  We analyzed properties of waves excited by mildly relativistic electron beams
propagating along magnetic field with a ring-shape perpendicular momentum
distribution in neutral and current-free solar coronal plasmas. These plasmas
are subject to both the beam and the electron cyclotron maser (ECM)
instabilities driven by the positive momentum gradient of the ring-beam
electron distribution in the directions parallel and perpendicular to the
ambient magnetic field, respectively. To explore the related kinetic processes
self-consistently, 2.5-dimensional fully kinetic particle-in-cell (PIC)
simulations were carried out. To quantify excited wave properties in different
coronal conditions, we investigated the dependence of their energy and
polarization on the ring-beam electron density and magnetic field. In general,
electrostatic waves dominate the energetics of waves and nonlinear waves are
ubiquitous. In weakly magnetized plasmas, where the electron cyclotron
frequency $\omega_{ce}$ is lower than the electron plasma frequency
$\omega_{pe}$, it is difficult to produce escaping electromagnetic waves with
frequency $\omega > \omega_{pe}$ and small refractive index $|c k / \omega| <
1$ ($k$ and $c$ are the wavenumber and the light speed, respectively). Highly
polarized and anisotropic escaping electromagnetic waves can, however, be
effectively excited in strongly magnetized plasmas with
$\omega_{ce}/\omega_{pe} \geq 1$. The anisotropy of the energy, circular
polarization degree (CPD), and spectrogram of these escaping electromagnetic
waves strongly depend on the number density ratio of the ring-beam electrons to
the background electrons. In particular, their CPDs can vary from left-handed
to right-handed with the decrease of the ring-beam density, which may explain
some observed properties of solar radio bursts (e.g., radio spikes) from the
solar corona.
