D. J. Southwood

Some features of field line resonances in the magnetosphere

Abstract Field line resonances in the magnetosphere have received much attention. By using an extremely simplified model we examine the circumstances under which finite disturbance amplitude solutions of the coupled wave equation can be obtained in the vicinity of the resonant field line. General features of solutions are noted and the relevance of recent experimental work to the problem is pointed out. The observed latitude dependence of polarisation provides strong evidence of the role of resonating field lines as

The Cluster Magnetic Field Investigation

The Cluster mission provides a new opportunity to study plasma processes and structures in the near-Earth plasma environment. Four-point measurements of the magnetic field will enable the analysis of the three dimensional structure and dynamics of a range of phenomena which shape the macroscopic properties of the magnetosphere. Difference measurements of the magnetic field data will be combined to derive a range of parameters, such as the current density vector, wave vectors, and discontinuity normals and curvatures, using classical time series analysis techniques iteratively with physical models and simulation of the phenomena encountered along the Cluster orbit. The control and understanding of error sources which affect the four-point measurements are integral parts of the analysis techniques to be used. The flight instrumentation consists of two, tri-axial fluxgate magnetometers and an on-board data-processing unit on each spacecraft, built using a highly fault-tolerant architecture. High vector sample rates (up to 67 vectors s-1) at high resolution (up to 8 pT) are combined with on-board event detection software and a burst memory to capture the signature of a range of dynamic phenomena. Data-processing plans are designed to ensure rapid dissemination of magnetic-field data to underpin the collaborative analysis of magnetospheric phenomena encountered by Cluster.

The hydromagnetic stability of the magnetospheric boundary

Abstract The hydromagnetic Kelvin-Helmholtz stability problem is studied for an infinite plane interface between compressible infinitely conducting fluids. The critical value of the relative streaming velocity for stability is studied by use of the equations for marginal stability without making the simplifying physical assumptions used by previous authors. In application to the magnetosphere boundary we find we can make some predictions without too precise a knowledge of all the parameters involved. At middle and low latitudes the first growing modes propagate across the Earths field with a very low phase velocity and wave fronts closely aligned to meridian planes. The modes tend to exhibit circular polarisation in a plane almost perpendicular to the Earths field. This behaviour should also occur at high latitudes when the magnetosheath field is closely aligned to he Earths field.

What are flux transfer events

Abstract We argue that surges in the reconnection rate on the magnetopause give rise to bubble-like regions of plasma containing a twisted field with energetic streaming particles in the outer layers. We propose that flux transfer events observed on spacecraft are the result of passage through or nearby such bubbles. All important observational features of flux transfer events fit qualitatively well with the model. Predictions of phenomena in the ionosphere associated with FTEs based on our model are substantially different from those predicted on earlier models such as Russell and Elphics; events could have a much larger footprint than believed hitherto. Correspondingly, predictions of the electromotive force imposed around the terrestrial polar cap by FTE occurrence may need upward revision.

Theory of hydromagnetic waves in the magnetosphere

Many of the significant theoretical advances in understanding the origin and behaviour of low frequency hydromagnetic waves originating in the magnetosphere in the last decade are reviewed. Topics covered include wave generation mechanisms, wave damping, effects of inhomogeneity, signal behaviour in the ionosphere and atmosphere.

Mirror instability: 1. Physical mechanism of linear instability

The mirror instability is prevalent in planetary and cometary magnetosheaths and other high beta environments. We review the physics of the linear instability. Although the instability was originally derived from magnetohydrodynamic fluid theory, later work showed that there were significant differences between the fluid theory and a more rigorous kinetic approach. Here we point out that the instability mechanism hinges on the special behavior of particles with small velocity along the field. We call such particles resonant particles by analogy with other uses of the term, but there are significant differences between the behavior of the resonant particles in this instability and in other instabilities driven by resonant particles. We comment on the implications of these results for our understanding of the observations of mirror instability-generated signals in space.

Bounce resonant interaction between pulsations and trapped particles

Abstract The exchange of energy between a given wave and the energetic particle population is investigated, with emphasis on waves with a rapid East-West variation of phase. Reasons are given why these are of particular interest, but the field disturbance has yet to be computed in detail and here a physical account is presented leading to rough estimates for the damping or amplification. The techniques applied to gyroresonance are followed as closely as possible, but bounce resonance is more complicated, and great simplifications are made, particularly in neglecting harmonics. Transformation to the rotating frame of the wave gives a simple way of calculating the change in energy of a single particle for a given change in L . The resonance condition including the Doppler shift due to the East-West particle drift is a quadratic for the particle energy. Typically there is a low energy for which the bounce frequency almost equals the wave frequency and a high energy for which the Doppler shifted frequency is much higher than the wave frequency. The high energy resonance is more favourable for wave amplification by the kind of particle distribution known to exist. Typically the high energy electrons are relativistic and here attention is given to the high energy proton resonance. For westward travelling waves, protons which move out lose energy and the outer edge of a proton belt could supply energy to such a wave via bounce resonance. Calculations are based on the strongest known mechanism. This requires a disturbance in B r at the equator, which has been observed by Coleman. Because of ignorance of the wave fields it was found convenient to use quasilinear theory rather than linear theory in estimating the damping or growth rate. The resulting expressions for the exchange in energy are easier to interpret physically. The results show that the damping is quite powerful and the conditions required for amplification are discussed. It is pointed out that the East-West component of the group velocity of such waves is small and this may be important in applications to the magnetosphere.

The Heliospheric Magnetic Field Over the South Polar Region of the Sun

Magnetic field measurements from the Ulysses space mission overthe south polar regions of the sun showed that the structure and properties of the three-dimensional heliosphere were determined by the fast solar wind flow and magnetic fields from the large coronal holes in the polar regions of the sun. This conclusion applies at the current, minimum phase of the 11-year solar activity cycle. Unexpectedly, the radial component of the magnetic field was independent of latitude. The high-latitude magnetic field deviated significantly from the expected Parker geometry, probably because of large amplitude transverse fluctuations. Low-frequency fluctuations had a high level of variance. The rate of occurrence of discontinuities also increased significantly at high latitudes.

Damping of geomagnetic pulsations by the ionosphere

Abstract A significant sink of geomagnetic pulsation energy is due to Joule dissipation in the ionosphere. To investigate this we have computed the damping experienced by standing Alfven waves in a dipole magnetic field. Both the uncoupled poloidal and toroidal modes are considered with Joule dissipation being introduced through a boundary condition which relates the electric and magnetic field strengths at the ionosphere, viz: 4πΣ p E c = b, where Σ p is the height integrated Pederson conductivity. The damping rates are strongly dependent on the ionospheric conductivity and we find that typically the normalized damping rate, γ ω , is ∼0.1 for nightside values of conductivity and ∼0.01 for the dayside. This would account for the observed scale of bandwidths in pulsation signals. Away from regions of extreme damping we find γ ∝ L−1 Σp−1.

An illustration of modification of geomagnetic pulsation structure by the ionosphere

In this paper we illustrate the results of a companion paper (Hughes and Southwood, 1976) in which we describe how geomagnetic pulsations are screened by the atmosphere and ionosphere. Here, using Fourier synthesis, we map down to the earths surface the fields predicted in the vicinity of a resonating magnetospheric field line by Southwood (1975a). The results demonstrate that the ionosphere-atmosphere system screens out rapid horizontal variation and also provide an illustration of the ionospheric polarization rotation phenomenon. Comparison with recent experimental work is made.