Anisotropic Radio-wave Scattering and the Interpretation of Solar Radio Emission Observations

Abstract

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 ∼ 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.

A Review of Recent Solar Type III Imaging Spectroscopy

“…The Gaussian decay was in contrast to the exponential decay used in previous works (e.g., Aubier and Boischot, 1972;Barrow and Achong, 1975;Mel'Nik et al, 2011) although similar HWHM decay times were found. The decay time r decay ∝ f 0.89±0.15 compares very well with comparisons of decay times all the way down to kHz frequencies, shown by Kontar et al (2019), that could be fit with a power-law that had a spectral index close to 1. The rise and decay times also showed a very strong correlation, indicating that one process dominates both time scales at LOFAR frequencies.…”

“…Bian et al (2019) modeled the scattering process using a Fokker-Planck equation and were able to reproduce the time profile but not the inverse frequency dependence of the decay time, which they concluded was down to the exclusion of a large-scale refractive term. Kontar et al (2019) recently extended the work of Bian et al (2019) but treated the scattering in the anisotropic domain, with the dominant effect being perpendicular to the heliospheric radial domain (Kontar et al, 2017). As well as explaining temporal profiles, Kontar et al (2019) used ray-tracing simulations to explain the increase in source sizes, finding a scattering increase in the FWHM around 1.1 R ⊙ at 35 MHz, although this value will depend upon the size of the density fluctuations from event to event.…”

“…Kontar et al (2019) recently extended the work of Bian et al (2019) but treated the scattering in the anisotropic domain, with the dominant effect being perpendicular to the heliospheric radial domain (Kontar et al, 2017). As well as explaining temporal profiles, Kontar et al (2019) used ray-tracing simulations to explain the increase in source sizes, finding a scattering increase in the FWHM around 1.1 R ⊙ at 35 MHz, although this value will depend upon the size of the density fluctuations from event to event. Changing the anisotropy parameter strongly influences source sizes that are off the solar limb and less so at disc center.…”

“…It is apparent that the scattering of waves off density fluctuations significantly affect what we observe at Earth, in particular for low frequency fundamental emission. If we want to fully unlock the benefits of type III imaging spectroscopy we must be able to untangle these effects and significant efforts are already under way (e.g., Kontar et al, 2019). The variation in source parameters as a function of frequency can be used to constrain and improve the ray-tracing models that are being used to describe light transport.…”

The Frequency Drift and Fine Structures of Solar S-bursts in the High Frequency Band of LOFAR

“…Moreover, density fluctuations can cause scattering of the radio waves during propagation, which is also not included in the simulation. The duration of the radio emission at a given frequency can also be broadened by scattering (Kontar et al 2019). The increase of the duration may mix the segments of the discontinuity and weaken the significance of fine structures.…”