but not for resolving smaller features such as gravity waves. Examples were shown when the zonal wind component was dominated by the effects of the semi-diurnal tide e.g. September 1991. On other occasions (e.g. July 1993) the 2-day wave and the ter-diurnal component of the tide were the dominant modes. Some examples were presented where the observed wave activity could be explained by a beating between the 2-day wave and the semi-diurnal tide, but on other occasions the explanation was less simple, suggesting that the wave-wave interaction action region may be in the lower mesosphere or stratosphere.
The CUTLASS HF radar system recently deployed in Finland was proving to be an excellent system for studying large scale gravity waves. Professor T B Jones (University of Leicester) showed how the amplitude of the backscattered signals, the skip distance, the elevation and azimuth of the received signal and the line-of-sight Doppler shift can all be used as complementary diagnostics signatures of the structure and motion of gravity waves. He illustrated, both with a simple model and with some recent HF radar data, how the time variation of the skip distance and the backscatter power changed as a function of radio wave propagation angle with respect to the wave propagation direction.
In the final paper of the day, M A Balikhin, V. Kjrasnosel'skikh and L J C Wooliscroft (University of Sheffield) discussed the dynamics of charged particles in the presence of electromagnetic field gradients. The authors had used an Kolmogorov-Sinay entropy approach to investigate particle motion in quasi-perpendicular and quasi-parallel shocks. The presence of small electromagnetic field gradients leads to the violation of the drift approximation, but the adiabatic approximation can still be used by replacing the magnetic moment by another invariant. This approach can therefore be successfully used in the Earth's bow shock provided that the thermal velocity considerably exceeds the bulk velocity of the plasma.