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

Based on the medians of the F2-layer maximum electron concentration NmF2 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: Fobs is
the solar radio emission flux at a wavelength of 10.7 cm measured by ground-based radio telescopes and Fadj
is the value of Fobs 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 Fobs make it possible to obtain the
annual asymmetry index R for a fixed value of Fobs with allowance for a substitution of Fobs by cFobs 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 Fobs 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 Fobs 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 Fadj 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 NmF2.