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Authors: Perna, L.* 
Pezzopane, M.* 
Zuccheretti, E.* 
Fagundes, P. R.* 
de Jesus, R.* 
Cabrera, M. A.* 
Ezquer, R. G.* 
Title: Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations
Issue Date: 1-Aug-2014
Series/Report no.: 3/54(2014)
DOI: 10.1016/j.asr.2013.05.014
Keywords: Equatorial ionosphere
Electron density enhancement
Traveling ionospheric disturbance
Fountain effect
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::01.02.05. Wave propagation 
01. Atmosphere::01.02. Ionosphere::01.02.06. Instruments and techniques 
01. Atmosphere::01.03. Magnetosphere::01.03.02. Magnetic storms 
05. General::05.01. Computational geophysics::05.01.99. General or miscellaneous 
05. General::05.01. Computational geophysics::05.01.01. Data processing 
05. General::05.01. Computational geophysics::05.01.03. Inverse methods 
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: This paper is focused on unusual nighttime impulsive electron density enhancements that are rarely observed at low latitudes on a wide region of South America, under quiet and medium/high geomagnetic conditions. The phenomenon under investigation is very peculiar because besides being of brief duration, it is characterized by a pronounced compression of the ionosphere. The phenomenon was studied and analyzed using both the F2 layer critical frequency (foF2) and the virtual height of the base of the F region (h'F) values recorded at five ionospheric stations widely distributed in space, namely: Jicamarca (-12.0°, -76.8°, magnetic latitude -2.0°), Peru; Sao Luis (-2.6°, -44.2°, magnetic latitude +6.2°), Cachoeira Paulista (-22.4°, -44.6°, magnetic latitude -13.4°), and Sao Jose´ dos Campos (-23.2°, -45.9°, magnetic latitude -14.1°), Brazil; Tucumán (-26.9°, -65.4°, magnetic latitude -16.8°), Argentina. In a more restricted region over Tucumán, the phenomenon was also investigated by the total electron content (TEC) maps computed by using measurements from 12 GPS receivers. A detailed analysis of isoheight ionosonde plots suggests that traveling ionospheric disturbances (TIDs) caused by gravity wave (GW) propagation could play a significant role in causing the phenomenon both for quiet and for medium/high geomagnetic activity; in the latter case however a recharging of the fountain effect, due to electric fields penetrating from the magnetosphere, joins the TID propagation and plays an as much significant role in causing impulsive electron density enhancements.
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