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Adding together the northern and southern hemispherevalues for pairs of stations, the combined peak electron density <i>Nm</i>F2 isgreater in December-January than in June–July. The same applies to the totalheight-integrated electron content. This "F2-layer annual asymmetry" betweennorthern and southern solstices is typically 30%, and thus greatlyexceeds the 7% asymmetry in ion production due to the annual variation ofSun-Earth distance. Though it was noticed in ionospheric data almost seventyyears ago, the asymmetry is still unexplained.  Using ionosonde data and also values derived from the InternationalReference Ionosphere, we show that the asymmetry exists at noon and atmidnight, at all latitudes from equatorial to sub-auroral, and tends to begreater at solar minimum than solar maximum. We find a similar asymmetry inneutral composition in the MSIS model of the thermosphere. Numericalcomputations with the Coupled Thermosphere-Ionosphere-Plasmasphere (CTIP)model give a much smaller annual asymmetry in electron density and neutralcomposition than is observed. Including mesospheric tides in the model makeslittle difference. After considering possible explanations, which do notaccount for the asymmetry, we are left with the conclusion that dynamicalinfluences of the lower atmosphere (below about 30 km), not included in ourcomputations, are the most likely cause of the asymmetry

Why is there more ionosphere in January than in July? The annual asymmetry in the F2-layer