Physical mechanism and statistics of occurrence of an additional layer in the equatorial ionosphere

Abstract

Abstract.A physical mechanism and the location and latitudinal extent of an additional layer, called the F3 layer, that exists in the equatorial ionosphere are presented. A statistical analysis of the occurrence of the layer recorded at the equatorial station Fortaleza (4øS, 38øW; dip 9øS) in Brazil is also presented. The F3 layer forms during the morning-noon period in that equatorial region where the combined effect of the upward ExB drift and neutral wind provides a vertically upward plasma drift velocity at altitudes near and above the F2 peak. This velocity causes the F2 peak to drift upward and form the Fa layer while the normal F• layer develops at lower altitudes through the usual photochemical and dynamical effects of the equatorial region. The peak electron density of the Fa layer can exceed that of the Fu layer. The F3 layer is predicted to be distinct on the summer side of the geomagnetic equator during periods of low solar activity and to become less distinct as the solar activity increases. Ionograms recorded at Fortaleza in 1995 show the existence of an Fa layer on 49% of the days, with the occurrence being most frequent (75%) and distinct in summer, as expected. During summer the layer occurs earlier and lasts longer compared to the other seasons; on the average, the layer occurs at around 0930 LT and lasts for about 3 hours. The altitude of the layer is also high in summer, with the mean peak virtual height being about 570 kin. However, the critical frequency of the layer (foF3) exceeds that of the Fu layer (foF•) by the largest amounts in winter and equinox; foFa exceeds foF2 by a yearly average of about 1.3 MHz.

Features of theF3layer in the low-latitude ionosphere at sunset

“…The amount of the occurrence of the sunset F 3 layer increases as the solar activity increases from low solar activity years (1995-1996 and 2006-2009) which display a linear dependence on the solar flux as revealed by Fejer et al [1991]. Actually, the solar activity dependence can also be found in the altitude of the daytime F 3 layer with a model simulation at the June solstice corresponding to 1100 LT at 4°N magnetic latitude at the longitude of Fortaleza [Balan et al, 1998]. The visible virtual height of the F 3 peak increases from 500 to 600 km as F10.7 changes from 95 to 194.…”

“…The visible virtual height of the F 3 peak increases from 500 to 600 km as F10.7 changes from 95 to 194. In comparison, the virtual height of the F 3 peak during the daytime varies from about 430 km in June to about 625 km in January at subtropical regions [Balan et al, 1998], which should be attributed to the seasonal variation of the wind-lifting effect.…”

“…The stratification of the F 2 layer on the days of occurrence started around 0800 LT and occasionally extended as late as 1600 LT [Balan et al, 1998;Rama Rao et al, 2005]. However, there is usually an evening PRE of the eastward electric field in the dusk sector which occurs simultaneously in the equatorial E and F regions [Heelis et al, 1974].…”

“…[7] In this paper we report a type of the F 3 layer produced by the E × B prereversal enhancement (PRE), which was not fully discussed in previous work [e.g., Balan et al, 1998]. A statistical result with respect to the seasonal and solar activity dependence of the sunset F 3 layer is given on the basis of more than a solar cycle (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) of ionogram data recorded at Jicamarca (dip latitude ∼0.5°N).…”

“…This additional layer was later renamed the F 3 layer because it has the same chemical composition as the F region . After that, the subject of the F 3 layer received extensive investigations at low and equatorial latitudes concerning its spatial and temporal distribution [Jenkins et al, 1997;Balan et al, 1998;Hsiao et al, 2001;Batista et al, 2002;Rama Rao et al, 2005;Zain et al, 2008;Uemoto et al, 2007;Sreeja et al, 2010;Zhao et al, 2011], and also its storm time characteristics [Zhao et al, 2005;Paznukhov et al, 2007;Balan et al, 2008;Sreeja et al, 2009;Zong et al, 2010]. The mechanism of the storm time F 3 layer resembles that in quiet periods and was caused by a rapid upward E × B drift resulting from an eastward penetration electric field but sometimes can be caused by strong field-aligned diffusion due to the storm time meridional wind surges as proposed by Lin et al [2009aLin et al [ , 2009b.…”