Abstract
Abstract 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.
Highlights
Solar radio bursts are commonly considered to be a signature of acceleration and propagation of nonthermal electrons in the solar corona (e.g., Ginzburg & Zhelezniakov 1958; Dulk 1985)
For the case of Cq = 2300R -1, the source size changes from ∼280 to ∼430 in 0.5 s during the decay phase (blue dots at the bottom of Figure 3(a)), which agree with the values obtained from the LOw-Frequency ARray (LOFAR) observations shown by the red line in Figure 3 (Kontar et al 2017; Sharykin et al 2018)
We quantitatively investigated the way in which scattering of radio waves on random density fluctuations with a power-law spectrum affects the time profile evolution, sizes, and positions of the observed radio bursts emitted via the plasma emission mechanism
Summary
Solar radio bursts are commonly considered to be a signature of acceleration and propagation of nonthermal electrons in the solar corona (e.g., Ginzburg & Zhelezniakov 1958; Dulk 1985). In the standard type III solar radio burst scenario, nonthermal electrons propagate away from the Sun and generate Langmuir waves (e.g., Ginzburg & Zhelezniakov 1958; Goldman 1983; Yoon et al 2016), so that the radio emission is progressively produced at lower frequencies as the electrons responsible for radio emission propagate away from the Sun. The radio emission is produced at the fundamental and harmonic (twice the local plasma frequency) frequencies, so observations at the same frequency examine the harmonic emission from distances farther away from the Sun. At the same time, various propagation effects—including the refraction due to plasma density gradients and scattering by small-scale density fluctuations—significantly affect the apparent properties of the radio sources, including their time evolution, position, and size (Steinberg et al 1971; Arzner & Magun 1999; Kontar et al 2017). In this study, Monte Carlo simulations of radio-wave propagation are used to investigate the time evolution, positions, and sizes of the apparent solar radio burst sources at subsecond scales.
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