Type III radio bursts are radio emissions associated with solar flares. They are considered to be caused by electron beams travelling 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 nonlinear kinetic plasma processes of the generation of beam-related radio emissions in inhomogeneous plasmas by utilizing fully kinetic particle-in-cell 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 emission due to wave–wave interactions and so-called electron cyclotron maser emission (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.

中文翻译：

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密度不均匀性对电子束等离子体中无线电波发射的影响

III 型射电暴是与太阳耀斑相关的无线电发射。它们被认为是由从日冕传播到太阳风的电子束引起的。磁重联是太阳耀斑过程中可能的电子束加速器，因为它会导致不稳定的分布函数和密度不均匀性（空腔）。电子束在不均匀环境中的无线电发射特性仍然知之甚少。我们通过利用全动力学粒子细胞代码数值模拟来捕获在不均匀等离子体中产生与光束相关的无线电发射的非线性动力学等离子体过程。我们的模型考虑了初始电子速度分布函数 (EVDF)，因为它们应该是由磁重联产生的。我们将分析重点放在具有强磁场的低密度区域。假设的 EVDF 允许两种不同的无线电波发射机制：由波-波相互作用引起的等离子体发射和由直接波粒相互作用引起的所谓电子回旋微波激射器发射 (ECME)。我们研究了密度不均匀性对通过等离子体发射和 ECME 将电子束的自由能转换为静电和电磁波能量的影响，以及由光束 EVDF 中的垂直梯度引起的电子共振频移。我们最重要的发现是朗缪尔波的谐波数量由于密度不均匀性的存在而增加。朗缪尔波的附加谐波是由光束产生的朗缪尔波及其谐波的合并产生的。