P. P. Batista

An investigation of gravity wave activity in the low‐latitude upper mesosphere: Propagation direction and wind filtering

horizontal phase speeds of up to � 80 m s � 1 . The large-scale ‘‘band’’ wave patterns (horizontal wavelength between 10 and 60 km) exhibited a clear seasonal dependence on the horizontal propagation direction, propagating toward the southeast during the summer months and toward the northwest during the winter. The direction of propagation was observed to change abruptly around the equinox period in mid March and at the end of September. Using a numerical simulation of gravity wave propagation in a seasonally variable climatological wind field, we have determined that the observed anisotropy in the wave propagation directions can be attributed to a strong filtering of the waves in the middle atmosphere by stratospheric winds. INDEX TERMS: 0310 Atmospheric Composition and Structure: Airglow and aurora; 3332 Meteorology and Atmospheric Dynamics: Mesospheric dynamics; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; KEYWORDS: airglow, winds, gravity waves, wind filtering, imager

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.

Equatorial planetary wave signatures observed in mesospheric airglow emissions

AbstractTheuppermesosphericairglowemissions,OI557:7nm,NaD589:3nm,OH(6,2)andO 2 atmospheric(0,1)bandhavebeenmeasuredusingaground-basedmultichannelairglowphotometerintheequatorialregion,S˜aoJo˜aodoCariri(7 ◦ S,37 ◦ W),Brazil.Goodweatherconditionmadeitpossibletostudynocturnalanddaytodayvariationoftheemissionratescontinuouslyformorethan12dayspermonthfromJanuarytoDecember1998.Itisfoundthattherearedistinct2-and3.5-dayperiodoscillationsintheemissionrates,thosearemostprobablyduetoRossby-gravitywaveandKelvinwave,respectively.TheamplitudesofoscillationoftheKelvinwavearelargeinJuneandJuly,being40%forOI5577,23%forO 2 b(0,1)and26%forOH(6,2). c 2002ElsevierScienceLtd.Allrightsreserved. Keywords:Equatorialairglow;Planetarywaves 1.IntroductionIntheEarth’smiddleatmosphere,between20and90km,long period and large scale atmospheric waves are propa-gating along the longitudinal (east–west) direction. In themiddle to high latitude, they are called Planetary waves(Rossby waves), with the phase propagating towards thewest. In the equatorial zone, characteristics of the wavesaredierent.Matsuno(1966)predictedfromhismodelcal-culationthepresenceoftwowaves,onewestwardandtheothereastwardpropagating.Almostatthesametime,fromradiosondeobservation,YanaiandMaruyama(1966)iden-tiedawestwardpropagatingwave,withaperiodofaround5 days and horizontal scale of about 10;000km (wavenumber

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.

Response of the airglow OH emission, temperature and mesopause wind to the atmospheric wave propagation over Shigaraki, Japan

Simultaneous observations of the night airglow OH (6, 2) band emission intensity and rotational temperature, by a sky scanning airglow spectrophotometer, and meteor winds, by a middle and upper atmosphere radar (MU radar), were carried out at Shigaraki (34.9°N, 136.1°E), Japan, from October 29 to November 11, 1994, as the first phase of a campaign, and from July 25 to July 31, 1995 as the second phase. Horizontal structures in the OH emission intensity and rotational temperature were monitored optically, together with the background wind and its wave induced fluctuations, measured by MU radar. Since the MU radar makes a direct measurement of the vertical wavelength, and the OH spectrophotometer makes a direct measurement of the horizontal wavelength, the two techniques are mutually complementary to determine intrinsic wave parameters. Gravity waves with intrinsic periods of 2 to 9 hours, horizontal wavelengths of 500 to 3000 km and vertical wavelengths of 12 to 75 km were identified. Between the two different observation techniques, there is a reasonable agreement in the inferred wave characteristics.

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.

An unusual airglow wave event observed at Cachoeira Paulista 23° S

Abstract An all-sky CCD airglow imager has been used to obtain a wealth of data on the MLT airglow emissions from Cachoeira Paulista, Brazil (22.7° S, 45.0° W). Measurements of the NIR OH, OI(557.7 nm), O2(0,1) and the thermospheric OI(630.0 nm) emissions have been made since October 1998. In addition, simultaneous measurements of the airglow zenith intensities and rotational temperatures, and vertical wind structure between 80 and 100 km were made using a co-located multi-channel photometer and a meteor radar. On the night of July 13, 1999, the CCD images showed active wave events exhibiting a variety of short period wave structures. The activity started with ripple patterns over the whole sky, lasting for almost one hour. Later the patterns changed to extensive band-like structures that lasted for several hours. After local midnight a bore-like wave-front passed overhead propagating towards the North. When the front was in the zenith the image data showed an increase in both the OI (5577 nm) and O2 (0,1) emissions, whilst a decrease in the OH intensity was observed, which is opposite to that observed by Taylor, (1995a). The co-located photometer revealed an exceptionally large intensity (40%) and temperature (∼12%) perturbation throughout the night that appears to be associated with a large-scale gravity wave perturbation with significant vertical phase shift between the different emissions.