D. M. Simonich

A long‐term trend in the height of the atmospheric sodium layer: Possible evidence for global change

An analysis of a long series of lidar measurements of the vertical distribution of atmospheric sodium shows the existence of a long-term trend in the centroid height of the layer. After making allowance for the sampling effects of the mean seasonal and diurnal variations in centroid height, it is found that the height of the layer fell by approximately 700 meters between 1972 and 1987. A regression analysis indicates a mean rate of fall of 49 ±12 m yr−1, with a correlation coefficient of −0.33, significant at the 99.99% level. The observed change is consistent with long term trends in mesospheric temperatures detected by other techniques.

Experimental evidence for photochemical control of the atmospheric sodium layer

On May 31, 1992, a rocket payload equipped with 10 airglow photometers was launched from the Alcântara Launch Center in northern Brazil. The payload measured sodium, hydroxyl, atomic, and molecular oxygen airglow emissions, and a sodium lidar, operating at the launch site, provided simultaneous vertical profiles of atmospheric sodium density. The airglow profiles, in conjunction with the sodium density distribution, are used to derive vertical profiles for atomic oxygen, ozone and hydrogen in the 80 to 100 km region. These profiles are then used as inputs to a photochemical model for the sodium layer. Good agreement is achieved between the modeled and experimental profiles of sodium and Na D line airglow, and the results indicate that the branching ratio for the production of Na(2P) in the reaction NaO + O → Na(2P, 2S) + O2 must be between 0.05 and 0.20.

Long-term and solar cycle changes in the atmospheric sodium layer

Abstract A new analysis of atmospheric sodium measurements, made at Sao Jose dos Campos (23 °S, 46 °W) since 1972, confirms the previously detected trend in the centroid height of the layer (near 92 km altitude) and shows the existence of a 10-yr solar-cycle related oscillation. The centroid height fell at an average rate of 37 ± 9m yr−1 between 1972 and 1994, and the 10-yr cycle has an amplitude of 170 ± 110 m. An analysis of the vertical distribution of atmospheric sodium shows that the fall in height of the centroid is not caused by a simple vertical displacement of the sodium profile, but by the growth of a bulge on the bottomside of the layer.

Horizontal structures in sporadic sodium layers at 23°S

During 1979 and 1980 the INPE lidar located at Sao Jose dos Campos, Brazil was operated in a steerable mode, measuring the sodium profiles sequentially at three points in the sky. Twelve sporadic sodium layer events (SSLs) which occurred in this period are studied in the present work. The evolution of the sporadic peaks at the three positions shows short time structures generally separated by consistent time lags in almost all events. On some occasions, the time evolution at one position is very different than at the other two. These data indicate that short duration SSLs have cloud-like structures which are advected over the measuring station by the horizontal winds, and the long duration layers show a patchy and wave-like structure. In no case did we observe rapid growth in the sodium density to occur simultaneously at all 3 measuring points. On this basis we believe that there is no evidence for fast production of sodium, and consequently that there is no need for theories for the formation of SSLs to be consistent with such fast production.

An evaluation of the evidence for ion recombination as a source of sporadic neutral layers in the lower thermosphere

Sporadic layers of metal atoms (Ns), occurring in the same height range as ionospheric sporadic-E layers, were first detected by lidar some 20 years ago. Ns layers have typical thicknesses of a few hundred meters to a few km, peak atom concentrations several times that of the ambient background layer, and are sometimes seen to grow and decay over time scales as short as a few minutes. Layers have been detected in Na, Fe, K and Ca, but it seems likely that they exist in other meteoric metals such as Mg. Despite a great deal of excellent experimental work over the past decade, the source of Ns layers is still an open question. Mechanisms suggested include direct meteor deposition, release from aerosol particles, chemical reduction of appropriate metal compounds, redistribution of existing atoms, and recombination of ions. The last-named of these mechanisms, although capable of explaining many of the observed characteristics of Ns layers, including their strong correlation with Es, has generally been rejected in the past, at least in the case of Na, because mass spectrometer measurements of Na+ have mostly shown concentrations too small to explain the observed sporadic sodium layers. However, recent laboratory measurements of the relevant recombination processes, and a re-evaluation of the rocket-borne mass-spectrometer measurements, suggest that ion recombination is in fact the strongest contender.

Simultaneous lidar observation of a sporadic sodium layer, a “wall” event in the OH and OI5577 airglow images and the meteor winds

Abstract Rare and sporadic events, caused by unusual conditions in the MLT region may occur such as the sporadic metal layers, observed by lidars, and the more recently detected sharp front in the nightglow emissions imaged with CCD all sky cameras. Few events of this latter kind have been reported in the literature and no simultaneous occurrence of both. In this work, we report the observation of a sporadic sodium layer measured by lidar in Sao Jose dos Campos, Brazil (22.5° S , 46° W ) on the night of July 13–14, 1999 which was followed by a rare “wall” event observed at Cachoeira Paulista (CP) (23° S , 45° W ) in the OH NIR and OI5577 by a CCD imager, and zenith tilting filter photometers. The wind field was also observed by a SkiYmet meteor radar system also operated at CP. At around 23:45 the sodium layer had an abrupt change in its shape. At the same time unusual situation also had occurred in the airglow data taken in CP both with the imager and with the zenith photometer. A sequence of all sky images for the OI 557.7 nm and OH NIR showed very clear lines dividing the sky into bright and dark areas, with the first propagating from the NE to SW at ∼70 m / s and the second propagating to NNE. Zenith photometer also showed a rapid decrease of the OH(6,2) intensity at around 23:45 and a very large increase in the rotational temperature of O2(0,1) (at ∼94 km ) from 23:00 to 24:00 (∼40 K ) . The meridional and zonal wind structure measured by the meteor radar in CP showed that from 21:00 to 01:00 a wind shear was present both in the meridional and zonal winds with the phase propagating from ∼97 to 80 km during this time which were very well correlated with the vertical displacement of the sporadic peak. Large-amplitude gravity wave was also detected. The temperature enhancement above 90 km could be responsible for the sodium density increase and also for the set up of the conditions for the airglow events.

Atomic hydrogen and ozone concentrations derived from simultaneous lidar and rocket airglow measurements in the equatorial region

Nighttime atomic hydrogen and ozone concentrations are derived from simultaneous measurements of the vertical profiles of upper mesospheric airglow emissions and atmospheric sodium. The airglow profiles were obtained in a sounding rocket experiment launched from Alcântara (2.5°S, 44.2°W) on May 31, 1992. A lidar operating at the launch site was used to measure sodium at the time of the rocket experiment. A total of 10 airglow photometers, 6 forward looking and 4 side looking, observed the OI 557.7 nm, O 2 Herzberg and O 2 atmospheric (0,0) bands, sodium D lines, OI 630 nm, OH(8,3) band, and the airglow continuum. The simultaneous ground-based sodium lidar and onboard sodium airglow measurements made it possible to derive the ozone concentration at heights between 85 and 100 km. The hydrogen concentrations were then calculated from the O 2 atmospheric (0,0), OH(8,3), and the ozone profiles. The results suggest that the hydrogen concentration varied from 1 × 10 9 cm -3 at 85 km to 1 × 10 8 cm -3 at 100 km, values much higher than those suggested by recent model atmospheres and by some rocket observations at middle and high latitudes. Although the method of obtaining the concentrations of the minor constituents in the upper atmosphere is an indirect optical technique, this is the first time that these concentrations have been measured by rocket in the equatorial region.

Negligible long‐term temperature trend in the upper atmosphere at 23°S

[1] Measurements of the vertical distribution of atmospheric sodium, made at Sao Jose dos Campos (23°S, 46°W) from 1972 to 2001 show a negligible net long-term trend in the layer over this time period. In view of the fact that cooling of the upper atmosphere, as indicated by some experimental studies and predicted by models, would be expected to influence the vertical distribution of sodium, this result can be used to estimate the limits of such cooling. In order to investigate the sensitivity of the sodium layer to changes in the atmospheric temperature profile, and thus estimate an upper limit to the changes it should be possible to detect, we have used a comprehensive model for the mesosphere and lower thermosphere (65-110 km) that includes all species and reactions thought to be relevant to the sodium chemistry. On the basis of this study we find that the sodium layer observations are consistent with model calculations for the global effects of increased CO 2 but are not consistent with many of the experimental studies, which show cooling trends much larger than expected from theoretical studies.

Formation of sporadic sodium layers

A study of sporadic sodium (Ns) layers observed at Sao Jose dos Campos (23°S, 46°W) shows that during their occurrence the form of the background sodium layer is different from that which it normally takes when Ns layers are absent. During Ns events, peak sodium in the background layer typically occurs below 90 km, whereas the peak of the average layer observed at our location is around 93 km. The observed change could be caused either by a loss of sodium on the topside of the layer or by a displacement of sodium to lower heights. The consistency of these two mechanisms with our observations, and with the known properties of Ns layers, is examined, but we are unable to determine which of the mechanisms is responsible for the observed phenomenon.

Atomic oxygen concentrations from rocket airglow observations in the equatorial region

Abstract A rocket payload designed to measure mesospheric sodium, hydroxyl and oxygen nightglow emissions, in addition to electron density and temperature, was launched from the Alcantara Launch Center (2°S, 44°W), Brazil, at 23:52 LST on 31 May 1992. The height profiles of the atomic oxygen OI557.7 nm and molecular oxygen Atmospheric (0-0) band emissions showed maxima at 100±3 km and 98±3 km, respectively. The emission data are used to calculate the atomic oxygen concentration profiles. The results show the validity for the equatorial region of the empirical parameters proposed by McDade et al. (1986).