Attractive and repulsive ponderomotive forces in space- and astrophysical plasmas
Swedish Institute of Space Physics
Ponderomotive forces are time-averaged, nonlinear forces acting on a media in the presence of oscillating electromagnetic fields. Ponderomotive forces are also conservative, thus representing a useful analytical tool to describe plasma acceleration. An intriguing aspect of ponderomotive forces is that they are not necessarily acting in the direction of the wave Poynting flux. Theory implies for instance that the force is unidirectional for magnetized plasmas in a diverging magnetic field (magnetic moment pumping, MMP). The direction of the gradient (Miller) force in a magnetized plasma is frequency dependent, attractive at frequencies below the ion gyro-frequency and repulsive at higher frequencies (classical radiation pressure).
In this report we address some interesting aspect of ponderomotive forces in space- and astrophysical plasmas. For instance, external irradiation of magnetized plasma immersed in a magnetic dipole field (e.g. a planetary ionosphere) leads to upward escape of the plasma (MMP force). An interesting new aspect is the non-linear cross-over from attractive to repulsive Miller force in space and astrophysical plasmas. This implies that a highly unstable cross-over may exist in a radiating plasma immersed in a dipole magnetic field (e.g. a star), the plasma imploding on one side and exploding on the other. Examples of space- and astro-plasma applications where the Miller-force instability may play a central role will be shown.
Created 2008-02-06 09:03:22 by Mats HolmstrÃ¶m Last changed 2008-02-28 08:47:56 by Mats HolmstrÃ¶m