Maser radiation from electrons accelerated by magnetised collisionless shock waves

In this paper we describe a model of electron energization and cyclotron-maser emission applicable to astrophysical magnetised collisionless shocks. It is inspired by the work of Begelman, Ergun and Rees who argued that the cyclotron maser instability occurs in localised magnetised collisionless shocks such as those expected in Blazar jets. We report on two recent laboratory experiments and numerical simulations carried out to investigate electron acceleration at collisionless shocks and the maser radiation mechanism. We describe how electrons accelerated by lower-hybrid waves at collisionless shocks generate cyclotron-maser radiation when the accelerated electrons move into regions of stronger magnetic fields. Magnetic compression and conservation of magnetic moment lead to the formation of an electron velocity distribution having a horseshoe or ring shape as the electrons are accelerated along the magnetic field. We show that under certain conditions the horseshoe or ring electron velocity distribution is unstable to the cyclotron maser instability.