B. R. Clemesha

Sporadic neutral metal layers in the mesosphere and lower thermosphere

Abstract Although the existence of thin ionized layers at heights around 100 km has been known for many years, it is only much more recently that thin neutral metal layers have been observed. Such layers, initially sodium and more recently calcium and iron, have been detected by lidar. The layers, with thicknesses between about 100m and several kilometres, and concentrations between about 10 2 and 10 5 cm −3 , occur most frequently between 90 and 100 km, and are normally superimposed on a background layer about 10 km thick. The occurrence of thin neutral layers appears to be latitude dependent, and is strongly linked to the appearance of Es on ionograms. Several causative mechanisms have been suggested, none of which appears to be capable of providing an altogether satisfactory explanation for the formation of the layers.

Predominant semi-annual oscillation of the upper mesospheric airglow intensities and temperatures in the equatorial region

Abstract The upper mesospheric and lower thermospheric airglow emissions, OI 557.7 nm, NaD 589.3 mn and the OH (9,4) band and its rotational temperature have been measured using a ground-based multichannel airglow photometer located at Fortaleza (3.9°S, 38.4°W) since 1986. The observed emission intensities show predominantly semi-annual oscillations with maxima at the equinoxes and minima during the solstices. The amplitudes of the oscillations are larger than those observed from the low latitude station, Cachoeira Paulista (22.7°S, 45.0°W). The OH rotational temperature, which represents a gas kinetic atmospheric temperature at around 85–95 km, also shows a strong semi-annual oscillation, 18 K peak to peak, with an. average value around 10 K higher than that observed from Cachoeira Paulista. These results do not agree with model atmospheres presently available. It is suggested that the differences result from the effects of seasonal variations in vertical eddy transport and/or meridional circulation.

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.

Atmospheric sodium measurements at 23°S

Abstract Measurements of the nighttime atmospheric sodium layer have been made over a period of 4 months at Sao Jose dos Campos (23°S, 46°W), using the laser radar technique. Peak sodium concentration is found at a height of 95 km and the topside scale height of the layer is usually less than 2 km. On six occasions a secondary peak has been observed at heights between 100 and 105 km. Detailed structure in the layer is frequently conserved from night to night and it is suggested that such structure may be the result of atmospheric tidal oscillations. Comparison with twilight measurements indicates that the average total abundance of sodium shortly after sunset is between 0.38 and 1.25 of the twilight value.

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.

Equatorial f-region oi 6300 å and oi 5577 å emission profiles observed by rocket-borne airglow photometers

Abstract OI 6300 A (1D-3P) and OI 5577 A (1S-1D) airglow emission profiles were measured in the equatorial region. Natal (5.8°S, 35.2°W), by photometers on board a sounding rocket. The two emissions showed similar profiles in the F-region, peaking at around 230 km, with widths of 50–60 km. The overhead zenith intensities below the thermospheric emission layers were 52 Rayleighs (R) for 01 5577 A and 310 R for OI 6300 A, giving an intensity ratio of 0.17. The ratio between the volume emission rates was found to vary with height from 0.25 at 200 km to 0.09 at 310 km. The ratio between the quantum yields f(1S) and f(1D), also found to be a function of height and varying from 0.02 at 200 km to 0.08 at 270 km, showed a good correlation with the simultaneously observed electron density profile. This suggests that the quantum yield f(1S) of the O(1S) production channel of the dissociative recombination process is related in some degree to the electron density.

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.

Equatorial mesospheric and F-region airglow emissions observed from latitude 4° south

Abstract Simultaneous ground-based measurements of the atmospheric airglow emissions OI 5577, 6300 and 7774 A, NaD 5893 A, OH (9, 4) band and O 2 atmospheric (0, 1) band at 8645 A have been made at an equatorial station, Fortaleza (3.9°S, 38.4°W, geomag. 2.1°S), Brazil, since November 1986. A microprocessor controlled multichannel tilting filter type photometer was constructed for this purpose. Hydroxyl rotational temperatures in the range 190–210 K were obtained from the OH (9, 4) Q and R branch measurements. Surprisingly low OI 5577 A and NaD intensities, about 40% and 25%, respectively, of those observed at low latitude (23°S), were observed. The OH (9, 4) band intensities, however, did not show any such difference. Time lagged nocturnal intensity variations of the mesospheric emissions observed provide evidence of dynamical effects in the equatorial upper atmosphere.