Dependence of NmF 2 Local Annual Asymmetry Index on Local Time and Solar Activity

An analysis of the dependence of the local annual asymmetry R index on local time and solar activity has been performed for 1963–2013 based on the data of the medians of the maximum electron density of F 2-layer NmF 2 at the pair of the ionospheric stations Boulder–Hobart where the R index is the January–July ratio of the total NmF 2 concentration (for this pair of stations) at a fixed local time. As an indicator of solar activity for the median NmF 2, the F index is used, i.e. the 81-day average value of the solar radio emission flux at a wavelength of 10.7 cm, which is centered on the middle of the given month. It was found that for the dependence of the R index on the local time LT, a semidiurnal mode prevails with maxima near noon and midnight and minima in the morning and evening. The lowest R values, R = 1, are observed at low solar activity in a narrow interval of 19–19.5 LT. Annual asymmetry in the NmF 2 median exists ( R > 1) for all other hours of local time at any level of solar activity. Near noon the R index increases with solar activity with a tendency to saturation at a high level of this activity. Around midnight for the dependence of the R index on F there is a maximum near F = 140, above which R decreases with growth of F . High R index values at noon and midnight are mainly due to relatively high values of NmF 2 in January in the Northern Hemisphere (local winter, Boulder) at noon and in the Southern Hemisphere (local summer, Hobart) at midnight.