When we examine space there are many phenomena that are hard to explain using the concept of
magnto-hydrodynamics and I have recently covered the many flaws with this model. It works well in
approximating in some scenarios but by no means can be used in all cases. Hannes Alfven was one
of the pioneers in developing this approach but realised later on that this approximation
was limiting our ability to understand what was actually going on. He had studied plasmas
in the laboratory for years and saw that much of astronomy was simply ignoring how plasma actually
behave. One of these was the double layer. He felt it was so fundamentally important that it should
be considered a celestial object in its own right. Due to this, I want to cover this in three parts.
In the first, we will examine the general properties of double layers. In part 2 we
will look at laboratory experiments and finally, we will examine examples of cosmic double layers.
Part 1: General Properties of Double Layers
It was Langmuir who first discovered what is now termed as a double layer.
A double layer is created in order to separate a plasma from its environment.
It can be considered analogous to a biological cell wall.
Whenever an electrical discharge is produced between a cathode and an anode, double layers
will be formed. One near the cathode called a cathode sheath. This accelerates electrons which
carry a current through a plasma. At the same time, a double layer is set up near the anode,
protecting the plasma from this electrode. A space charge constitutes the border between the
double layer and the plasma. All these double layers carry electric currents.
The lateral limitation of the plasma is also produced by double layers, which reduce and
slow down the escape of the rapid electrons and accelerated the positive ions outwards so that an
ambipolar diffusion is established. If the plasma is enclosed in a vessel, its walls get a negative
charge and a positive space charge is set up which is the border between the double layer
and the plasma. If the discharge constricts itself the walls can be taken away with no
influence on the space charge they carry. In these double layers, the net electric current is zero.
If the cathode itself emits electrons through for example a photoelectric emitter, the sign
of the cathode fall may be reversed, so that the double layer is limited by a negative space
charge which acts as a virtual cathode. The anode fall may also be reversed.
The lateral double layer may also change sign.
This occurs in a dusty plasma if the dust is negatively charged.
This could happen if the dust absorbs most of the electrons. In this case we have a
reversed plasma where the positive ions form the lighter component compared to the negative ones.
If a plasma is inhomogenous so that the chemical composition, density,
and/or electron temperature differs in different parts of the plasma, the plasma may set up double
layers which split the plasma into two or more regions, each of which becomes more homogeneous.
There are innumerable variations and complications of these simple cases
but this is not different to how biological cell walls
show innumerable variations. If we try to increase the current by increasing the applied voltage,
the plasma may produce a double layer. This takes up part of the voltage so that the plasma
current density does not exceed a certain value. Hence the plasma divides itself into two cells,
analogous to what a biological cell does when it gets a large energy input.
The mechanism of a Double Layer
Electrons and ions will move in opposite directions. Because of this counterflow
instabilities are set up within the current. The net result is a reconfiguration of the
current-carrying charges over a narrow region so as to cause a potential drop.
Due to this potential drop a beam of electrons is accelerated to the right
while a beam of ions is accelerated to the left.
The classic double layer is an electrostatic structure which
can appear within a current-carrying plasma and sustain a significant net potential difference.
Four populations of particles are required to produce the space charge distribution needed to
support the double layer potential. These are: 1. the current-carrying streaming electrons
2. the current-carrying streaming ions 3. Trapped electrons on the downstream
side of the streaming electrons
4. Trapped ions on the downstream side of the streaming ions
The streaming electrons and ions are present because the field-aligned current
is an essential ingredient of a double layer. Due to the acceleration of the streaming particles by
the double-layer potential, the density of each streaming particle population decreases on the
downstream side. Therefore, trapped particles must be produced to ensure overall charge neutrality.
The double-layer acts as an electrical load dissipating energy at a particular rate,
which is transformed to the direct kinetic energy of accelerated particles. Thus the DL exhibits
inertial resistance. This should be contrasted with the effect of ohmic resistance, which is to
transform electrical energy to thermal or random particle motions. Since the DL acts as a load,
there had to be an external source maintaining the potential difference and driving the current.
Noise in Double Layers
There is one property of a double layer which is often neglected. Double layers often if not always
produce noise and fluctuations. Lindberg studied the noise in a stationary fluctuating double layer
and demonstrated that it broadens the energy spectrum of the electrons.
The plasma may expand perpendicular to the magnetic field. The electrons in the beam which
is produced in the double layer are scattered much more by the noise than by collisions.
Noise is often associated with strong currents through the plasmas.
Langmuir proposed that random impulses strongly scatter electrons in a gas discharge.
This noise is often incorrectly called turbulence.
It is fundamentally important to the property of plasmas that theories which do not take
this into consideration run some risk of being irrelevant. Any computer simulations
which do not produce noisy double layers should be regarded with some scepticism.
Peratt et al. have been able to simulate in detail,
experiments on the interaction of two plasma filaments produced by exploding wires.
Both electrostatic and electromagnetic fluctuations are implicitly included.
Since the start of thermonuclear research, plasma theories have absorbed a large part of
the energies of the best physicists of our time. The progress of this field is much less than was
originally expected. The reason may be that from the point of view of the traditional theoretical
physicist, a plasma looks immensely complicated. We may express this by saying that when,
by an immense number of vectors and tensors and integral equations, theoreticians have
prescribed what a psalm must fo, the plasma - like a naughty child - refuses to obey.
One way to try and address this problem is through plasma experiments.
If we confine ourselves to cosmic plasmas, there are two different ways of doing this:
By performing scale model experiments in the laboratory.
This requires a sophisticated technique, which in part borrows from thermonuclear physicists
but also requires methods to translate laboratory results to cosmic situations.
By using space as a laboratory and performing the experiments in space
Field and Particle Aspects of Plasmas
Space measurements of magnetic fields are relatively easy, whereas direct measurements
of electric currents are very difficult and in many cases impossible. Hence it is natural to
present the results of space exploration with pictures of the magnetic field configuration.
Furthermore, in magnetohydrodynamic theories, it is convenient to eliminate the current.
This method is acceptable in the treatment of a number of phenomena.
However, there are also a number of phenomena which cannot be treated in this way.
These require an approach in which the electric current is taken into account.
Phenomena which cannot be understood without explicitly accounting for the current are:
The formation of double-layer The occurrence of explosive
events such as solar flares, magnetic substorms, internal ionisation in comments and stellar flares
Formation of filaments in the solar atmosphere, in the ionosphere of Venus
and in the tails of comets and in interstellar nebulae.
Formation of current sheets which may give space a cellular structure.