Middle atmosphere dynamical sources of the semiannual oscillation in the thermosphere and ionosphere

• Published in: Geophysical research letters Vol. 44; no. 1; pp. 12 - 21
• Main Authors: Jones, M., Emmert, J. T., Drob, D. P., Siskind, D. E.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16.01.2017
• Subjects: Air; Atmosphere; Atmospheric circulation; Atmospheric density; Atmospheric dynamics; Atmospheric research; Atmospherics; Atomic oxygen; atomic oxygen transport; Circulation; Climatology; Computer simulation; Continuity (mathematics); Continuity equation; Density; Diagnosis; Diffusion; Dye dispersion; Dynamics; Earth; Earth atmosphere; Earth gravitation; Eddies; Eddy diffusion; Electrodynamics; Environmental impact; General circulation; General circulation models; Gravitational waves; Gravity; Gravity waves; Ionosphere; Lower thermosphere; Mass; Mathematical models; Mesosphere; Middle atmosphere; middle atmosphere dynamics; Nonlinearity; Oxygen; Regions; Remote sensing; Satellites; Scientists; Semiannual oscillation; Space debris; Spaceborne remote sensing; Thermosphere; thermosphere‐ionosphere; Transport; Turbulent diffusion; Vortices
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2016GL071741

The strong global semiannual oscillation (SAO) in thermospheric density has been observed for five decades, but definitive knowledge of its source has been elusive. We use the National Center of Atmospheric Research thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) to study how middle atmospheric dynamics generate the SAO in the thermosphere‐ionosphere (T‐I). The “standard” TIME‐GCM simulates, from first principles, SAOs in thermospheric mass density and ionospheric total electron content that agree well with observed climatological variations. Diagnosis of the globally averaged continuity equation for atomic oxygen ([O]) shows that the T‐I SAO originates in the upper mesosphere, where an SAO in [O] is forced by nonlinear, resolved‐scale variations in the advective, net tidal, and diffusive transport of O. Contrary to earlier hypotheses, TIME‐GCM simulations demonstrate that intra‐annually varying eddy diffusion by breaking gravity waves may not be the primary driver of the T‐I SAO: A pronounced SAO is produced without parameterized gravity waves. Plain Language Summary Within the transition region from Earth's atmosphere to space (called the “thermosphere”), scientists have known about a twice‐a‐year fluctuation in density (referred to as the “semiannual” fluctuation) for over 50 years. However, the primary cause of this thermospheric semiannual density fluctuation has eluded scientists until now. This paper is the first to show that the semiannual density fluctuation in the thermosphere originates some 150 to 200 miles below it (or an atmospheric region called the “mesosphere‐lower thermosphere”). Air density in the upper regions of Earth's thermosphere is mostly from atomic oxygen (O), which originates from a product of complex chemical processes in the atmospheric regions below Earth's thermosphere. We show that O in the atmospheric region directly below Earth's thermosphere, the “upper mesosphere,” also has a semiannual fluctuation, which then gets transported vertically, into Earth's thermosphere, causing the semiannual fluctuation in thermospheric air density. Our results have important implications for satellite operators, as air density fluctuations such as the semiannual fluctuation have a noticeable impact on the environment in which satellites and hazardous space debris fly. Key Points The TIME‐GCM simulates a realistic semiannual oscillation (SAO) in the thermosphere and ionosphere (T‐I) from first principles Resolved‐scale variations in oxygen transport in the upper mesosphere drive the T‐I SAO simulated by TIME‐GCM Model runs indicate that intra‐annually varying eddy diffusion is not the primary driver of the T‐I SAO