Plasma

The Fundamental State of Matter

Definitions

When one or more of the outer (valence) electrons are stripped away from an atom we say the atom has become 'ionized'.  It then exhibits a net positive electrical charge, and is called a 'positive ion'.  On the other hand, if an extra electron is added onto a neutral atom, the combination then carries a net negative charge and is referred to as a 'negative ion'.  The electrical forces between dissimilar ions are orders of magnitude stronger than any mechanical force such as that produced by gravity.  An electrical plasma is a cloud of ions and electrons that, under the excitation of applied electrical and magnetic fields, can sometimes light up and behave in some unusual ways.  The most familiar examples of electrical plasmas are the neon sign, lightning, and the electric arc welding machine. The ionosphere of Earth is an example of a plasma that does not emit visible light.  Plasma permeates the space that contains our solar system. The cloud of particles that constitutes the solar 'wind' is a plasma.  Our entire Milky Way galaxy consists mainly of plasma.  In fact 99% of the entire universe is plasma!

History

During the late 1800's in Norway, physicist Kristian Birkeland explained that the reason we could see the auroras was that they were plasmas. Birkeland also discovered the twisted corkscrew shaped paths taken by electric currents when they exist in plasmas.  Sometimes those twisted shapes are visible and sometimes not - it depends on the strength of the current density being carried by the plasma. Today these streams of ions and electrons are called Birkeland Currents. The mysterious sprites, elves, and blue jets associated with electrical storms on Earth are examples of Birkeland currents in the plasma of our upper atmosphere.
In all three modes of operation, plasmas emit measurable electromagnetic radiation (radio frequency noise). At any given time, the current density (Amps per square meter) existing in the plasma, determines which particular mode a plasma is operating in.  The atomic structure of the gas that became ionized to form the plasma in the first place also is a factor in this.
Double Layers
One of the most important properties of any electrical plasma is its ability to "self-organize" - that is, to electrically isolate one section of itself from another. The isolating wall is called a double layer (DL). When a plasma is studied in the lab, it is usually contained in a closed cylindrical glass tube. Electrodes are inserted into the ends of the tube - one electrode (called the anode) is maintained at a higher voltage than the electrode at the other end (the cathode). If such a voltage difference is applied, then ionization will be initiated and current will start to flow through the plasma. Positive ions (atoms with one or more electrons stripped off) will migrate away from the anode, and negative ions (atoms carrying one or more extra electrons) will move toward the anode.  The mathematical sum of these two oppositely directed flows constitutes the total current in the plasma.

If the voltage difference from one electrode to the other becomes large enough, a DL will form in a narrow cross-section somewhere in the middle of the tube. Almost all the voltage drop that is applied across the electrodes will fall across this DL. The plasma on one side of the DL (the side toward the anode) will have approximately the same voltage as the anode. The plasma on the cathode side of the DL will have essentially the same voltage as the cathode.  The two halves of the plasma are then electrically isolated from one another by the DL. No electrostatic force is felt by particles on one side of the DL due to charges on the other side of the DL.  The total electric current, however, is the same throughout the plasma (on both sides of the DL).  Plasmas are excellent conductors and, therefore, there will not be a significant voltage drop across them while they are carrying current - thus the need for the presence of the DL that 'takes' most of any externally applied voltage.  In other words, the DL is where the strongest electric fields in the plasma will be found.

If a foreign object is inserted into a plasma, a DL will form around it, shielding it from the main plasma.  This effect makes it difficult to insert voltage sensing probes into a plasma in order to measure the electric potential at a specific location.  This is a well known property of plasmas.  Various methods have been developed in the laboratory to overcome it.

In space, it is impossible to send a spacecraft to measure the voltage of the solar plasma at some point.  Voltage is a relative measure (like velocity, for example); it must be measured with respect to some datum.  A spacecraft will start out having the same voltage as the surface of Earth.  As it penetrates the plasmasphere and enters the solar plasma it will slowly accumulate charge and thus alter its voltage.  The strength of an electric field, however, can be measured in space.

The Z-Pinch
Electric current, passing through a plasma, will take on the corkscrew (spiral) shape discovered by Birkeland. These Birkeland currents most often occur in pairs. There is a tendency for these pairs to compress between them any material (ionized or not) in the plasma. This is called the "z-pinch" effect.  The ability of Birkeland currents to accrete and compress even non-ionized material is called "Marklund convection".

Hannes Alfven and the 'Frozen-in' Magnetic Fields

For years it was assumed that plasmas were perfect conductors and, as such, a magnetic field in any plasma would have to be 'frozen' inside it.

The technical explanation is as follows: One of Maxwell's equations is that the curl of E is equal to  -dB/dt. Consequently, if the electric field, E, in a region is everywhere zero valued, then any magnetic field in that region must be time invariant (have a constant value).  So if all plasmas are ideal conductors (and so cannot have electric fields - that is to say, voltage differences - inside them), then any magnetic fields inside a plasma must be frozen - i.e., cannot move or change in any way.

Now we know that there can be slight voltage differences between different points in plasmas.  Plasma engineer Hannes Alfvén pointed out this fact in his acceptance speech while receiving the Nobel Prize for physics in 1970.  The electrical conductivity of any material, including plasma, is determined by two factors: the density of the population of available charge carriers (the ions) in the material, and the mobility of these carriers. In any plasma, the mobility of the ions is extremely high. Electrons and ions can move around very freely in space. But the concentration (number per unit volume) of ions available to carry charge may not be at all high if the plasma is a very low pressure (diffuse) one.  So, although plasmas are excellent conductors, they are not perfect conductors.  Weak electric fields can exist inside plasmas.  Therefore, magnetic fields are not frozen inside them.

Currents in Cosmic Sized Plasmas

Because plasmas are good (but not perfect) conductors, they are equivalent to wires in their ability to carry electrical current.  It is well known that if any conductor cuts through a magnetic field, a current will be caused to flow in that conductor.  This is how electric generators and alternators work.  Therefore, if there is any relative motion between a cosmic plasma, say in the arm of a galaxy, and a magnetic field in that same location, Birkeland currents will flow in the plasma.  These currents will, in turn, produce their own magnetic fields.

Plasma phenomena are scalable. That is to say, their electrical and physical properties remain the same, independent of the size of the plasma. Of course dynamic phenomena take much less time to occur in a small laboratory plasma than they do in a plasma the size, say, of a galaxy. But the phenomena are identical in that they obey the same laws of physics.  So we can make accurate models of cosmic sized plasmas in the lab - and generate effects exactly like those seen in space.  In fact, electric currents, flowing in plasmas, have been shown to produce most of the observed astronomical phenomena that are inexplicable if we assume that the only forces at work in the cosmos are magnetism and gravity.

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The Electric Sky (Mikamar Pub.)