Long-term trend of f_oF_2 at a West African equatorial station linked to greenhouse gas increase and dip equator secular displacement

摘要

Long-term variability of f_oF_2 at Ouagadougou station (12.4°N, 358.5°E; dip, 1.45°), a West African equatorial station, is studied, analyzing an annual mean data series at 12 LT for the period 1966-1998. After filtering the solar activity effect using Rz as a solar activity proxy, a downward trend of -0.015 MHz/yr is obtained, that is a ～5% decrease during the period of 33 years here considered, equivalent to a ～0.2%/yr. The downward trend is qualitatively consistent with a decreasing trend expected from increasing greenhouse gas concentration but greater than the ～0.003%/yr that would result from the 20% increase in CO_2 that actually took place during the period of analysis. The f_oF _2 decreasing trend is not in agreement with the trend expected from the secular variation of the dip angle at the location of Ouagadougou, but here an additional mechanism is considered, that is the secular movement of the dip equator toward Ouagadougou. This implies an approaching of the trough of the equatorial ionization anomaly and thus an f_oF_2 decrease at Ouagadougou that is qualitatively in agreement with the observed trend.