Oscillations of global mean TEC

Abstract

The global mean of TEC (total electron content) is retrievable from signals of the global navigation satellite system (GNSS) received by the world‐wide network of GNSS ground stations. The time series of global mean TEC show dominant fluctuations with periods of 11‐year, 1‐year, 0.5‐year, and 27‐day. The annual variation of global mean TEC confirms the unsolved puzzle that “there is more ionosphere in January than in July”. Global mean TEC varies from +12.5% in January to −12.5% in July while the annual variation of solar irradiance due to the orbital eccentricity of the Earth is only ±3.3%. The oscillations of global mean TEC can be related to oscillations of the ionizing, solar EUV (extreme ultraviolet) irradiance as represented by the Mg II index. Comparing the dominant spectral components of EUV and global mean TEC, we find different amplification factors ΔTEC/ΔMg II for each spectral component. Depending on periodicity, a 1% ‐variation of solar EUV irradiance can induce variations of global mean TEC from 3.8 to 22%. While the 27‐day oscillation of EUV perfectly correlates with the 27‐day oscillation of global mean TEC (r = 0.90), we find a weak correlation (r = 0.31) between the 13.5‐day oscillations of global mean TEC and EUV. The amplitude modulations of the spectral components of global mean TEC during solar cycle‐23 are derived, and influences of atmospheric dynamics are discussed. The typical response of global mean TEC to solar flares is derived by a superposed epoch analysis of 240 major flares of solar cycle‐23 and gives new insights into the Sun‐Earth connection chain.