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

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 ( δ N / N ) in the corona is derived as a function of height ( R ). Combined with the archival low frequency data, δ N / N values across ≈1.5–2.2 R ⊙ agree within a few factors. The burst decay time ( τ decay ) 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 τ decay 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.