Critical frequencies, sunspots, and the Sun's ultra‐violet radiation

Abstract

Monthly values of Rƒ0, the critical frequency relative to the critical frequency at zero sunspots, have been averaged for a number of stations and tabulated for the eleven‐year period 1937–47, The variations of Rƒ0 for the E‐, F1‐, and F2‐regions have been correlated with the solar data for sunspot‐numbers, bright Hα and Ca flocculi, faculae, and the coronal‐line intensity. The variations correlated are found to lag behind the sunspot‐number variations by the following parts of a month: Hα flocculi, 0.15; Ca flocculi, 0.31; Rƒ0E, 0.43; faculae, 0.56; Rƒ0F1, 0.66; Rƒ0F2, 0.68; coronal line, 0.87; and this is interpreted as giving the order of increasing mean life of the relevant solar features.In the expression RS = (1 + bR) = (Rƒ0)n relating relative ultra‐violet intensity RS to sunspot‐number R and critical frequency, the adopted values of n and b for the regions E, F1, and F2 respectively are n = 4, 4, and 2, and b = 0.0097, 0.0124, and 0.0200. Values of n determined from the relation between critical frequency and the Sun's zenith‐distance were 3.72 for the E‐region, and 4.90 for F1.The values of Rƒ0F2 are shown to vary with time, season, and station. The noon values however do not vary greatly and are generally close to the diurnal mean. A method of choosing stations reasonably free from the F2‐anomaly is given, and the recombination and electron‐decay coefficients are determined from these stations. The number of quanta per cm2 per sec outside the Earth's atmosphere at zero sunspots is given as: E, 5 × 108; F1, 1.9 × 109; and F2, 2.3 × 109.