Longitudinal variability of low‐latitude total electron content: Tidal influences

Abstract

[1] Recently, nighttime ultraviolet (UV) observations obtained by IMAGE FUV and TIMED GUVI instruments have revealed a longitudinal wave number four pattern in the nighttime airglow intensity and in the position of the equatorial anomalies during equinox and high solar flux conditions. In the present study, we have extended this work and determined the longitudinal variability of the low-latitude total electron content (TEC) climatology during different geophysical conditions with a special emphasis on the longitudinal wave number four structure in the low-latitude ionosphere. We have used more than 5 million low-latitude TOPEX TEC observations covering the entire 13 years of TOPEX TEC data from August 1992 until October 2005. This data set was used to determine the local time, seasonal, solar cycle, and geomagnetic activity dependence of the longitudinal variability of TEC at equatorial and low latitudes, and in particular, to address the existence and evolution of the wave number four longitudinal pattern under these conditions. Our study shows that the wave number four pattern is created during the daytime hours at equinox and June solstice but is absent, or washed out by other processes, during December solstice. During equinox the wave number four pattern is created around noon with well-defined longitudinal enhancements in the low-latitude TEC. These enhancements, which are symmetric about the geomagnetic equator during this season, last for many hours and can be clearly seen past midnight. The longitudinal patterns are found to be nearly identical between the vernal (March/April) and autumnal (September/October) equinoxes and largely independent of the solar cycle conditions. The wave number four pattern is also observed during geomagnetically active conditions, indicating that the processes that create this pattern are also present during active times. The variations between the well-defined longitudinal maxima and minima are of the order of 20%. During June solstice, the wave number four pattern is also observed in the afternoon hours but, in contrast to the equinox cases, it exhibits a strong hemispheric asymmetry and is not observed during the night. The low-latitude TEC exhibits clear longitudinal variations during December solstice, with large daytime enhancements over the east Asian and Pacific regions and a third enhancement emerging in the afternoon over the Atlantic Ocean, but a clear wave number four pattern is not observed during this season. Although the equatorial and low-latitude TEC values exhibit clear longitudinal patterns during all seasons, a significant amount of scatter remains in the TEC data that is not accounted for by changes in the solar cycle, the season, or the local time or by the longitudinal variability. This remaining scatter is largest near the poleward edges of the anomalies and is of the order of 40%.

Vertical connection from the tropospheric activities to the ionospheric longitudinal structure simulated by a new Earth's whole atmosphere-ionosphere coupled model

“…The amplitude of wave 4 is about 4% in our simulation, which is a degree similar to the total electron content observation by TOPEX (e.g., Figure 8 of Scherliess et al [2008]) but significantly smaller than the electron content integrated over 400-450 km obtained by the FORMOSAT-3/COSMIC observation (e.g., Figure 1 of Lin et al [2007], which shows an amplitude of more than 10%). As for the wave 1 structure, most observations do not show a clear wave 1, except for the plasma density observation at a 400 km height made by the CHAMP satellite (e.g., Figure 1 of Liu and Watanabe [2008]).…”

“…[13] Compared with observations of the F-region plasma density, the wave 4 structure becomes dominant only in the Southern Hemisphere in our simulation, while the wave 4 structure is observed in both hemispheres for a similar situation (September equinox, daytime, moderate solar activity) [Lin et al, 2007;Kil et al, 2008;Liu and Watanabe, 2008;Scherliess et al, 2008]. The amplitude of wave 4 is about 4% in our simulation, which is a degree similar to the total electron content observation by TOPEX (e.g., Figure 8 of Scherliess et al [2008]) but significantly smaller than the electron content integrated over 400-450 km obtained by the FORMOSAT-3/COSMIC observation (e.g., Figure 1 of Lin et al [2007], which shows an amplitude of more than 10%).…”

“…[10] Using the present version of GAIA, described in section 2, we have carried out a 30 day consecutive run in September, which is one of the months when the ionospheric four-peak longitudinal structure becomes dominant according to the observations [e.g., Kil et al, 2008;Liu and Watanabe, 2008;Scherliess et al, 2008;Wan et al, 2008]. As the initial condition, we used results from noncoupled simulations using each atmospheric GCM and ionospheric model.…”

The role of the vertical <I><B>E</B></I>×<I><B>B</B></I> drift for the formation of the longitudinal plasma density structure in the low-latitude F region

“…Oh (oh@spweather.com) 2007; Scherliess et al, 2008). The observations of similar periodic structures in the daytime equatorial electrojet (England et al, 2006a;Mouël et al, 2006) and equatorial vertical ion velocity on the topside (Hartman and Heelis, 2007;Kil et al, 2007) supported the association of the longitudinal density structure with the daytime vertical drift of equatorial plasma.…”

The International Reference Ionosphere 2012 – a model of international collaboration

“…A good example is the recently discovered four maxima structure in the longitudinal variation of F-peak electron density and of ionospheric electron content that was first observed with IMAGE/EUV observations (Immel et al 2006), and then confirmed with data from CHAMP (Lühr et al 2007) and TOPEX (Scherliess et al 2008), and that is thought to be caused by nonmigrating, diurnal atmospheric tides that are driven by tropospheric weather in the tropics. While theoretical models still struggle to include this phenomenon in their modeling framework, inspection of the longitudinal variation of the F-peak density value NmF2 in IRI revealed that IRI already reproduces this phenomenon (McNamara et al 2010).…”