Plasma Zoo: Gyroresonant Scattering

In a background magnetic field, represented by the cyan arrows, two electrons are propagating to the right, executing identical gyromotion. A circularly polarized electromagnetic wave approaches the upper electron from the left.

As the upper electron begins to interact with the wave, the electric field of wave is initially oriented in a way that removes energy from the particle, slowing its speed perpendicular to the magnetic field, which makes the pitch angle (the angle between the particle velocity vector and the magnetic field vector) and gyro-radius smaller.

As the wave field decelerates the electron, its gyro-frequency gets closer matched to the electromagnetic wave. If the electric field is directed radially with the gyromotion, the particle is no longer accelerated by the electric field and the particle 'surfs' the wave, the energy no longer changing (and the particle trail is white).

After a few more orbits, the particle gyromotion is again decelerated (trail turns blue) but also picks up some speed along the field line, moving it ahead of the particle outside the wave.

The increasing magnetic field gradient moves the particle gyrofrequency well past the resonance condition. Then the particle oscillates between losing small amounts of energy (blue trail) and gaining small amounts of energy (red trail), with little changes to total energy as it propagates in the wave.

The particle in the wave is now well ahead of the particle outside the wave.

An oblique view of the interaction, with the camera moving ahead of the particles and looking back on their motion. This view makes the gyromotion of the particles, and the circular polarization of the waves more obvious.

Color bar for the particle trails. White means no change in particle energy, blue means losing energy and red means gaining energy.