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The ionospheric response to the geomagnetic storm of 15 May 2005 has been studied over midlatitude stations in the dayside and nightside, simultaneously. In the day side the ionospheric response has been studied using the ground-based GPS and ionosonde measurements from the stations POL2 and Alma-Ata, respectively. The dayside total electron content (TEC) and f o F 2 variations are characterized by two well-separated enhancements. Of which the first enhancement in both the parameters is attributed to the episode of prompt penetration electric field caused by the sudden southward turning of interplanetary magnetic field (IMF) B z around 0600 UT. The second enhancement which was also superposed by wave like modulations has been attributed to the storm-induced winds. The maximum peak-to-peak amplitude of modulation in TEC is found to be 5 TECU (total electron content unit, 1 TECU = 10 16 el m À2). The enhanced plasma density observed during the daytime at midlatitudes is found to be locally produced and not transported from the equatorial ionization anomaly region because the time of enhanced plasma density at midlatitude is earlier than that observed at low latitudes. During the storm main phase, the nightside GPS observations from the midlatitude station ALGO (Algonquin Park, Canada) show moderate to large TEC fluctuations and short duration depletions that occur in a narrow latitude zone. These fluctuations and depletions in TEC have been attributed to the combined effect of storm-induced equatorward movement of the midlatitude ionospheric trough due to the expansion of auroral oval and the storm time-enhanced density. The maximum amplitude of the TEC depletions is found to be of the order of 20 TECU. Rate of TEC Index is also found to be high with a maximum value of 2.

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