Properties of Decameter IIIb–III Pairs

Abstract

A large number of Type IIIb-III pairs, in which the first component is a Type IIIb burst and the second one is a Type III burst, are often recorded during decameter Type III burst storms. From the beginning of their observation, the question of whether the components of these pairs are the first and the second harmonics of radio emission or not has remained open. We discuss properties of decameter IIIb-III pairs in detail to answer this question. The components of these pairs, Type IIIb bursts and Type III bursts, have essentially different durations and polarizations. At the same time their frequency drift rates are rather close, provided that the drift rates of Type IIIb bursts are a little larger those of Type III bursts at the same frequency. Frequency ratios of the bursts at the same moment are close to two. This points at a harmonic connection of components in IIIb-III pairs. At the same time there was a serious difficulty, namely, why the first harmonic had fine frequency structure in the form of striae and the second harmonic did not have it. Recently Loi, Cairns, and Li (Astrophys. 790, 67, 2014) succeeded in solving this problem.The physical aspects of observational properties of decameter IIIb-III pairs are discussed and pros and cons of harmonic character of Type IIIb bursts and Type III bursts in IIIb-III pairs are presented. We conclude that practically all properties of IIIb-III pair components can be understood in the framework of the harmonic relation of components of IIIb-III pairs.

Book of Proceedings Tenth Workshop Primorsko, Bulgaria 2019

“…From the histogram of spikes distribution on the polarization we can conclude that degree of polarization of spikes changes from 0 up to 100% with average value about 50% and 20% for the bursts with right and left polarization respectively. The former value is close to that obtained for the decameter Type IIIb bursts which were observed simultaneously with Type III bursts and composed so called IIIb-III pairs [Melnik V. N., et al, 2018]. The mean value of Type IIIb bursts polarization is 40% -60% depending on the time of observations.…”

“…Duration of these bursts increases with observational frequency decrease. For example, at frequency 16 MHz duration of Type III bursts is about 10s [Melnik V. N., et al, 2018].…”

On the Observational Properties of the Decameter Striae

“…They identified them as chains of short (≈ 1 second), narrow-band (15 -100 kHz) striae bursts drifting similarly to the usual Type III bursts. It is currently well known that Type IIIb bursts are observed in the wide frequency range from 10 MHz up to 200 MHz [1][2][3][4][5][6][7][9][10][11][12]. The occurrence rate of the elementary striae increases toward the lower frequencies [2].…”

“…The individual striae in most cases have no frequency drift. The polarization of the elementary striae is circular and can reach 80 -100 % [1,12,13]. Type IIIb bursts are observed either as isolated bursts or accompanied by Type III bursts forming well known IIIb-III pairs.…”

“…Up to now the relation between components of the IIIb-III pairs is not fully understood. Some authors inclined that Type IIIb bursts are precursors for the Type III bursts [1,6] and another suppose that IIIb-III pairs are harmonically related where Type IIIb bursts are the first harmonic, and Type III bursts the second one [3,5,6,12]. The similarity and association of the Type IIIb bursts and Type III bursts lead to the assumption that Type IIIb bursts are generated by the plasma emission mechanism.…”