Characterising coronal turbulence using snapshot imaging of radio bursts in 80–200 MHz

A Mohan

doi:10.1051/0004-6361/202142029

Abstract

Context. Metrewave solar type-III radio bursts offer a unique means to study the properties of turbulence across coronal heights. Theoretical models have shown that the apparent intensity and size of the burst sources evolve at sub-second scales under the influence of local turbulence. The properties of the evolution vary with observation frequency. However, observational studies remained difficult due to the lack of high fidelity imaging capabilities at these fine temporal scales simultaneously across wide spectral bands. Aims. I present a spectroscopic snapshot imaging (0.5 s, 160 kHz resolution) study of a type-III burst event across the 80–200 MHz band. By modelling the temporal variability of the source sizes and intensity at every observation frequency, the characteristics of coronal turbulence are studied across a heliocentric height range of ≈1.54–1.75 R⊙. Methods. To understand the morphological evolution of the type-III source, a 2D Gaussian fitting procedure is used. The observed trends in the source area and integrated flux density are analysed in the framework of theoretical and data-driven models. Results. The strength of density fluctuations (δN/N) in the corona is derived as a function of height (R). Combined with the archival low frequency data, δN/N values across ≈1.5–2.2 R⊙ agree within a few factors. The burst decay time (τdecay) and the full width at half maximum of the source showed a power-law dependence with frequency, roughly consistent with the results from data-driven models. However, the values of τdecay across frequencies turned out higher than the expected trend. The intrinsic sizes of the burst source were derived, correcting for scatter broadening. This roughly matched the expected size of flux tubes at the coronal heights explored. I also report the observation of an intrinsic anti-phased pulsation in the area and flux density of the source.

Highlights

Type-III bursts are triggered by supra-thermal electron beams produced at various particle acceleration sites in the solar corona, especially at active regions (Wild 1950)
Langmuir wave turbulence generated by the instability triggers various wavewave and wave-particle interactions that lead to coherent plasma radiation from the regions along the beam trajectory at the respective local plasma frequencies and their harmonics (2νp) (e.g., Ginzburg & Zhelezniakov 1958; Tsytovich & Kaplan 1969; Melrose & Sy 1972)
The observed emission is heavily influenced by the stochastic density fluctuations in the medium, primarily owing to the fact that the frequency of the radiation (ν) is close to the local νp

Summary

Introduction

Type-III bursts are triggered by supra-thermal electron beams produced at various particle acceleration sites in the solar corona, especially at active regions (Wild 1950) These particle beams move along nearby open magnetic field structures, tracing a range of coronal heights and generating two-stream instability along the trajectory. The analytical expressions for these trends encapsulate the basic features of the local coronal turbulence, such as the density fluctuation index (δN/N) and the width of the strong scattering region. To apply this model to data and make reliable estimates of the properties of coronal turbulence, a sub-second-scale imaging capability with sufficient imaging fidelity is required. The modern interferometric arrays, including the Murchison Widefield Array (MWA; Tingay et al 2013; Wayth et al 2018) and the LOw Frequency ARray (LOFAR; van Haarlem et al 2013), have facilitated such studies

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Discussion

Conclusion