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Using OI6300 airglow images collected over São João do Cariri (7.4° S, 36.5° W) from 2000 to 2007, the equatorial plasma bubble (EPB) zonal drifts were calculated. A strong day-to-day variability was observed in the EPB zonal drifts due to the complexity in the dynamics of the nighttime thermosphere-ionosphere system near the equator. The present work investigated the contribution of the semidiurnal lunar tide M2 for the EPB zonal drifts. On average, the M2 contributes 5.6 % to the variability of the EPB zonal drifts, presenting an amplitude of 3.1 m/s. The results showed that the M2 amplitudes in the EPB zonal drifts were solar cycle and seasonal dependents. The amplitude of the M2 was stronger during the high solar activity reaching over 10 % of the EPB zonal drift average. Regarding the seasons, during the southern hemisphere summer, the M2 amplitude was twice larger (12 %) compared to the equinox ones. The seasonality agrees with other observations of the M2 in the ionospheric parameters such as vertical drifts and electron concentration, for instance. On the other hand, the very large M2 amplitudes found during the high solar activity must be further investigated.

In this study, OI 630.0 nm nightglow image data obtained from an all-sky imaging station located at Sao Joao do Cariri (7.4°S, 36.5°W), have been used to study the occurrence of equatorial plasma bubbles (EPBs) over the Brazilian equatorial region. The observations, which took place from September 2000 to December 2010 (almost eleven years), covered a significant part of solar cycle 23 (descending phase of SC23) and the very beginning of solar cycle 24 (ascending phase of SC24). There were 1337 nights of observations with 666 nights with EPB events, which translates to an occurrence rate of ∼ 49.8%. Of these nights, 1290 were considered quiet (Dst ≥ -50 nT) with ∼ 50% plasma bubbles occurrence rate. The occurrence of EPBs was studied under three solar activity phases – high solar activity phase (HSA, September 2000 – March 2004), moderate solar activity phase (MSA or descending phase, March 2004 – October 2006) and low solar activity phase (LSA, October 2006 – December 2010). The low solar activity phase is part of the peculiar solar cycle 24, considered the weakest in over a century with most sunspotless days (2008–2009). The maximum occurrence of bubbles, equal to 54.2%, was found during the HSA phase, with percentages characterizing MSA and LSA being respectively 52.4% and 45.8%. The analysis also showed clear seasonal variation in the EPB occurrence with maximum rates in summer, spring, autumn, and the minimum rates in winter for all solar activity phases. Overall, there was observed solar cycle variation in each season with maximum occurrence in HSA followed by MSA and then LSA except in autumn where higher occurrence rate was observed in LSA phase than in MSA phase.

OI 630.0 nm airglow observations, from a new observatory at Bom Jesus de Lapa, were used to study the interaction between EPBs (Equatorial Plasma Bubbles) and the MSTID (Medium-Scale Traveling Ionospheric Disturbance) over the Northeast region in Brazil. On the night of September 16 to 17, 2020, an EPB was observed propagating eastward, in an apparent fossil stage, until it interacted with a dark band electrified MSTID (eMSTID). After the interaction, four EPBs merged, followed by an abrupt southward development and bifurcations. Analysis of the data suggests that an eastward polarization electric field, induced by the dark band eMSTID, forced the EPB into an upward drift, growing latitudinally along the magnetic field lines and then bifurcating.

Nighttime airglow images observed at the low-latitude site of Sao Joao do Cariri (7.4S, 36.5W) showed the presence of a medium-scale atmospheric gravity wave (AGW) associated with the...

Using airglow data from an all-sky imager deployed at São João do Cariri (7.4∘ S, 36.5∘ W), the start times of equatorial plasma bubbles was studied in order to investigate the day-to-day variability of this phenomenon. Data from a period over 10 years were analyzed from 2000 to 2010. Semimonthly oscillations were clearly observed in the start times of plasma bubbles from OI6300 airglow images during this period of observation, and four case studies (September 2003, September–October 2005, November 2005 and January 2008) were chosen to show in detail this kind of modulation. Since the airglow measurements are not continuous in time, more than one cycle of oscillation in the start times of plasma bubbles cannot be observed from these data. Thus, data from a digisonde at São Luís (2.6∘ S, 44.2∘ W) in November 2005 were used to corroborate the results. Technical/climate issues did not allow one to observe the semimonthly oscillations simultaneously by the two instruments, but from October to November 2005 there was a predominance of this oscillation in the start times of the irregularities over Brazil. Besides, statistical analysis for the data in the whole period of observation has shown that the lunar tide, which has semimonthly variability, is likely the main forcing for the semimonthly oscillation in the start times of equatorial plasma bubbles. The presence of this oscillation can contribute to the day-to-day variability of equatorial plasma bubbles.

Occurrences of equatorial plasma bubbles (EPBs) and medium-scale traveling ionospheric disturbances (MSTIDs) were studied using GPS satellite data-based total electron content mapping, ionograms, and 630 nm all-sky airglow images observed over the South American continent during the period of 2014–2015. In many cases, we observed a close relationship between the inter-bubble distance and the horizontal wavelength of the MSTIDs. The MSTIDs followed by EPBs occurred primarily in the afternoon to evening period under strong tropospheric convective activities (cold fronts and/or intertropical convergence zones). The close relationship between EPBs and MSTIDs suggests that MSTIDs could be one of the seeding sources of EPBs.

Mesospheric airglow measurements of two or three layers were used to characterize both vertical and horizontal parameters of gravity waves. The data set was acquired coincidentally from a multi-channel filter (Multi-3) photometer and an all-sky imager located at São João do Cariri (7.4∘ S, 36.5∘ W) in the equatorial region from 2001 to 2007. Using a least-square fitting and wavelet analysis technique, the phase and amplitude of each observed wave were determined, as well as the amplitude growth. Using the dispersion relation of gravity waves, the vertical and horizontal wavelengths were estimated and compared to the horizontal wavelength obtained from the keogram analysis of the images observed by an all-sky imager. The results show that both horizontal and vertical wavelengths, obtained from the dispersion relation and keogram analysis, agree very well for the waves observed on the nights of 14 October and 18 December 2006. The determined parameters showed that the observed wave on the night of 18 December 2006 had a period of ∼43.8±2.19 min, with the horizontal wavelength of 235.66 ± 11.78 km having a downward phase propagation, whereas that of 14 October 2006 propagated with a period of ∼36.00±1.80 min with a horizontal wavelength of ∼195±9.80 km, and with an upward phase propagation. The observation of a wave taken by a photometer and an all-sky imager allowed us to conclude that the same wave could be observed by both instruments, permitting the investigation of the two-dimensional wave parameter. Keywords. Atmospheric composition and structure (airglow and aurora) – electromagnetics (wave propagation) – history of geophysics (atmospheric sciences)

On 3 October 2005 a mesospheric front was observed over São João do Cariri (7.4∘ S, 36.5∘ W). This front propagated to the northeast and appeared in the airglow images on the west side of the observatory. By about 1.5 h later, it dissipated completely when the front crossed the local zenith. Ahead of the front, several ripple structures appeared during the dissipative process of the front. Using coincident temperature profile from the TIMED/SABER satellite and wind profiles from a meteor radar at São João do Cariri, the background of the atmosphere was investigated in detail. On the one hand, it was noted that a strong vertical wind shear in the propagation direction of the front produced by a semidiunal thermal tide was mainly responsible for the formation of duct (Doppler duct), in which the front propagated up to the zenith of the images. On the other hand, the evolution of the Richardson number as well as the appearance of ripples ahead of the main front suggested that a presence of instability in the airglow layer that did not allow the propagation of the front to the other side of the local zenith. Keywords. Atmospheric composition and structure (airglow and aurora) – meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides)

Periodic waves were observed in the OI6300 airglow images over São João do Cariri (36.5∘ W, 7.4∘ S) from 2012 to 2014 with simultaneous observations of the thermospheric wind using two Fabry–Pérot interferometers (FPIs). The FPIs measurements were carried out at São João do Cariri and Cajazeiras (38.5∘ W, 6.9∘ S). The observed spectral characteristics of these waves (period and wavelength) as well the propagation direction were estimated using two-dimensional Fourier analysis in the airglow images. The horizontal thermospheric wind was calculated from the Doppler shift of the OI6300 data extracted from interference fringes registered by the FPIs. Combining these two techniques, the intrinsic parameters of the periodic waves were estimated and analyzed. The spectral parameters of the periodic waves were quite similar to the previous observations at São João do Cariri. The intrinsic periods for most of the waves were shorter than the observed periods, as a consequence, the intrinsic phase speeds were faster compared to the observed phase speeds. As a consequence, these waves can easily propagate into the thermosphere–ionosphere since the fast gravity waves can skip turning and critical levels. The strength and direction of the wind vector in the thermosphere must be the main cause for the observed anisotropy in the propagation direction of the periodic waves, even if the sources of these waves are assumed to be isotropic. Keywords. Meteorology and atmospheric dynamics (waves and tides)

Using data from airglow an all sky imager and a coherent backscatter radar deployed at São João do Cariri (7.4° S, 36.5° W) and São Luís (2.6° S, 44.2° W), respectively, the start time of equatorial Spread-F were studied. Data from a period of over 10 years was investigated from 2000 to 2010. The semimonthly oscillations were clearly revealed in the start time of plasma bubbles from Oi6300 airglow images during three periods (September 2003, September–October 2005, November 2005 and January 2008). Since the airglow measurements are not continuous in time, more than one cycle of oscillation in the start time of plasma bubbles cannot be observed from these data. Thus, coherent backscatter radar data appeared as an alternative to investigate the start time of the ionospheric irregularities. Semimonthly oscillation were observed in the start time of plumes (November 2005) and bottom type Spread-F (November 2008) with at least one complete cycle. Technical/climate issues did not allowed to observe the semimonthly oscillations simultaneously by the two instruments, but from September to December 2005 there was a predominance of this spread-F start time oscillation over Brazil. The presence of this oscillation certainly contribute to the day-to-day variability of spread-F.

Airglow and wind measurements from the Brazilian equatorial region were used to investigate the presence and the effects of the 3–4-day ultrafast Kelvin waves in the MLT. The airglow integrated intensities of the OI557.7 nm, O2b(0-1) and OH(6-2) emissions, as well as the OH rotational temperature, were measured by a multichannel photometer, and the zonal and meridional wind components between 80 and 100 km were obtained from a meteor radar. Both instruments are installed in the Brazilian equatorial region at São João do Cariri (7.4∘ S, 36.5∘ W). Data from 2005 were used in this study. The 3–4-day oscillations appear intermittently throughout the year in the airglow. They were identified in January, March, July, August and October–November observations. The amplitudes induced by the waves in the airglow range from 26 to 40 % in the OI557.7 nm, 17 to 43 % in the O2b(0-1) and 15 to 20 % in the OH(6-2) emissions. In the OH rotational temperature, the amplitudes were from 4 to 6 K. Common 3–4-day oscillations between airglow and neutral wind compatible with ultrafast Kelvin waves were observed in March, August and October–November. In these cases, the amplitudes in the zonal wind were found to be between 22 and 28 m s−1 and the vertical wavelength ranges from 44 to 62 km. Evidence of the nonlinear interaction between the ultrafast Kelvin wave and diurnal tide was observed. Keywords. Atmospheric composition and structure (airglow and aurora) – meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides)

The present work reports seasonal characteristics of small- and medium-scale gravity waves in the mesosphere and lower thermosphere (MLT) region. All-sky images of the hydroxyl (NIR-OH) airglow emission layer over São João do Cariri (7.4∘ S, 36.5∘ W; hereafter Cariri) were obtained from September 2000 to December 2010, during a total of 1496 nights. For investigation of the characteristics of small-scale gravity waves (SSGWs) and medium-scale gravity waves (MSGWs), we employed the Fourier two-dimensional (2-D) spectrum and keogram fast Fourier transform (FFT) techniques, respectively. From the 11 years of data, we could observe 2343 SSGW and 537 MSGW events. The horizontal wavelengths of the SSGWs were concentrated between 10 and 35 km, while those of the MSGWs ranged from 50 to 200 km. The observed periods for SSGWs were concentrated around 5 to 20 min, whereas the MSGWs ranged from 20 to 60 min. The observed horizontal phase speeds of SSGWs were distributed around 10 to 60 m s−1, and the corresponding MSGWs were around 20 to 120 m s−1. In summer, autumn, and winter both SSGWs and MSGWs propagated preferentially northeastward and southeastward, while in spring the waves propagated in all directions. The critical level theory of atmospheric gravity waves (AGWs) was applied to study the effects of wind filtering on SSGW and MSGW propagation directions. The SSGWs were more susceptible to wind filtering effects than MSGWs. The average of daily mean outgoing longwave radiation (OLR) was also used to investigate the possible wave source region in the troposphere. The results showed that in summer and autumn, deep convective regions were the possible source mechanism of the AGWs. However, in spring and winter the deep convective regions did not play an important role in the waves observed at Cariri, because they were too far away from the observatory. Therefore, we concluded that the horizontal propagation directions of SSGWs and MSGWs show clear seasonal variations based on the influence of the wind filtering process and wave source location. Keywords. Atmospheric composition and structure (airglow and aurora) – electromagnetics (wave propagation) – history of geophysics (atmospheric sciences)

Using airglow images observed at São João do Cariri (7.4ºS, 36.5ºW; dip angle: -11ºS) from September 2000 to April 2007, plasma bubblezonal drifts for ninety-seven geomagnetically quiet nights (Dst index higher than -30 nT) were calculated. The plasma bubble eastward averaged drifts presented an increase from 18:00 to 22:00 local time (LT), and followed by a deceleration. The plasma bubbles observed during high solar activity were faster after 22:00 LT. Eastward drifts observed during the summer, after 21:00 LT, were higher than drifts observed during the equinox months. Few nights had plasma bubble eastward drifts higher than 100 m/s and almost all bubbles disappeared after 02:00 LT. Averaged zonal drifts observed between -9.5 and -5.5ºS were almost constant, primarily,around 23:00 LT. Usando imagens de aeroluminescência coletadas em São João do Cariri (7,4ºS, 36,5ºW; latitude de dipolo magnético: -11ºS) no período de setembro de 2000 a abril de 2007, foi possível calcular a deriva zonal de bolhas de plasma em noventa e sete noites geomagneticamente calmas (índice Dst maior que -30 nT). As derivas zonais médias das bolhas de plasma aumentaram das 18:00 até as 22:00 horas locais (HL) e desaceleraram em seguida. As bolhas de plasma observadas durante a atividade solar alta foram mais rápidas após as 22:00 HL. As derivas zonais de bolhas de plasma no verão, depois das 21:00 HL, foram maiores que nos meses de equinócio. Poucas noites apresentaram bolhas de plasma com derivas superiores a 100 m/s e quase todas as bolhas desapareceram depois das 02:00 HL.As derivas médias zonais observadas entre -9,5 e -5,5ºS foram praticamente constantes, principalmente, por volta das 23:00 HL.

Neste trabalho, o parâmetro η de Krassovsky, foi utilizado para investigar as amplitudes e fases das variações periódicas das emissões de airglow do O2(0-1) e do OH(6-2). As observações foram realizadas por meio de fotômetro em Rikubetsu (43,5ºN; 143,8ºE), Japão, no período compreendido entre março de 2004 e agosto de 2005, em colaboração com o grupo do Laboratório do Ambiente Solar-Terrestre da Universidade de Nagoya (STEL-LAB). Os valores médios obtidos foram de |η| = 5,3 ± 1,7 e Φ = -40,2º± 30,6º para a emissão do O2; e |η| = 8,0 ± 4,7 e Φ = -41,9º± 41,4º para a emissão do OH. Em ambas as emissões, houve uma boa concordância dos resultados obtidos com aqueles presentes na literatura. Porém, deve ser enfatizado que os modelos concordam melhor com os valores de |η| do que para Φ. Para a emissão do OH, foram obtidos valores de |η| mais elevados do que normalmente é verificado em trabalhos experimentais. Esses resultados se ajustam muito bem ao modelo de Schubert et al. (1991), para uma onda de gravidade com comprimento horizontal de 100 km. In this work, the Krassovsky's η parameter, was used to investigate the amplitudes and phases of periodic variations of O2(0-1) and OH(6-2) airglow emissions. The data set was obtained from photometer measurements at Rikubetsu (43.5ºN, 143.8ºE), Japan, from March 2004 to August 2005, under collaboration with STEL-LAB (Solar-Terrestrial Environment Laboratory), Nagoya University. The results showed |η| = 5.3 ± 1.7 and Φ = -40.2º± 30.6º for O2 emission, and|η| = 8.0 ± 4.7 and Φ = -41.9º± 41.4º for OH emission. There is a good agreement between the results obtained in this work with the previous results. But, it must be emphasized that the models for Φ don't predict the results as good as the models for |η|. It was found the existence of larger |η| values for the OH emission, so that there are few results in literature concerning of this high values. Comparing it with the models of Schubert et al. (1991), it was observed that they have good correlation with the model for a gravity wave with a horizontal wavelength of 100 km.

There is a great deal of interest in long-term change in the upper atmosphere and its possible relationship with human activities, but it is clear that we do not yet have a consistent picture of solar cycle and long-term trends in the MLT region. The long series of rocket measurements made from the early 1960s through to the mid-1990s, over a wide range of latitudes, have been interpreted by some as showing strong long-term trends, but there are considerable doubts as to the reliability of these measurements for heights above about 50 km. Measurements made by various other techniques, such as airglow rotational temperature, VLF reflection height, and satellite-borne radiometers, provide conflicting results. At low latitudes, published results from satellite and sodium lidar measurements indicate negligible trends, but new results from OH rotational temperature measurements, made a 23 ∘ S , show consistent solar cycle variations in very close synchronization with the F10.7 flux, although with a linear trend much larger than expected.

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.

In this paper we demonstrate how ground-based measurements of selected airglow emission features may be used to determine the vertical distribution of atomic oxygen in the upper mesosphere and lower thermosphere. The technique used is based on a method first suggested by Sharp and McDade [1996] and exploits differences between the kinetics of specific oxygen airglow features to determine key parameters related to the atomic oxygen vertical distribution. The technique is applied to ground-based measurements of the OI 557.7 nm green line, the O 2 Atmospheric (0-1) band, and the OH Meinel (9-4) band emissions made at Cachoeira Paulista, Brazil (23°S, 45°W) from January 1987 to June 1994. In order to compare the results obtained here with observations, the derived atomic oxygen density profiles are used to forward calculate hydroxyl nightglow profiles which are then compared with profiles measured by the Wind Imaging Interferometer (WINDII) experiment on board the Upper Atmosphere Research Satellite (UARS). This comparison shows good agreement for the seasonally averaged nocturnal behavior of both the peak altitude and the peak volume emission rate.

This study is an extension of previous statistical studies (Sobral et al., 1990, 1991, 1999) of both the local time and latitude variations of the zonal drift velocities of ionospheric plasma depletions, over the Brazilian low latitude station Cachoeira Paulista — CP (22.54°S, 45.00°W). The past studies were based on OI 630 nm scanning photometer data and the present one is based on digital OI 630 nm airglow images obtained by an all-sky imager system. These data were gathered between October 1998 and October 1999, at CP. The present results show that, in general, the velocities clearly tended to decrease with local time. Such a decrease should be associated with decreasing intensity of the vertical component of the ambient electric field which, in turn can be accounted for by recombination. All zonal drifts obtained for the 18 nights were eastwards. During equinox, the velocities clearly tended to decrease with local time at lower rates as compared with spring and summer. The highest and lowest zonal drift velocities, from all three seasons considered here, were observed to be in the summer ≈180 ms−1 at 21:45 LT, and in the spring ≈25 ms−1 at 03:15 LT, respectively. Ionospheric plasma bubbles were detected out to the maximum extra-tropical geographical latitude of ≈28° S, which was the highest latitude position analyzed in this study.

Gravity wave effects in the nocturnal thermospheric F -region domain are seldom detected in the intertropical region by optical (airglow) techniques, especially during geomagnetically quiet times, in part because the low inclination of the magnetic field, as opposed to the case of the mid-latitude region, does not favor significant vertical excursions of ionospheric plasma in response to meridional winds. Such difficulty in detecting gravity wave signatures in the F -region by means of optical techniques tends to increase in the absence of geomagnetic storms because of the lack of strong forcing mechanisms necessary to generate high intensity gravity waves. The purpose of this work is to show that during the quiet day of 9 August 1999, the Terminator may have been a source region of wave-like disturbances in the nocturnal F -region at the low-latitude station Cachoeira Paulista (22°41'S; 45°00W, dip 30°). A digital all-sky OI 630nm imager system located at that station has shown propagating wave-like spatial structures in the airglow intensity near the Terminator. This observation supports a previous study on the evidence of the presence of gravity waves during the post-sunset period at the same location by means of a scanning photometer system (1997, Sobral, J. Atmos. Terr. Phys. 59 , 1611–1623). The absence of range-type spread- F as monitored by a local digisonde and the absence of radio wave scintillation as monitored by a local GPS receiver, excludes the hypothesis that the wave-like airglow structures are associated with the occurrence of the ionospheric plasma bubbles. Downwards motion of the iso-density real height contours at 22.0 ms −1 and 33.1 ms −1 are observed. The wave detection by the imager system is reported and discussed here.

The first Brazilian scientific micro-satellite, SACI-1, is scheduled to be launched in the 2nd semester of 1999. It will be placed in a sun-synchronous polar orbit at a height of 750 km. Four scientific payloads, airglow photometer, high energy particle telescopes, plasma probes and high sensitive magnetometer, are on board to monitor the Earth's upper atmosphere environment. The main scientific goal is focused on upper atmospheric dynamics, ionospheric plasma irregularities, anomalous cosmic ray and field-aligned electric current system in the magnetosphere. International collaboration will be necessary, and this will be a key point in order to maximize the utilization of the data.

Measurements of the hydroxyl nightglow from the Wind Imaging Interferometer (WINDII) satellite-borne experiment and from ground-based photometers are compared and combined in order to characterize the hydroxyl airglow nocturnal behavior at 23°S and the equatorial region. Ground-based observations at Cachoeira Paulista (23°S, 45°W) and at Fortaleza (4°S, 38°W) from 1987 to 1994 and WINDII data from March 1992 to January 1994 are selected, and two periods are studied: the March/April equinox and the December/January solstice. The WINDII results for the equinox/solstice difference and for the nocturnal behavior of the vertically integrated emission rate of the hydroxyl nightglow are found to be in very good agreement with the ground-based observations. The WINDII measurements revealed two general characteristics of the hydroxyl nightglow altitude variation at the latitudes above. The hydroxyl nocturnal variations are predominantly occurring below 90 km, and the vertically integrated emission rate and the peak height of the volume emission rate show an inverse relationship with seasonal, latitudinal, and inter annual effects. The interpretation of these results in terms of atmospheric tides is then discussed.

In December 1995, a campaign was carried out to study the day-to-day variability in precursor signatures to large-scale ionospheric F-region plasma irregularities, using optical diagnostic techniques, near the magnetic equator in the Brazilian sector. Three instruments were operated simultaneously: (a) an all-sky (180° field of view) imaging system for observing the OI 630 nm nightglow emission at Alcântara (2.5°S, 44.4°W); (b) a digisonde (256-Lowell) at São Luis (2.6°S, 44.2°W); and (c) a multi-channel tilting filter-type zenith photometer for observing the OI 630 nm and mesospheric nightglow emissions at Fortaleza (3.9°S, 38.4°W). During the period December 14-18, 1995 (summer in the southern hemisphere), a good sequence of the OI 630 nm imaging observations on five consecutive nights were obtained, which are presented and discussed in this study. The observing period was geomagnetically quiet to moderate (Kp = 0+ to 5+; Dst = 18 nT to -37 nT). On four nights, out of the five observation nights, the OI 630 nm imaging pictures showed formations of transequatorial north-south aligned intensity depletions, which are the optical signatures of large-scale ionospheric F-region plasma bubbles. However, considerable day-to-day variability in the onset and development of the plasma depleted bands was observed. On one of the nights it appears that the rapid uplifting of the F-layer in the post-sunset period, in conjunction with gravity wave activity at mesospheric heights, resulted in generation of very strong plasma bubble irregularities. One of the nights showed an unusual formation of north-south depleted band in the western sector of the imaging system field of view, but the structure did not show any eastward movement, which is a normal characteristic of plasma bubbles. This type of irregularity structure, which probably can be observed only by wide-angle imaging system, needs more investigations for a better understanding of its behaviour.Key words. Atmospheric composition and structure (airglow and aurora) · Ionosphere (equatorial ionosphere; ionospheric irregularities)

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.

The upper atmospheric airglow emissions, OI 557.7 nm, NaD 589.3 nm, OH (9,4) and O2 atmospheric (0,1) bands and their rotational temperatures have been measured using ground-based multichannel airglow photometers, one located near the equator at Fortaleza (3.9 S, 38.4 W) and the other at lowmiddle latitude Cachoeira Paulista (22.7 S, 45.0 W). Monthly averaged nocturnal variations calculated from the data from 1987 to 1993 were used to study the influence of atmospheric dynamical processes on these emissions. Harmonic analysis revealed that the Cachoeira Paulista data are mainly represented by a 12 hour period oscillation. Phase differences between the different emissions reveal that the vertical phase velocity is about 4 km/h and the vertical wavelength about 50 km. Fortaleza data on the other hand showed longer period, 24 hours, slow vertical propagation velocity, 1.1 km/h, and short vertical wavelength, about 30 km. These facts lead us to conclude that the Cachoeira Paulista data are mainly controlled by the semidiurnal tidal oscillation and the Fortaleza data are controlled by the diurnal tide.

Simultaneous rocket measurements of the OH Meinel (8-3) band and O2 Atmospheric (0-0) band emission profiles are used to investigate the OH (v = 8) loss in the equatorial mesosphere due to collisions with O2 and N2. The O2 Atmospheric (0-0) band emission profile is used to obtain the atomic oxygen concentration. The derived branching ratios, A(8)A(8-3), and quenching parameters are found to be about a factor of two smaller than those obtained by McDade et al. (1987). Adopting branching ratios A(8)A(8-3) from the literature, the best agreement between the emission profiles of oxygen and OH is obtained using the Meinel band transition probabilities of Turnbull and Lowe (1989).

Simultaneous measurements of the nightglow profiles of the O 2 Herzberg I bands, the O 2 Atmospheric (0-0) band and the atomic oxygen green line, carried out at Alcântara (2.3 °S; 44.4 °W), are presented and discussed. This is the first measurement of the vertical profile of the Herzberg I band to be carried out at equatorial latitudes. The atomic oxygen concentration derived from the O 2 Atmospheric (0-0) band emission profile is used to test a photochemical scheme to explain the measured O 2 ( A 3 Σ u + − X 3 Σ g − ) nightglow emission based on atomic oxygen recombination. The kinetic analysis is carried out assuming the emission ratio Herzberg I: Herzberg II = 5:1, 10:1 and the extreme case where Herzberg II brightness = 0. For all three cases studied, the quenching rates required to explain the measurements are higher than the values derived from previous airglow measurements.

The MU radar (middle and upper atmosphere radar) has been used since 1984 to study the dynamics of the mesosphere and lower thermosphere (MLT) as well as the dynamics of the lower atmosphere and the ionosphere. Gravity waves in the mesosphere have been studied extensively over various time and vertical scales during the daylight hours using backscatter from turbulent irregularities. Scatter from meteor trails can be used during day or night to study the dynamics of the MLT, and techniques to apply the MU radar for meteor scatter observations have been developed and recently improved. The meteor observation mode of the MU radar can detect about 15,000–20,000 echoes/day and 10,000 echoes/day are from underdense trails with arrival angle determination. This high meteor echo rate enables us to determine the wind velocities and molecular diffusion coefficient with the time/height resolutions of 30 min × 1 km at 80–100 km for 24 hours a day. From the horizontal distribution of the meteor echoes, horizontal gradients of the wind fields can also be detected, as well as vertical shears of horizontal winds. These high resolution meteor echo observations were made simultaneously with OH spectrometer observations. These combined observations yielded information on both the horizontal and vertical structure of the gravity waves. Comparisons were made between the temperature variations determined by OH airglow and determined by the diffusion coefficient of the meteor echoes and excellent agreement was found. Cooperative observations with sodium lidars, FPI, airglow imagers, and an MF radar are also being carried out.

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).

The MULTIFOT airglow photometer payload was launched from Alcântara (2.5°S, 44.4°W) on a SONDA III rocket at 23:52 hrs local time on 31 May 1992. A total of ten photometers, six forward-looking and four side-looking, measured the height profiles of the airglow emissions O 2 Herzberg band system, 01557.7 run, NaD 589.3 nm, 01630.0 nm, OH(8,3) band R branch at 724.0 nm, O 2 Atmospheric (0,0) band at 762.0 nm and the sky background at 578 nm and 710 nm. At the time of launch, a ground-based airglow photometer observed the intensity variations of these emissions, together with the rotational temperature of the OH(9,4) band, and a sodium lidar measured atomic sodium concentration from 80 to 110 km.

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.

Average nightly intensities series for the NaD, OI 5577-E, OH(9,4) airglow and the OH(9,4) rotational temperatures from 1977 to 1986 obtained at Cachoeira Paulista, Brazil (23° S, 45° W) have been correlated to solar 10.7 cm flux. Generally there is a positive correlation for all parameters with F10.7 at a high significance level. The series are also divided in the west and est phase of the QBO oscillation in the tropical stratosphere and no significant effect is found. This positive correlation agrees with many previous studies, but the temperature dependence disagrees with recent results obtained between 50°N and 70°N.

During the Spread FEx campaign, under the NASA Living with a star (ILWS) program which was carried out in the South American Magnetic Equatorial region from September to November 2005, we observed formation of the bottom-type spread F and simultaneous occurrence of mesospheric gravity wave events. The events were monitored by the ionosonde, coherent radar and airglow OI 630.0 nm and OH imager. It is found that the bottom-type scattering layer has a wave form generated most probably by local gravity waves. Reverse ray-tracing of the observed gravity waves indicate their possible sources in the troposphere or thermosphere. Forward ray-tracing indicates their penetration into the ionosphere. The present work summarizes the observational evidence and results of the data analysis and discusses the mesosphere–ionosphere coupling processes.

On May 31, 1992, a rocket payload equipped with 10 airglow photometers was launched from the Alcântara Launch Center in Northern Brazil. The payload was designed to measure sodium, hydroxyl, atomic and molecular oxygen airglow emissions, in addition to electron density and temperature. At the time of launch, a sodium lidar was operated at the same site, giving vertical profiles of atmospheric sodium density. The measured vertical profiles of the Na D line emission and atomic sodium density are remarkably similar, with sharp boundaries at 81 km and 99 km. On the assumption that the Chapman mechanism is responsible for the Na D emission, the measurements imply an almost constant ozone density of 4×107 cm−3 between 87 and 99 km. The unexpectedness of this height independent ozone concentration suggests that our understanding of the sodium excitation mechanism might be inadequate.

Simultaneius meausurements of the night airglow O 2 atmospheric (0,1) band (864.5 nm) and OH(9,4) band (774.6 nm) emissions have been carried out at Cachoeira Paulista (22.7 o S, 45.0 o W) since 1983. We present a study of gravity wave induced variations in the emission intensities and rotational temperatures. For both the emissions, the intensity variations are normally delayed with respect to the temperature variations, and the phase difference is larger for OH(9,4) than for O 2 (0,1). The ratio between the amplitudes of fluctuation of the intensity and temperature, expressed as η = ( δ I/I/( δ T/T) has been calculated for both the emissions. The mean value of η for OH(9,4) was 4.9 with a range of 1.03 to 7.9, in agreement with a recent model. For the O 2 A(0,1) band, varied from 3.4 to 16.1, with a mean of 7.1. The relation between η and the fluctuation period is also different for the two emissions.

Two rocket observations of the OI λ557.7 nm nightglow emission in the lower thermosphere have been carried out from the equatorial region at Natal (5.8°S, 35.2°W), one on 9 December 1985 and the other on 31 October 1986. The volume emission rate obtained from these observations showed a maximum at a height of 97 ± 1 km with a half-width of 6 km for the first experiment and 98 ± 1 km with a half-width of 7 km for the second experiment. The observed emission profiles showed large variability, suggesting dynamical control of the atomic oxygen. The atomic oxygen density profiles deduced from the present data are compared with atmospheric models and currently available experimental data.

The quantum yields ƒ( 1 D) and ƒ( 1 S) of the atomic oxygen excited states O( 1 D) and O( 1 S), respectively, from dissociative recombination in the nocturnal F-region are determined utilizing rocket airglow (OI 630 nm and OI 557.7 nm) and electron density data obtained in two experiments that were carried out at Natal (geogr. 5.8°S, 35.2°W), Brazil, on 11 December 1985 and 31 October 1986. Using the 557.7 and 630 nm airglow data from the second experiment Takahashi et al. (1990, Planet. Space Sci . 38 , 547) calculated the ratio ƒ( 1 S)/tf( 1 D). From a different approach we utilize in this paper the airglow volumetric emission rate and electron density to calculate ƒ( 1 S) and ƒ( 1 D), individually, which are compared with the previously published results from in situ and laboratory measurements. Our results show ƒ( 1 S) to be height dependent. In the second experiment it varied from 1.96 × 10 −2 at 190 km to 1.33 × 10 −1 at 315 km with an average value of 0.053. The average value for the first experiment was 0.041. The ƒ( 1 D) magnitude was also found to vary significantly with altitude, its value increasing both upwards and downwards from 250 km where it attains a minimum of about 0.77.

An analysis of a long time-series of measurements of the airglow emissions, OH(9, 4) 775.0 nm, NaD 589 nm, OI 557.7 nm and O 2 (0, 1) 864.5 nm atmospheric band, shows that the nocturnal mean intensities of all these emissions are highly correlated with the nocturnal temperature means, as determined from the rotational structure of the OH(9, 4) emission. After removing seasonal and long-term variations, the closest correlation is observed between the OH (9, 4) emission intensity and the corresponding rotational temperature, followed by the NaD, and the OI 557.7 nm and O 2 A emissions. The regression coefficients, ( d I I ) ( d T T ) , where I is intensity and T is temperature, are between 8 and 9 for the NaD and oxygen emissions, and about 5.5 for the OH band. It is clear that these regression coefficients do not reflect the temperature dependences of the various reactions involved. It seems probable that the observed correlations are the result of correlated temperature and density changes in the upper atmosphere.

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.

Since 2001, the formation of plasma bubbles observed by ionospheric airglow OI 630.0 nm and OI 577.0 nm emissions have been registered at Southern Space Observatory, located at Sao Martinho da Serra, RS, south of Brazil (29° S, 53° W). A photometer to monitor the zenith intensity and an all-sky imager to observe the spatial variation of the emissions were used. The observed zenith intensities showed strong seasonal variation, maxima in the equinox season (March-April and September-October) and minima in solstice (June-July and December- January). The Ol 630.0 intensity depletions caused by formation of the plasma bubble along the magnetic field line were detected by the imager. The depletions were observed more frequently in the period from September to March during the summer season in Southern Hemisphere.

The airglow layers are very useful to study and monitor the temperature and wave activity in the mesosphere region. All-sky cameras have been used to observe the nightglow emissions and to investigate the gravity waves characteristics and horizontal structures. The present paper describes the methodology used to estimate the gravity wave parameters such as the horizontal wavelength, phase speed, period and propagation direction, observed in the airglow images.

A aeroluminescência do sódio mesosférico ocorre quando o átomo de sódio decai do estado excitado ²P para o estado ²S emitindo na linha D em lambda = 589.3 nm. Medidas da intensidade noturna da emissão têm sido realizadas por um fotômetro multicanal instalado em São João do Cariri (7,38ºS; 36,54ºw) desde 1998. Neste trabalho foi analisada uma série de dados obtidos no período de 1998 a 2003, com o objetivo de estudar o comportamento sazonal da emissão e suas possíveis relações com a emissão do OH (6,2) e com a atividade solar. As análises mostram que a emissão caracteriza-se por uma forte variação semi-anual com máximos nos equinócios e mínimos nos solstícios, estando estes resultados em concordância com outros estudos realizados em regiões equatoriais. A dependência da emissão do NaD com a atividade solar mostrou uma concordância durante a ascensão e o declínio do ultimo ciclo solar, porém, durante o pico da atividade solar houve discrepâncias. Por outro lado, as emissões do NaD e do OH (6,2) mostraram ter aspectos semelhantes nas suas respectivas variações sazonais. The mesospheric sodium airglow emission occurs when the sodium atom decays from the excited level ²P to the level ²S, emitting at the line D in lambda = 589.3 nm. Measurements of the emission's nocturnal intensity has been carried by a multichannel photometer installed in São João do Cariri (7,38ºS; 36,54ºw) since 1998. This work analyzed a data set corresponding to a period from 1998 to 2003, in order to study the emission's seasonal comportment. The analyses showed a strong seasonal variation. The results are in accordance to other studies completed at equatorial regions. The relationship between solar activity and NaD emission showed a correspondence during ascension and descension of the solar activity, but discrepancies arise during peak of the solar activity. On the other hand, the NaD and OH (6-2) showed similar aspects in their respective seasonal variation.

In this work we present a methodology to calculate the temperature in the upper mesosphere using a spectral OH (6-2) airglow temperature imager. The temperature imager system consists of a CCD camera and an optical head (including an interferometer filter, Fresnel lens and objective lens) which was developed by Airglow group of INPE in São José dos Campos, Brazil. The instrument calibration was performed at National Institute of Polar Research, Japan, using a 2 m diameter integrating sphere. The airglow test observations were carried out at Cachoeira Paulista Observatory (22.7ºS), Brazil, from March to May 2005, and the results were compared to that obtained by a tilting-filter zenith photometer (MC2) at the same place. Some results are presented and compared with MSISE-90 atmospheric temperature model, which showed a good agreement in the nocturnal mean values, while exhibiting a different nocturnal variability. Neste trabalho será apresentada uma metodologia para calcular a temperatura na alta mesosfera usando a emissão da luminescência atmosférica do OH (6-2). O sistema imageador de temperatura consiste de uma câmera CCD e um sistema óptico (incluindo um filtro de interferência, uma lente Fresnel e a objetiva), instrumento este que foi desenvolvido pelo grupo de Luminescência Atmosférica do INPE em São José dos Campos, Brasil. A calibração do instrumento foi realizada no Instituto Nacional de Pesquisa Polar, Japão, usando uma esfera integradora de 2 m de diâmetro. As primeiras observações de temperatura e intensidade do OH (6-2) foram feitas no Observatório de Cachoeira Paulista (22.7ºS), Brasil, de Março a Maio de 2005, e os resultados foram comparados com os obtidos por um fotômetro zenital de filtro inclinado (MC2) que operou no mesmo local. Alguns resultados são apresentados e comparados com o modelo atmosférico de temperatura, MSISE-90, mostrando uma boa concordância com os valores médios noturnos, porém diferindo com relação à variabilidade noturna.

A variety of optical remote sensing techniques have now revealed a rich spectrum of wave activity in the upper atmosphere. Many of these perturbations, with periodicities ranging from ~ 5 min to many hours and horizontal scales of a few tens of km to several thousands km, are due to freely propagating atmospheric gravity waves and forced tidal oscillations. Passive optical observations of the spatial and temporal characteristics of these waves in the mesosphere and lower thermosphere (MLT) region ( ~ 80-100 km) are facilitated by several naturally occurring, vertically distinct nightglow layers. This paper describes the use of state-of-the-art ground-based CCD imaging techniques to detect these waves in intensity and temperature. All-sky (180º) image measurements are used to illustrate the characteristics of small-scale, short period ( < 1 hour) waves and to investigate their seasonal propagation and momentum impact on the MLT region. These results are then contrasted with measurements of mesospheric temperature made using a new temperature mapping imaging system capable of determining induced temperature amplitudes of a large range of wave motions and investigating night-to-night and seasonal variability in mesospheric temperature. O espectro de ondas na alta atmosfera tem sido revelado por uma variedade de técnicas de sensoriamento remoto ótico. Muitas destas perturbações apresentam períodos variando de 5 minutos a algumas horas e escalas horizontais que variam de dezenas a milhares de quilômetros e são devidas a livre propagação de ondas de gravidade e marés atmosféricas. Observações óticas das características espacial e temporal destas ondas na região ( ~ 80-100 km) da mesosfera e baixa termosfera (MLT) utilizam a variação da aeroluminescência atmosférica. Este trabalho descreve o estado da arte das técnicas que utilizam o imageamento via CCD para detectar estas ondas através das variações provocadas na intensidade e temperatura. Medidas de um imageador de céu todo (All-sky, 180º) são usadas parar determinar as características de ondas de pequena escala e curto período ( < 1 hora) e investigar a sazonalidade da direção propagação destas ondas, como também o depósito de momento na região da MLT. Estes resultados são comparados com medidas da temperatura atmosférica realizadas usando um novo sistema de imageamento de mapeamento da temperatura mesosférica capaz de determinar variações nas amplitudes da temperatura provocadas pelas ondas de gravidade através da investigação da variabilidade noturna da temperatura mesosférica.

Continuos observation of airglow emissions obtained by an all-sky imager (INPE), installed at Cachoeira Paulista (230 S), have been used to investigate the characteristics on the major low frequencies oscillations. The images of the Near Infrared OH emissions produce keograms constructed from a north-south and east-west slice from the middle of every image joined over the whole night. The keograms are a convenient way to reveal wave activities over the whole night, on the other hand, the individual images are the best way to see short period gravity waves.