N.R. Teixeira

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.

Observations of thermospheric neutral winds at 23°S

Abstract Observations of night-time thermospheric neutral wind velocities from measurements of Doppler shifts of the OI 630.0 nm airglow emission line, using a Fabry-Perot interferometer, have been carried out from 23°S geographic latitude in the Brazilian sector, during the period March 1988–December 1989. The observing location is situated inside both the equatorial ionospheric and South Atlantic magnetic anomalies. In this paper we present and discuss salient features of the average nocturnal variations of the thermospheric meridional and zonal wind velocities during autumn, winter, spring and summer seasons. The results are compared with the wind velocities predicted by the HWM-87 and HWM-90 models. The observed and predicted meridional and zonal wind velocities from the HWM-87 and HWM-90 models show a reasonable agreement in terms of tendencies of nocturnal variations during different seasons. However, some discrepancies are observed with regard to the magnitudes and detailed variation in the nocturnal patterns. The observed zonal winds are stronger than the meridional winds. Also the summer zonal winds are stronger than in winter, contrary to the HWM-87 model predictions. The observed thermospheric wind velocities, presented here, are comparable with those reported from other low latitude stations.

Airglow intensity and temperature response to atmospheric wave propagation in the mesopause region

Abstract Simultaneous measurements of the O 2 (0,1) atmospheric band (864.5 nm) and OH(9,4) band (774.6 nm) emissions in the night airglow, using a tilting filter photometer, have been carried out at Cachoeira Paulista (22.7 deg. S, 45.0 deg. W), Brazil, since 1983. The band intensities of the O 2 (0,1) band, I(O2) and OH(9,4) band, I(OH), and their rotational temperatures, T(O2) and T(OH) have been obtained. In the nocturnal variations, time-lagged wave-like co-variations between the intensities and rotational temperatures were frequently observed. Correlation studies reveal that, in general, the rotational temperature variations, T(O2) and T(OH), lead the intensity variations, I(O2) and I(OH), respectively. It is found that the time lag between T(O2) and T(OH) is less, by about 0.5 hour, than that between I(O2) and I(OH), which is about 1.5 hours, indicating that the airglow intensity response to atmospheric wave propagation is different from the temperature response.

Rocket measurements of the equatorial airglow: MULTIFOT 92 database

Abstract 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.

Solar cycle and seasonal variations of the low latitude OI 630 nm nightglow

Abstract Regular zenith measurements of the OI 630 nm nightglow emission have been carried out at Cachoeira Paulista (22.7°S, 45.0°W; geomag. 11.9°S), Brazil, since 1975. The long series of observations during the period 1975–1982, including the ascending phase of the last solar cycle, permitted studies of solar cycle effects and seasonal variations. A large intensity increase, about seven times, from low solar activity to high solar activity has been observed. Also, the seasonal-nocturnal intensity variations show large changes between years of low and high solar activity. The characteristics of the variations observed are closely related to the equatorial electric field variations, since the observation site is under the southern equatorial ionospheric anomaly crest.

Tropical twilight HeI 10830 emission

Abstract A further years observation of twilight Hel 10830 A emission made in Brazil confirms the trend of seasonal variation found earlier. The 10830 intensities taken with concurrent ionosonde data are discussed in the context of recent model calculations of the emission rates. An increase with K P is attributable to changing production and loss rates of He 2 3 S with changing thermospheric temperature. The morning intensities being greater than those of the evening can be due to several effects, an important one of which appears to be the much greater loss rate of photoelectron energy to ambient electrons in the evening, arising from the very high evening electron concentrations at the Appleton Anomaly ionization crests.

Observations of thermospheric temperatures at 23°S

Abstract Observations of night-time thermospheric temperatures from measurements of Doppler widths of the OI 630.0 nm airglow emission line, using a Fabry-Perot interferometer, have been carried out from 23°S geographic latitude in the Brazilian sector, during the period March 1988–October 1989. The observing location is situated inside both the equatorial ionospheric and South Atlantic magnetic anomalies. In this paper we present and discuss salient features of the average nocturnal variations of the thermospheric temperature during equinox and winter seasons. The results are compared with the thermospheric temperatures predicted by the MSIS-86 model. The observed temperatures are in good agreement compared with the MSIS-86 model temperatures. They are comparable with those reported from other low latitude stations and no anomalous high thermospheric temperatures have been observed in this region of the South Atlantic magnetic anomaly.

Multispectral optical observations of ionospheric F-region storm effects at low latitude

Abstract Simultaneous measurements of the OI 7774 A and 6300 A, N 2 + 3914 A and Hβ 4861 A nightglow emissions have been carried out at Cachoeira Paulista (22.7°S, 45.0°W; geomag. 11.9°S) since 1982, to study the response of the low-latitude ionospheric F -region to magnetic storms. The observations obtained during three magnetic storms (12–13 June 1983, 7–8 August 1983 and 28–29 March 1984) are presented and discussed. The N 2 + 3914 A and Hβ 4861 A emissions excited by energetic particle precipitation were observed during the main phase of strong magnetic storms. In contrast to the observations reported from mid-latitude stations by other investigators, no enhancements in the OI 7774 A emission due to energetic particle precipitation were evident at our latitude. Even during strong magnetic storms, the OI 7774 A intensity variations observed follow the variation of ( f 0 F 2) 4 , suggesting radiative recombination as the main excitation mechanism. The OI 6300 A emission, on 7–8 August and 28–29 March, showed periodic intensity variations, which are associated with vertical oscillations of the ionospheric F -region plasma, as shown by the periodic height variations of the F -region seen from the ionograms obtained at the same location. Also, the OI 6300 A North-South scanning observation on one occasion showed no meridional and longitudinal phase change, indicating the absence of propagation.

Plasma drifts inferred from thermospheric neutral parameters during geomagnetic storms at 23°S

Abstract Nighttime thermospheric neutral winds and temperatures are derived from measurements of Doppler shifts and widths of the OI 630.0 nm airglow emission line, respectively, using a Fabry-Perot interferometer at Cachoeira Paulista (23°S, 45°W, 16°S dip latitude), Brazil. The observed nighttime East–West and North–South temperature gradients and neutral wind velocity variations are presented for two geomagnetic storm periods 7–10 July 1991 and 4–9 August 1991. Zonal plasma drifts inferred during an intense geomagnetic storm, from the nighttime neutral wind velocities and temporal temperature gradient variations, suggested that a strong zonal plasma drift (−240 m⧹s) on 9–10 July 1991 changed the direction of the zonal winds to westward at 01:30. Also it was noticed that, during the geomagnetic storm, the zonal temperature gradients were not well defined like those observed during quiet time. The meridional wind was towards the pole for the whole night during the geomagnetic storm of 9–10 July, a behaviour which is unusual during winter time (southern hemisphere) at Cachoeira Paulista. However, the meridional plasma drifts inferred did not show significant differences during the quiet and disturbed periods. The method presented to infer plasma drifts using the observed neutral parameters showed results comparable with those observed by incoherent scatter radar measurements at Arecibo.

A comparative study of low-latitude ionospheric and OI 630nm nightglow observations with the SLIM and IRI models

Measurements of ionospheric F-region peak electron densities, peak heights and atomic oxygen 630 nm airglow emission column intensities, carried out at a low-latitude station located at Cachoeira Paulista (22.7°S, 45.0°W; dip latitude 11.9°S), in Brazil, are analyzed and compared with the ionospheric parameters predicted by the SLIM and by the IRI ionospheric models. The MSIS neutral atmosphere model is used with the IRI model to calculate the 630 nm nightglow emission intensities. This comparative analysis considers data representative of the summer, winter and equinox seasons, obtained during the high solar activity period around 1980. Reasonably good agreement is obtained between the local time variations of the ionospheric and airglow data, and the IRI model predicted parameter variations, but the SLIM model overestimates the F-region peak heights especially during the late afternoon and nighttime hours. These height discrepancies are explained in terms of the strong longitudinal dependence of the electromagnetic E × B vertical plasma drifts and thermospheric neutral winds, which greatly influences the dynamics of the low-latitude ionospheric plasma and airglow emissions. Due to the importance of the relative alignement condition between terminator and magnetic meridian, with its longitudinal and seasonal dependence, on the vertical plasma drifts around sunset, it is suggested that the SLIM ionospheric model should separately consider the longitudinal zones of positive, negative or zero magnetic declination.