Welcome back to part 3 of Understanding Double Layer.
In the first two parts, we explored the main concepts of double layers
and how experiments led to a circuit approach to understanding these plasma phenomena. In this
part, we will examine how double layers could help explain a variety of different phenomena.
The basic properties of a plasma are likely to be the same in different regions of cosmic plasmas.
The linear dimensions of plasma vary by 10^27 in three jumps of 10^9
From the laboratory plasma - 0.1m To the magnetospheric plasmas - 10^8m
To interstellar plasmas 10^17m Up to the Hubble distance - 10^26m
Results from laboratory plasma physics and in situ measurements by spacecraft in the magnetospheres
make sophisticated plasma diagnosis possible out to the reach of spacecraft. Plasmas at
larger distances should to a large extent be investigated by extrapolation. This is possible
because of our increased knowledge of how to translate results from one region to another.
Hannes Alfven goes on to state. The necessary paradigm transition leads to theories based
on experiments and observations. It started in the laboratory in the 1960s.
In situ measurements in the magnetospheres caused a similar paradigm transition there. This can be
depicted as a knowledge expansion, which so far has stopped at the reach of spacecraft.
The results of laboratory and magnetospheric research should be extrapolated further out.
When this knowledge is combined with direct observations of interstellar and intergalactic
plasma phenomena, we can predict that a new era in astrophysics will begin,
largely based on the plasma universe model. And so here we are nearly 40 years
after Alfven wrote this paper and it really feels like we have not moved much further forward.
So let’s move on and examine some examples of cosmic double layers
Auroral Circuit
This is by far the best know example.
It is derived from a large number of measurements in the magnetosphere and in the ionosphere which
were pioneered by the Applied Physics Laboratory at John Hopkins University.
Zmuda and Armstrong observed that the average magnetic field in the magnetosphere
had superimposed on its transverse field which they interpreted as due to hydromagnetic waves.
Inspired by a discussion with Falthammer, Dessler suggested that the transverse field
components instead indicated electric currents essentially parallel to the magnetic field lines.
This means that it was Dessler who discovered the electric currents which Birkeland had predicted.
Dessler called them Birkeland currents.
In the auroral current system, the central body maintains a dipole field. B1 and B2 are magnetic
field lines from the body. C is a plasma cloud near the equatorial plane moving in the sunward
direction producing an electromotive force which gives rise to a current in the circuit c1,
a1, a2, c2. The circuit may contain a double later which is essentially used for accelerating
auroral electrons. The energy is transferred from C to the double layer and is not caused
by magnetic reconnection. It is a property of the electric circuit.
Heliospheric current
Hannes Alfven viewed that the Sun acts as a unipolar inductor producing a current which
during odd solar cycles goes outward along the axes in both directions
and inward in the equatorial plane. The current closes at a large distance, the exact location is
currently unknown. The equatorial current layer is often very inhomogenous. Further, it moves up and
down like the skirt of a ballerina. In even solar cycles the direction of the current is reversed.
By analogy with the magnetospheric circuit, we may expect the heliospheric circuit to have
double layers. They should be located at the axis of symmetry, but perhaps preferentially in those
solar cycles when the axial current is directed away from the sun. They should produce high energy
electrons and synchrotron radiation which should make them observable. Since Hannes wrote this
paper we have made many more measurements and in particular, both Voyager probes
have now left the Heliopause and detected signals from beyond this. When you examine
some of the radio sources they detected you find that they do indeed detect a strange radio source
which is coming from the region above the axis of the sun. Could these be from the sun’s DL?
Double Radio Sources
Hannes Alfven felt that if you replaced the rotating magnetised sun in heliospheric circuit by
a galaxy, which is also magnetised and rotating. Then there should be a similar current system,
but magnified by about 9 orders of magnitude. Inside the galaxy, the current may flow in the
plane of symmetry similar to the current sheet in the equatorial plane of the sun.
The EMF which is derived from the galactic rotation
is applied to two circuits in parallel, one to the north and one to the south.
In the magnetosphere, the current flowing out from the ionosphere produces double layers
at some distance from the earth. Because of the similarity of the plasma configuration,
we may expect double layers at the axis of a galaxy, and a large release of energy in them.
These locations may coincide with the double radio sources found in some active galaxies.
There is a flow of thermal electrons to the layer from the outside and when passing a series of
double layers the electrons are accelerated to very high energies. Hence a beam of very
high energy electrons is emitted from the double layer along the axis towards the central galaxy.
In analogy with the current in the magnetotail, the current in the equatorial plane of a galaxy
may also produce double layers, which may be associated with large releases of energy.
Although the electrons are primarily accelerated in the direction of the magnetic field,
they will be scattered by magnetic inhomogeneities and spiral in such a way that
they emit synchrotron radiation. The accelerated electrons will be more like an extremely hot gas
than a beam. With increasing distance from the double layer, the electrons will spread
and their energy and hence their synchrotron radiation will decrease.
Ions passing the double layer in the outwards
direction will be accelerated to the same energy as the electrons.
Because of their larger rest mass, they will not emit much synchrotron radiation.
It should be stressed that, just as in the magnetosphere and in the laboratory,
the energy released in the double layer derives from the circuit energy and is transferred to
it by electric currents which essentially consist of relatively low energy particles.
There is no need for a beam of high energy particles to be shot out from
the central galaxy. On the contrary, the central galaxy may be bombarded by high
energy electrons which have obtained their energy from the double layer.
Solar Prominence Circuit - Solar Flares
The circuit consists of a magnetic flux tube above the photosphere.
Hannes Alfven thought the generator in this circuit was due to a whirl motion in the
sunspots magnetic field. In the electric sun model, this would come from the imbalance of
charge due to the sun being electron deficient. When the generator output increases the circuit
energy can be dissipated in two different ways: When current density surpasses a critical value
an exploding double layer is produced in which most of the circuit energy
is released. This causes a solar flare Under certain circumstances, the electromagnetic
pressure of the current loop may produce a motor which gives rise to a rising prominence.
Magnetic substorms
According to Bostrom, an explosion of the transverse current in the magnetotail
gives an attractive mechanism for the production of magnetic substorms.
Bostrom showed that an equivalent magnetic substorm circuit is a way of presenting
the substorm model. The onset of a substorm is due to the formation of a double layer,
which interrupts the cross-tail current so that it is redirected to the ionosphere.
Currents and Double Layers in Interstellar space.
As it is relatively easy to measure magnetic fields, it is natural that the first description
of the electromagnetic state of interstellar and intergalactic space is based on magnetic
field descriptions. However, as no one claims - at least not explicitly - that the magnetic
fields are curl-free, we must have a network of currents. As investigations of double layers
require explicit pictures of electric currents, it is essential to apply these pictures.
Filamentary structures have been observed for some time
and maybe observed everywhere where sufficient accurate observations can be made.
Many of these filaments may produce double layers and some of these may explode.
These explosions may account for x-ray and gamma-ray bursts.