Response of equatorial ionosphere to the September 2017 geomagnetic storm: a case study using digisonde observations at Fortaleza

The present study investigates the ionospheric responses of the September 7–8, 2017 geomagnetic storm over the southern hemisphere equatorial station, Fortaleza (3.9∘ S, 321.6∘ E). The study uses ionospheric observations obtained using a digital ionosonde (DPS-4) in the Brazilian ionosonde network. The storm time response of the equatorial ionosphere is influenced mainly by electric field perturbations and Travelling Ionospheric Disturbances (TIDs). This study attempts to identify the role of Prompt Penetration Electric Field (PPEF), Disturbance Dynamo Electric Field (DDEF), and TIDs on the equatorial ionosphere, though it is difficult to separate them. The ionospheric parameters-Vertical (Vz), Zonal (Vy), and Meridional (Vx) plasma drift velocities, F2 layer Critical frequency (foF2), and F2 layer height at peak density (hmF2)-are analyzed. A Prompt Penetration Eastward Electric Field Model (PPEEFM), developed at the University of Colorado, is also used to compare the observations in order to validate the prompt penetration effects. Variations in vertical plasma drift observed over Fortaleza at dawn of September 7, 2017, before the commencement of the geomagnetic storm are correlated to the prompt penetration electric fields generated by the re-orientations of the Interplanetary Magnetic Field (IMF). The PPEEFM model data also provides evidence of prompt penetration electric fields during this time period. The effect of the disturbance dynamo is reflected in the plasma drift values (Vx,Vy & Vz), foF2, and hmF2 during the daytime and evening of September 7, 2017. Large oscillations observed in the vertical and zonal plasma drift simultaneously on the night of September 8, 2017, after the commencement of the geomagnetic storm, can also be attributed to the effect of prompt penetration. Oscillations in plasma drift (Vz & Vy) and wave-like patterns in foF2 and hmF2 values, observed after the occurrence of the second progressive phase of the geomagnetic storm (super substorm), are attributed to the effect of large-scale TIDs.