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Charged Particle Motion in Earth's Magnetosphere

Drift Motion of Charged Particles

If a uniform electric field vector E is added the magnetic field vector B the charged particle experiences a force:

in addition to the magnetic force:

The total force on the particle is thus:

Only the electric force can do work on the particle since the magnetic force is always perpendicular to the particle velocity. If both vector v and vector E are perpendicular to vector B,

acceleration by the electric field changes the circular motion shown on the preceding page.

In the segment of its orbit when the particle is moving in the direction of the electric force, it accelerates, and its gyroradius expands. When it moving against the electric force, it decelerates and its gyroradius contracts. These distortions of the gyroradius cause the particle to drift in a direction perpendicular to both vector E and vector B.

Notice in the following animation how the velocity increases during the particle's downward swing, reaching its maximum at the base of the orbit. The veclocity vector has a rightward component during the half of the orbit when it is strongest, while it has a leftward component during the half when velocities are weakest. The net effect is a rightward drift.

Animation courtesy the COMET and HAO programs at UCAR/NCAR.

These distortions of the gyroradius cause the particle to drift in a direction perpendicular to both vector E and vector B. The average drift velocity is given by:

This kind of drift can be induced by any effect that modifies a particle’s gyroradius over the course of its orbit.

Last modified May 17, 2005 by Randy Russell.

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