Wave Excitation by Energetic Ring-distributed Electron Beams in the Solar Corona

Abstract

We re-considered the properties of electromagnetic waves excited by ring-distributed, in the magnetic-field-perpendicular velocity space, electron beams in the solar atmosphere as they are caused by magnetic reconnection. Ring-beam electrons contain additional free energy due to the positive gradient of their velocity space distribution in the direction perpendicular to the magnetic field, i.e., d f /du ⊥ > 0 in addition to the beam-related free energy u ·d f /du > 0 in the direction parallel to the magnetic field. As a result, not only beam instability but also so called electron cyclotron maser (ECM) instability can be excited, which may generate escaping electromagnetic waves. In order to understand the properties of the waves generated in the course of the propagation of such beams, we investigated the intensity and polarization properties of these excited waves in dependence on the evolving beam density and coronal magnetic field strength. For this sake, we utilized 2.5-dimensional particle-in-cell (PIC) code Corresponding author: X. Zhou zhouxw@pmo.ac.cn arXiv:1907.12958v1 [physics.plasm-ph] 28 Jul 2019 2 Zhou et al.numerical simulations. We separated the intensities of the different wave modes according to their dispersion properties. We found that predominantly electrostatic plasma waves are generated but also highly anisotropic and polarized electromagnetic whistler, Z, O and X-mode waves. Their intensity anisotropy strongly depends on the number density of the ring-beam electrons compared to the density of the background electrons. Circular polarization degree (CPD) and spectrogram of the escaping electromagnetic waves with ω > ω pe and |ck/ω| < 1 are also strongly anisotropic, but becoming more symmetric about the wave propagation direction θ = 90 • for denser ring-beam electron population. Meanwhile, with denser ring-beam electron population, escaping waves are predominantly left-handed polarized over a wide range of propagation directions. We discuss the consequences of our findings for using the solar radio burst observations to diagnose the beam and plasma conditions at the sites of their generation.

The effects of density inhomogeneities on the radio wave emission in electron beam plasmas

“…In such an environment the ECME instability could generate radio waves. Even though this frequency ratio is not common for the solar coronal plasma, it applies to cavity ducts in which the density is lower than that in the surrounding coronal plasma (Morosan et al 2016; Zhou et al 2020). The thermal speed of the background electron plasma is , which corresponds to a coronal plasma temperature and results in a Debye length of .…”

“…Note that our study focuses on a local generation mechanism of type III SRBs due to magnetic reconnection generating superthermal electrons in solar flares. While some aspects of this process have already been investigated previously (Wu et al 2014; Treumann & Baumjohann 2017; Zhou et al 2020), we analysed here the generation mechanisms of fundamental Langmuir and electron cyclotron waves as well as their harmonics when the electron beams propagate through small-scale density gradients in the ambient plasma. We show that density inhomogeneities fundamentally change the properties of the resulting waves.…”

“…In order to understand the different aspects of plasma emission and ECME, a number of studies have already been carried out (see e.g. Melrose (2017) and Zhou et al (2020) and references therein). Those studies usually focused on EVDFs with positive gradients either in the parallel direction (for plasma emission) or in the perpendicular direction (for ECME) to the magnetic field.…”

“…A linear stability analysis of ring-beam distributions was carried out, for example, by Vandas & Hellinger (2015). The nonlinear evolution of the wave excitation due to ring-beam distribution functions was investigated in detail by Zhou et al (2020) among others. Those authors derived the properties of waves generated by ring-beam distribution functions in the solar coronal plasma, like the polarization of escaping waves.…”

“…While harmonics of Langmuir waves were claimed to be found in some numerical experiments (e.g. Rhee et al 2009), other numerical studies of wave generation failed to find conclusive evidence for the generation of harmonics due to the standard plasma emission mechanism (Ganse et al 2012; Zhou et al 2020). Fully kinetic PIC simulations of plasma emission leading to the generation of higher-order harmonic(s) of the plasma frequency waves are very rare.…”