Journal
ANNALES GEOPHYSICAE
Volume 24, Issue 12, Pages 3293-3311Publisher
COPERNICUS GESELLSCHAFT MBH
DOI: 10.5194/angeo-24-3293-2006
Keywords
ionosphere; ionosphere-atmosphere interactions; mid-latitude ionosphere; atmospheric composition and structure; thermosphere-composition and chemistry
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Adding together the northern and southern hemisphere values for pairs of stations, the combined peak electron density NmF2 is greater in December-January than in June-July. The same applies to the total height-integrated electron content. This F2-layer annual asymmetry between northern and southern solstices is typically 30%, and thus greatly exceeds the 7% asymmetry in ion production due to the annual variation of Sun-Earth distance. Though it was noticed in ionospheric data almost seventy years ago, the asymmetry is still unexplained. Using ionosonde data and also values derived from the International Reference Ionosphere, we show that the asymmetry exists at noon and at midnight, at all latitudes from equatorial to sub-auroral, and tends to be greater at solar minimum than solar maximum. We find a similar asymmetry in neutral composition in the MSIS model of the thermosphere. Numerical computations with the Coupled Thermosphere-Ionosphere-Plasmasphere (CTIP) model give a much smaller annual asymmetry in electron density and neutral composition than is observed. Including mesospheric tides in the model makes little difference. After considering possible explanations, which do not account for the asymmetry, we are left with the conclusion that dynamical influences of the lower atmosphere (below about 30 km), not included in our computations, are the most likely cause of the asymmetry.
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