Nocturnal variability of total electron content (TEC) at Koudougou (Burkina Faso) during three geomagnetic storms of solar cycle 24: implications for equatorial ionospheric irregularities

Résumé

Abstract. This study analyses the nocturnal variability of GPS-derived total electron content (TEC) at Koudougou, Burkina Faso (12°15’N, 2°20’W), during three geomagnetic storms with Sudden Storm Commencement (SSC) that occurred during solar cycle 24 (22 June 2015, 14 July 2012 and 8 October 2013). The objective is to identify the geophysical conditions favourable to the generation or suppression of equatorial ionospheric irregularities. In this work, nocturnal variability is quantified by the hourly nocturnal standard deviation σ_TEC (20:00–05:00 LT) and by the relative perturbation δTEC (%). The reference TEC is constructed from the median of five geomagnetically quiet days (Kp < 2) of the month of each storm. A Superposed Epoch Analysis (SEA) is applied to the three events aligned at T = 0, corresponding to the SSC time. The storms of 22 June 2015 (SYM-H_min = −198.8 nT) and 14 July 2012 (SYM-H_min = −114.2 nT) are dominated by the negative phase (δTEC reaching −91.4 % and −99.5 % respectively), with σ_TEC classified as moderate to reduce during the main phase (3.38 and 3.79 TECU), characteristic of the disturbance Dynamo. In contrast, the storm of 8 October 2013 (SYM-H_min = −64 nT) exhibits a prolonged positive phase (δTEC_max = +77.6 %, 16 hours of positive phase) and an intense σ_TEC over the three analysed nights (5.0 to 7.9 TECU), the signature of a dominant Prompt Penetration Electric Field (PPEF). SEA of the three events reveals a transition from a brief median positive phase (T = −21 h, +29.7 %) to a prolonged median negative phase (T = +1 h to +34 h, minimum −80.3 % at T = +32 h). These results quantitatively highlight, over the West African sector, the generation/suppression duality of nocturnal ionospheric irregularities during disturbed periods, illustrating the antagonistic role of PPEF and the disturbance Dynamo.

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