What is a plasma?
A plasma is an ionized
, electrically conducting gas
of charged particles
. For an ionized gas
to qualify as a plasma
of charged particles
must simultaneously satisfy two important criteria
: (i) the density
should be sufficiently high that the long range Coulomb
force be a significant
factor in determining the statistical properties
of the particles
; (ii) it should be low enough that the Coulomb force
of a near neighbour particle
much less than the cumulative long range Coulomb
force exerted by the many distant particles
. The most characteristic aspect of the plasma state
is perhaps that the particles exhibit collective behaviour
because of the long range nature of the Coulomb force
Above a temperature of about 100,000 degrees Kelvin most matter exists in an ionized state. For this reason the plasma state is frequently called the fourth
state of matter. That is, if one adds heat to a solid one obtains a liquid, add heat to a liquid and one obtains a gas, add sufficient heat to a gas and
the atoms themselves become ionized and one obtains a plasma. Such high temperatures are, however, not necessary, for a plasma to exist.
Provided there is a mechanism for ionizing the gas and the density is sufficiently low for recombination to be slow, a plasma can exist at relatively
low temperatures. This is frequently the case in laboratory produced plasmas and, indeed, in the Earth's own ionosphere--an example of a
plasma produced by photoionization of the tenous outer layers of the atmosphere.
By some estimates 99% of the observable universe is in the plasma state. Why then is there so little natural plasma on Earth? The answer is
simply that the temperature here on Earth is too low and the density of matter is too high. However, as we have mentioned, as we begin to leave
the Earth environment, e.g., in the upper layers of the atmosphere, we meet plasma. Still further up one would come across our nearest example
of an astrophysical plasma: the solar wind. The solar wind is a tenuous plasma of ejected solar material that streams toward the earth and fills
much of interstellar space. We are shielded from these energetic particles by our own Earth's magnetic field, which helps to divert the flow of the
solar wind around us. However, during solar storms energetic solar particles still reach earth through the magnetic "funnels" at the poles and we
observe these as aurorae. Our high altitude satellites, on which we are becoming increasingly reliant for day to day living (e.g., satellite TV and
phones; global positioning systems), are, however, at some risk. For this reason a detailed knowledge of the sun and the solar wind plasma is
required so that satellites can be rotated prior to the onset of a severe solar storm to avoid damage to delicate instrumentation. This has lead to
the development of so-called space weather forecasting. Space weather forecasting combines plasma physics with a detailed knowledge of
processes going on in our Sun, and is a modern application of plasma physics.
Kudos to Wintersweet, who gave me the link the above came from.