MIST meeting, London, 22 November 2002
Abstracts

First session

J Eastwood, A Balogh, E Lucek (Imperial College, London) I Dandouras and C Mazelle (CESR, Toulouse, France). Cluster observations of waves and particles in the terrestrial foreshock

The terrestrial foreshock exhibits a wide variety of wave activity, in particular ultra low frequency waves observed at frequencies well below the ion cyclotron frequency. Of interest is the way in which these waves are generated and subsequently interact with the ion populations in the foreshock. We present observations of ULF foreshock waves based on magnetic field data from the 4 spacecraft Cluster mission, analysing the plasma frame properties of such waves using multi-spacecraft techniques. Details of the associated plasma properties are also discussed.

G A Abel, A J Smith (British Antarctic Survey, Cambridge, UK), N P Meredith (Mullard Space Science Laboratory, University College London, UK), and R R Anderson (University of Iowa, USA). The evolution of substorm enhanced whistler mode waves: Why is electron energy dispersion not reflected in the waves?

By assuming a first order gyroresonance interaction between electrons and whistler mode waves, and that the waves are parallel propagating, it is possible to express the spectrum of the whistler waves in terms of an equivalent parallel resonant energy. By examining wave spectra measured by the CRRES spacecraft between 02:00 and 06:00 MLT we have previously shown that the general evolution of the equivalent parallel resonant energy of substorm enhanced whistler mode waves is constant with time. This implies that the well known energy dispersion seen in substorm injected electrons is not reflected in the substorm enhanced whistler waves, an assumption previously used to explain the frequency dispersion seen in Substorm Chorus Events on the ground. In this paper we consider the first order resonance curves in energy space, and show why the electron energy dispersion is not necessarily expected to be reflected in the waves.

N P Meredith (Mullard Space Science Laboratory, University College London, UK), R M Thorne (Department of Atmospheric Sciences, Univeristy of California at Los Angeles, USA), R B Horne (British Antarctic Survey, Cambridge, UK), D Summers (Department of Mathematics and Statistics, Memorial University of Newfoundland, Canada), B J Fraser (Department of Physics, University of Newcastle, Australia) and R R Anderson (Department of Physics and Astronomy, University of Iowa, USA). Statistical analysis of relativistic electron energies for cyclotron resonance with EMIC waves observed on CRRES

Electromagnetic ion cyclotron (EMIC) waves which propagate at frequencies below the proton gyrofrequency can undergo cyclotron resonant interactions with relativistic electrons in the outer radiation belt and cause pitch-angle scattering and electron loss to the atmosphere. Typical storm-time wave amplitudes of 1-10 nT cause strong diffusion scattering which may lead to significant relativistic electron loss at energies above the minimum energy for resonance, Emin. A statistical analysis of over 800 EMIC wave events observed on the CRRES spacecraft is performed to establish whether scattering can occur at geophysically interesting energies (<= 2 MeV). While Emin is well above 2 MeV for the majority of these events, it can fall below 2 MeV in localised regions of higher plasma density (fpe/fce > 10), for wave frequencies just below the hydrogen or helium ion gyrofrequencies. These lower energy scattering events, which are mainly associated with resonant L-mode waves, are found within the magnetic local time range 13 < MLT < 18, for L > 4.5. The average wave spectral intensity of these events (4-5 nT2/Hz) is sufficient to cause strong diffusion scattering. The spatial confinement of these events, together with the limited set of these waves that resonate with <= 2 MeV electrons, suggest that these electrons are only subject to strong scattering over a small fraction of their drift orbit. Consequently, drift-average scattering lifetimes are expected to lie in the range of several hours to a day. EMIC wave scattering should therefore significantly affect relativistic electron dynamics during a storm. The waves that resonate with the ~MeV electrons are produced by low energy (~keV) ring current protons, which are presumably injected into the inner magnetosphere during enhanced convection events.

Y Taroyan and R Erdelyi (Space and Atmosphere Research Group, Department of Applied Mathematics, The University of Sheffield, Sheffield, UK). Steady state excitation of field line resonances by global Waveguide modes in the magnetosphere

A major shortcoming of theories of long-period magnetic pulsations is the separate treatment of the problems of wave excitation and resonant coupling. This could account for many substantial discrepancies between the waveguide/cavity mode theories and observations. A unified approach leading to a new type of field line resonance (FLR) excitation mechanism is presented. It is shown that in a steady state the unmediated coupling of the waveguide modes to the local field line oscillations provides a natural and a very efficient transport of energy from the magnetosheath flow to the shear Alfvén waves even in the ideal MHD limit. The role of ionospheric dissipation is discussed and many well-known observational features are recovered.

A Dobbin and A Aylward (Atmospheric Physics Laboratory, University College London). Revisiting the basics of middle atmosphere flow using CMAT

The CMAT (Coupled Middle Atmosphere And Thermosphere) model is an extension of the UCL-Sheffield-SEL thermosphere ionosphere model that goes down to 30km altitude. The basic model has been presented by Harris (2000), and at previous MIST meetings. One of the objectives of developing CMAT was to investigate the coupling between the upper atmosphere and the lower atmosphere, given the importance such studies have assumed in investigations of solar variability and climate change, and in potential ground-level effects due to "solar weather". A key element in this is "getting the mesosphere right". It is known that at the mesopause the summer temperatures are lower than the winter temperatures and a number of explanations have been put forward for this. This seems to be tied in with the fact the zonal jets in the lower mesosphere are closed off at mesopause altitudes, and there is a "reverse" winter-to-summer meridional flow at that height. The "closing of the zonal jets" has been ascribed to gravity wave momentum dumping at that altitude, although a similar effect can be ascribed to momentum transfer from the auroral zone. As part of the validation of CMAT some simple experiments have been performed to understand the global effects of the various energy and momentum inputs as far as the MLT (Mesosphere and Lower Thermosphere) is concerned. Runs without auroral momentum and energy inputs and without gravity wave drag have been compared to the staged turn-on of each of these effects. It has been possible with these runs to understand the relative importance of the different effects. As one by-product of this work, some two-dimensional energy and momentum inputs can be defined for use in 2-d models of this region meant to investigate the longer term aspects of climate change.

N Balan, H Alleyne (Control Engineering, University of Sheffield, Sheffield, UK), S Kawamura and S Fukao (RASC, Kyoto University, Kyoto 611, Japan). Mean wind velocity and tidal parameters in the mid latitude thermosphere

Solar activity (F10.7) variations of the diurnal mean wind velocity, and amplitudes and phases of the tidal components of periods 24-, 12-, 8-, and 6- hours in the mid latitude thermosphere are studied using MU radar (35°N, 135°E) incoherent scatter (IS) observations of duration up to 96 hours conducted for 15 years since 1986. The wind velocity averaged over 220-450 km altitudes in the meridional direction is derived from the plasma drift velocity measured parallel to the geomagnetic field lines following a 'layer wind' method [Oliver et al., 1998]. The study shows that the mean wind velocity and tidal amplitudes decrease with increasing solar activity, with the rate of decrease being faster for stronger tides. These variations could mainly be due to the simultaneously calculated ion-drag that increases with solar activity. The data also shows the presence of waves of period near 16.5 hours and 2 days. The mean wind velocity and tidal amplitudes and phases obtained from the theoretical global scale wave model (GSWM), empirical horizontal wind model (HWM93), and Millstone Hill IS radar data show similar solar activity dependence as those obtained from the MU radar though there are significant quantitative differences.

S England and N Arnold (University of Leicester, Leicester, UK). The impact of gravity waves on the dynamics of the mesosphere-lower thermosphere region - recent model results

A new zonally averaged mechanistic model of the atmosphere from the stratosphere to the thermosphere (the Stratosphere to Thermosphere Energy Variability Experiment) has been developed to study coupling between the middle and upper atmosphere. This model has been used to test a number of gravity wave parameterisations. The impact of gravity waves on the dynamics of the MLT region has been investigated. The model has been run through several annual cycles under different solar conditions, the results of which will be presented.

J A Wild, S E Milan, S W H Cowley (Department of Physics & Astronomy, University of Leicester, Leicester, UK), C J Owen (Mullard Space Science Laboratory, University College London, UK) and H Frey (University of California, Berkeley, USA). Space- and ground-based observations of the structure and dynamics of the auroral ionosphere during an interval of BY dominated interplanetary magnetic field

During the interval 12-17 UT on 8th December 2001, the IMF was generally oriented in the dawnward direction with BY varying between -3 and -6 nT. The BZ component was typically smaller throughout (|BZ|<=4nT) and negative until around 14:30 UT (lagged time) when it switched to positive values. This northward turning resulted in an IMF clock angle of -80°, which endured until the end of the interval. This talk will present SuperDARN, Cluster and IMAGE WIC observation of the auroral oval and the high-latitude ionospheric dynamics during an interval in which the solar wind-magnetosphere-ionosphere coupling mechanism departed markedly from the ubiquitous southward IMF paradigm.


Second session

R W Sims and S E Pryse (University of Wales, Aberystwyth, UK). ESR line-of-sight velocities and the open/closed boundary during northward IMF in summer

Results are presented from a multi-instrument investigation of the summer dayside ionosphere under positive IMF Bz. The EISCAT Svalbard Radar observed converging line-of-sight ion flows, whose latitude varied with IMF Bz near magnetic noon. Increased ion temperatures were observed north of the region of convergence that is attributed to opened flux tubes being convected rapidly away from the high-latitude reconnection site. Radio tomography imaged the background ionospheric electron density over an extended latitudinal region, whilst the CUTLASS radars measured the changing convection pattern. Particle detectors onboard DMSP and NOAA satellites identify the open/closed boundary and the Tsyganenko magnetic field line model provides information on its motion. The observations are discussed in light of the Tsyganenko magnetic field model.

J P Dewhurst, C J Owen and A N Fazakerley (Mullard Space Science Laboratory, University College London, UK). Cluster PEACE observations of thinnings and expansions of the substorm plasma sheet

During the course of a geomagnetic substorm, magnetic flux is first stored in the magnetotail during the growth phase and then subsequently removed following the onset of the reconnection at a near-Earth neutral line. Concurrent to these actions, the mid-tail plasma sheet is thought to thin during the growth phase and then rapidly expand after onset. Cluster's Plasma Electron And Current Experiment (PEACE) allows 4-point observations of the plasma sheet - lobe boundary as this interface passes over the Cluster tetrahedron. The relative timings of the boundary passage at each spacecraft allow a determination of its speed and direction of motion. We have performed this analysis for over 150 boundary crossings associated with substorm-related plasma sheet thinnings and expansions during the Cluster magnetotail pass of 2001. Average plasma sheet - lobe boundary normal vectors and normal component velocities are presented. Their significance to the debate on the triggering mechanism of the substorm onset is discussed.

B Hnat, S C Chapman and G Rowlands (Space and Astrophysics Group, University of Warwick, Warwick, UK). Intermittency, scaling and the Fokker-Planck approach to fluctuations of the solar wind bulk plasma parameters as seen by WIND

The solar wind provides a natural laboratory for observations of MHD turbulence over extended temporal scales. Here, we apply a model independent method of differencing and rescaling to identify self-similarity in the Probability Density Functions (PDF) of fluctuations in solar wind bulk plasma parameters as seen by the WIND spacecraft. Whereas the fluctuations of speed v and IMF magnitude B are multi-fractal, we find that the fluctuations in the ion density $\rho$, energy densities B2 and $\rho$ v2 as well as MHD-approximated Poynting flux vB2 are mono-scaling on the timescales up to ~26 hours. The single curve, which describes the fluctuations PDF of these four quantities up to this timescale, is non-Gaussian. We use a Fokker-Planck approach to model this PDF and compere it with the Castaing distribution that arises from a model for the intermittent turbulent cascade. We also derive the transport coefficients and associated Langevin equation for the fluctuations.

S C Chapman, B Hnat, G Rowlands (Space and Astrophysics Group, University of Warwick, Warwick, UK), N W Watkins and M P Freeman (British Antarctic Survey, Cambridge, UK). Scaling of solar wind epsilon and the AU, AL and AE indices as seen by WIND

We apply the finite size scaling technique to quantify the statistical properties of fluctuations in AU, AL and AE indices and in the $\epsilon$ parameter that represents energy input from the solar wind into the magnetosphere. We find that the exponents needed to rescale the probability density functions (PDF) of the fluctuations are the same to within experimental error for all four quantities. This self-similarity persists for time scales up to ~4 hours for AU, AL and $\epsilon$ and up to ~2 hours for AE. Fluctuations on shorter time scales than these are found to have similar long-tailed (leptokurtic) PDF, consistent with an underlying turbulent process. These quantitative and model-independent results place important constraints on models for the coupled solar wind-magnetosphere system.

S W Ellacott and W P Wilkinson (Division of Mathematical Sciences, University of Brighton, Brighton, UK). The heating of directly transmitted ions at perpendicular shocks: a new approach based on Lagrangian dynamics

The heating of directly transmitted ions at low Mach number quasi-perpendicular collisionless shocks is rapid, greater than adiabatic, and exhibits a distinct T^>T// anisotropy. In this paper we present a theoretical study of the evolution of the ion velocity distribution across a stationary one-dimensional perpendicular model shock profile. A Lagrangian/Hamiltonian formulation of the ion equations of motion is introduced. A detailed study of Liouville's equation then reveals that although an analytical solution is not possible, several interesting properties of the distribution can be proved. Assuming a Maxwellian incident velocity distribution, it is possible to obtain the analytical form for the distribution through the shock in terms of functions that are independent of the incident temperature. We recover the observed results that the distribution is stretched across the magnetic field direction as it passes through the shock and that it rotates around the field in the downstream region. We are able to show that in the low-temperature limit, the shape of the distribution remains Gaussian, but that this is not the case for higher temperatures. A strict lower bound and an estimate upper bound for the variance of the downstream velocity and therefore the heating are obtained.

K Nykyri (The Blackett Laboratory, Imperial College, London, UK) and A Otto (The Geophysical Institute, University of Alaska Fairbanks, USA). Influence of Hall effects on the Kelvin-Helmholtz instability

The Kelvin-Helmholtz (KH) instability in its nonlinear stage can develop small scale filamentary field and current structures at the flank boundaries of the magnetosphere. It has been shown previously with MHD simulations that reconnection can occur inside these narrow current layers resulting in plasma transport from the solar wind into the magnetosphere. During periods of northward IMF the time scale of this transport in consistent with satellite observations of the cold, dense plasma sheet. However, when the length scales of these narrow current layers approach the ion inertia scale, the MHD approximation is not valid anymore and the Hall term in the Ohm's law must be included. We will study the influence of the Hall term on the KH instability with 2-D Hall-MHD simulations and compare our results with corresponding MHD simulations.

A N Fazakerley, C J Owen and J P Dewhurst (Mullard Space Science Laboratory, University College London, UK). Cluster PEACE observations of X-line Hall current electrons in the Earth's magnetotail?

Theoretical studies of the reconnection process suggest that in the ion diffusion region around a magnetic X-line (where the ions but not the electrons decouple from the magnetic field) we may expect to see characteristic Hall current signatures in the magnetic field. The electrons carrying these Hall currents are predicted to flow in specific directions. Rare examples of such characteristic magnetic field perturbations have apparently been observed near X-lines, both at the magnetopause by Geotail and in the magnetotail by WIND. These events have been interpreted as evidence supporting such Hall current models. In the WIND study (Oieroset et al. Nature, 2001) a case is made that the electrons carrying the Hall current are also briefly observed. However the interpretation of the data in these single spacecraft cases depends heavily on an assumption that the spacecraft took a particular trajectory through a structure that is assumed to have been present, based on theoretical work. With Cluster we are now able to examine such regions using four spacecraft, which allow a more severe test of the models. We report on a Cluster study of one such event, and the search for the Hall current signatures in the electrons and the more commonly observed Hall perturbations to the magnetic field.


Third session

J Scuffham, A Balogh and G Giampieri (Imperial College, London). Modelling Mercury's magnetosphere

The imminent return of spacecraft to the planet Mercury has sparked a new drive in efforts to model its magnetosphere. We present a brief summary of the data returned by the Mariner 10 magnetic fields experiment and the models that were based on these observations. We also present the results of work that is being carried out to modify existing models of the Earth's magnetosphere to investigate their applicability to Mercury. We show how the significantly smaller dipole moment, the absence of a ring current and the differences in boundary conditions are likely to affect the magnetosphere of Mercury.

J D Nichols, S W H Cowley and E J Bunce (Radio & Space Plasma Physics Group, University of Leicester, Leicester, UK). Magnetosphere-ionosphere coupling currents in JupiterÆs middle magnetosphere: Dependence on ionospheric Pedersen conductivity and Iogenic plasma mass outflow rate

The dynamics of Jupiter's plasma environment is dominated by the outflow of material originating from the moon Io, which orbits deep within the magnetospheric cavity. Breakdown of corotation associated with this radial transport results in the formation of a magnetosphere-ionosphere coupling current system which transfers angular momentum to the magnetospheric plasma and has been linked to the formation of the main jovian auroral oval. We discuss solutions for this current system based on steady plasma outflow from the Io torus, and consider how they depend on the values of the effective jovian ionospheric Pedersen conductivity and iogenic plasma mass outflow rate. We present a full normalised solution for a simple dipole planetary field and develop a normalised analytic approximation for a realistic current sheet model. We compare the analytic approximation with the full numeric current sheet solution and find it to be excellent for the parameter range of interest at Jupiter.

P Hanlon, M Dougherty and G Giampieri (Imperial College, London). Jovian plasma sheet dynamics

The dynamics and field structure of the Jovian magnetosphere are dominated by the presence of a vast current carrying plasma sheet centred about the magnetic equator. The magnetic field due to the predominantly azimuthal current carried by this plasma sheet, extends the field lines of the middle magnetosphere radially outward producing the elongated structure that is characteristic of the system. We employ magnetic field data gathered by the Galileo spacecraft in order to investigate the structure of the plasma sheet with radial range.

G Giampieri and M K Dougherty (Imperial College, London). Modelling of the ring current in Saturn's magnetosphere

The existence of aá ring current inside of Saturn's magnetosphere was first suggested by Ness et al. [1981,1982], in order to explain various features in the magnetic field observations from the Voyager 1 and 2 spacecraft. Connerney et al. [1983] have formalized the equatorial current model and estimated its parameters from the two Voyager data sets. Here, we further investigate the model, by reconsidering the same data from the two Voyager spacecraft as well as including the Pioneer 11 flyby data set. First, we obtain, in closed form, an analytic expression for the magnetic field produced by the ring current. We then fit the model to the external field, that is the difference between the observed field and the internal magnetic field, considering all the available data. We point out differences among the model's parameters for the three flybys, and also investigate possible deviations from the axial and planar symmetries assumed in the model.

I C F Mueller-Wodarg (Atmospheric Physics Laboratory, University College London, UK), R V Yelle (Lunar and Planetary Laboratory, University of Arizona, USA), M Mendillo (Center for Space Physics, Boston University, USA) and A D Aylward (Atmospheric Physics Laboratory, University College London, UK). Eclipses on Titan: Effects on its upper atmosphere

Every 15 years Titan's orbit around Saturn lies within the equatorial plane and is eclipsed when passing behind Saturn, spending up to 6 hours per day in total darkness. This affects temperatures, dynamics and composition in its atmosphere. The condition for eclipse persists for around 23 Titan days. For the first time, a General Circulation Model has been used to calculate the global effect of such an eclipse on Titan and revealed a strong response from the thermosphere, with temperatures dropping by around 20 K and waves propagating globally. These effects could be observable in an extended mission of the Cassini spacecraft, from year 2009 onwards, when Titan approaches equinox.

T Moffat, A Aylward and I Mueller-Wodarg (Atmospheric Physics Laboratory, University College London). Battles with the god of war: Taming the UCL Mars model

The UCL three dimensional numerical model of the Martian thermosphere has recently been updated to allow for more flexibility in specifying the spatial resolution, a more realistic description of the physical processes, and an improved structure for future developments. Results from this updated model are presented for different seasons and compared with output from other models as well as measurements. We discuss these results and where they are leading in the development of the model.

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