T. Dudok de Wit

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.

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.

Wavelet bicoherence analysis of strong plasma turbulence at the Earth’s quasiparallel bow shock

Nonlinear interactions in strong plasma turbulence have been investigated in the vicinity of the Earth’s quasiparallel bow shock by making use of magnetometer data from the AMPTE–UKS (Active Magnetospheric Particle Tracer Explorers–United Kingdom Subsatellite) spacecraft. Using a new and more robust estimator of the bicoherence, it is shown that the large‐amplitude monolithic magnetic structures observed upstream the shock front interact with the whistler wave trains that accompany them. A unified picture emerges, in which the whistlers progressively grow from these soliton‐like structures through nonlinear interaction, similarly to the dynamical evolution of step‐like profiles in the Korteweg–de Vries equation.

Nonlinear decay of foreshock Langmuir waves in the presence of plasma inhomogeneities: Theory and Cluster observations

[1]xa0The intense high-frequency electrostatic waves observed in the terrestrial foreshock often have a form of a superposition of two monochromatic waves close to the plasma frequency. We suggest an interpretation of these spectra as signatures of nonlinear decay of Langmuir waves to electron-sound and ion-sound secondary waves. This decay instability is known to have different properties in inhomogeneous plasma, namely the threshold amplitude of this instability is inversely proportional to the scale of the inhomogeneity. We show that the observed dependence of the wave amplitude on the modulation scale of the wave packets is consistent with this property and the theory of absolute decay instability in inhomogeneous plasma can be applied to explain the satellite observations. In this study we used electric field data from the Wide Band Data instrument on board Cluster satellites.

Determination of the electron anomalous mobility through measurements of turbulent magnetic field in Hall thrusters

Measurements of the turbulent magnetic field in a Hall thruster have been carried out between 1kHz and 30MHz with the aim of understanding electron transport through the magnetic field. Small detecting coils at the exit of the accelerating channel and outside of the ionic plume were used to characterize various instabilities. The characteristic frequencies of the observed power spectral densities correspond to known classes of instabilities: low frequency (20–40kHz), transit time (100–500kHz), and high frequency (5–10MHz). A model of the localized electron currents through a magnetic barrier is proposed for the high-frequency instability, and is found to be in good quantitative agreement with the observations. Based on the measured high-frequency turbulent magnetic field, the turbulent electric field is estimated to be about 1V∕cm outside of the plume and ranges from 10to102V∕cm at the channel midradius at the exit of the thruster. The “anomalous” electron collision frequency, related to the high-frequenc...

The FIELDS Instrument Suite for Solar Probe Plus

NASA’s Solar Probe Plus (SPP) mission will make the first in situ measurements of the solar corona and the birthplace of the solar wind. The FIELDS instrument suite on SPP will make direct measurements of electric and magnetic fields, the properties of in situ plasma waves, electron density and temperature profiles, and interplanetary radio emissions, amongst other things. Here, we describe the scientific objectives targeted by the SPP/FIELDS instrument, the instrument design itself, and the instrument concept of operations and planned data products.

Non-Gaussian statistics in space plasma turbulence: fractal properties and pitfalls

Statistical properties of collisionless plasmas in the vicinity of the Earths bow shock are investigated with the aim to characterize the intermittent behaviour of non- magnetohydrodynamic turbulence. The structure functions of the fluctuating magnetic field reveal an increasing departure from Gaussianity at small scales, which is similar to that observed in solar wind turbulence and is surprisingly little affected by the abrupt shock transition. While these results may be the signature of a multifractal process, a deeper inspection reveals caveats in such an interpretation. Several effects, including the anisotropy of the wavefield, the violation of the Taylor hypothesis and the occasional occurrence of coherent wave packets, strongly affect the higher order statistical properties. Most of the small differences observed between the up- and downstream sides of the shock can be ascribed to the occurrence of discrete whistler wavetrains, while the wavefield itself is much less intermittent. It is also shown how the finite length of the records prohibits a reliable estimation of structure functions beyond the fourth order. These results preclude an unambiguous identification of underlying models for intermittency.

Identifying nonlinear wave interactions in plasmas using two-point measurements: A case study of Short Large Amplitude Magnetic Structures (SLAMS)

Two fundamental quantities for characterizing nonlinear wave phenomena in plasmas are the spectral energy transfer associated with the energy redistribution between Fourier modes, and the linear growth rate. It is shown how these quantities can be estimated simultaneously from dual-spacecraft data using Volterra series models. We consider magnetic field data gathered upstream the Earths quasiparallel bow shock, in which Short Large Amplitude Magnetic Structures (SLAMS) supposedly play a leading role. The analysis attests the dynamic evolution of the SLAMS and reveals an energy cascade toward high-frequency waves. These results put constraints on possible mechanisms for the shock front formation.

Waveforms of Langmuir turbulence in inhomogeneous solar wind plasmas

Modulated Langmuir waveforms have been observed by several spacecraft in various regions of the heliosphere, such as the solar wind, the electron foreshock, the magnetotail, or the auroral ionosphere. Many observations revealed the bursty nature of these waves, which appear to be highly modulated, localized, and clumped into spikes with peak amplitudes typically 3 orders of magnitude above the mean. The paper presents Langmuir waveforms calculated using a Hamiltonian model describing self-consistently the resonant interaction of an electron beam with Langmuir wave packets in a plasma with random density fluctuations. These waveforms, obtained for different profiles of density fluctuations and ranges of parameters relevant to solar type III electron beams and plasmas measured at 1 AU, are presented in the form they would appear if recorded by a satellite moving in the solar wind. Comparison with recent measurements by the STEREO and WIND satellites shows that their characteristic features are very similar to the observations.

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.