The Equatorial Background Solar Corona during Solar Minimum

Abstract

We report two-frequency (51 and 77 MHz) radio observations of the equatorial brightness distribution of the “undisturbed” solar corona, close to the minimum between sunspot cycles 22 and 23. The contributions from different discrete structures in the observed one-dimensional profiles were identified and removed through an iterative multi-Gaussian least-squares curve fitting technique, and the “background” corona was obtained for each observation. Its average east-west (E-W) diameter is and at 51 and 77 MHz respectively. This indicates that the plasma levels corresponding to the above two frequencies are located at radial distances (r, measured from center of the Sun) of ≈1.35 and 1.19 R; and the electron density () there is 3.2 and cm−3, respectively. Using the above values, we derived the corresponding density gradient, r−6.5. We also calculated the integrated flux densities (S) of emission from the background corona; the average values are ≈0.46 and 1.12 sfu (1 sfu = solar flux unit = 10−22 W m−2 Hz−1) at the above two frequencies. The corresponding brightness temperatures () are 3.85 and K. We estimated the spectral dependence of the former, finding that it scales as ν2.2, where ν is the frequency of observation. The day-to-day values of the E-W diameter show significant variation at both 51 and 77 MHz, even after the removal of contributions from discrete sources.