Annual and Semiannual Oscillations of Thermospheric Composition in TIMED/GUVI Limb Measurements

Abstract

The Global UltraViolet Imager (GUVI) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite provides a data set of vertical thermospheric composition (O, N 2 , and O 2 densities) and temperature profiles from 2002-2007. Even though GUVI sampling is limited by orbital constraint, we demonstrated that the GUVI data set can be used to derive the altitude profiles of the amplitudes and phases of annual oscillation (AO) and semiannual oscillation (SAO), thereby providing important constraints on models seeking to explain these features. We performed a seasonal and interannual analysis of GUVI limb O, O 2 , and N 2 densities and volume number density ratio O/N 2 at constant pressure levels. These daytime observations of O and O/N 2 in the lower thermosphere show a strong AO at midlatitudes and a clear SAO at lower latitudes. The global mean GUVI O/N 2 number density ratio shows the AO, with slightly larger values in January than in July and a SAO with O/N 2 greater during equinoxes than at the solstices. O and N 2 densities on fixed pressure levels in the upper thermosphere are anticorrelated with solar extreme ultraviolet flux. On the other hand, O/N 2 is smaller during solar minimum and larger during solar maximum. The thermospheric AO and SAO in composition have a constant phase with altitude throughout the thermosphere.

Seasonal Variation of O/N2 on Different Pressure Levels From GUVI Limb Measurements

“…The hemispheric asymmetry of annual variations in amplitudes (A 12 ) and phases (F 12 ) shown in Figures 5 and 6 may be associated with the hemispheric differences in the tides and waves originating in the lower atmosphere, which are probably due to different land and ocean distributions in the NH and SH. This may also result from hemispheric differences in the geomagnetic field configuration (Lei et al, 2013; Qian et al, 2009; Yue et al, 2019).…”

“…O/N 2 are calculated using the O and N 2 altitude profiles. Meanwhile, we transform the altitude profiles to pressure levels using the observed profiles of O, N 2 , and O 2 number densities and temperature based on the ideal gas law (Yue et al, 2019). The O/N 2 on different constant pressure levels are calculated, corresponding to the logarithm of pressure (log P) values from −13.5 to −6 with a step width of 0.5, namely, 16 constant pressure surfaces.…”

“…However, there is little attention to the seasonal variations of O/N 2 volume density ratio at different heights due to the lack of the observations of the altitude profiles of composition. ∑O/N 2 cannot provide information on the neutral composition profiles, and the height at which O and N 2 number density make major contribution to ∑O/N 2 is around 140–180 km due to the densities' exponential decay with height (Strickland et al, 1995; Yu et al, 2019, 2020). Therefore, there is an uncertainty in using ∑O/N 2 as an indicator of the compositional disturbances at other heights (Kil et al, 2011; Meier et al, 2005).…”

“…Thus, the O/N 2 volume density ratio at different heights can be determined from these data. Yue et al (2019) demonstrated the interannual and seasonal variations of GUVI limb O, N 2 , and O 2 number densities, as well as the O/N 2 volume density ratio on two constant pressure surfaces. Due to the limited data coverage in local times and latitudes, the limb data were averaged over all available local times to present the distributions, and the variations at different latitudes were not shown continuously.…”

Comparison of Reference Heights of O/N2 and ∑O/N2 Based on GUVI Dayside Limb Measurement

“…Stephan et al () also found them in strong agreement during a geomagnetic storm. Yue et al () demonstrated their consistent seasonal variations from GUVI limb and disk ∑O/N2 during geomagnetically quiet periods. Emmert et al () presented the first validation of mass density from GUVI limb retrievals by comparing with that from satellite orbit data.…”