Keywords:wave-particle interaction, nonlinear process, plasma_pause
We study a possible nonlinear mechanism which could be responsible for generation of the non-thermal continuum radiation. We propose a theoretical model and perform numerical simulations using a 2-D electromagnetic particle code (KEMPO2)  to verify the mechanism . In the simulation system, we have density gradient and ring beam electrons in the dense region as a source of electrostatic waves through ring-beam instability. These electrostatic waves cause non-gyrotropy of the ring velocity distribution which is necessary for generation of the harmonic waves. Velocity component Vy together with magnetic field Bz of the generated Z-mode waves creates Lorentz force in the x-direction (along the magnetic field). Hence, oscillating energetic electrons work as an anntenna emiting O-mode waves to the low density region. Electromagnetic O-mode waves are emitted at harmonics of the cyclotron frequency up to the maximal plasma frequency of the high density region, and they are observed as non-thermal continuum radiation in the low density region.
 M. Horky, Y. Omura, and O. Santolik, Particle simulation of electromagnetic emissions from electrostatic instability driven by an electron ring beam on the density gradient, Physics of Plasmas 25, 042905, doi: 10.1063/1.5025912, 2018.
 M. Horky, and Y. Omura, Novel nonlinear mechanism of the generation of non-thermal continuum radiation, Physics of Plasmas 26, 2019.