The effects of the St. Patrick’s Day geomagnetic storms of 2013 and 2015 in the equatorial and
low-latitude regions of both hemispheres in the 100°E longitude sector is investigated and compared with the
response in the Indian sector at 77°E. The data from a chain of ionosondes and GPS/Global Navigation Satellite
Systems receivers at magnetic conjugate locations in the 100°E sector have been used. The perturbation in
the equatorial zonal electric field due to the prompt penetration of the magnetospheric convective under
shielded electric field and the over shielding electric field gives rise to rapid fluctuations in the F2 layer parameters.
The direction of IMF Bz and disturbance electric field perturbations in the sunset/sunrise period is found to play a
crucial role in deciding the extent of prereversal enhancement which in turn affect the irregularity formation
(equatorial spread F) in the equatorial region. The northward (southward) IMF Bz in the sunset period inhibited
(supported) the irregularity formation in 2015 (2013) in the 100°E sector. Large height increase (hmF2) during
sunrise produced short-duration irregularities during both the storms. The westward disturbance electric field on
18 March inhibited the equatorial ionization anomaly causing negative (positive) storm effect in low latitude
(equatorial) region. The negative effect was amplified in lowmidlatitude by disturbed thermospheric composition
which produced severe density/total electron content depletion. The longitudinal and hemispheric asymmetry of
storm response is observed and attributed to electrodynamic and thermospheric differences.
