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

Y. SAHAI; F. BECKER-GUEDES; P. R. FAGUNDES; R. DE JESUS; A. J. DE ABREU; L. J. PAXTON; L. P. GONCHARENKO; C. BRUNINI; M. GENDE; A. S. FERREIRA; N. S. LIMA; F. L. GUARNIERI; V. G. PILLAT; J. A. BITTENCOURT; C. M. N. CANDIDO

JOURNAL OF GEOPHYSICAL RESEARCH

American Geophysical Union

Año: 2009 vol. 114 p. 1 - 19

0148-0227

The Sun was very active in the early part of November 2004. During the period of8?10 November 2004, intense geomagnetic disturbances with two superstorms wereobserved. In this paper, we have investigated the generation and suppression ofequatorial ionospheric irregularities and the daytime changes in the F region electrondensity in the Latin American sector during the period of intense geomagneticdisturbances. We present the ionospheric sounding observations carried out at Manausand Sao Jose dos Campos, Brazil, during this geomagnetically disturbed period. Also,GPS observations obtained from several stations in Brazil, Argentina, and St. Croix,U.S. Virgin Islands, during the disturbed period are presented. During the main phase ofthe first superstorm, around the prereversal enhancement time (night of 7?8 November),prompt penetration of electric field was observed and the presence of equatorialionospheric irregularities was detected from St. Croix, U.S. Virgin Islands (in thenorthern hemisphere) to Bahia Blanca, Argentina (in the southern hemisphere). Theionospheric sounding observations at Manaus indicate inhibition of prereversalenhancement on the nights of 9?10 and 10?11 November, possibly due to the disturbedthermospheric winds or disturbance electric fields. Virtually no phase fluctuations on thenights of 9?10 and 10?11 November were observed in the Latin American sector.During the daytime on 8 November, the vertical total electron content (VTEC)observations show a negative storm phase at Porto Alegre (Brazil) and Bahia Blanca(Argentina). Again during the daytime on 10 November, the VTEC observations show anegative storm phase from Brasilia (Brazil) to Bahia Blanca. These negative stormphases are associated with a decrease in the O/N2 ratio. During the daytime on 9November, the VTEC observations show a positive storm phase extendingfrom St. Croix to Porto Alegre, and again on 10 November, VTEC observationsshow a positive storm phase. These positive storm phases observed are possibly due tochanges in large-scale wind circulation and an increase in the O/N2 ratio