Different long-term trends of the oxygen red 630.0 nm line nightglow intensity as the result of lowering the ionosphere F2 layer

Abstract

have been analyzed. It is shown that a decrease in the long-term trend of the mean annual red 630.0 nm line intensity from the premidnight value (+0.770±1.045 R/year) to its minimum negative value (−1.080±0.670 R/year) at the midnight/after midnight is a possible result of the observed lowering of the peak height of the ionosphere F2 layer electron density hmF2 (−0.455±0.343 km/year). A theoretical simulation is carried out using a simple Chapman-type layer (damping in time) for the height distribution of the F2 layer electron density. The estimated values of the lowering in the hmF2, the increase in the red line intensity at pre-midnight and its decrease at midnight/after midnight are close to their observational ones, when a negative trend in the total neutral density of the upper atmosphere and an increase in the mean northward wind (or its possible consequence -a decrease in the southward one) are assumed.

Long-term changes in the nightly behaviour of the oxygen red 630.0 nm line nightglow intensity and trends in the thermospheric meridional wind velocity

“…Figure 1 also demonstrates the longterm increase in the red-line intensity before morning twilight (pre-sunrise): for Local Time (LT) 03-05 h, the average intensity of the 630.0 nm line is greater for 1980 to 1992 than for 1958 to 1970. The positive trend in the red-line intensity I 630 before morning twilight was not noted in earlier papers (Givishvili et al 1996, Gudadze et al 2008. The values of long-term changes in monthly mean nightly behaviour of the 630.0 nm line intensity (see figure 1) are different for different months and could result in different monthly/seasonal trends.…”

“…For midnight, it is reversed: during the first period (1958 to 1970), the 630 nm intensity is greater than during the second period (1980 to 1992). Figure 1 shows the long-term increase of the post-twilight and the decrease at midnight/after midnight in the mean annual red-line intensity (Gudadze et al 2008). Figure 1 also demonstrates the longterm increase in the red-line intensity before morning twilight (pre-sunrise): for Local Time (LT) 03-05 h, the average intensity of the 630.0 nm line is greater for 1980 to 1992 than for 1958 to 1970.…”

“…Recently reported positive trend values in the mid-latitude red-line nightglow intensity after twilight (post-sunset) and negative values during midnight at Abastumani for 1957 to 1993 (Gudadze et al 2007(Gudadze et al , 2008 show the importance of the investigation of long-term changes in the nightly behaviour of the 630 nm line intensity. The red-line intensity data were obtained during cloudless nights at lunarless conditions using two photometers: the first one with a time resolution of 12-15 min (operated from 1957 to 1977) (Fishkova 1983, Gudadze et al 2007).…”

“…The study is the development of the recent results by Gudadze et al (2008) showing that the positive trend observed after twilight and negative value during midnight/after midnight in the annual mean red-line intensity at Abastumani is caused by the long-term decrease of the ionosphere F2 layer peak height, which was observed at the Tbilisi ionosphere station, neighbouring Abastumani. This interpretation of the observed decrease in the hmF2, in addition to the cooling of the upper atmosphere, needs the assumption of the long-term increase in the northward wind velocity (or decrease in the southward wind velocity).…”

“…Recent investigations show a global decrease in the upper atmosphere neutral gas density (Keating et al 2000, Emmert et al 2004, Marcos et al 2005, Emmert et al 2008), but negative trends in the hmF2 (the lowering of the ionosphere F2 layer) and critical frequency foF2 (or NmF2) have not been observed for all ionosphere stations on the globe (Bremer 1998, Marin et al 2001, Bremer et al 2004 mid-latitude ionosphere F2 layer is controlled by ambipolar diffusion of ions, recombination and thermospheric meridional wind. Different trend values of hmF2 observed at different ionosphere stations may possibly reveal long-term dynamical changes accompanying global cooling in the upper atmosphere (Jarvis et al 1998, Gudadze et al 2008, Jarvis 2008, Bencze 2009). …”