A new cyclotron maser radiation mechanism in space plasmas

In this paper, we discuss a new cyclotron maser type instability driven by a crescent or horseshaped electron distribution function. Such distribution functions are easily created by an electron beam moving into a stronger magnetic field region, where conservation of the first adiabatic invariant causes an increase in their pitch angle. This produces a broad region on the distribution function where ∂fe/∂v⊥ > 0. Planetary dipole magnetic fields are examples of where these types of distributions can be found. We examine the stability of these electron horseshoe distribution functions for right-hand extraordinary mode (R–X mode) radiation close to the electron cyclotron frequency propagating perpendicular to the magnetic field using both non-relativistic and relativistic beams.