Unusually low coronal radio emission at the solar minimum

Abstract

We present two-dimensional observations of the quiet Sun at 73.8, 50.0, and 38.5 MHz obtained with the Clark Lake Radioheliograph during the sunspot minimum period of September 1986. The observed peak brightness temperatures during the entire period of sunspot minimum are found to be extremely low, lying in the range (0.6 × 105 K − 2.5 × 105 K). It is shown that these low values cannot be explained by the generally adopted models for N e and T e in a homogeneous corona. The effect of scattering by random density fluctuations is introduced in order to decrease the values of predicted T b . The value of peak T b is computed as a function of relative r.m.s. density fluctuations e = <ΔN e >/N e ; and it is found that ɛ should be in the range from 0.07 to 0.19, 0.1 to 0.25, and 0.15 to 0.35, respectively, at 38.5, 50.0, and 73.8 MHz, respectively, to explain the observed low brightness temperatures.