Decameter Type III Bursts with Changing Frequency Drift-Rate Signs

TL;DR

This paper investigates decameter Type III solar radio bursts that exhibit changing drift rate signs, providing new insights into their propagation mechanisms and challenging traditional interpretations of positive drift rates.

Contribution

It introduces the observation and analysis of Type III bursts with multiple drift rate sign changes, proposing a novel explanation involving group velocities in the solar corona.

Findings

Type III bursts can change drift sign multiple times.

Positive drift rates can occur even with outward electron beams.

Observed drift rates range from -5.56 MHz/s to 12 MHz/s.

Abstract

We discuss properties of type III bursts which change sign of their drift rate from negative to positive and vice versa. Moreover such bursts may change sign of their drift rates more than once. These specific type III bursts were observed simultaneously by radio telescopes UTR-2, URAN-2 and NDA in frequency band 8-41 MHz. The negative drift rates of these bursts are close to those of usual decameter type III bursts and variate from -0.84 MHz/s to -5.56 MHz/s. The positive drift rates of specific type III bursts vary in the wider range from 0.44MHz/s to 12 MHz/s. Unlike inverted U-bursts these type III bursts still drift from the high frequencies to the low frequencies in spite of the change of the drift rates signs. Our basic explanation of the positive drift rate of these type III burst differs from the common assumption that positive drift rates of Type III bursts are connected with electron beam propagation towards the Sun. We propose that, even if electron beams move outward from the Sun, they can generate type III bursts with positive drift rates if in some regions of the solar corona the group velocities of type III radio emissions are lower than the velocities of the electron beams.