各向异性无线电波散射与太阳射电观测的解释
Anisotropic Radio-Wave Scattering and the Interpretation of Solar Radio Emission Observations
摘要：
观察到的性质（即，在非均匀湍流日冕中，通过等离子体过程在局部等离子体频率附近产生的太阳射电辐射的源大小、源位置、持续时间、衰减时间）以及因此对太阳射电爆发的解释受到传播效应的强烈影响。基于无线电波在各向异性电子密度起伏介质中传输的Fokker-Planck和Langevin方程，给出了一种三维随机传播过程的描述。使用基于这个模型的数值处理，我们调查的特征源的大小和爆发衰变时间的III型太阳射电爆发。模拟结果与太阳射电爆发观测结果的比较表明，在太阳日冕主要垂直的密度波动是必需的，与各向异性因子$\sim 0.3$在30 MHz左右观察到的源。模拟结果还表明，光子在主辐射区附近是各向同性的，但波随后被大尺度折射聚焦，导致等离子体无线电辐射方向性，其特征是在30 MHz附近的半高半宽约为40 °。结果适用于各种太阳射电爆发通过等离子体发射产生。
英文摘要：
The observed properties (i.e., source size, source position, time duration, decay time) of solar radio emission produced through plasma processes near the local plasma frequency, and hence the interpretation of solar radio bursts, are strongly influenced by propagation effects in the inhomogeneous turbulent solar corona. In this work, a 3D stochastic description of the propagation process is presented, based on the Fokker-Planck and Langevin equations of radio-wave transport in a medium containing anisotropic electron density fluctuations. Using a numerical treatment based on this model, we investigate the characteristic source sizes and burst decay times for Type III solar radio bursts. Comparison of the simulations with the observations of solar radio bursts shows that predominantly perpendicular density fluctuations in the solar corona are required, with an anisotropy factor $\sim 0.3$ for sources observed at around 30~MHz. The simulations also demonstrate that the photons are isotropized near the region of primary emission, but the waves are then focused by large-scale refraction, leading to plasma radio emission directivity that is characterized by a half-width-half-maximum of about 40~degrees near 30~MHz. The results are applicable to various solar radio bursts produced via plasma emission.
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