Nighttime ionospheric irregularity during intense geomagnetic storm events over the Europe-African longitudinal sector

Radio communication and navigation systems can be severely impacted by irregularities in the ionosphere. There is still much to learn about how geomagnetic storms affect the occurrence of these irregularities. Ionosphere studies in different regions, particularly the equatorial and low-latitudes, are necessary to enhance the forecasting of this phenomenon. This study investigates the effect of intense geomagnetic storm events of August 27, October 7 and December 22, 2015, on nighttime ionospheric irregularities. Data collected from the receivers of the Global Navigation Satellite Systems (GNSS) in specific longitudinal sector of 15^∘W - 0^∘, 0^∘ - 15^∘E, 15^∘E - 30^∘E and 30^∘E - 45^∘E, as well as from the Swarm constellations in the longitude range of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup><mml:mrow><mml:mn>30</mml:mn></mml:mrow> <mml:mrow><mml:mo>∘</mml:mo></mml:mrow> </mml:msup> </mml:math> W - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup><mml:mrow><mml:mn>70</mml:mn></mml:mrow> <mml:mrow><mml:mo>∘</mml:mo></mml:mrow> </mml:msup> </mml:math> E and the latitude range of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup><mml:mrow><mml:mn>50</mml:mn></mml:mrow> <mml:mrow><mml:mo>∘</mml:mo></mml:mrow> </mml:msup> </mml:math> S - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup><mml:mrow><mml:mn>50</mml:mn></mml:mrow> <mml:mrow><mml:mo>∘</mml:mo></mml:mrow> </mml:msup> </mml:math> N, have been used. The rate of change of the total electron content (TEC) index (ROTI) and the rate of change of the electron density index (RODI) were analyzed. In the main phases of the storms, at equatorial and low-latitude regions we observed ionospheric irregularities in the African longitudinal sectors of 15^∘W - 0^∘, 0^∘ - 15^∘E and 30^∘E - 45^∘E in the three storm events. The observed ionospheric irregularities were more pronounced in the western than eastern regions. These irregularities were possibly driven by the Prompt Penetration Electric Fields (PPEFs) that point east to west during nighttime. Ionospheric irregularities were inhibited in the selected storm periods over the middle latitudes in the African sector. In the longitudinal sector of 15^∘E - 30^∘E, we obtained inhibition of irregularity in the study periods. In the case of the top-side ionosphere, we observed enhanced and depleted electron density during the storm in the low latitude and equatorial regions. In the low latitude and equatorial regions, there were electron density fluctuations within the range of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup><mml:mrow><mml:mn>15</mml:mn></mml:mrow> <mml:mrow><mml:mo>∘</mml:mo></mml:mrow> </mml:msup> </mml:math> N and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup><mml:mrow><mml:mn>15</mml:mn></mml:mrow> <mml:mrow><mml:mo>∘</mml:mo></mml:mrow> </mml:msup> </mml:math> S, indicating significant top-side irregularities.