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

Abstract

Metrewave solar type-III radio bursts offer a unique means to study the properties of turbulence across the 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 varies 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. I present a spectroscopic snapshot imaging (0.5 s, 160 kHz resolution) study of a type-III burst event across 80 - 200 MHz band. By modelling the temporal variability of the source sizes and intensity at every observation frequency, the characteristics of coronal turbulence is studied across a heliocentric height range of ~1.54 - 1.75 $R_{\odot }$. 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 ($\delta N/N$) in the corona is derived as a function of height (R). Combined with the archival low frequency data, $\delta N/N$ values across ~1.5 - 2.2 $R_{\odot }$ agree within a few factors. The burst decay time (${\tau }_{decay}$) and the FWHM of the source showed a power-law dependency with frequency, roughly consistent with the results from data driven models. However,the values of ${\tau }_{decay}$ across frequency are higher than expected. 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 area and flux density of the source.

Accepted for publication in A&A

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