  This work presents, for the first time, the analysis of the occurrence of
ionospheric irregularities during geomagnetic storms at Tucum\'an-Argentina, a
low latitude station in the Southern American longitudinal sector ( 26.9 {\deg}
S, 294.6 {\deg} E; magnetic latitude 15.5 {\deg} S), near the southern crest of
the equatorial ionization anomaly (EIA). Three geomagnetic storms occurred on
May 27, 2017 (a month of low occurrence rates of spread-F), October 12, 2016 (a
month of transition from low to high occurrence rates of spread-F) and November
7, 2017 (a month of high occurrence rates of spread-F) are analyzed using
Global Positioning System (GPS) receivers and ionosondes. The rate of change of
total electron content (TEC) Index (ROTI), GPS Ionospheric L-band
scintillation, the virtual height of the F-layer bottom side (h'F) and the
critical frequency of the F2 layer (foF2) are considered. Furthermore, each
ionogram is manually examined for the presence of spread-F signatures. The
results show that, for the three events studied, geomagnetic activity creates
favorable conditions for the initiation of ionospheric irregularities,
manifested by ionogram spread-F and TEC fluctuation. Post-midnight
irregularities are developed due to the presence of eastward disturbance dynamo
electric fields (DDEF). For the storm of May, an eastward over-shielding prompt
penetration electric field (PPEF) is also acting. This PPEF is added to the
DDEF and produces the uplifting of the F region that helps trigger the
irregularities. What is more, during October and November, strong GPS L band
scintillation is observed associated with strong range spread-F (SSF), that is,
irregularities29 extending from the bottom-side to the topside of the F region.
