Jorvik MIST 2001
Abstracts

Monday 09 April

Session 1

A N Fazakerley, P J Carter, A J Coates, C J Owen, S Szita, P Travnicek, G Watson, R Wilson (MSSL/UCL) M Goldstein (GSFC) C. Gurgiolo (Bitteroot) C H Perry (RAL), S J Schwartz (QM), K Svenes (NDRE) and J D Winningham (SRI).

First results from Cluster PEACE.

The two PEACE (Plasma Electron And Current Experiment) sensors onboard each Cluster spacecraft each sample the electron velocity distributions from the full 4 pi solid angle range, and the energy range from about 1 eV to 26 keV with a time resolution of 4 seconds. Measurements can also be made with a time resolution of 2 seconds over a reduced energy range. The first few months of operations have seen Commissioning Activities in the magnetotail, the dusk flanks of the magnetosphere and the magnetosheath, followed by Science Operations as the apogee swings round towards Noon MLT. We present an overview of the multipoint PEACE observations made to date, and introduce some of our early results.

E A Lucek, T S Horbury, A Balogh, M W Dunlop, P Cargill, and the FGM team (IC).

Cluster magnetic field observations of the magnetopause and the magnetosheath.

The Cluster satellites have made the first four point measurements across the magnetopause and in the magnetosheath. During the first few months of operation the Cluster orbit moved from the dusk flank towards noon, and we present magnetic field observations from this interval on occasions when all four spacecraft entered the magnetosheath. We examine the occurrence of wave activity in the magnetosheath and its location relative to the magnetopause. In particular we look for mirror structures and electromagnetic ion cyclotron (EMIC) wave occurrence as an indication of magnetosheath structure. Four-point measurements of mirror structures allow, in principle, an estimate to be made of their scale sizes parallel and perpendicular to the field. We present the first results in these areas.

M Dunlop, A Balogh, P Cargill and the FGM team (IC).

Cluster observes the Earth's magnetopause: co-ordinated four-point magnetic field measurements.

The four-spacecraft Cluster mission has provided high-time resolution measurements of the magnetic field from closely maintained separation distances (200-600 km). Four-point coverage of the Earth's magnetopause began on the 9th and 10th November 2000 when all spacecraft first exited the dusk-side magnetosphere at about 1900 LT, providing extensive coverage of the near, flank magnetosheath and magnetopause boundary layer on re- entry to the magnetosphere. The magnetopause traversal on this occasion was caused by the arrival of an intense solar storm at the Earth, which produced a large compression of the magnetopause and high magnetic activity. A large number of subsequent magnetopause crossings, for local times covering the whole of the dusk, day-side magnetopause have now occurred and we will summarise the initial results from these. The magnetopause traversals represent an unprecedented data set allowing detailed analysis of the local magnetic structure (gradients) and dynamics of the magnetopause boundary. The orientation and motion of the magnetic discontinuity at the boundary is investigated in the light of the spacecraft configuration at each crossing. The detailed structure of the magnetopause boundary layer, revealed by the multi-point data, is also investigated.

C J Owen, A N Fazakerley, P J Carter, A J Coates, I C Krauklis, S Szita, M G G T Taylor, P Travnicek, G Watson, R J Wilson (UCL/MSSL).

Cluster PEACE observations of electrons at the dayside magnetopause.

During the first quarter of 2001 the apogees of the CLUSTER spacecraft quartet will precess through midday local times. This provides the first opportunity for 4 spacecraft studies of the bow shock, magnetosheath and the dayside magnetopause current layer and boundary layers. In this paper, we present observations of electrons in the energy range  1 eV - 26 keV made by the Plasma Electron And Current Experiment (PEACE) at the magnetopause boundary. During this period, the spacecraft have separations in the range a few hundred to  1000 km. This scale size is ideally suited, for example, to study the 3-D structure, substructure and evolution of the magnetopause current layer and the electron boundary layers. In addition, the spacecraft separation scale is of order or less than the typical size of flux transfer events (FTEs), which are expected to be observed following a burst of reconnection on the dayside magnetopause. It is known that FTEs contain plasma from both the magnetosphere and the magnetosheath [Thomsen et al., 1987], but that the distributions are modified compared to those found in these bounding regions. With CLUSTER PEACE data, the 3-D configuration of electron populations within these FTE structures may be determined for the first time. Results from initial studies of these regions and events will be presented.

J P Eastwood, A Balogh and M W Dunlop (IC).

Observations of the Motion of a Discontinuity in the Magnetosheath with Cluster.

Despite its relatively recent launch, Cluster has already provided a wealth of new information about the Earth's magnetic environment. In this paper, magnetic field data from Cluster's passage through the magnetosheath is investigated using the discontinuity analyser technique, and a transient event is identified. This event is characterised in terms of its velocity and planarity, and illustrates the power of 4 spacecraft observations to describe the motion of the discontinuity.

M G G T Taylor, A Fazakerley, P Carter, S Szita (MSSL/UCL), P Travnicek (MSSL and IAP), G Watson, C J Owen, A J Coates, I C Krauklis and R J Wilson (MSSL/UCL).

PEACE in the Cusp.

We present an overview of the first 3 months of PEACE data, concentrating on the Northern cusp region of the Magnetosphere.

I Krauklis, A Fazakerley, C Owen, M Taylor, P Carter, G Watson and S Szita (MSSL/UCL).

Preliminary observations of the mid-altitude cusp region by the cluster spacecraft.

On the 1st October 2000 Cluster spacecraft Samba (3) and Tango (4) made an outbound crossing of the mid-altitude (4.7Re) cusp region. The IMF measured by ACE was southward at this time. Both spacecraft encountered the LLBL and cusp regions at different times. The equatorward edge of the LLBL reduced by 0.33 degrees of invariant latitude between the two spacecraft. In contrast the equatorial edge of the cusp did not vary in invariant latitude.

Session 2

T Horbury, E Lucek, A Balogh, M Dunlop and P Cargill (IC).

Cluster magnetic field measurements of the bowshock.

Cluster II has recently provided the first four spacecraft measurements of the Earth's bowshock. We will present a preliminary analysis of several bowshock crossings, both quasi-parallel and quasi-perpendicular using magnetic field data. Four spacecraft measurements enable estimates of orientations, velocities and hence spatial scales of structures within and around the bowshock, as well as correlation lengths of such structures. The results of such an analysis will be presented, and implications discussed.

I Bates, H St. C K Alleyne, K Yearby (Sheffield), M Andre (SISP).

Multi-point Study of Magnetosheath Waves.

The magnetosheath, being the coupling region between the solar wind and the magnetosphere, is important in the solar-terrestrial interaction. The magnetosphere contains turbulence, the study and full understanding of which is extremely difficult, containing non-linear phenomena in many cases far from equilibrium. However a full experimental picture can be built by analysing multi-point measurements. Recent correlative measurements, such as those from AMPTE, ISEE and Cluster satellite missions, allow the comprehensive experimental study of magnetosheath waves. Recent results of these studies are presented in this paper.

J A Wild, S W H Cowley, J A Davies, M Lester, S E Milan, G Provan, T K Yeoman (Leicester), A Balogh (IC), A. Fazakerley (MSSL/UCL) and H. Réme (CNRS).

Coordinated Cluster and ground-based observations of an outbound magnetopause crossing on 2nd Feb 2001.

On 2nd Feb 2001, data acquisition by the Cluster spacecraft commenced at approximately 08 UT whilst the spacecraft were located at an altitude of  6 RE above the northern polar region. During the 5 hours that followed the spacecraft continued on an outbound trajectory, passing through the cusp and crossing the magnetopause in the post-noon sector at approximately 15 UT. During this interval the northern hemisphere magnetic field line footprint of the Cluster quartet was located within the fields-of-view of the CUTLASS pair of HF coherent-scatter radars and in the vicinity of the EISCAT incoherent-scatter radar located on Svalbard. Ground-based observations of the ionospheric conditions during this interval are compared with in-situ observations of the near-Earth plasma environment made by the Cluster spacecraft.

I R Mann (York), E Donovan (Calgary), M Dunlop (IC), I Voronkov (Alberta), T K Yeoman (Leicester), D K Milling (York), J Wild (Leicester), A Balogh (IC), A Viljanen (FMI), H J Opgenoorth (SISP).

Global Magnetospheric Oscillations Driven by Magnetopause Kelvin-Helmholtz Instability: Co-ordinated Ground-based and Cluster II Observations During a Fast Solar Wind Speed Interval.

We present observations from the 9th and 10th December 2000 when the solar wind speed exceeded 600 km/s, and when both flanks of the magnetosphere show evidence of energy injection by the Kelvin-Helmholtz instability. On the dusk flank, between 2200UT and 0200UT, the CANOPUS magnetometers observe very large amplitude (150 nT p-p) Pc5 ULF pulsations. During the same interval, waves with similar period are detected by the Prince George and Saskatoon SuperDARN radars, as well as in CANOPUS Riometer and meridian scanning photometer data. On the dawn flank, two large amplitude Pc5 wave packets are observed between 0100UT-0200UT and 0330-0400UT in the IMAGE and SAMNET magnetometer data from the European sector, coincident with ULF pulsating backscatter power and velocities in the STARE Norway radar. During this interval, Cluster was on an outbound pass from the northern dusk side magnetospheric lobe into the magnetosheath, local-time conjugate to the CANOPUS magnetometer array. In-situ magnetic fluctuations observed by Cluster FGM show evidence of periodic magnetopause motion at the same period as the ULF waves seen on the ground. Our observations support the hypothesis that during intervals of fast solar wind speed, the K-H instability can bathe the flank magnetosphere with ULF wave power.

I J Coleman and M P Freeman (BAS).

An empirical study of magnetic field line draping in the dayside magnetosheath.

We present results of a multi-spacecraft study of the relationship between the magnetic field in the solar wind and in the magnetosheath. Specifically, we address the issue of magnetic field line draping, and determine empirically the relationship between the interplanetary magnetic field incident on the magnetosheath, and the resulting field near the magnetopause.

B M A Cooling (QM) C J Owen (MSSL) S J Schwartz (QM).

Open flux tube motion at the magnetopause.

We have developed a simple 3D model which predicts the mtoin of open flux tubes along the magnetopause for a given IMF of arbitrary orientation which may be used as an aid to interpretation of data. Our model may also be used to identify likely reconnection sites on the dayside and near-Earth nightside magnetopause and to identify possible locations for steady-state reconnection. The model highlights the importance of sub-Alfvenic magnetosheath flow at the magnetopause in allowing steady-state reconnection.

N W Watkins and M P Freeman(BAS).

How do we know the magnetosphere is a non-equilibrium system: and why does it matter? Chang [1992,1999] and Consolini [1997] have drawn our particular attention to the importance of the non-equilibrium nature of the Earth's magnetosphere. Following Nicolis and Prigogine [1989] we first use a simple example - Benard convection - to illustrate the qualitative difference between non-equilibrium and equilibrium systems. We then show how several well established structural features of the magnetosphere can only be accounted for by its out-of-equilibrium nature. This motivates the search for other features, in particular long-range spatial and temporal correlation. We discuss the evidence for such long range correlation, and the constraints this places on the classes of model which can be used to model the magnetosphere, in particular sandpiles and shell models of turbulence.

M P Freeman, D J Riley and N W Watkins (BAS).

The effect of the driver on the scaling properties of energy dissipation in a turbulent system.

Scale-free distributions have been observed for the size, duration and waiting times of energy dissipation events in the sun, solar wind and magnetosphere. These could be modelled by fully-developed turbulence, amongst other possibilities. However, the magnetosphere is driven by the solar wind and the solar wind is driven in turn by the sun. So, is the scaling we see in the magnetosphere simply due to the scaling we see in the solar wind driver or is it due to the nonlinear cross-scale coupling within the magnetosphere? We address this question by examining whether the scaling properties of energy dissipation in a 1-D shell model of fully- developed turbulence are influenced by the scaling properties of the driver. We find that a periodic driver does not alter the scale-free distributions of fully-developed turbulence and that the dominant time scale of the driver is absent in the output. Thus if we were to observe the magnetospheric energy dissipation during an interval of periodic solar wind driving then we might be able distinguish whether the scaling properties of the magnetosphere were due to those of the solar wind or intrinsic to the magnetosphere.

Tuesday 10 April

Session 3

D Rees (Utah/Hovemere), Mark Conde(Alaska), Åke Steen, Urban Brändström(Kiruna) and Nikita Shumilov(Tromso/Hovemere).

Daytime ground-based optical imaging of the aurora.

It has proved possible recently to image passive airglow emissions from the upper atmosphere during full daytime by using, for example, efficient narrow-band optical filters based on Fabry-Perot etalons. Rees et al., (2000) were able to obtain the first true visible-light image of the aurora in the light of OI at l630-nm wavelength during daytime from ground level during a campaign at the Swedish Inst. For Space Physics (IRF), Kiruna, Sweden, in spring 1999 using a prototype `Daytime Auroral Imager'. In this, and in subsequent experimental programmes at Kiruna, an imaging spectrometer, based on a double-etalon Fabry-Perot interferometer, featuring excellent spectral resolution and out-of-band rejection, has been used. Such passive measurements, however, are extraordinarily difficult by day, owing to the overwhelming background of sunlight scattered by the lower atmosphere. The new technique holds great promise for future ground-based aeronomy studies. Certain auroral plasma processes are unique to the sunlit ionosphere. These auroral plasma processes, in addition to the morphology of the summer time and daytime aurora, can now be explored in detail. There are also considerable possibilities that the techniques may be further exploited at equatorial and low latitudes as a method of obtaining real two-dimensional images of structures of the daytime F-region ionosphere, since a major component of the OI l630-nm emission is due to dissociative recombination, and is thus related to NmF2.

S C Robertson(Soton), I Furniss (UCL), B S Lanchester(Soton), I McWhirter(UCL), M H Rees(Soton), A D Aylward(UCL). T S Trondsen(Calgary),

An observation of shock induced proton aurora, with supporting in situ satellite data.

Observations from Longyearbyen (Norway, 78.2N, 15.8E) on the 26 November 2000 from 06:20UT, showed a proton event resulting from an interplanetary shock. A variety of instrumentation, both ground based and satellite borne, recorded this event. Hydrogen beta spectra, produced by the arriving protons, were recorded by the Southampton/UCL HiTIES platform in the magnetic zenith. Spectral analysis, with the aim of defining the proton energy distribution, are presented. The results of this analysis are placed in the context of the large scale cusp precipitation.

S R Marple (Lancaster).

Sky-map at 38.2 MHz as measured by IRIS.

A new 38.2 MHz map of the northern hemisphere sky has been produced using data recorded by Imaging Riometer for Ionospheric Studies (IRIS). A riometer (relative ionospheric opacity meter) is designed to measure ionospheric absorption by observing the deviation of the received power from the normal `quiet day' level (Quiet Day Curve). Thus the QDC is the locus around a sky-map at constant declination. IRIS is capable of measuring cosmic noise in 49 different directions simultaneously, but due to symmetry of the beam pattern, only 28 unique declinations are available for the generation of a sky-map. The sky-map generated by IRIS is unique since it is accompanied by a map of associated error values, a feature made possible only due to a large volume of data recorded by this facility.

C F del Pozo, F Honary (Lancaster) and K S C Freeman (RAL).

Auroral absorption and electric field during unstable e-region conditions.

This paper discusses the correspondence between absorption, electric field, and conductivities during substorm activity and E-region instability conditions. The study employs IRIS and EISCAT observations between 1997 and 1999 covering a number of substorm events with onsets near Tromsø. The minimum time resolution for the analysis is of the order of 30 sec-to-1 min thus greater than the recombination times in the E and D regions. Absorption, electric field and conductivity data are organized in function of the various phases of substorm activity, the precipitation level and the electric field strength. There is an overall, inverse dependence between the electric field and the auroral absorption (AA) that may be explained by the conservation of the ionospheric current across the region of enhanced conductivity. AA is basically determined by the high-energy side of the precipitation spectrum, thus only if both the ‘soft’ and ‘hard’ components of the energy spectrum are part of the same particle distribution, we may have this one-to-one correspondence between increased conductances and AA. If it were a separate component of energetic particles, absorption may increase independently of the electric field strength. The study looks in some detail at this overall statistical dependence as well as at the cases where a separated beam of energetic electrons or the ‘stretching’ of the tail of the particle distribution may occur, particularly at substorm onset. It also discusses the relative strength of absorption as a function of the electric field, for comparable levels of precipitation and during the various phases of substorm.

J K Hargreaves (Lancaster), A Ranta (Sodankyla), J D Annan and J C Hargreaves (Bidston).

Fine structure in auroral-absorption spike events.

The "spike event" is the shortest known feature of auroral radio absorption. Most examples occur at the beginning of a substorm but last for only a minute or two. It has now been discovered that there is a fine structure within the spike event, having periodicity in the range 15 - 60 s and modulating the absorption by some +/-10 flux of precipitating electrons. The fine structure has been studied by a wavelet method, and was detected within those 7 events (out of 9 studied) which moved in a poleward direction. Magnetic fluctuations in the Pi1-2 range are well known to occur at the beginning of a substorm, and in several cases the magnetic and absorption fluctuations appear to be related. It seems possible that both originate in the thin current sheet that occurs in the magnetotail at substorm onset.

R S Dabas (on leave from National Physical Laboratory, India) and L Kersley (Aberystwyth).

Adaptive modelling of the ionosphere over Europe using radio tomographic images.

Models of the ionosphere, used in applications for the prediction or correction of propagation effects on practical radio systems, are often inadequate in their representation of the structure and development of large-scale features in the electron density. Over northern Europe characterization of the main trough presents particular problems for such empirical models and hence for radio propagation forecasting and ionospheric mapping. Results are presented from a study aimed at investigating the possible role of radio tomographic imaging in adapting models to yield a better representation of the ionosphere over Europe. It is shown that use of radio tomography gives better agreement with actual ionosonde data than can be obtained from the best available model alone, indicating a possible future role for the technique in near-real time mapping of ionospheric conditions.

H Rishbeth (Soton) and I C F Mueller-Wodarg (UCL).

The January/July asymmetry in the F2-layer: tides or Buonsanto's oxygen hypothesis?

The ionospheric F2 layer contains about 20 in July, a much greater difference than can be directly attributed to the variation of Sun-Earth distance. Work in progress suggests that the asymmetry is linked to atmospheric tides. An alternative suggestion by Buonsanto, that considers the increased dissociation of molecular oxygen in January as compared to July, is discounted.

M J Harris, A L Aruliah (UCL), S C Solomon (Colorado). A D Aylward (UCL),

Modelling 0I 630nm nightside emission using the new CMAT model.

Some first results are presented for a new Coupled Middle Atmosphere and Thermosphere (CMAT) general circulation model. CMAT was able to simulate winter solstice trends in [0I] 630 nm integrated column brightness, as observed by the University College London Fabry-Perot Interferometers at Kiruna (Sweden), and Longyearbyen (Svalbard). The variation of night-time emission trends with solar cycle and geomagnetic activity is presented. The simulated 630 nm emission at these sites is mainly due to the two classical production mechanisms, the impact of energetic electrons and the dissociative recombination of 02+. The relative importance of these mechanisms is controlled by both direct particle precipitation and background electron density, which is in turn influenced by thermospheric dynamics. The UT maximum of the integrated column brightness did not necessarily correspond to the maximum in electron precipiation. A parameterisation for the night-time activation of 0(3P) to form 0(1D) due to energetic precipiatating electrons has been formulated using the GLOW auroral modal. This parameterisation is presented for possible application in other thermospheric general circulation models.

Session 4

T K Yeoman, P J Chapman, S Crooks, M Lester, K A McWilliams, S E Milan, M J Parsons, D Payne, E C Thomas, J T Thornhill, and N M Wade (Leicester).

Stereo-CUTLASS - two radars for the price of one?

Stereo-CUTLASS is a major new capability for the CUTLASS radar system. In stereo mode the radar utilises spare duty cycle in its transmitters to transmit two independent multipulse trains at different frequencies. These multipulse sequences are then processed in two separate receiver units, allowing the radar to operate two beams simultaneously. Stereo has been implemented in the Finland radar since November 2000. Latest results from Stereo will be presented, and the implications for data quality arising as a consequence of processing two simultaneous pulse sequences will be explained.

M P Freeman (BAS).

A unified model of the response of ionospheric convection to changes in the interplanetary magnetic field.

We reconcile apparently contradictory observations of the response of ionospheric convection to changes in the interplanetary magnetic field within the common framework of the expanding-contracting polar cap (ECPC) model. By deriving analytical solutions to the ECPC model for three postulated idealised cases, we show that the variety of observed convection responses can all be realised within this framework. In particular, we show that real phase motions along the polar cap boundary due to an expansion of the reconnection X-line at the magnetopause would be identified by some observing methods but not by others, thereby explaining why authors disagree on whether such phase motions actually exist.

K A McWilliams, T K Yeoman (Leicester), J B Sigwarth, L A Frank (Iowa) and M. Brittnacher (Washington).

Particle, Auroral, and Convection Responses to Magnetic Reconnection Following a Southward Turning of the Interplanetary Magnetic Field.

Following an isolated southward turning of the interplanetary magnetic field, anti-sunward ionospheric flow bursts were measured in the postnoon sector by the CUTLASS HF radar pair, the easternmost of the northern hemisphere SuperDARN radars. These are the first two-dimensional electric field measurements within the footprint of newly reconnected magnetic flux. Initially, the phase velocity of the reconnected flux was double the convection speed, and this interval was interpreted as one of prolonged reconnection during which layers of magnetic flux are successively peeled from the magnetopause. The DMSP-F7 spacecraft measured a dispersed ion signature, associated with the cusp and reconnection, in the vicinity of the radar footprint of reconnection. Space-bourne images showed that the ultraviolet aurora expands in concert with the directly measured convection in the postnoon sector and that the ultraviolet aurora agrees very well with the global scale convection pattern deduced from the SuperDARN map potential technique. Near 18 MLT the auroral oval expanded at the same speed as the convecting plasma in the ionosphere, indicating that this is a closed magnetospheric boundary with no flow across it. Estimates reveal that during this interval the cross polar cap potential difference increased by several tens of kilovolts. This can be accounted for by bursts of reconnection at the mangetopause, making flux transfer events the primary mechanism for flux transfer from the solar wind to the magnetosphere during this interval. Conjugate radar measurements reveal pulsed anti-sunward flows in the southern prenoon sector, consistent with the expected asymmetry of the convection pattern due to the upstream orientation of the y component of the interplanetary magnetic field.

G Chisham , I J Coleman, M P Freeman, M Pinnock (BAS).

The ionospheric signature of a split reconnection x-line during imf By dominated conditions: strong evidence for the anti-parallel merging hypothesis.

Using the SuperDARN HF radar network we present measurements of the convection electric field in the winter hemisphere dayside ionosphere during intervals when the IMF has a negative Bz component and a significant By component. These conditions have been predicted to give the most distinctive difference in the ionospheric projection of the magnetopause reconnection X-line between the anti-parallel merging and subsolar merging hypotheses. The radar observations show evidence for two distinct high-latitude merging regions on the dayside magnetopause that is in favour of the anti-parallel merging hypothesis and contradicts the subsolar merging hypothesis. We examine the stability and temporal correlation of the two merging regions.

A Grocott and S W H Cowley (Leicester).

Substorm-Excited Flows In The High-Latitude Ionosphere.

It is well accepted that during intervals of southward IMF, reconnection at the dayside magnetopause can drive flows in the high-latitude ionosphere. In two previous studies we have found evidence of plasma convection driven in the nightside ionosphere during intervals of northward IMF. Here, we examine SuperDARN data from November 02, 1999, in which a number of enhancements to the nightside convection pattern can be seen during an extended period of predominantly northward IMF. Within this interval, we observe the ionospheric response to an isolated substorm, and the subsequent recovery over an extended period of geomagnetic inactivity.

E E Woodfield (Leicester), K Hosokawa (Kyoto), S E Milan (Leicester), N Sato (NIPR), M Lester (Leicester).

An inter-hemispheric, statistical study of nightside spectral width distributions.

Data from the Iceland East and Syowa East SuperDARN coherent HF radars have been used in a statistical analysis of nightside spectral width distributions. The distributions from the two hemispheres show marked similarities in their dependence upon latitude, season and magnetic local time. The forms of the distributions are similar in nature to those observed in the cusp and low-latitude boundary layer. Preliminary results are given.

G A Abel and M P Freeman (BAS).

On the Temporal Character of Ionospheric Flows and Currents in the Low Latitude Boundary Layer and Cusp.

This paper presents an attempt to characterize ionospheric flows at the footpoint of the low latitude boundary layer and the cusp in terms of its power spectrum. We have studied intervals of CUTLASS Finland HF radar data previously identified by McWilliams et al. (Annales Geophysicae, 2000) as containing signatures of pulsed ionospheric flow bursts. Pulsed ionospheric flow bursts are believed to be the ionospheric signature of flux transfer events. Selecting 2-hour periods of uninterrupted line of sight velocity measurements at 100 or 120 s resolution, we have attempted to characterize the ionospheric flows by calculating the power spectrum using a Hanning- windowed Fourier transform. The average spectrum of the ionospheric flows is power law-like with a best-fit slope over its whole range of approximately -0.5. We have compared the radar velocity spectrum to that of the magnetic field measured by the Svalbard magnetometers of the IMAGE network, which lie under the radar field of view. We find all three components of the magnetic field have power law-like spectrums with slopes of approximately -1.3. The difference between the velocity and magnetic field spectra appears to come from the spatial averaging effect of the magnetometers. We find that spatial averaging of the radar velocity causes the power spectrum slope to steepen. The small value (i.e. <1) of the power law of the velocity power spectrum indicates that the ionospheric flow cannot be characterized simply as classic turbulence. As such, Fourier analysis may not be a suitable technique to apply and we are investigating further methods of characterising the flows.

T J Stubbs (IC), M Lockwood (RAL), P Cargill (IC), P Anderson (Aerospace Corp), W Denig (Phillips Lab), M Grande, B Kellett (RAL), M Lester (Leicester), I McCrea, C Perry (RAL) and T Yeoman (Leicester).

A multi-instrument study of the northern polar cusp for northward and southward IMF conditions.

We report observations of the northern polar cusp made by the EISCAT (European Incoherent Scatter) Svalbard Radar (ESR), the coherent scatter CUTLASS radars and the POLAR, WIND and ACE spacecraft and the fleet of low-altitude sun-synchronous DMSP spacecraft. We use data from the CAMMICE MICS particle instruments aboard POLAR. WIND and ACE used as an upstream solar wind monitor. We discuss two contrasting bservational periods on the 1st and 16th of April 2000. The ESR and CUTLASS observation were planned to coincide with times when POLAR would be passing through the cusp, i.e.at invariant latitudes from 65 deg to 85 deg while at magnetic local times ranging from 11:00 to 13:00. This translated to radar observing runs are centred at 09:15 UT, when Svalbard is under the cusp. The run on the 1st April 2000 is for mainly northward IMF ( 3 nT) causing the auroral oval to shrink poleward of the ESR and CUTLASS radar fields of view. The ESR observes slow flow velocities and unstructured electron densities and temperatures consistent with quiet conditions. The DMSP spacecraft observe high ion/electron densities within the polar cap, again consistent with quiet northward IMF. On the 16th April 2000, the IMF Bz component is strongly southward at  -10nT, this coincides with an expanding auroral, which is observed to move from  77 deg to  72 deg geographic latitude by the CUTLASS Finland radar. The field-aligned ESR radar observes intermittent enhancements of electron density, and DMSP observations of cusp precipitation revealed transient reconnection pulses. The data support the association of structured polar cap plasma and pulsed magnetopause reconnection.

H Khan, M Lester and J A Davies (Leicester), P E Sandholt (Oslo).

Multi-Instrument Case Study of the Dynamic Cusp.

We present high-time resolution data obtained from the EISCAT Svalbard radar taken on the 4th December 1999, in conjunction with data from the CUTLASS Finland HF radar and meridian scanning photometer at Ny Alesund. Poleward moving features observed in the Finland radar in the dawn and pre- noon sectors coincide with electron density and ion temperature enhancements in the ESR data. The optical data indicate the presence of a series of poleward-moving transients in the red line, which in the past have been associated with pulsed dayside reconnection generating new open flux. The combined observations provide a means of further identifying pulsed reconnection events using different instrumentation and illustrate how these features propagate in the plasma. The field-aligned measurements of the ESR and the scanning capabilities of the HF radar allow the features to be observed both spatially and temporally, giving a very good indication as to how these features evolve.

S K Morley (Soton) and M Lockwood (RAL).

Regarding a different origin of sawtooth cusp ion dispersions.

During substorm growth on August 22, 1998, the Defense Meteorological Satellite Program (DMSP) craft F-14 crossed the cusp just to the south of Longyearbyen, returning on the following orbit. The two passes by the DMSP F-14 satellite have similar trajectories and the open-closed field line boundary (OCB)crossings as determined from the SSJ/4 precipitating particle data and Polar UVI images imply a similarly shaped polar cap, yet the cusp ion dispersion signatures differ substantially. The cusp crossing at  0854 displays a stepped ion dispersion typical of a meridional pass, whereas the crossing at  1038 is a sawtooth form ion dispersion, considered typical of a satellite travelling longitudinally with respect to the OCB. It is shown that this change of dispersed ion signature is likely to be due to a change in the reconnection rate.

Session 5

A N Wright (St. Andrews), W Allan (NIWAR), M S Ruderman, (Sheffield), R C Elphic (LANL).

The dynamics of current carriers in standing Alfven waves: Parallel electric fields in the auroral acceleration region.

The acceleration of current carriers in an Alfven wave current system is considered. The model incorporates a dipole magnetic field geometry, and we present an analytical solution of the two-fluid equations by successive approximations. The leading solution corresponds to the familiar single- fluid toroidal oscillations. The next order describes the nonlinear dynamics of electrons responsible for carrying a few miroAmps/m² field aligned current into the ionosphere. The solution shows how most of the electron acceleration in the magnetosphere occurs within 1 RE of the ionosphere, and that a parallel electric field of the order of 1 mV/m is reponsible for energising the electrons to 1 keV. The limitations of the electron fluid approximation are considered, and a qualitative solution including electron beams and a modified $E\parallel$ is developed in accord with observations. We also find that the electron acceleration can be nonlinear, (ve|| grad||)ve|| > omega*ve||, as a result of our nonuniform equilibrium field geometry even when $ve\parallel$ is less than the Alfven speed.

L G Ozeke and I R Mann (York).

On the generation of asymmetric standing poloidal Alfvén waves by the drift-bounce resonance mechanism

The effect of asymmetric ionospheric conductivities on the properties of standing poloidal Alfven field line oscillations have been determined by numerically solving the guided poloidal wave equation. Introducing asymmetric ionospheric conductivities causes the waves electric field to become asymmetric about the equatorial plane. In particular, as the ionospheric conductivities in the northern and southern hemispheres become highly asymmetric a half wavelength poloidal wave can develop into a quarter wavelength mode. Poloidal field line oscillations may be generated by ring current ions via the drift-bounce resonance mechanism. When the ionospheric conductivities are symmetric then fundamental field aligned harmonic mode waves can only be driven by even N resonances, whilst second harmonic mode waves can only be driven by odd N resonances. However by calculating the drift-bounce motion of particles through the asymmetric electric fields of poloidal waves we show that both fundamental and second harmonic mode waves can be generated by both even and odd N resonances. Our calculations reiterate the possible importance of the drift-bounce resonance mechanism as a ring current decay process.

L J Baddeley, D M Wright, T K Yeoman (Leicester)and K J Trattner (Lockheed).

Energy sources for particle-driven ULF waves: A survey of Polar TIMAS ion distribution functions.

Recent data from the SP-UK-OUCH and DOPE experiments have revealed significant new populations of particle driven ULF waves in the morning and afternoon sectors. A case study by Wright et al. 2001, identified the particle population responsible for driving a Pg pulsation, as observed at the ground by the DOPE Sounder and IMAGE magnetometer chain. Data from the Polar CAMMICE (MICS) and TIMAS energetic particle instruments were utilised to derive the ion distribution functions (idfs) which characterised the wave energy source in the energetic ion population. Since then we have undertaken a statistical survey of all dayside idfs measured on L-shells 6 to 9 by the TIMAS instrument in 1996 and 1998, identifying the occurrence of non maxwellian distribution functions, which have positive gradients ('bump on tail' distributions). The data have been binned in MLT and compared with the ground observations of the particle driven ULF waves by the CUTLASS HF Radars and DOPE HF Sounder. In addition we will discuss the evolution of these unstable ion distributions, their possible sources and potential mechanisms by which they couple to the wave modes.

J T Mathews, I R Mann (York) and T K Yeoman (Leicester).

Co-ordinated Radar and Ground Based Magnetometer Observations of a Latitudinally Localised Pc5 ULF Wave in the Afternoon Sector.

Magnetometer and radar measurements of a field line resonance are presented, along with evidence that the driving force behind the event was internal to the magnetosphere. Both sets of instruments indicate that the resonance is a Pc5 wave on the L=6 shell, with a frequency of 3.3 mHz and an m value of approximately 23. ACE data for the time period of the event show that the solar wind is unlikely to be the driving force, and this is further supported by the fact that the event is in the dusk sector, and exhibits a westward phase (i.e. it propogates in the sunward direction). The parameters of the event are compared to the model of Ozeke and Mann (JGR, 2001), and fit well with the idea that the FLR was probably caused by an N=0 drift resonance with ring current ions.

Z C Dent, I R Mann (York), F W Menk (Newcastle), J Goldstein (Rice) and L G Ozeke(York).

A comparison of ground-based magnetometer and IMAGE satellite observations of plasma density during a geomagnetically quiet interval.

The cross-phase technique compares magnetometer data from two meridionally spaced stations to obtain the eigenfrequency of the field line whose footprint lies at their mid-point. Such eigenfrequencies can be used with a geomagnetic field line model to produce estimates of plasma density in the equatorial plane. Magnetometer data from the SAMNET, IMAGE and BGS arrays in the European sector have been used to remote-sense plasma density profiles spanning the plasmasphere, plasmapause and plasmatrough regions during a geomagnetically quiet interval. These ground-based observations are compared to those measured in-situ by the IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) satellite.

J M Rees, C R Wilford and R J Moffett (Sheffield).

Analysis of magnetometer data with the aid of wavelet transforms.

Eigenfrequencies of magnetospheric field lines can be determined by analysing the spectral difference between the signals received by two closely spaced meridional ground-based magnetometer stations. Conventionally this has been achieved using windowed Fourier transforms (WFT). However, the WFT is an inaccurate and inefficient method of time- frequency analysis due to aliasing of high- and low-frequency components that do not fall into the frequency range of the window. In addition to the geomagnetic field line resonances, unfiltered magnetometer data comprises low-frequency trend and high-frequency, localised structures. We demonstrate that both of these features can be effectively removed using a wavelet-based filter. The resulting cross-phase spectra allow easier identification of the resonant frequency.

A J Smith, G A Abel (BAS) and N P Meredith (MSSL).

Substorm chorus events: wave characteristics, source regions, and their time dependence following the expansion phase onset.

Clouds of energetic electrons injected near midnight during substorm expansion phase onset drift eastward toward dawn and generate bursts of whistler mode chorus waves which can propagate to the ground. The resulting "substorm chorus events" (SCEs) serve as a wave ground signature of substorm onsets alongside the more traditionally used ones based upon optical and magnetic field data. Although the phenomenon has been known for decades, it is only relatively recently that modern instrumentation and data processing techniques have allowed it to be used in a systematic way for studies of the substorm process. Previous MIST presentations have concentrated on using large numbers of SCE onset times at Halley, Antarctica, to investigate statistically the properties of the "typical" substorm as observed at a near auroral ground stations. In contrast, we here present the results of case studies of a relatively small number of SCEs. The detailed wave spectra, and the apparent motion of the wave source region as viewed jointly from Halley station and from the network of BAS Automatic Geophysical Observatories between Halley and the South Pole, are used to infer details of the wave-particle interactions which occur during the expansion phase of the substorm. We also report the results of a search for SCEs in satellite wave data.

C E J Watt, R B Horne and M P Freeman (BAS).

Resistivity due to ion-acoustic waves in collisionless plasmas

Resistivity can play an important role in regions of collisionless space plasmas where the ideal MHD condition breaks down. The resistivity in these regions is provided by wave-particle interactions between current-driven instabilities and the plasma particles. Electrostatic current-driven ion- acoustic waves are an example of one such instability. We present results from a one-dimensional Vlasov simulation code which models ion-acoustic waves. A comparison is made between a commonly-used analytical method which estimates the resistivity using quasilinear theory, and the values of resistivity calculated in the simulation. It is found that the resistivity calculated using the simulation code is at least four orders of magnitude higher than the analytical estimates at the same stage of instability evolution.

Session 6

S R Cash, T R Robinson, D M Wright and E L Kolesnikova (Leicester).

Using FAST satellite data to characterise the effects of the 3Hz modulated ionospheric heating experiments at Tromso.

FAST satellite data collected during an ionospheric modification experiment at Tromso shows a 3Hz wave in the perpendicular electric field and downward electron flux data. This talk will discuss the processes that occur during 3Hz modulated heating experiments that lead to the 3Hz wave detection by FAST. The phase relationships between electron flux energy channels are consistent with a simple model describing the height of the acceleration region. Natural electron acceleration events associated with the same ionospheric conditions are compared and contrasted with the characteristics of heating events.

F Honary, M J Kosch (Lancaster), M T Rietveld(MPAe) and T K Yeoman (Leicester).

Heater-induced airglow emission.

One of the most striking effects of ionospheric modification experiments is the enhancement of the 630 nm (and to a lesser extent the 557.7 nm) atomic oxygen lines of the airglow. The airglow enhancement is believed to be due to the excitation of 1D state of atomic oxygen by energetic electrons. There are two different theoretical explanations for the possible mechanisms of electron energization. One is due to the acceleration of electrons by Langmuir waves which are excited by the pump wave. Another mechanism is electron heating due to anomalous absorption of HF pump wave. Recent observations of Heater-induced airglow emission will be reported and the implications for the possible electron energization mechanism will be discussed.

N P Meredith (MSSL/UCL), R B Horne (BAS), R H A Iles (MSSL/UCL), R M Thorne (UCLA), R R Anderson (Iowa), D Heynderickx (BISAe).

Evidence for enhanced substorm activity and whistler mode chorus associated with electron acceleration to relativistic energies.

Intense interest currently exists in determining the physical mechanism(s) responsible for the acceleration of electrons to relativistic energies during/following geomagnetic storms. Enhanced storm-time convection electric fields can provide a seed population of electrons with energies of the order of a few hundred keV. Wave-particle interactions involving whistler mode chorus then provide a mechanism for accelerating this seed population to relativistic values. Here we present plasma wave and particle data from the CRRES satellite during three case studies to see if they can be explained in terms of this energisation mechanism. We first consider a strong geomagnetic storm that contains prolonged substorm activity during its 3.25 day recovery phase. The recovery phase is characterised by electron injections at sub-relativistic energies, enhanced chorus amplitudes and a gradual increase in the flux of relativistic electrons to values of the order of 10 times the pre-storm values, over the entire outer zone. We next consider a strong geomagnetic storm that contains very little substorm activity during its 3 day recovery phase. Here the recovery phase is characterised by a lack of electron injections at sub-relativistic energies, reduced chorus amplitudes and a reduction in the flux of relativistic electrons. Finally, we examine a period of prolonged substorm activity in the absence of a major geomagnetic storm. This period is characterised by electron injections at sub relativistic energies, enhanced chorus amplitudes and a gradual increase in the flux of relativistic electrons in the region 4 < L < 6.5. These results suggest that the gradual acceleration of electrons to relativistic energies during geomagnetic storms can be effective only when there are periods of prolonged substorm activity following the main phase of the geomagnetic storm. Furthermore, the results suggest that it may be possible to obtain electron acceleration to relativistic energies by periods of prolonged substorm activity in the absence of a significant storm signature. These results are consistent with an acceleration mechanism involving the energisation of a seed population of electrons with energies of the order of 100 keV to relativistic energies by wave-particle interactions involving whistler mode chorus.

R B Horne (BAS), N P Meredith, R H A Iles (MSSL/UCL), R M Thorne (UCLA), R R Anderson (Iowa) and D. Heynderickx (BIRAe).

Observations of Storm-Time Energetic Electron Pitch Angle Distributions During an Electron Acceleration Event in the Outer Radiation Belt.

We present observations of the electron pitch angle distributions in the outer radiation belt (3 < L < 7) using data from the CRRES spacecraft. The data were taken during a magnetic storm at energies between 150 keV and 1.5 MeV when the spacecraft outbound legs were in the midnight-early morning MLT sector. In this local time sector the MeV electron flux decreases initially, and then increases during the recovery phase to values that exceed the pre-storm level by a factor of about 10. The pitch angle distribution at L=5 is almost isotropic at the beginning of the event, and then becomes peaked near 90 degrees at energies of a few hundred keV, and depleted at pitch angles < 30o (> 150o) at MeV energies during the flux drop-out. The flux subsequently increases first at the lower energies and then at higher energies. During the flux increase the pitch angle distribution evolves to a rounded distribution at lower energies and a top hat at higher energies (MeV), and then to a top hat distribution at all energies, but a top hat that is broader in pitch angle at MeV energies. We compare the evolution at L=5 with the evolution of the pitch angle distribution at L = 4. We discuss these results in relation to loss processes such as pitch angle scattering, and acceleration mechanisms such as radial diffusion, VLF waves and ULF pulsations.

A S Rodger, M M Lam (BAS), D Murr, W J Hughes (Boston), S P Slinker (NRL), H Frey (UCLA) and A Weatherwax (Maryland).

The magnetosphere-ionosphere system: how it responds to pressure changes at the dayside magnetopause.

Although the structure and dynamics of the magnetosphere and the ionosphere under various solar wind dynamic pressure conditions is well known, the time-dependent response is not well understood or described. For example, field-aligned currents couple the magnetosphere to the ionosphere when pressure changes impact the magnetopause but the factors controlling the number, location, intensity, scale size and motion of the currents are cause for considerable debate. We shall use a wide variety of observational data to provide new facts about the current systems. Also we shall show output from a sophisticated MHD computer model to provide new insight into the mechanisms causing the observed ionospheric effects of solar wind dynamic pressure changes.

M Pinnock, R Horne (BAS) and H U Frey (California).

Discovery of a New Auroral Signature of Solar Wind Pressure Pulses.

Noon sector aurora data from South Pole, Antarctica reveal a previously unreported phenomena associated with solar wind dynamic pressure increases impacting the magnetopause. These solar wind events trigger a longitudinally narrow ( 200 km) auroral emission which leaps equatorward, often spanning 10 degrees of latitude within the time resolution of the experiment (60 s). Event sequences show that this response precedes (by  240 s) the longitudinally extended equatorward bulge of the auroral oval which has previously been associated with solar wind dynamic pressure increases. A similar signature has been found to occur during hot flow anomaly events. The mechanism producing this signature is explored in terms of dynamic pressure variations impacting the dayside magnetopause and enhanced pitch angle scattering of particles in the magnetosphere.

A J Kavanagh (Lancaster/RAL), J B Blake (Aerospace), E Nielson (MPAe), F Honary (Lancaster) and I W McCrea (RAL).

IRIS response to Solar Wind Pressure Pulses.

On 4 May 1998 the Earth's magnetosphere encountered an increase in pressure in the solar wind at 0257UT ( 0530 MLT) . This resulted in both an increase in geomagnetic field strength and high latitude electron precipitation. The precipitation was observed as a patch of increased absorption by the IRIS instrument at Kilpisjärvi. A magnetometer at Kagoshima, Japan, observed deflections in the H component which closely followed the temporal pattern of the IRIS absorption patch. Geostationary and polar orbit satellite observations revealed the increased fluxes of electrons that occured as the pressure pulse hit the magnetosphere. Simultaneous observations by the STARE radar indicated that the precipitated electrons moved with velocities comparable with the ExB drift velocity.

Wednesday 11 April

Session 7

M J Jarvis, M A Clilverd and P J Espy (BAS).

An introduction to the BAS Geospace-Atmosphere Transfer Functions Programme.

This talk gives a brief introduction to one of the ten Programmes comprising the new BAS science portfolio "Antarctic Science in the Global Context 2000-2005". Geospace-Atmosphere Transfer Functions will exploit the unique advantages provided by the extreme atmospheric driving forces over Antarctica to characterise and quantify the energy transfer processes in the mesosphere and lower thermosphere. It is aimed at achieving a better understanding of the global upper atmosphere circulation, temperature balance, short-term variability and long-term change (see www.antarctica.ac.uk/gatf ).

G O L Jones (BAS), F T Berkey, C S Fish (Utah), W K Hocking (Ontario).

Mesospheric winds over Bear Lake by IDI and meteor radar.

A meteor radar for monitoring mesospheric winds has recently been deployed, on a temporary basis, at the Bear Lake Observatory in Utah. This has allowed a comparison of wind measurements with a co-located Imaging Doppler Interferometer (IDI) which has been operating at the site for the past two years. The wind patterns from both instruments are found to agree very well confirming the reliability of the IDI observations against the more recognised meteor radar winds. A comparison of mesospheric tidal and planetary wave activity will be presented, and the flexibility of the IDI technique will be discussed.

S Chew, M A Clilverd and G O L Jones (BAS).

PMSE or not PMSE: that is the question.

The study of Polar Mesosphere Summer Echoes (PMSE) has become important in recent years because of their links with the cold, summer mesosphere where temperatures depart significantly from radiative equilibrium. These cold temperatures, and their associated effects of radar echoes (PMSE) and ice particle formation (NLC), may provide an early indicator of climate change. Although PMSE are widely seen at VHF frequencies, and occasionally at UHF, observations within the MF/HF band (1-30MHz) are rare and not clear-cut. Sightings have been reported from MF spaced-antenna wind radars but the precise nature of the radar scattering is inconclusive. This presentation will focus on PMSE-like features in data from the MF radar at Rothera in the Antarctic and will discuss the importance of PMSE observations in the southern hemisphere.

R E Hibbins, J G Keir and N J Farnell (BAS).

Extraction of gravity wave parameters from all-sky image data.

The collection and analysis of data from the BAS-Utah State University CCD all-sky imaging camera operating at Halley Station, Antarctica in 2000 are discussed. Continuous observations throughout the Austral winter have generated  90Gb of 512×512 TIFF files. Semi-automated image processing and summary data generation techniques are presented that enable the user to discard unwanted data, identify periods where atmospheric gravity waves are observed and extract the relevant parameters.

A R Lawrence (Cambridge/BAS), M J Jarvis, P J Espy (BAS), S D Eckermann(NRL).

Simulation of Gravity Waves in Antarctica using a Ray-Tracing Model.

The characteristics and propagation effects of mesospheric gravity waves in the high-latitude southern hemisphere are investigated using a gravity wave ray-tracing model. The Gravity-wave Regional or Global Ray Tracer (GROGRAT) model was used with HWM-93 and ECMWF background wind fields, to simulate horizontal and vertical propagation of gravity waves from the surface up to mesospheric altitudes. The ray paths and average momentum flux of the waves are examined to help determine energy propagation and possible source regions for these waves. These results are then correlated with observations of gravity wave signatures, obtained from a high- resolution airglow imager recently installed at Halley, Antarctica.

P J Espy (BAS), M J Taylor (Utah), M A Clilverd, R E Hibbins and M J Jarvis (BAS).

An overview of mesospheric wave parameters observed from Halley research station, Antarctica.

As part of a co-operative BAS-Utah State University programme, a CCD all- sky imaging camera was operated at Halley Research Station, Antarctica (75.6S, 26.4W) during austral 2000 observing season. The camera was equipped with narrow and broadband filters to observe the Na-D, O2 Atmospheric and OH Meinel night-airglow emissions with a temporal resolution of  30 sec. Observations of atmospheric gravity waves passing through the 87 to 96 km region were obtained throughout the austral winter season during new moon periods. Due to the large volume of data obtained, semi-automated processing algorithms were employed to extract relevant wave parameters such as horizontal wavelength, phase velocity and propagation direction. A survey of these results as well as their seasonal behaviour will be presented and compared to similar northern hemisphere observations.

G H Millward(UCL), T J Fuller-Rowell (SEC), A.D. Richmond (HAO/NCAR) M.V. Codrescu (SEC).

Storm-Time Changes in the Upper Atmosphere at Low Latitudes.

A three-dimensional coupled model of the thermosphere, ionosphere, plasmasphere and electrodynamics has been used to investigate the dynamic and electrodynamic response at low latitudes during a geomagnetic storm. A storm was simulated at solstice, and high solar activity, and was characterized by a 12-hour enhancement of the high latitude magnetospheric electric field and auroral precipitation. The deposition of energy at high latitudes heats the thermosphere and drives equatorward wind surges, and changes the global circulation. The first wave arrives at the equator, 3.5 hours after storm onset. The change in the global circulation drives downwelling at low latitudes, which decreases molecular species, and causes a slight positive ionospheric phase. By far the dominant driver of the low latitudes is due to the changes in electrodynamics. The dynamo effect of the altered wind circulation opposes the normal diurnal variation, with downward ion drift during the day and upward drift at night. On the dayside, the equatorial ionization anomaly becomes weaker, the height is lowered, and the eastward zonal winds are reduced. At night the anomaly is strengthened, the ionosphere is raised, and zonal winds accelerate. The global electrodynamic changes are consistent with earlier results, but the speed of the response was unexpected. The model results showed an equatorial response within 2 hours of the storm onset, well before the first gravity waves arrived at the equator. The dynamo action of the mid- latitude wind surges, drive an F-region dynamo, that can redistribute charge around the globe very quickly. The meridional winds act as the driver of the low latitude storm response by changing the dynamo action of the winds. In contrast, the zonal winds respond to the redistribution of charge brought about by the electrodynamic changes, rather than acting as a driver the change.

R Balthazor and R Erdelyi (Sheffield).

Modelling of high speed flows: thermospheric consequences.

Recent improvements to the Coupled Thermosphere Ionosphere Plasmasphere model have permitted first detailed modelling of high speed ion flows and their effects on the polar thermosphere. The contribution to the thermospheric energy budget from transonic viscous heating is found to be significant.

Y Z Su, G J Bailey (Sheffield), M H Denton (Aberystwyth) and R A Heelis (Texas).

Plasmapause Signatures in the Subauroral Topside Ionosphere During a Severe Geomagnetic Storm.

It has been suggested that the subauroral electron temperature enhancement in the ionosphere is a signature of the plasmapause. However, the energy source of the temperature enhancement has not been determined. In this talk, we present observations of electron temperature, ion temperature and plasma density made by the DMSP F10 satellite at an altitude of around 800 km during a severe geomagnetic storm which commenced on 24 March 1991. The observations show that the geomagnetic storm gives rise to strong effects in the subauroral topside ionosphere. The behaviour of the topside ionosphere suggests that the plasmapause moves to very low L shell and that there is a huge input of energy around the plasmapause during the storm period. SUPIM (Sheffield University Plasmasphere Ionosphere Model) has been used to assist the understanding of the satellite observations.

Session 8

M M Lam and A S Rodger (BAS).

The effect of Forbush decreases on tropospheric parameters in the Antarctic.

Egorova et al. (2000) conclude that Forbush decreases in galactic cosmic rays have a significant effect on the atmosphere at Antarctic station Vostok (78.5 S, 106.9 E) via the mechanism of electrofreezing. We review the results of Egorova et al. (2000) and present the results of a similar study at South Pole (90.0 S), located about 1000 km from Vostok, which has been conducted in order to examine the spatial extent of this phenomenon. No observable effect of Forbush decreases on the atmosphere over South Pole is found. We discuss possible reasons for the disagreement between the results of the two studies and conclude that it is due to crucial differences in methodology. Egorova et al. (2000), Journal of Atmospheric and Solar-Terrestrial Physics 62, 955-966.

M J Kosch (Lancaster), K Cierpka, A Kohsiek, T Hagfors and K Schlegel (MPAe).

How good are horizontal thermospheric wind measurements from ground-based Fabry-Perot interferometers?

Thermospheric neutral winds have long been measured using ground-based Fabry-Perot interferometers. Horizontal wind estimates are made by combining Doppler shift observations of the OID 630 nm emission at 30-45 deg. elevation from several azimuths, which are sequentially scanned. The horizontal radius covered is typically 500-900 km. Several problems exist with the method: (1) There is a vertical wind component to the Doppler shift measurement for horizontal winds. The vertical wind can usually only be measured directly overhead of the FPI; (2) It takes time for the FPI to cycle through one scan, during which the horizontal winds may change; and (3) There may be spatial gradients in both the horizontal and vertical winds, a likely possibility at auroral latitudes. A test has been developed to estimate the "goodness" of the assumptions made in extracting thermospheric horizontal winds for a single FPI. This involves computing the vertical wind from the horizontal wind estimates and comparing to the actual vertical wind measurement. The method has been tested using data from the FPI at Skibotn (69.36 N, 20.33 E), Norway, for active (9/2/97 and 9/11/98) and quiet (8/2/97 and 10/11/98) geomagnetic periods.

M Mendillo, S Smith (Boston), H Rishbeth (Soton), and D. Hinson (DEE).

Ionospheric Variability: A Comparison of Earth and Mars.

A planet's ionosphere offers an upper atmospheric environment to study that is a system highly sensitive to processes that couple energetics and dynamics. On Earth, four distinct layers result from a mix of solar, geomagnetic and neutral atmosphere effects over a broad altitude range. On Mars, two layers appears in an ionosphere that spans a rather limited height range. In this paper, we will explore the contributions to ionospheric variability that are driven by solar, solar wind and neutral atmosphere processes acting at Earth and Mars. Long term ionosonde measurements on Earth provide ample data to define variability patters. Recent radio occultation experiments using the Mars Global Surveyor (MGS)satellite provide the first multi-day sets of data ever taken for ionospheric studies on Mars.

A J Coates, A M Rymer, D R Linder (MSSL), M Grande (RAL) , K Svenes, B T Narheim (NDRE), M F Thomsen (LANL) and D T Young (Michigan).

CAPS electron spectrometer observations at Jupiter.

The Cassini spacecraft passed the planet Jupiter on 30 December 2000. The CAPS electron spectrometer on board has given exciting new data on the Jovian bow shock and magnetopause during the days before and weeks following the closest approach. With complementary data on board, and from the Galileo spacecraft and the Hubble space telescope, an excellent new dataset exists. In particular the first Cassini shock crossings were at relatively large distances from the planet due to solar wind conditions. Here we present the observations and draw preliminary conclusions.

M K Dougherty (IC), S W H Cowley (Leicester) and the Cassini Magnetometer team.

Cassini magnetometer observations from Jupiter.

The flyby of the Cassini spacecraft past Jupiter took place on 30 December 2000. An overview of the magnetic field observations from this flyby will be described. Cassini's trajectory along the dusk flank resulted in the spacecraft spending long periods of time in the magnetosheath as well as, on a number of occassions entering into the magnetosphere proper. Initial comparisons with observations from the Galileo spacecraft which was inside of Jupiter's magnetosphere during this period will also be discussed.

M J Owens and P J Cargill (IC).

Correlation of Magnetic Field Intensities and Solar Wind Speeds of Events Observed by ACE.

For a selection of magnetic clouds, a relationship between peak magnetic field intensity and peak velocity has been shown to exist [Gonzalez et al. 1998]. Using data from NASA's ACE spacecraft, this correlation has be been shown to extend to general events within the solar wind. However, the degree of correlation between maximum magnetic field intensity and the maximum velocity varies strongly with the criterion used to select events. This dependence is investigated, as is the nature of the events leading to maximum correlation. For the purposes of space weather prediction, individual magnetic field components (in particular Bz, the southward magnetic field component) have also been analysed.

A R Breen (Aberystwyth), A J Lazarus (MIT), P Riley (SAIC), R A Fallows, A Canals (Aberystwyth).

The large-scale structure of the solar maximum solar wind from interplanetary scintillation and in-situ measurements.

Measurements of solar wind velocity from spacecraft and interplanetary scintillation observations during the third whole sun month (August- September 1999), May 2000 and the solar maximum solar wind study interval (September 2000) are used to study the three-dimensional structure of the solar maximum solar wind. MHD modelling is used to investigate the relationship between structures in the corona and the solar wind.

A Rees and R J Forsyth (IC).

A Survey of Magnetic Clouds in Ulysses Data (1996-2000) : Preliminary Results.

From 1996 until late 2000 solar activity has been on the increase. During this period Ulysses has travelled from approximately 50°N, down through the ecliptic to over the south pole of the Sun at latitude of 80°S. A survey has been carried out on the unique magnetic field data set obtained by Ulysses during this time, searching for smooth field rotations of the order of days in duration. These smooth rotations, combined with field magnitude enhancements, are common signatures of magnetic clouds (MCs). We present here the preliminary results of this survey.

J M Gloag and A Balogh (IC).

Limitations of shock parameter calculations in describing heliospheric shock waves.

The classification of discontinuous events observed in the solar wind as shock waves requires the calculation of a number of parameters which can also be used to distinguish between different types of shock waves. These parameters depend on properties of the interplanetary magnetic field and the solar wind plasma, all of which are measured or can be derived from measurements made by in situ spacecraft. In the work presented here over a hundred shock waves are considered, all of which appear on the Ulysses shock list between mid August 1996 and the end of 1999. It is found that more than half of the shock waves appear to have an entropy decrease from upstream to downstream, according to the established methods of calculation. This is in conflict with the second law of thermodynamics which asserts that there must be an entropy increase due to the irreversible dissipation processes occurring. These results suggest that the idealizations and approximations present in the MHD treatment are not always appropriate in the case of low Mach number collisionless shock waves. The more extended nature of interplanetary shocks needs to be taken into account by considering how different processes related to the shock transition occur over different time scales.

M Y Gulamali, and P J Cargill (IC).

A Numerical Study Of The Interaction Between Alfvén Waves And Shocks.

The turbulent magnetic field and plasma fluctuations in the solar wind are thought to arise through two main mechanisms. Firstly, Alfvenic fluctuations, generated near or within the solar corona, are believed to be convected out from the Sun by the solar wind. Secondly, local instability mechanisms in interplanetary space e.g. stream-shears, reconnection etc. are believed to produce a host of plasma wave modes which interact in a non-linear manner to produce a turbulent energy cascade. We study the interaction of Alfven waves with shock structures such as those found bounding corotating interaction regions, in an effort to understand in what proportion the mechanisms above contribute to the general turbulent nature of the solar wind. We present preliminary results of our numerical simulations and relate these to previous work concerning Ulysses' observations of magnetohydrodynamic turbulence in corotating interaction regions.

Posters

A M Buckley and M P Gough (Sussex) H Alleyne and K H Yearby (Space Instrumentation Group, ACSE, Sheffield).

Electron Modulation Measured by the Cluster Particle Correlators.

As part of the DWP (Digital Wave Processing experiment) within the Cluster WEC (Wave Experiment Consortium) a particle correlator instrument operates which measures the time structuring of the electrons as a function of energy on each of the four spacecraft. The Cluster particle correlators are implemented by generating on board ACFs (Auto Correlation Functions) taking raw input of electron counts from the PEACE HEEA sensor. The information on the electron flux modulation covers the frequency range DC up to 41 kHz (encompassing typical electron plasma frequencies and electron gyro frequencies and their harmonics), the electron energy range is that covered by the PEACE HEEA sensor ( 0.6 eV to 26 keV). Initial results are presented for both natural plasma measurements (in particular magnetopause/bow shock traversals, where the electron counting statistics are high and turbulent plasma conditions exist) and also measurements during periods when the active Cluster experiments (electron beam emission / active wave sounding) are operating.

M H Denton (Aberystwyth), R L Balthazor, C R Wilford (Sheffield), I K Walker (Aberystwyth), R J Moffett and G J Bailey (Sheffield).

Modelling the high-latitude ionosphere for comparison with experimental observations.

The aim of the collaboration between Sheffield and Aberystwyth is to combine modelling studies with experimental observations of the high- latitude ionosphere. The radio tomography experiment operated by the UWA group is designed to provide spatial images of large-scale structures in the electron density of the high-latitude ionosphere, with observations from successive satellite passes providing information on the temporal development of the features. The Sheffield University version of the Coupled-Thermosphere-Ionosphere-Plasmasphere Model (CTIP) is being used in an attempt to replicate the experimental observations, with a view to understanding the complex interaction of the physical processes controlling the structuring of the plasma. The initial testing of the concept has yielded promising results, but the demands on computing time required for the extended runs for the model have made systematic study impractical. At Sheffield, CTIP is run currently on UNIX-based workstations (SUN/DEC), hardware that is not available at UWA. However, with the rapid increase in computer-processing power and the low cost of PCs in comparison to workstations, the transfer to a PC-architecture running the LINUX operating system has been investigated, with encouraging results. The stability and robust nature of the LINUX-OS has made porting of the model successful, with only minor modifications, while the run-time has been reduced substantially. It is believed that further significant improvements can be achieved that will enable detailed study to be made of the mechanisms controlling the development of the dayside trough and other regular features of the high-latitude ionosphere found in the tomographic images.

M Dunlop, A Balogh (IC)and K-H Glassmeier (TUB).

First application of multi-point magnetic field analysis tools: the curlometer and discontinuity analyser.

The Cluster spacecraft have recently completed multiple spacecraft coverage of the dusk-side tail magnetosphere, magnetosheath and near solar wind. For the first time, full, 3-D information on magnetic field structures at small to medium scales in the near-Earth environment has been collected. Strongly time dependant events, however, can not be simply resolved by Cluster unless some knowledge of their structure or time dependence is pre- determined. It is critical to know, for instance, what are the relative scales between the spacecraft array and the structures present, as well as the measurement errors of each. A variety of analysis techniques, utilising the combined, four spacecraft information in different ways, are therefore required, which may not all apply well to any particular event or situation. Advanced methods of analysis often need to be supported by (pre- )analysis of the individual spacecraft data to provide some classification of event properties. The resulting data regimes (having characteristic variations in time and space) define different classes of technique: spatial gradients, geometry and motion of structures, and spectral properties. We focus on the curlometer (direct estimation of the electric current density from spatial gradients) and DA (analysis of the geometry and motion of magnetic discontinuities) techniques. The applicability of these multi-point techniques is tested for selected events within the data set. The analysis is placed in the context of various mission constraints (such as position, timing and experiment accuracy).

S Foster (Soton) and M Lockwood (RAL).

Long-term changes in the solar photosphere associated with changes in the coronal source flux.

Using sunspot observations from Greenwich and Mount Wilson, we show that the latidinal spread of sunspot groups has increased since 1874, in a manner that closely mirrors the long-term ( 100 year) changes in solar flux, Fs, as inferred from solar activity. This latitude spread is shown to be well correlated with the flux emergence rate required by the model of the coronal source flux variation by Solanki et al (2001). The time constant for decay of this open flux is found to be $3.6\pm0.8$ years. using this value, and quantifying the photospheric flux emergence rate using the latitudinal spread of sunspot groups, the model reproduces the observed coronal source flux variation. The ratio of the 100-year drift to the solar cycle amplitude for the flux emergence rate is found to be half of the same ratio for Fs.

D K Milling, I R Mann (York) and F W Menk (Newcastle).

Some effects of the plasmapause on cross-phase signatures.

Using cross-phase analysis on data from a closely spaced array of magnetometers, we have obtained meridional frequency-latitude profiles of field line resonances from the inner plasmasphere, across the plasmapause and into the plasmatrough. The location of the plasmapause is indicated by a discontinuity in the frequency-latitude profile. We show that the cross- phase response from stations separated by <0.5 L is inhibited when the plasmapause is overhead. The time development of the cross-phase spectra allows us to track the motion of the plasmapause during the day. In addition we observe anomalous frequency values obtained from more widely separated station pairs spanning the plasmapause. An attempt is made to identify the source of this anomalous signal.

A M Rymer, A J Coates and D R Linder (MSSL).

Cassini Electron Spectrometer (ELS) observations of an interplanetary shock.

Preliminary study of interplanetary shock seen at Cassini 18 November 2000. We study the ELS counts/energy, density and temperature data during the shock. This interplanetary shock is related to observations of a solar CME event observed by SOHO (Lasco C2/C3) on 8 November 2000, the effects of which were also observed by ACE, WIND, GOES and Polar at Earth during the period 8-10 November 2000.

Y Taroyan and R Erdelyi (Sheffield).

Resonant flow instability of MHD surface waves in low-beta plasmas.

The effect of velocity shears on the spectrum of MHD surface waves is studied in cold (plasma-beta zero) and low-beta plasmas. A non-uniform steady intermediate region is sandwiched by a homogeneous static and steady magnetic semi-infinite plasmas. The presence of the non-uniform steady intermediate layer can cause the surface wave to be subject to resonant absorption (slow and/or Alfvenic resonances). This model can serve as an approximation for MHD wave propagation and instabilities for different solar or magnetospheric configurations (e.g., magnetopause, coronal loops, coronal plumes, heliopause, etc.). Analytical solutions are derived in and around the resonant surfaces in dissipative (resistive) MHD. Using these analytical solutions, the effect of the velocity shear on the damping rate of the surface wave can easily be investigated. The presence of the flow can both increase and decrease the efficiency of resonant absorption. We also show how the resonance can lead to instabilities of the global surface modes for certain values of the velocity shear. The resonant flow instabilities, which are physically distinct from the non-resonant Kelvin-Helmholz instabilities can occur for velocity shears significantly below the Kelvin-Helmholz threshold. Although resonant absorption is present as a dissipation mechanism, the amplitude of the surface modes grow in time. The resonant flow instability can be explained in terms of negative energy waves. Observational consequences in the magnetosphere are discussed.

C R Wilford, R J Moffett, J M Rees (Sheffield). R A Heelis (Texas),

A Modified CTIP model and Comparisons with DMSP Satellite Data.

The coupled thermosphere-ionosphere-plasmasphere model (CTIP)is a three-dimensional model that solves equations of continuity, momentum and energy balance to produce quantities such as ion and electron concentrations, temperatures and field-aligned velocities. The Sheffield version of the CTIP model has been modified in order to include He+. Results from the modified model show good agreement with the original CTIP model. Data from the Defence Meteorlogical Satellite Program (DMSP) is used to compare and validate the results from the modified CTIP model.

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