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A very intense geomagnetic storm (superstorm) began with storm suddencommencement (SSC) at 08:03 UT on 20 November 2003, as a result of thecoronal mass ejection (CME) by sunspot 484 hurled into space on 18 November2003. The geomagnetic storm attained |Dst|max=472 nT at20:00 UT (20 November). In this paper we present the simultaneous ionosphericsounding observations, using the Canadian Advanced Digital Ionosondes(CADIs), carried out from Palmas (PAL; 10.2° S, 48.2° W; diplatitude 5.5° S; a near equatorial station) and São José dosCampos (SJC; 23.2° S, 45.9° W; dip latitude 17.6° S; stationlocated under the crest of equatorial ionospheric anomaly), Brazil. Inaddition, total electron content (TEC) measurements from several GPSreceiving stations in the Brazilian sector during this storm are presented.The simultaneous ionospheric sounding observations carried out at SJC andPAL, and TEC observations on 3 consecutive days viz., 19 November (quiet),20 November (disturbed) and 21 November (recovery phase) are presented.Salient features from the ionospheric observations in the Brazilian sectorduring the superstorm are discussed. The difference in the observedionospheric response at the two stations (PAL and SJC) is considerable. Thisis not surprising given that PAL is close to the magnetic equator and SJC isnear the crest of the equatorial ionospheric anomaly (EIA). It should bepointed out that soon after the SSC (about 4 h later), the F-regioncritical frequency (foF2), the F-region peak height (hpF2), and variationsof virtual heights at different frequencies (iso-frequency plots) all showwavelike oscillations of the F-region during daytime at both the ionosphericsounding stations. Unusual rapid uplifting of F-region at PAL was observedduring both the main and recovery phases of the storm

The ionospheric response in the Brazilian sector during the super geomagnetic storm on 20 November 2003