Properties of Magnetized Dusty Plasmas
Dusty plasmas on a new wavelength
Neil Cramer and
Sergey Vladimirov
have found that waves in
magnetized plasmas behave very differently when dust is present.
In a plasma which is threaded by a magnetic field, the field
lines wiggle or twist if part of the plasma is displaced in some
manner; these disturbances can propagate away from their origin,
along the magnetic field lines, as ``Alfvén'' waves. The
properties of Alfvén waves in ``ordinary'' plasmas have been
thoroughly investigated and are well known. Understanding these
properties is crucial in comprehending the behavior of any
dynamic plasma, because the waves transport energy and momentum
through the plasma and in this way communicate any applied forces.
Neil Cramer and
Sergey Vladimirov
have investigated Alfvén wave
behavior in dusty plasmas and they have uncovered some
fundamental new results.
One particularly interesting feature is the discovery that
low-frequency Alfvén waves can be efficiently absorbed in dusty
plasmas. In the absence of dust, negatively charged electrons and
positively charged protons generate equal and opposite electrical
currents as low frequency waves pass by. However, dust grains are almost
immobile (because they are exceedingly massive in comparison with
electrons and protons) and a large proportion of the electrons
reside on the dust grains, so in dusty plasmas there is an imbalance in
the electrical currents generated by the passage of an Alfvén wave.
The net current can then excite large oscillations in the
magnetic field lines where the wave energy is dumped and the
plasma is heated.
Thus in attempting to shove around a lump of dusty plasma, a lot of
energy is dissipated as heat within the plasma where the Alfvén waves
are absorbed. This property is especially important in understanding
the collapse of dusty interstellar clouds into protostars
Neil and
Sergey's
work leads us to expect quite rapid collapse
as the applied gravitational forces are efficiently dissipated into
heat.
Left: A micrograph of a typical plasma dust grain magnified 500 thousand times.
Courtesy of Professor Yukio Watanabe, Kyushu U.
Right: Sketch of the constituents of a dusty plasma, showing the relative
dearth of electrons which have been absorbed by the dust grains.
In reality the dust grains can be one billion times bigger than the ions,
and the electrons are essentially point-like.
Dusty reconnection in magnetized
plasmas
Everyone is familiar with matter in its three everyday states of
solid, liquid and gas but most of the universe is comprised of matter in a
fourth state, plasma. Magnetic field structures in a magnetized plasma can
rearrange themselves by a process called reconnection.
Neil Cramer and
Sergey Vladimirov
have investigated reconnection in dusty magnetized plasmas.
Plasmas are essentially gases of electrically charged particles, containing
equal numbers of positive (+) and negative (-) particles so they are
electrically neutral overall. In high-temperature plasmas, as occur in a
fluorescent light tube and in many astrophysical situations, the charged
particles are primarily positively charged atomic nuclei and single electrons.
Magnetic fields threading a plasma can guide electric currents along intricate
paths. As movement of the plasma pushes the magnetic field lines around, the
field lines can ``reconnect'' or rearrange themselves to form new structures.
Two current filaments can sometimes merge into one as a result of reconnection,
and the current paths are rearranged as the magnetic field structure changes.
Reconnection can be both rapid and violent, producing shock waves, energetic
particles, and tremendous heating of the plasma.
It has been realised that ``dust'' can occur in the low temperature plasmas
that occur in interstellar molecular clouds, in protostellar clouds where star
formation occurs, and in the atmospheres of cool stars. This dust is made up of
heavy compounds, such as silicates, which have coagulated to form particles a
millionth of a millimeter across. The dust grains soak up electrons - which are light and
very mobile - and become highly negatively charged. This causes asymmetries in
the plasma, because the dust particles are very massive and move very slowly
compared with electrons and nuclei. Neil Cramer,
Sergey Vladimirov and Jun-Ichi
Sakai (University of Toyama) have found that these asymmetries change the behavior of shock
waves and of the reconnection process itself. The dust causes long-wavelength
oscillations in the magnetic field upstream or downstream of a shock rather than
those of much shorter wavelengths which occur in the absence of dust. Charged
dust grains change the reconnection process by causing the current filaments to
twist and rotate about each other like a giant catherine wheel, or to merge
asymmetrically. Such effects may change the timescale for star formation and
have other observable consequences.
The panels show two current filaments (red regions) merging divided by a
current sheet (blue region). The filaments are initially symmetrical (left
panel) but develop lop-sidedly because of the dust, as can be seen in the panel
on the right.
S.Vladimirov@Physics.usyd.edu.au
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