Date: Thu Feb 28 17:40:30 2002
Posted By: John W. Weiss, Grad Student in Planetary Science
Area of science: Physics
The answer to this question is actually very interesting. Here's why.
Classically speaking (that is, let's ignore quantum mechanics for now),
matter does not have a frequency as such. For instance, the apple on my
desk (which you probably can't see, of course) doesn't really have a
frequency. A plasma is just another state of matter, so it doesn't have a
frequency either. Light, on the other hand, is inherently an oscialltion
and all photons have some frequency associated with them.
"Ah-ha!" you say, "What about bells, tuning forks and bridges over the
Indeed, all matter has certain frequencies at which it
likes to vibrate. But just like bells aren't always ringing,
matter isn't always vibrating. (In some bits of matter, like my very tasty
apple, the vibrations are quickly damped out so that apples are seldom
found ringing.) So the question is, do plasmas have some preferred
frequencies? Do they depend on the type of plasma? What plasma parameters
The answer to the first question is "yes." There are several intertesting
frequencies in a plasma.
In addition to that frequency, there are special speeds in a plasma. Once
you put the plasma together and set the size scales (say, the edges of a
box or the ends of a loops of plasma on the Sun), these lead to preferred
frequencies as well:
- First, the charged particles really love to circle about field lines.
They circle with angular (divide by 2 pi for the more usual frequency)
frequency q B/m c, where q is the charge on the particle, B is the magnetic
field strength, m is the particle mass and c is the speed of light. (Note,
you'll need to be using centimeters, grams and second units, so measure the
magnetic field in Guass, not Teselas.)
Plasmas will have other characteristic frequencies as well, depending on
the situation. But these are the big ones that come to my mind. If you
would like to learn more about these waves, I've been using The Physics
of Fuilds and Plasmas. by Arnab Chaoudhuri, and Introduction to
Space Physics, by M. Kivelson and C. Russell.
- The sound speed. Plasmas are fluids, and usually they respond to
pressure (compression) waves just like air. In air, these are called sound
waves, so we call them sound waves in plasmas, too. You only get real
sound waves along the direction of the magnetic field, since the magnetic
field messes things up if you try to send the waves across it. The sound
speeds depends on how the plasma pressure responds to changes in the density.
- The magnetosonic speed. This is a mix of gas pressure and magnetic
pressure causing waves. Magnetic field lines do not like to be squished
together, so if you push on some of them, they others will push back. This
results in waves perpindicular to the field lines that are much like sound
waves, but with magnetic pressure added in.
- The Alfvén speed. If you imagine plucking a field line, like
plucking a violin string, that wave you create will travel along the field
line at the Alfvén speed. The Alfvén speed goes like B/sqrt(4 pi
rho), where B is again the magnetic field strength and rho is the density
of the plasma.
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