Features of additional stratification in ionospheric F₂ layer observed for half a solar cycle over Indian low latitudes

Abstract

The features of the additional stratification in the ionospheric F2 layer often referred to as the F3 layer observed over an Indian low‐latitude station Waltair (17.7°N, 83.3°E, magnetic latitude (Mag. Lat.) 8.2°N) during the period 1997–2003 are presented along with the data from two other Indian stations. The observations grossly confirm those reported earlier in the occurrence and seasonal variability of the F3 layer. From an analysis of half a solar cycle ionosonde data (1997–2003), it is observed that the layer appeared more frequently during the summer solstice months of low solar activity period and persisted for longer durations during this season compared with equinox and winter solstice. The best stratification is seen between 10–12 hours IST. The occurrence of the F3 layer does not seem to depend on magnetic activity but the percentage of occurrence decreased with increasing solar activity. The solar activity dependence over Waltair confirms the model predictions of Balan et al. (1998) that the layer becomes less distinct and less frequent as solar activity increases. The ionosonde data for the same period (1997–2003) from an equatorial station, Trivandrum (8.4°N, 76.9°E, Mag. Lat. 0.47°N) and another low‐latitude station, SHAR (14°N, 80°E, Mag. Lat. 6.8°N) are also analyzed with a view to examine the effect of the equatorial plasma dynamics on the occurrence of such events.

Features of theF3layer in the low-latitude ionosphere at sunset

“…This mechanism adequately explains the high occurrence of the F 3 layer observations at Fortaleza, Kwajalein, and Vanimo in summer, and the low occurrence at Jicamarca. Similar observational results were also obtained at other magnetic equatorial stations [Jenkins et al, 1997;Rama Rao et al, 2005;Uemoto et al, 2007].…”

“…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].…”

“…efficient to raise the morning F 2 plasma to the topside altitude to form an F 3 layer during solar maximum [Balan et al, 1998]. Then the occurrence of the F 3 layer is higher during periods of low solar activity than under periods of high solar activity as revealed by long-term observational ionogram data [Batista et al, 2002;Rama Rao et al, 2005]. Considering the availability, integrity, and quality of the ionograms, data from [14] Figure 5 shows the monthly occurrence probability of the F 3 layer versus local time at five stations.…”

“…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.…”

Features of the F3 layer occurrence over the equatorial location of Trivandrum

“…Based on the average upward drift (Fejer et al, 1991) and the Horizontal Wind Model (HWM) (Hedin et al, 1991), a distinct F3 layer was predicted to occur on the summer side of the geomagnetic equator (Balan et al, 1998). However, observations show that the layer occurs even during the winter months (Batista et al, 2002;Rama Rao et al, 2005), which was explained as due to the variability in the driving forces like the drift, wind, tides and waves (Balan et al, 1998). There had been several observations of the F3 layer from various equatorial locations.…”