Mesospheric Na layer at extreme high latitudes in summer

Abstract

Abstract. Summertime observations of the mesospheric Na layer at high latitudes are reported from the 1993 Airbome Noctilucent Cloud (ANLC-93) campaign in the Canadian Arctic and at the Amundsen-Scott South Pole Station. Measurements at the South Pole reveal a layer that has a smaller column abundance and is significantly higher and thinner than at midlatitudes. Using a model that was essentially optimized to wintertime conditions at high northern latitudes, the South Pole layer can be modeled satisfactorily if the rate coefficient for the reaction between sodium bicarbonate and atomic hydrogen is set to k(NaHCO3 + H • Na + H20 + CO2) = 1.1 x 10 -• exp (-910/T) cm 3 molecule 4 s 4. In particular, the model is able to reproduce the small scale height of about 2 km observed on the underside of the layer. It is then shown that this steep gradient in the atomic Na mixing ratio can be sustained against vertical eddy diffusion because of the sufficiently rapid chemical cycling between Na its major reservoir, NaHCO3. This justifies the assumption in the model that the vertical transport of Na species can be treated in terms of a single continuity equation describing total sodium. The observations from the campaigns in both hemispheres show that the Na abundance has a temperature dependence of about 2 x 108 cm -2 K 4 at temperatures below 170 K, in good accord with the model. About 40% of this dependence appears to be caused by the activation energies of the reactions that partition sodium between atomic Na and NaHCO3, and the remainder by the temperature dependence of the odd-oxygen/odd-hydrogen chemistry in the upper mesosphere.

Statistics of the sodium layer parameters at low geographic latitude and its impact on adaptive-optics sodium laser guide star characteristics

“…Because of the long residence time (∼7 days) of ablated sodium in the region between 80 and 100 km, and the comparatively short lifetimes of the rate-determining chemical reactions that convert sodium between atomic Na and these reservoirs, the chemistry reaches a photochemical steady-state on the timescale of vertical transport by eddy or molecular diffusion. The chemical reactions responsible for the conversion of Na to NaHCO 3 occur rapidly and have small temperature dependencies (Plane 2004(Plane , 1998. NaHCO 3 is converted back to Na by the reaction with H. This reaction has a large activation energy, that is at higher temperatures it becomes much faster, and the steady-state balance shifts from NaHCO 3 to Na (Cox et al 2001).…”

A study of the reaction between NaHCO3 and H: Apparent closure on the chemistry of mesospheric Na

“…observation; furthermore, the steep underside of the Na layer indicates that a fairly rapid recycling of Na must occur [Plane et al, 1998]. Hence a measurement of k4 provides a rather stringent test of the current understanding of mesospheric sodium chemistry.…”

“…Since only Na atoms and Na + ions can bc observed (by lidar and rocket-borne mass spcctromctry, respectively), the mcsosphcric chemistry of sodium has had to bc unraveled through laboratory kinetic studies of elementary reactions, coupled with modeling. Significant progress has been made over the past decade, with models that arc now able to reproduce the characteristic features of the Na layer remarkably well, over a range of latitudes and seasons [Helmer and Plane, 1993;McNeil et al, 1995;Plane et al, 1998Plane et al, , 1999.…”

The terrestrial potassium layer (75–110 km) between 71°S and 54°N: Observations and modeling

“…He proposed that the seasonal differences between Na and K might be explained by the kinetics of key potassium reactions having smaller temperature dependences than their sodium analogues. However, the sodium model that he used was quite Plane et al, 1998Plane et al, , 1999.…”