Simon N. Walker

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.

THEMIS observations of mirror structures: Magnetic holes and instability threshold

Nonpropagating mirror-mode structures are commonly observed in many regions of natural plasma such as solar wind, planetary magnetosheaths, in cometary plasma, Io wake, terrestrial ring current and even on the outskirts of solar system. Mirror structures are typically observed in the shape of magnetic holes or peaks. Fast survey mode plasma data from the THEMIS satellites are used to solve the puzzle of how mirror structures in the form of dips can be observed in the regions of mirror stable plasma. THEMIS data also show that for mirror structures with spatial scales that considerably exceed ion Larmor radius the perpendicular temperature anticorrelates with the strength of the magnetic field. This contradiction with the conservation of adiabatic invariants is explained by the role of trapped particles. Citation: Balikhin, M. A., R. Z. Sagdeev, S. N. Walker, O. A. Pokhotelov, D. G. Sibeck, N. Beloff, and G. Dudnikova (2009), THEMIS observations of mirror structures: Magnetic holes and instability threshold, Geophys. Res. Lett., 36, L03105, doi:10.1029/ 2008GL036923.

Ramp nonstationarity and the generation of whistler waves upstream of a strong quasiperpendicular shock

A number of models have been proposed to explain the observation of low frequency (LF) whistler waves in the upstream region adjacent to the ramp of the quasiperpendicular part of the Earths bow shock. One such model relates these waves to the nonlinear dynamics of the ramp itself. Two aspects of this model, the plasma frame spectrum of the turbulence and the occurrence of nonlinear waves propagating away from the shock in the upstream region have already been shown to be in agreement with observations. A third aspect, the interaction and energy transfer between these nonlinear waves and the whistler turbulence, is studied by applying high order spectral analysis to Interball magnetic field data. Evidence for the nonlinear coupling between these two types of waves and the possible transfer of energy are presented.

An Ultra-high Speed Public Key Encryption Processor

This paper describes the architecture and design of a public key encryption processor which implements the RSA algorithm with key lengths of512 bits. The chip, which is 6.2 by 4.2 millimetres, has been designed in a 0.7 micron CMOS, silicon on insulator process and has a target clock speed of 15OMHz. It is a self contained subsystem which interfaces directly to standard microprocessors and will be capable of encrypting at rates well in excess of 64k baud (for contractural reasons we are unable, at this time, to disclose the emct speed of operation).

Dual-spacecraft observations of standing waves in the magnetosheath

An unambiguous determination of the wave modes observed in plasma turbulence requires the determination of the dispersion characteristics of the wave. The method adopted for the determination of the wave dispersion relation is based upon the study of the phase difference of the waves at two closely separated spacecraft [Balikhin and Gedalin, 1993]. It is applied to observations of waves in the magnetosheath magnetic field made by AMPTE UKS and AMPTE IRM. We show that the observed waves are propagating in the sunward direction but are convected toward the magnetopause by the plasma flow. As a result, the observed waves are quasi-standing in the flow. These waves have been previously identified as either slow or mirror modes depending the procedure adopted for their analysis. They may also be an experimental observation of the mirror and slow (MIAOW) waves identified near the magnetopause boundary in the hybrid simulations of Omidi and Winske [1995]. The observed waves possess short wavelengths (λ) such that RBi/λ ≈ 1 (where RBi is the ion Larmor radius). Thus they may only be studied analytically within the framework of the kinetic approximation. Both the nonlinear amplitudes and possibly the strong underlying inhomogeneity are responsible for the significant differences between the properties of the observed modes and those resulting from linear homogeneous kinetic theory.

Ion sound wave packets at the quasiperpendicular shock front

Electric field measurements from a single spacecraft have been used to study ion-sound turbulence observed within the Earths bow shock. The observed frequency of the ion-sound waves can be both lower and higher than the local electron cyclotron frequency depending upon the direction of wave propagation in the plasma rest frame. The ion-sound waves observed upstream of the ramp can not be generated either by an instability related to the gradient in the electron temperature or an electric current within the ramp. A comparison of wave vectors for distinctive wave packets indicate that non-stationary, short scale current layers formed in the processes of the ramp evolution might be the source of the free energy for such waves.

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.

Experimental determination of the dispersion relation of magnetosonic waves

Magnetosonic waves are commonly observed in the vicinity of the terrestrial magnetic equator. It has been proposed that within this region they may interact with radiation belt electrons, accelerating some to high energies. These wave-particle interactions depend upon the characteristic properties of the wave mode. Hence, determination of the wave properties is a fundamental part of understanding these interaction processes. Using data collected during the Cluster Inner Magnetosphere Campaign, this paper identifies an occurrence of magnetosonic waves, discusses their generation and propagation properties from a theoretical perspective, and utilizes multispacecraft measurements to experimentally determine their dispersion relation. Their experimental dispersion is found to be in accordance with that based on cold plasma theory.

On the fine structure of dipolarization fronts

Measurements from the closely spaced Cluster spacecraft are used to study the structure of the magnetic and electric fields within the magnetic ramp of dipolarization fronts (DF) observed close to the neutral sheet and the midnight meridian (YGSM<3RE). The spacecraft separation was small enough (<300 km) to treat the magnetic ramp of the DF front as a planar structure as indicated from variance analysis. The finite value of the magnetic field along the minimum variance direction for the events studied indicates that the dipolarization front structure was distinct from a tangential discontinuity. In addition to the main increase of the magnetic field in the maximum variance component, strong oscillations were observed in the intermediate component. The presence of this oscillatory structure results in an expansion of the region in which a change of magnetic pressure occurs, the size of which is typically an ion Larmor radius or greater. This widening is important in maintaining the pressure balance at the edge of the DF. This phenomenon resembles observations of intense current sheets in the magnetotail and also laboratory experiments of current sheet formation, in which a similar widening of the ramp region has been observed. In this paper we argue against the idea that an electron temperature anisotropy, resulting in electron curvature currents, can explain the formation of the oscillatory structures observed at DFs. These oscillations can be explained as eigenmode waves of the plasma that propagate away from the disturbance (DF) that is moving at subsonic speeds. Oscillations observed within the magnetic ramp indicate field-aligned currents that are expected to be associated with DF.