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Wave phenomena in dusty plasmas

Introduction

The study of dusty plasmas is a relatively new area of research in the field of plasma physics. Recent interest in such plasmas has increased because of observation of dust in the vicinity of comets and in laboratory devices, but much of the physics of dust in space is still unknown. A dusty plasmas consists of dust grains (mainly micron sized) embedded in a tradional (e.g. electron-ion) plasma. The plasma particles will hit the dust grains, they become charged by different mechanisms (primary charging, photo-electron effects, secondary electron emission, field emission, ...) and are electromagnetically coupled to the plasma. Different examples are known in the solar system: the rings of the major planets, asteroids, tails and comae of comets and even the lower magnetosphere of the earth. The grains are highly charged, 10.000 electron charges are typical, but their masses are even higher, of the order of 1.000.000 proton masses and higher.

One of the most striking features, explained by the physics of dusty plasmas are the spokes encountered in the B-ring of Saturn. These spokes (shown in the figure as grey radial structures in the dark background in the right lower corner of the photograph) are explained by dust grains that are levitated from large boulders in the ring plane.

Part One: Waves in dusty plasmas

When we look at the literature for the papers on the nature of the charging of dust grains embedded in a plasma, almost no publication links the model to the data from recent space missions.

Therefore the start of the research was collecting data, from the recent space missions to the major planets and comets. We restricted this part by looking at the most promising applications: the ring systems of the giant planets, comae and tails of comets, and dusty plasmas in laboratories. Furthermore we use the data to re-evaluate the important charging mechanisms in the different applications. 

The wave behaviour of a dusty plasmas differs from the behaviour of usual plasmas, because of several reasons:

  1. Characteristic frequencies of the dust components are much smaller than those corresponding with electrons or ions, and therefore the most interesting dusty plasma effects occur for low frequencies.
  2. The mass of the dust grains is much higher than the mass of the plasma particles, and therefore in some cases the gravity forces come into play. This is the case in the generalized Jeans instability.
  3. The number of free electrons is less than the number of ions, because some of the eletrons are captured by the grains, and are therefore immobilized by the high dust masses.
  4. The charge of the dust grain depends on the local plasma conditions (temperature and plasma density), which will vary with the waves coming by and therefore the dust grain charge has to be taken into account as an extra independent variable.
  5. In most space applications the grain size is not fixed, but one encounters a power law for the grain size distribution. This induces a whole continous range of different charge over mass ratios, whereas these ratios are fixed for usual plasmas.
For waves in homegenous dusty plasmas in the presence of an ambient magnetic field and with fixed dust charges (wave frequency much faster than the dust charging frequency), a nonlinear treatment of the waves can be carried out by using a multi-fluid treatment. The results can be used for general multi-species plasmas, but as a special case for dusty plasmas. As a rule, one recovers the Kortweg-de Vries equation or the Nonlinear Schrodinger equation, and their corresponding soliton solutions. 

When we take the variable dust grain charge into account, the theory becomes quickly very complicated, and a better understanding of the charging model becomes a priority. 

Furthermore, we looked into the influence of a dust size distribution on different wave modes, and we showed that new kind of instabilities can occur, due to the dust size distribution (dust distribution instability). 

Part Two: Dust detection methods and the Radio Dust Analyzer

For the planetary and interplanetary medium, some techniques have been developed to measure dust characteristics. 

One of them is the dust detector which provides in situ measurements of the mass and the velocity vector of the dust grains. Another are the plasma wave and radio science experiments on board of several interplanetary spacecraft, detecting broad-band noise, in the ring planes of the outer giant planets and in tails of comets. This noise is believed to be caused by small dust grains bombarding the body of the spacecraft. The few kilometer per second relative velocity between the spacecraft and the dust grains is sufficient to fully vaporize the impacting grains and in part ionize the produced gas. The expanding plasma cloud causes the detected noise, and leaves an ionization signal. This signal has the interest of being local, but they have a limited space coverage since it can only be made along a trajectory of available space probes. Additional data can be obtained by images taken through filters at various phase angles. They can be a source of data for the spatial and size distribution of the dust particles, but they can only reveal properties integrated along the line of sight. 

Recently, we proposed a new technique to derive the dust grain characteristics with the help of a wire dipole antenna. Charged dust grains passing by the antenna induce an electric potential change for the time of the flyby. These ``waveforms'' were studied as a function of the characteristics of the dust grain (its charge and velocity vector) and the plasma parameters. The thermal noise level due to flyby, emission, and impacts of the ambient plasma electrons is calculated and compared with the magnitude of the dust signal. This recently proposed dust-detection technique has been called the Radio Dust Analyzer (RDA).Under construction.

Author: P. Meuris   Curator: XXX  XXX@oma.be

 

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