New ACP Letter: Interhemispheric transport of metallic ions within ionospheric sporadic E layers by the lower thermospheric meridional circulation

18 March 2021

Long-lived metallic ions in the Earth's ionosphere (the electrified portion of the upper atmosphere) have been studied for many decades. One long-standing mystery is the mechanism driving the marked seasonal dependence of metallic ions within the ionospheric sporadic E (E_s) layers, which was first observed in the 1960s. The concentration of metallic ions in the upper atmosphere has a maximum in summer and a minimum in winter.

The authors have found a new phenomenon of a winter-to-summer seasonal transport of ionospheric irregularities induced by the long-lived metallic ions between 90 and 130 km from satellite radio occultation measurements. The long-term ground-based observations from five ionosondes along the Greenwich meridian (using data from the Rutherford Appleton Laboratory, UK) and five ionosondes along 120° E (obtained by the Chinese Meridian Project, China) were analysed to try to identify the signature of the large-scale horizontal transport of metallic ions in E_s layers. The mechanism proposed in this study is based on the now widely accepted wind shear theory. It explains the formation of mid-latitude E_s layers through vertical ion convergence driven by vertical shears of horizontal neutral winds. By extending this original theory to three dimensions they have shown that this theory can also explain the observed seasonal anomalies. The model simulations reveal that the winter-to-summer meridional circulation transports the long-lived metal ions from the winter hemisphere to the summer hemisphere. The Fe⁺ ion flux is southward in January and northward in July.

This result has resolved a previously unexplained aspect of the seasonal dependence of E_s layers and emphasizes the influences of atmospheric meridional circulation on the large-scale interhemispheric transport of composition in the atmosphere and ionosphere. In addition, it provides strong support for the lower thermospheric meridional circulation predicted by the model, which is difficult to measure and characterize directly because of the considerably smaller amplitudes than tides.

Contact: Xianghui Xue (xuexh@ustc.edu.cn) and Christopher J. Scott (chris.scott@reading.ac.uk)