Abstract
Critical frequency foF2 long-term trends at Dakar station (14.4°N, 342.74°E) located near the crest of the equatorial ionization anomaly EIA, are analysed taking into account geomagnetic activity, increasing greenhouse gases concentration and Earth’s magnetic field secular variation. After filtering solar activity effect using F10.7 as a solar activity proxy, we determined the relative residual trends slopes α values for three different levels of geomagnetic activity. For example, at 1200 LT, the value of α goes from ǂ.27%/year for very magnetically quiet days to -0.19%/year for magnetically quiet days and to -0.13%/year for all days. It appears from the slopes α obtained, that they increase with the level of geomagnetic activity and their negative values are qualitatively consistent with the expected decreasing trend due to the increase in greenhouse gases concentration but are greater than 0.003%/year which would result from a 20% increase in CO2 emissions which actually took place during the analysis period. Regarding Earth’s magnetic field magnitude, B secular variation and the dip equator secular movement, Dakar station is located near the crest of the equatorial ionization anomaly, Earth’s magnetic field magnitude, B decreases there and the trough approaches the position of Dakar during the period of analysis. These two phenomena induce a decrease in foF2 which is in agreement with the decreasing trend observed at this station.
Highlights
IntroductionThe ionosphere, in particular its F layer, plays an important role in radio communications (HF) and the propagation of GPS (Global Positioning System) signals, and most studies on its long-term variations use the critical frequency of its F2 layer (foF2) directly linked to its electronic density
At 1200 LT, the value of α goes from −0.27%/year for very magnetically quiet days to −0.19%/year for magnetically quiet days and to −0.13%/year for all days. It appears from the slopes α obtained, that they increase with the level of geomagnetic activity and their negative values are qualitatively consistent with the expected decreasing trend due to the increase in greenhouse gases concentration but are greater than 0.003%/year which would result from a 20% increase in CO2 emissions which took place during the analysis period
Regarding Earth’s magnetic field magnitude, B secular variation and the dip equator secular movement, Dakar station is located near the crest of the equatorial ionization anomaly, Earth’s magnetic field magnitude, B decreases there and the trough approaches the position of Dakar during the period of analysis
Summary
The ionosphere, in particular its F layer, plays an important role in radio communications (HF) and the propagation of GPS (Global Positioning System) signals, and most studies on its long-term variations use the critical frequency of its F2 layer (foF2) directly linked to its electronic density. Several possible mechanisms [15] [20] [30] [37] are used to explain the long-term trends of foF2, among others: 1) solar cycle long term variation, 2) long term variation in geomagnetic activity, 3) increasing greenhouse gases concentration and 4) secular Earth’s magnetic field variation.
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