  Type III radio bursts are radio emissions associated with solar flares. They
are considered to be caused by electron beams traveling from the solar corona
to the solar wind. Magnetic reconnection is a possible accelerator of electron
beams in the course of solar flares since it causes unstable distribution
functions, and density inhomogeneities (cavities). The properties of radio
emission by electron beams in an inhomogeneous environment are still poorly
understood. We capture the non-linear kinetic plasma processes of generation of
beam-related radio emissions in inhomogeneous plasmas by utilizing
fully-kinetic Particle-In-Cell (PIC) code numerical simulations. Our model
takes into account initial electron velocity distribution functions (EVDFs) as
they are supposed to be created by magnetic reconnection. We focus our analysis
on low-density regions with strong magnetic fields. The assumed EVDFs allow two
distinct mechanisms of radio wave emissions: plasma emissions due to wave-wave
interactions and so-called electron cyclotron maser emissions (ECME) due to
direct wave-particle interactions. We investigate the effects of density
inhomogeneities on the conversion of free energy from the electron beams into
the energy of electrostatic and electromagnetic waves via plasma emission and
ECME, as well as the frequency shift of electron resonances caused by
perpendicular gradients in the beam EVDFs. Our most important finding is that
the number of harmonics of Langmuir waves increases due to the presence of
density inhomogeneities. The additional harmonics of Langmuir waves are
generated by a coalescence of beam-generated Langmuir waves and their
harmonics.

Plasma Physics;Solar and Stellar Astrophysics;Computational Physics