Dependence of the Annual Asymmetry Local Index for the NmF2 Median on Solar Activity

Abstract

Based on the medians of the F 2-layer maximum electron concentration NmF 2 for two ionospheric stations, Boulder and Hobart, in 1963–2013, an analysis of the dependence of the annual asymmetry local index R at noon on solar activity has been performed, where the R index is the January/July ratio of the summary NmF2 concentration for this pair of stations. Solar activity indices averaged over 81 days are used: F_obs is the solar radio emission flux at a wavelength of 10.7 cm measured by ground-based radio telescopes and F_adj is the value of F_obs reduced to a fixed distance from the Sun of one astronomical unit. It is found that the regression equations that manifest the NmF2 medians dependence on F_obs make it possible to obtain the annual asymmetry index R for a fixed value of F_obs with allowance for a substitution of F_obs by cF_obs in these regression equations, the c coefficient is equal to 1.03 and 0.97 for January and July, respectively. The c = 1 case corresponds to neglecting the annual asymmetry in the F_obs index due to the ellipticity of the Earth’s orbit. For the c = 1 version, the R index increases with a solar activity increase from 1.2 under low activity up to almost 1.4 under high activity. An additional allowance for the annual asymmetry in F_obs leads to an increase in the R index by approximately 0.1 almost independently of the solar activity level. Apparently, this conclusion is being drawn for the first time. The F_adj index also makes it possible to obtain a correct estimate of the R index, because the annual asymmetry in the solar radiation flux is indirectly taken into account via the experimental values of NmF 2.