  The recent Parker Solar Probe (PSP) observations of type III radio bursts
show that the effects of finite background magnetic field can be an important
factor in the interpretation of data. In the present paper, the effects of
background magnetic field on the plasma emission process, which is believed to
be the main emission mechanism for solar coronal and interplanetary type III
radio bursts, are investigated by means of the particle-in-cell simulation
method. The effects of ambient magnetic field are systematically surveyed by
varying the ratio of plasma frequency to electron gyro-frequency. The present
study shows that for a sufficiently strong ambient magnetic field, the
wave-particle interaction processes lead to a highly field-aligned longitudinal
mode excitation and anisotropic electron velocity distribution function,
accompanied by a significantly enhanced plasma emission at the second harmonic
plasma frequency. For such a case, the polarization of the harmonic emission is
almost entirely in the sense of extraordinary mode. On the other hand, for
moderate strengths of the ambient magnetic field, the interpretation of the
simulation result is less than clear. The underlying nonlinear mode coupling
processes indicate that to properly understand and interpret the simulation
results require sophisticated analyses involving interactions among magnetized
plasma normal modes including the two transverse modes of the magneto-active
plasma, namely, extraordinary and ordinary modes, as well as
electron-cyclotron-whistler, plasma oscillation, and upper-hybrid modes. At
present, a nonlinear theory suitable for quantitatively analyzing such complex
mode-coupling processes in magnetized plasmas is incomplete, which calls for
further theoretical research, but the present simulation results could provide
a guide for future theoretical efforts.
