Longitudinal Variation of the Ionospheric Response to the 26 August 2018 Geomagnetic Storm at Equatorial/Low Latitudes

We have studied the ionospheric response at near-equator latitudes during the geomagnetic storm of 26 August 2018, which was the strongest geomagnetic disturbance that year (minimum Dst value: −174 nT). For the analysis, we considered the F2-layer critical frequency (foF2) and peak height (hmF2), as well as total electron content (TEC) data for Jicamarca (geographic coordinates: 12° S, 283.2° E; geomagnetic coordinates: 2.26° S, 4.09°  W), Saoluis (geographic coordinates: 2.6° S, 315.8° E; geomagnetic coordinates: 5.94°  N, 28.5° E), Guam (geographic coordinates: 13.4°  N, 144.8° E; geomagnetic coordinates: 5.73°  N, 143.2°  W) and Libreville (geographic coordinates: 0.39°  N, 9.45° E; geomagnetic coordinates: 1.64°  N, 82.6°  W). First, we observed pre-storm improvements, which could be due to previous moderate geomagnetic activity. Second, only one station (Jicamarca) clearly revealed a prompt penetration electric field (PPEF) effect when Bz turned strongly negative during the initial phase of the storm. Over the stations Saoluis and Guam, a PPEF effect was not evident. These stations presented pre-storm enhancements in foF2. In this case study, disturbed dynamo electric fields appear not to have played a crucial role in increasing electron density near equatorial regions during the recovery phase because the observed disturbances did not correspond with those produced by these electric fields, that is, negative (positive) storm effects on the dayside (night). Third, the increases in electron density observed during recovery are most likely caused by neutral composition changes.