Observations of the Type U Burst in within 10–70 МHz with the GURT Radio Telescope

Abstract

The results of the first observations of solar sporadic radio emission within 10-70 MHz with the Giant Ukrainian Radio Telescope (GURT), now under construction, are presented. Observations in such a wide range of frequencies allowed considerable increase of the probability of harmonic pairs recording, where a harmonic pair means the pair of bursts generated at the fundamental and second harmonics of the electron plasma frequency.

The solar U-burst observed on 8 August, 2012 is analyzed. It was recorded at the fundamental (25 MHz) and second (50 MHz) harmonics of plasma frequency. The burst’s key features have been determined. Among these, the time delay between the fundamental and harmonic emissions was of especial interest. The fundamental emission delayed for 7.5 s with respect to the harmonic. The model of occurrence of such a delay is proposed, in which the emission source is located inside the magnetic loop containing the plasma of increased density. In this case, the delay appears due to the difference in group velocities of electromagnetic waves at fundamental and harmonic frequencies.