Waves and plasma emissions excited by ring-beam energetic electrons interacting with weakly magnetized plasmas

Abstract

Ring-beam energetic electrons can drive two different kinetic instabilities, including the bump-on-tail instability by the beam component and the electron cyclotron maser instability by the ring component. It is critical to understand how the two instabilities develop and interact, and how their competition for free energy affects the wave modes and further plasma emission process. Here, we present fully kinetic particle-in-cell simulations of the ring-beam interaction with weakly magnetized plasmas of coronal conditions. We found that both the beam-Langmuir (BL) mode and the upper-hybrid (UH) mode can be excited efficiently, along with significant fundamental and harmonic plasma emissions. In comparison with the corresponding pure-beam case, in the ring-beam case, the BL mode has a suppressed range of k⊥, slower damping and thus stronger intensity, and the fundamental emission is considerably stronger, while the harmonic emission has comparable intensity yet being more directional. We further investigated the effect of the pitch angle (α) of the ring-beam distribution, found that the (1) BL mode is more limited in the range of k⊥ with larger α and (2) the UH mode is excited in localized blobs that can be clustered with a straight line whose slope is ∼cot α. Most of these intriguing results can be interpreted with the linear kinetic theory.