Atomic Oxygen SAO, AO and QBO in the Mesosphere and Lower Thermosphere Based on Measurements from SABER on TIMED during 2002–2019

Using version 1.0 of TIMED/SABER nighttime O(3P) density data in the mesosphere and lower thermosphere (MLT) retrieved from 2.0 and 1.6 μm radiances, we conducted a study on the semiannual oscillation (SAO), annual oscillation (AO) and quasi-biennial oscillation (QBO) of the atomic oxygen volume mixing ratio at 96 km, from 40° S to 40° N, for 2002–2019. We first analyzed the altitude profiles of the atomic oxygen volume mixing ratio and kinetic temperature, and chose to study the daily average of the atomic oxygen volume mixing ratio at 96 km. For the analysis of SAO and AO, we fitted two sinusoidal functions with periods of 6 and 12 months to the daily mean atomic oxygen volume mixing ratio to obtain the annual and semiannual amplitude. The SAO amplitudes had two peaks of 1.68 × 10−3 and 1.63 × 10-3 at about 25° S and 25° N, and displayed a clear hemispheric symmetry. The AO amplitude increased with the latitude and showed distinct minima (valleys) of 3.36 × 10−4 around the equator, as well as a clear hemispheric asymmetry. The correlation coefficient between the atomic oxygen volume mixing ratio QBO with equatorial stratospheric QBO was higher in the tropics than the mid latitudes.

Intra-Annual Variation of Eddy Diffusion (k zz ) in the MLT, From SABER and SCIAMACHY Atomic Oxygen Climatologies

Atomic oxygen (O) in the mesosphere and lower thermosphere (MLT) results from a balance between production via photo-dissociation in the lower thermosphere and chemical loss by recombination in the upper mesosphere. The transport of O downward from the lower thermosphere into the mesosphere is preferentially driven by the eddy diffusion process that results from dissipating gravity waves and instabilities. The motivation here is to probe the intra-annual variability of the eddy diffusion coefficient (k _zz ) and eddy velocity in the MLT based on the climatology of the region, initially accomplished by Garcia and Solomon (1985, https://doi.org/10.1029/JD090iD02p03850). In the current study, the intra-annual cycle was divided into 26 two-week periods for each of three zones: the northern hemisphere (NH), southern hemisphere (SH), and equatorial (EQ). Both 16 years of SABER (2002-2018) and 10 years of SCIAMACHY (2002-2012) O density measurements, along with NRLMSIS^® 2.0 were used for calculation of atomic oxygen eddy diffusion velocities and fluxes. Our prominent findings include a dominant annual oscillation below 87 km in the NH and SH zones, with a factor of 3-4 variation between winter and summer at 83 km, and a dominant semiannual oscillation at all altitudes in the EQ zone. The measured global average k _zz at 96 km lacks the intra-annual variability of upper atmosphere density data deduced by Qian et al. (2009, https://doi.org/10.1029/2008JA013643). The very large seasonal (and hemispherical) variations in k _zz and O densities are important to separate and isolate in satellite analysis and to incorporate in MLT models.

Transport of Nitric Oxide Via Lagrangian Coherent Structures Into the Top of the Polar Vortex

The energetic particle precipitation (EPP) indirect effect (IE) refers to the downward transport of reactive odd nitrogen (NOx = NO + NO2) produced by EPP (EPP-NOx) from the polar winter mesosphere and lower thermosphere to the stratosphere where it can destroy ozone. Previous studies of the EPP IE examined NOx descent averaged over the polar region, but the work presented here considers longitudinal variations. We report that the January 2009 split Arctic vortex in the stratosphere left an imprint on the distribution of NO near the mesopause, and that the magnitude of EPP-NOx descent in the upper mesosphere depends strongly on the planetary wave (PW) phase. We focus on an 11-day case study in late January immediately following the 2009 sudden stratospheric warming during which regional-scale Lagrangian coherent structures (LCSs) formed atop the strengthening mesospheric vortex. The LCSs emerged over the north Atlantic in the vicinity of the trough of a 10-day westward traveling planetary wave. Over the next week, the LCSs acted to confine NO-rich air to polar latitudes, effectively prolonging its lifetime as it descended into the top of the polar vortex. Both a whole atmosphere data assimilation model and satellite observations show that the PW trough remained coincident in space and time with the NO-rich air as both migrated westward over the Canadian Arctic. Estimates of descent rates indicate five times stronger descent inside the PW trough compared to other longitudes. This case serves to set the stage for future climatological analysis of NO transport via LCSs.

New insights for mesospheric OH: multi-quantum vibrational relaxation as a driver for non-local thermodynamic equilibrium

The question of whether mesospheric OH(υ) rotational population distributions are in equilibrium with the local kinetic temperature has been debated over several decades. Despite several indications for the existence of non-equilibrium effects, the general consensus has been that emissions originating from low rotational levels are thermalized. Sky spectra simultaneously observing several vibrational levels demonstrated reproducible trends in the extracted OH(υ) rotational temperatures as a function of vibrational excitation. Laboratory experiments provided information on rotational energy transfer and direct evidence for fast multi-quantum OH(high-υ) vibrational relaxation by O atoms. We examine the relationship of the new relaxation pathways with the behavior exhibited by OH(υ) rotational population distributions. Rapid OH(high-υ) + O multi-quantum vibrational relaxation connects high and low vibrational levels and enhances the hot tail of the OH(low-υ) rotational distributions. The effective rotational temperatures of mesospheric OH(υ) are found to deviate from local thermodynamic equilibrium for all observed vibrational levels.

Diurnal variation of the potassium layer in the upper atmosphere

Measurements of the diurnal cycle of potassium (K) atoms between 80 and 110 km have been made during October (for the years 2004-2011) using a Doppler lidar at Kühlungsborn, Germany (54.1°N, 11.7°E). A pronounced diurnal variation is observed in the K number density, which is explored by using a detailed description of the neutral and ionized chemistry of K in a three-dimensional chemistry climate model. The model captures both the amplitude and phase of the diurnal and semidiurnal variability of the layer, although the peak diurnal amplitude around 90 km is overestimated. The model shows that the total potassium density (≈ K + K⁺ + KHCO₃) exhibits little diurnal variation at each altitude, and the diurnal variations are largely driven by photochemical conversion between these reservoir species. In contrast, tidally driven vertical transport has a small effect at this midlatitude location, and diurnal fluctuations in temperature are of little significance because they are small and the chemistry of K is relatively temperature independent.