Nov 15, 2005
Electric Currents Big and Small
The novelty plasma ball demonstrates many of the properties of
plasma that can be seen in the Sun, in nebulae, and in galaxies.
Blaise Frazier's beautiful photo of the spherical
electrode in the centre of a plasma ball shows a blue filamentary
streamer as it wavers during the 8-second exposure. Thousands of
volts of electricity ionize the gas in the globe, ripping electrons
from molecules and atoms. As electrons recombine with the ions, the
gas gives off light. The colors depend on the kind of gas filling
The plasma ball illustrates some of the fundamental
characteristics of plasma. The blue streamer looks flat, but that is
an illusion of the photography. In reality, the filament is as thin
as the light blue edges. It flickers between the two sides and
extends from the electrode in a thin tube to the outer glass sphere
of the ball.
Sometimes called a plasma cable, or plasma rope, the
filament is the result of electrons and ions flowing through the
plasma (i.e., an electric current). The current generates a magnetic
field that surrounds the filament like hoops around a beer barrel.
The magnetic field pinches the current and keeps it collimated (or
When we take off a nylon sweater in a dark room and
we see tiny sparks fly, we are seeing similar but smaller
filamentary discharges. We see larger filamentary discharges as
lightning in storms. The flares we see erupting from our Sun are
even larger filaments. Because of the greater distances involved and
the different magnetic fields and particle densities, flares appear
to move in slow motion. And finally, filaments the size of solar
systems make up nebulae.
Although non-neutral plasmas can be created in the
laboratory, space plasmas generally contain equal numbers of
negatively charged electrons and positively charged ions. They are
considered to be electrically neutral. And since plasmas are also as
electrically conductive as a lightning rod, this suggests that if a
charge build-up should occur, then it would be neutralised quickly.
But this view, like that of the “flat” filament in the plasma ball,
is an illusion.
In 1831, Michael Faraday invented the homopolar
motor/generator (also called a Faraday disk). It consists of a
conducting disk in an axial magnetic field. The disk may be rotated
in the magnetic field to generate an electric current between its
axis and its edge. Or electric current may be passed from axis to
edge to cause the disk to rotate in the magnetic field. In that way,
the conducting atmosphere of the Sun is caused to rotate faster at
the equator than at higher latitudes, when we should expect the
expulsion of the solar wind to slow it.
Hannes Alfvén described the circuit for this Faraday
motor many decades ago, but he didn’t consider its link with a
greater galactic circuit. The current "disk" of the Sun is the
heliospheric current sheet that flows between the Sun and the outer
reaches of our solar system (see image Thunderbolts, Oct 31, 2005).
And the axial circuit consists of Birkeland currents flowing along
the "open" polar magnetic field lines.
Similarly, the galactic disk is the disk of a
Faraday motor, caused to rotate by Birkeland currents flowing along
the axis and out along the spiral arms. Stars in the spiral arms
receive their power from those galactic currents. Galaxies, in turn,
are threaded like "Catherine wheels" on intergalactic Birkeland
currents. The cosmic circuitry of Birkeland currents can be traced
by their magnetic fields.
So, far from being electrically sterile, cosmic
plasma is awash with electric current filaments. And just like
Frazier's plasma ball, we see the same beauty and evidence of
currents in astronomical nebulae, glowing hydrogen HII regions, and
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