Ion Cyclotron Waves (ICWs) are transverse shear waves produced as energy is released from a higher energy anisotropic distribution of ions (a.k.a. a “ring distribution” in velocity space) to a lower energy isotropic distribution. Specifically, the anisotropy arises when neutrals are ionized and accelerated along the magnetic field. The ICWs will release energy from the plasma and scatter the pick-up ions’ velocity back to thermal equilibrium.

T_par  describes the ions’ temperature distribution parallel to the magnetic field and T_perp describes the distribution perpendicular to it.

The charged particles themselves will drift along the magnetic field in a helical trajectory where the radius (gyroradius) and angular frequency (Ion Cyclotron Frequency/Gyrofrequency) of the orbit depends on the mass and charge of the specific ion.

\Omega = \frac{qB}{mc}

where \Omega is the gyrofrequency, q is the charge, B is the magnitude of the magnetic field, m is the mass of the ion, and c is the speed of light.

This q/m dependence for the frequency of the ICW allows them to be used in space physics as a proxy mass spectrometer and aids in the identification of specific breeds of pick-up ions (e.g. ICWs produced by Na+ will have a larger frequency than those produced by Cl- in the same magnetic environment). Similar q/m ions in the environment can reabsorb the ICWs produced by the pick-up ions which can dampen their overall wave power.

ICWs are circularly polarized where the direction of polarization (Left Handed or Right Handed) depends on the charge of the ion because the charge defines the direction of the orbit around the magnetic field.

Left-handed waves are produced by (+) ions and right-handed waves are those produced by (-) ions.

The polarization is also referred to as the ellipticity of the wave where

1. Linear Polarization = Zero Ellipticity
2. Left Handed Circular Polarization = -1 Ellipticity
3. Right Handed Circular Polarization = 1 Ellipticity

 

Application of topic: “Wave Activity in Europa’s Wake: Implication for Ion Pickup” — M. Volwerk, M. G. Kivelson, K. K. Khurana, Journal of Geophysical Research, 2001