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Effects observed in the Latin American sector ionospheric F-region during the intense geomagnetic disturbances in the early part of November 2004


The Sun was very active in the early part of November 2004. During the period of 8-10 November 2004, intense geomagnetic disturbances with two superstorms and two intense geomagnetic storms were observed. In this paper, we present the ionospheric sounding observations carried out at Manaus and Sao Jose dos Campos, Brazil, during this geomagnetically disturbed period. Also, GPS observations obtained from several stations in Brazil and Argentina, and St. Croix, Virgin Island, U.S.A., during the disturbed period are presented. During the main phase of the first superstorm, around the pre-reversal enhancement time (night of 07-08 November), prompt penetration of electric field was observed and the presence of equatorial ionospheric irregularities was detected from St. Croix, Virgin Is. (in the northern hemisphere) to Bahia Blanca, Argentina (in the southern hemisphere). During the daytime on 08 November, a negative storm phase was observed at Porto Alegre (Brazil) and Bahia Blanca (Argentina), possibly associated with a decrease in the ratio O/N2 . The ionospheric sounding observations at Manaus indicate inhibition of pre-reversal enhancement on the nights of 09-10 and 10-11 November, possibly due to the disturbed thermospheric winds. Virtually no spread F (phase fluctuations) on the nights of 09-10 and 10-11 November were observed in the Latin American sector investigated. The VTEC observations during the daytime on 10 November show a positive storm phase at St. Croix, Virgin Is. (near mid-latitude station in the northern hemisphere), no change at Belem, Brazil (a near equatorial station), and a negative storm phase from Brasilia, Brazil, to Bahia Blanca, Argentina (covering low- to mid-latitude regions in the southern hemisphere), possibly due to storm-time changes in the ratio O/N2 in the northern and southern hemispheres. (AU)

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