Subsecond Time Evolution of Type III Solar Radio Burst Sources at Fundamental and Harmonic Frequencies
Subsecond Time Evolution of Type III Solar Radio Burst Sources at Fundamental and Harmonic Frequencies
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DOI：
10.3847/1538-4357/abc24e
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发表时间：
2020-10
期刊：
影响因子：
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通讯作者：
Xingyao Chen;E. P. Kontar;N. Chrysaphi;N. Jeffrey;M. Gordovskyy;Yihua Yan;B. Tan
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文献类型：
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作者：
Xingyao Chen;E. P. Kontar;N. Chrysaphi;N. Jeffrey;M. Gordovskyy;Yihua Yan;B. Tan
天文射电望远镜的最新发展为亚秒级太阳射电爆发的成像和光谱学提供了新的机会。窄频带成像揭示了位置和源大小的时间变化，这些变化不符合第三类太阳射电爆发的标准图像，需要更好地了解无线电波传输。在本文中，我们利用三维蒙特卡罗射线跟踪模拟，占各向异性密度湍流的不均匀的日冕定量解释的基本（近等离子体频率）和谐波（双）等离子体发射在1032 MHz的图像动态观察。比较模拟与观测，我们发现，各向异性散射从一个瞬时发射点源可以占所观察到的时间分布，质心位置，和源的大小的基本组成部分的III型射电爆发（产生的地方f pe = 32 MHz）。当各向异性密度湍流波矢量的垂直分量与平行分量之比为0.25左右时，与观测结果的一致性最好。在相同频率下观察到的谐波发射源（频率为32 MHz，但在f pe = 16 MHz时产生）具有与基波发射产生的谐波发射源相当的明显大小，但表现出慢得多的时间演化。无线电波传播的模拟使得有可能定量地解释在亚秒时间尺度上基波和谐波发射的视源大小和位置的变化，并可用作诊断工具的等离子体湍流在上日冕。
Recent developments in astronomical radio telescopes opened new opportunities in imaging and spectroscopy of solar radio bursts at subsecond timescales. Imaging in narrow frequency bands has revealed temporal variations in the positions and source sizes that do not fit into the standard picture of type III solar radio bursts, and require a better understanding of radio-wave transport. In this paper, we utilize 3D Monte Carlo ray-tracing simulations that account for the anisotropic density turbulence in the inhomogeneous solar corona to quantitatively explain the image dynamics at the fundamental (near plasma frequency) and harmonic (double) plasma emissions observed at ∼32 MHz. Comparing the simulations with observations, we find that anisotropic scattering from an instantaneous emission point source can account for the observed time profiles, centroid locations, and source sizes of the fundamental component of type III radio bursts (generated where f pe ≈ 32 MHz). The best agreement with observations is achieved when the ratio of the perpendicular to the parallel component of the wavevector of anisotropic density turbulence is around 0.25. Harmonic emission sources observed at the same frequency (∼32 MHz, but generated where f pe ≈ 16 MHz) have apparent sizes comparable to those produced by the fundamental emission, but demonstrate a much slower temporal evolution. The simulations of radio-wave propagation make it possible to quantitatively explain the variations of apparent source sizes and positions at subsecond timescales both for the fundamental and harmonic emissions, and can be used as a diagnostic tool for the plasma turbulence in the upper corona.