Imaging spectroscopy of solar radio burst fine structures

E P Kontar,S Yu,A A Kuznetsov,N L S Jeffrey,P Subramanian,A G Emslie,B Alcock,V N Melnik,N H Bian

doi:10.1038/s41467-017-01307-8

Abstract

Solar radio observations provide a unique diagnostic of the outer solar atmosphere. However, the inhomogeneous turbulent corona strongly affects the propagation of the emitted radio waves, so decoupling the intrinsic properties of the emitting source from the effects of radio wave propagation has long been a major challenge in solar physics. Here we report quantitative spatial and frequency characterization of solar radio burst fine structures observed with the Low Frequency Array, an instrument with high-time resolution that also permits imaging at scales much shorter than those corresponding to radio wave propagation in the corona. The observations demonstrate that radio wave propagation effects, and not the properties of the intrinsic emission source, dominate the observed spatial characteristics of radio burst images. These results permit more accurate estimates of source brightness temperatures, and open opportunities for quantitative study of the mechanisms that create the turbulent coronal medium through which the emitted radiation propagates.

Highlights

Solar radio observations provide a unique diagnostic of the outer solar atmosphere
The radio burst on 2015 April 16 around 11:57 UT was simultaneously observed by one of the largest decameter arrays, the LOw Frequency ARray (LOFAR)[25] and by the URAN-226 (Ukrainian Radio interferometer of National Academy of Sciences)
The high frequency-time resolution of these observations have allowed us to image the radio waves as they emerge from the solar atmosphere

Summary

Introduction

Solar radio observations provide a unique diagnostic of the outer solar atmosphere. the inhomogeneous turbulent corona strongly affects the propagation of the emitted radio waves, so decoupling the intrinsic properties of the emitting source from the effects of radio wave propagation has long been a major challenge in solar physics. Imaging observations with the Culgoora radioheliograph[18,19] have revealed an interesting enigma for the events at the limb: sources of fundamental emission are radially shifted outwards with respect to harmonic emission (and are apparently situated at different heights in the solar atmosphere), the physics of the responsible coherent plasma emission mechanism requires that they are produced cospatially[3]. This is puzzling, since the refraction of radio waves shifts sources radially inwards and, since the fundamental component is refracted more than the second-harmonic component, the fundamental component should appear lower[19]. These observations with high spatial and temporal resolution demonstrate radio wave propagation effects in the solar corona

Methods

Results

Conclusion