# B Do force drifts occur in highly collisional plasmas

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1. Dec 30, 2017

### chandrahas

I've read on wikipedia that the force drifts of the guiding centers off particles in a magnetic field also occurs in cold plasma. But does it occur in a cold plasma (Partially Ionized ) in which the mean free time is less than the gyro frequency time?

I thought that the drift were present because of the asymmetry in the gyration. In order to observe this effect, the orbit has to be completed. At least partially. But if the mean free time is smaller than the time required to complete an orbit we wouldn't be observing this effect would we?

To sum it all up: Do force drift occur in Partially Ionized plasmas in which the mean free time is much smaller than the time of gyration?

Thanks

2. Jan 4, 2018

### PF_Help_Bot

Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.

3. Jan 5, 2018

### jasonRF

As you guessed, the guiding center drifts derived in the collisionless case do not apply if there are collisions. In order to properly include collisions, we usually look for stationary solutions of a multi-fluid model with explicit collision terms.

This approach is used to model the ionosphere, which is highly collisional at low altitudes and weakly collisional at high altitudes.

It turns out that the highly collisional plasma essentially behaves as if there is no magnetic field. For example, consider a plasma with a DC electric field perpendicular to a DC magnetic field. In the highly collisional case the particals drift parallel to the electric field. In the collisionless case you get the standard $\mathbf{E \times B}$ drift. In the intermediate case the drift is in some direction betwen those two extremes; when the collision frequency equals the gyrofrequency the drift is exactly 45 degrees from both $\mathbf{E}$ and $\mathbf{E \times B}$.

Jason