MadSci Network: Physics


Date: Thu Feb 28 17:40:30 2002
Posted By: John W. Weiss, Grad Student in Planetary Science
Area of science: Physics
ID: 1013984332.Ph

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 Tacoma Narrows?" 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 are important?

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: 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.

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