L. J. C. Woolliscroft

Determination of dispersion relations in quasi-stationary plasma turbulence using dual satellite data

The joint frequency-wavenumber spectrum is one of the basic quantities for analyzing plasma turbulence. It is shown how the full spectrum can be recovered from wavefields measured by two or more satellites via spectral methods based on wavelet transforms. Compared to standard cross-correlation techniques, different branches in the dispersion relation can be resolved and quasi-stationary wavefields can be accessed. Using this new approach, low frequency magnetic field data from the AMPTE-UKS and AMPTE-IRM spacecraft are investigated and the impact of nonlinear processes on wave propagation at the Earths foreshock is revealed.

The Digital Wave-Processing Experiment on Cluster

The wide variety of geophysical plasmas that will be investigated by the Cluster mission contain waves with a frequency range from DC to over 100 kHz with both magnetic and electric components. The characteristic duration of these waves extends from a few milliseconds to minutes and a dynamic range of over 90 dB is desired. All of these factors make it essential that the on-board control system for the Wave-Experiment Consortium (WEC) instruments be flexible so as to make effective use of the limited spacecraft resources of power and telemetry-information bandwidth. The Digital Wave Processing Experiment, (DWP), will be flown on Cluster satellites as a component of the WEC. DWP will coordinate WEC measurements as well as perform particle correlations in order to permit the direct study of wave/particle interactions. The DWP instrument employs a novel architecture based on the use of transputers with parallel processing and re-allocatable tasks to provide a high-reliability system. Members of the DWP team are also providing sophisticated electrical ground support equipment, for use during development and testing by the WEC. This is described further in Pedersen et al. (this issue).

Experimental determination of the dispersion of waves observed upstream of a quasi‐perpendicular shock

Highly-coherent waves in the frequency range 1-15 Hz are usually observed upstream of the ramp of supercritical quasi-perpendicular shocks. A few models have been proposed to explain their origin. In the present paper the wave vectors of these waves are determined using AMPTE UKS and AMPTE IRM data in order to differentiate between theoretical models.

The Wave Experiment Consortium (WEC)

In order to get the maximum scientific return from available resources, the wave experimenters on Cluster established the Wave Experiment Consortium (WEC). The WECs scientific objectives are described, together with its capability to achieve them in the course of the mission. The five experiments and the interfaces between them are shown in a general block diagram (Figure 1). WEC has organised technical coordination for experiment pre-delivery tests and spacecraft integration, and has also established associated working groups for data analysis and operations in orbit. All science operations aspects of WEC have been worked out in meetings with wide participation of investigators from the five WEC teams.

On the nature of low frequency turbulence in the foot of strong quasi-perpendicular shocks

Abstract Low frequency fluctuations of the magnetic field in the vicinity of the Earths bow shock are presented. Observations from the Prognoz-10 (or Intershock) and AMPTE UK satellites are analysed using cospectral techniques. For high Mach number quasi-perpendicular shocks the main low frequency fluctuations seem to be due to whistler waves which are generated by non-linear evolution of the shock front. This is in contradiction to previous proposals that these waves are generated by an instability driven by the reflected ions.

A study of the dispersion of the electron distribution in the presence of E and B gradients: Application to electron heating at quasi-perpendicular shocks

The dynamics of electrons in the presence of E and B field gradients is considered. The results are applied to electron heating in quasi-perpendicular shocks. The main focus is on the transitions between adiabatic and overadiabatic heating and from perpendicular to oblique geometry. Contrary to previous results, it is shown that the divergence of initially close electron trajectories always takes place in the quasi-perpendicular collisionless shock front. In the case of negligible electric field gradients at the shock front, the divergence rate depends only upon the magnetic field gradient and results in adiabatic heating. If the electric field gradient is not negligible, divergence rate depends on increasing electric field gradients. In this case, the divergence rate in the first part of the ramp will exceed the “adiabatic” divergence rate. This enhancement is significant for scales of the electric field of about 4–6 c/ωpe. Under the conditions of the Earths bow shock the magnetic field and particularly its gradients affect the electron trajectory divergence rate and thus the process of thermalization of electrons.

Demagnetization of electrons in inhomogeneous E ⊥ B: Implications for electron heating in shocks

We describe properties of perpendicular electron heating in inhomogeneous E ⊥ B fields. The heating is due to the demagnetization of the electrons when the electric field slope becomes large enough such that e |▿E|/meΩ² > 1, Ω = eB/me. Electrons can be efficiently accelerated across the magnetic field by the electric field. Such a phenomenon may occur in the ramp of a quasi-perpendicular collisionless shock, resulting in the effective transfer of the cross-shock potential energy into the downstream electron gyration energy. This energization should be observed as heating. Numerical analysis of electron dynamics in a model shock structure shows that experimentally observed dependencies are reproduced quite naturally. Efficiency of the proposed mechanism depends on the electric field scale in the shock front. Available observational data do not allow unambiguous conclusions about the presence of necessary scales. On the other hand, electron measurements and comparison with the proposed mechanism can help understand the shock fine structure.

Non-stationarity and low frequency turbulence at a quasiperpendicular shock front

Abstract Previous studies have shown that quasi-monochromatic waves in the frequency range 1–15 Hz are usually observed upstream of the ramp of supercritical quasi-perpendicular shocks. A number of models have been proposed to explain the origin of these waves. In order to differentiate between these models, one has to determine both the observed frequencies and also wave vectors of the measured waves. The present paper is devoted to the determination of the dispersion relation ω( k ) of these waves, using simultaneous data from AMPTE UKS and AMPTE IRM.

AMPTE-UKS observations of current sheets in the solar wind

Abstract Active current sheets or diamagnetic cavities in the solar wind have been observed both by the AMPTE-UKS spacecraft on a number of occasions, and independently from ISEE by Thomsen et al /1/. Preliminary results from one of these UKS observations have been discussed recently by Schwartz et al /2/. In this paper we consider these phenomena in more detail. Results are presented for the position of five events. High resolution plasma data and plasma wave activity associated with these current sheets are examined.

Current measurements in space plasmas and the problem of separating between spatial and temporal variations in the field of a plane electromagnetic wave

A description of an instrument for measuring a current density vector is presented. This instrument uses 3 orthogonal Rogovskii coils. The need for such and instrument is discussed. Together with magnetic measurements it is possible to determine the wave vector of a plane electromagnetic wave using the formula B × CurlB = kB2 = B × [j + ∂E∂t] using direct on-board processing and assuming that ∂E∂t can be neglected in this frequency range. The real frequency of the wave can be calculated using Dopplers law ω = Ω + kυ, where υ is the vehicle velocity and Ω the measured frequency. Test results for prototype coil are presented, together with a discussion of further developments.