Spectral characteristics of solar S bursts

Abstract

Observations of the solar radio spectrum have been made with high time and frequency resolution. Spectra were recorded over six 3-MHz bands between 30 and 82 MHz. The receivers used were capable of time and frequency resolutions of 1 ms and 2 kHz, respectively. A large number of radio bursts exhibiting a variety of find spectral structure were recorded.

The bursts, referred to here as S bursts, were observed throughout the 30–82 MHz frequency range but were most numerous in the 33–44 MHz band and were very rare at 80 MHz. On a dynamic spectrum the bursts appeared as narrow sloping lines with the centre frequency of each burst decreasing with time. The rate of frequency drift was about 1/3 that of type III bursts. Most bursts were observed over only a limited frequency range (< 5 MHz) but some drifted for more than 10 MHz. The durations measured at a single frequency and the instantaneous bandwidths of S bursts were small; Δt = 49 ± 34 ms and Δf = 123 ± 56 kHz for bursts observed near 40 MHz. A significant number had Δt ≤ 20 ms. Flux densities of S burst sources were estimated to fall in the range 10²³-5 × 10²¹ Wm⁻¹ Hz⁻¹.

A small proportion (1–2%) of bursts showed a fine structure in which the burst source apparently only emitted at discrete, regularly spaced frequencies causing the spectrogram to exhibit a series of bands or fringes. The fringe spacing increased with wave frequency and was δf ∼- 90 kHz for fringes near 40 MHz. The bandwidths of fringes was narrow, often less than 30 kHz and in some cases down to 10–15 kHz.