Solar radio astronomy at low frequencies

Abstract

Powerful radio radiation often originates from the Sun at decametric and kilometric wavelengths. Radiation from the quiet Sun is produced by the thermal mechanism of bremsstrahlung, and radio bursts of several kinds are produced by the non-thermal mechanisms of plasma radiation and, rarely, gyrosynchrotron radiation.

Radio bursts are frequent during years of high solar activity. They are produced by several mechanisms of excitation: i) streams of electrons, individual, in groups of ten or so, or in storms of tens of thousands, ii) shock waves, with the radiation coming either from near the shock or distant from it by electrons accelerated by the shock, and iii) configurations of magnetic field that contain energetic electrons. In the frequency range of interest here, the radiation is produced at a distance from the Sun that varies from R ≈ 2 R_⊙ at 20 MHz to R ≈ 10 R_⊙ at 1 MHz, and R ≈ 1 AU at 30 kHz. In that frequency range the observed flux densities range from S ∶ 10⁻¹⁷ to 10⁻¹⁴ W m⁻² Hz⁻¹, and values of brightness temperature range from T _b ∶ 10¹² to 10^15.5 K.

To the present time there have been almost no observations of solar radiation between about 20 MHz, the limit of observations from the ground, and about 2 MHz where most observations from spacecraft commence. Below about 2 MHz, dynamic spectra of flux densities of solar bursts have been recorded, and, by the use of (short) dipoles on spinning spacecraft, observations have been made of the directions of centroids and characteristic sizes of the emitting sources. Even in the absence of true imaging capabilities, this information has proved to be very valuable in studies of morphology, brightness temperatures, and physics of solar radio phenomena. Proper imaging of the sources will remove present ambiguities and allow new phenomena to be observed.