  According to the standard scenario of plasma emission, escaping radiations
are generated by the nonlinear development of the kinetic bump-on-tail
instability driven by a single beam of energetic electrons interacting with
plasmas. Here we conduct fully-kinetic electromagnetic particle-in-cell
simulations to investigate plasma emission induced by the ring-distributed
energetic electrons interacting with overdense plasmas. Efficient excitations
of the fundamental (F) and harmonic (H) emissions are revealed with radiation
mechanism(s) different from the standard scenario: (1) The primary modes
accounting for the radiations are generated through the electron cyclotron
maser instability (for the upper-hybrid (UH) and Z modes) and the thermal
anisotropic instability (for the whistler (W) mode); the F emission is
generated by the nonlinear coupling of the Z and W modes and the H emission by
the nonlinear coupling of the UH modes. This presents an alternative mechanism
of coherent radiation in overdense plasmas.
