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Authors: Pezzopane, M. 
Fagundes, P. R. 
Ciraolo, L. 
Correia, E. 
Cabrera, M. A. 
Ezquer, R. G. 
Title: Unusual nighttime impulsive foF2 enhancement below the southern anomaly crest under geomagnetically quiet conditions
Issue Date: 9-Dec-2011
Series/Report no.: /116 (2011)
DOI: 10.1029/2011JA016593
Keywords: equatorial ionosphere
travelling ionospheric disturbance
ionosphere-atmosphere interactions
instrument and techniques
Subject Classification01. Atmosphere::01.02. Ionosphere::01.02.99. General or miscellaneous 
01. Atmosphere::01.02. Ionosphere::01.02.02. Dynamics 
01. Atmosphere::01.02. Ionosphere::01.02.04. Plasma Physics 
01. Atmosphere::01.02. Ionosphere::01.02.05. Wave propagation 
01. Atmosphere::01.02. Ionosphere::01.02.06. Instruments and techniques 
05. General::05.07. Space and Planetary sciences::05.07.99. General or miscellaneous 
05. General::05.07. Space and Planetary sciences::05.07.01. Solar-terrestrial interaction 
05. General::05.07. Space and Planetary sciences::05.07.02. Space weather 
Abstract: An unusual nighttime impulsive electron density enhancement was observed on 6 March 2010 over a wide region of South America, below the southern crest of the equatorial anomaly, under low solar activity and quiet geomagnetic conditions. The phenomenon was observed almost simultaneously by the F2 layer critical frequency ( foF2) recorded at three ionospheric stations which are widely distributed in space, namely Cachoeira Paulista (22.4°S, 44.6°W, magnetic latitude 13.4°S), São José dos Campos (23.2°S, 45.9°W, magnetic latitude 14.1°S), Brazil, and Tucumán (26.9°S, 65.4°W, magnetic latitude 16.8°S), Argentina. Although in a more restricted region over Tucumán, the phenomenon was also observed by the total electron content (TEC) maps computed by usingmeasurements from 12 GPS receivers. The investigated phenomenon is very particular because besides being of brief duration, it is characterized by a pronounced compression of the ionosphere. This compression was clearly visible both by the virtual height of the base of the F region (h′F) recorded at the aforementioned ionospheric stations, and by both the vertical electron density profiles and the slab thickness computed over Tucumán. Consequently, neither an enhanced fountain effect nor plasma diffusion from the plasmasphere can be considered as the single cause of this unusual event. A thorough analysis of isoheight and isofrequency ionosonde plots suggest that traveling ionospheric disturbances (TIDs) caused by gravity wave (GW) propagation could have likely played a significant role in causing the phenomenon.
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