P.R. Fagundes

Vertical and zonal equatorial F-region plasma bubble velocities determined from OI 630 nm nightglow imaging

Abstract An all-sky (180°) imaging system for observations of the OI 630 nm nightglow emission is in routine operation at Cachoeira Paulista (22.7°S, 45.0°W; 16°S dip latitude), Brazil. This technique permits observations covering a large area, extending from the equatorial region to nearly mid-latitude from this observational site. During February 1995, a relatively good sequence of observations on six consecutive nights were obtained. On five out of the six nights, weak to strong nearly north-south aligned intensity depleted regions, which are the optical signatures of large-scale field-aligned equatorial F-region plasma depletions, were observed. Important features from this set of observations, including vertical and zonal velocities of the plasma bubbles, are presented and discussed in this paper.

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.

Solar flux and seasonal variations of the mesopause temperatures at 51°N

Abstract Observations of the OH (8-3) band airglow emission, using a multichannel tilting filter type photometer, have been carried out at Calgary (51°N, 114°W), Canada, since 1981. In this paper recent measurements of the nocturnal, seasonal and solar flux variations of the mesopause temperature, obtained from the rotational temperature of the OH (8-3) band observations, are presented. The data presented span the ascending phase of the present solar cycle viz. 1987–1988 (low solar activity) and 1990 (high solar activity). Good correlations (r = 0.73) between the OH (8-3) band rotational temperature and the 10.7 cm solar flux were observed. The mean temperature for the period investigated was about 210 K. The seasonally averaged nocturnal variations show only small irregular excursions, possibly associated with solar tides and the passage of gravity waves in the mesopause region. However, the observed rotational temperatures show considerable night-to-night changes.

Simultaneous observations of equatorial F-region plasma depletions and thermospheric winds

Abstract Simultaneous observations of the OI 630.0 nm nightglow emission using an all-sky imaging system, and thermospheric winds and temperatures using a Fabry-Perot interferometer at 630.0 nm, have been recently conducted at Cachoeira Paulista (22.7°S, 45.0°W, dip latitude 15.8°S). The all-sky imaging observations of the OI 630.Onm nightglow emission, which arises from the O 2 + ionospheric dissociative recombination process, are used to monitor the morphology and dynamics of transequatorial ionospheric plasma bubbles in the height range of the nightglow emissions. In this paper we present simultaneous observations from these two instruments obtained in April and August 1991, in which some nights showed the presence of ionospheric plasma bubbles. These observations are also complemented by ionospheric sounding data obtained at the equatorial station Fortaleza (3.9°S, 38.4°W, dip latitude 3.7°S). Significant features of the morphology and time evolution of the plasma depletions as seen through the OI 630.0 nm emission images and possible influence of the thermospheric winds on the generation of the plasma irregularities are analyzed and discussed.

Observations of gravity waves from multispectral mesospheric nightglow emissions observed at 23°S

Abstract Simultaneous measurements of the 015 57.7 nm, O2 atmospheric (0,1) band, NaD and OH (9,4) band emissions obtained during the period October November 1989 at Cachoeira Paulista (23°S, 45°W), Brazil, have been analysed to study gravity waves in the mesospheric region at a low-latitude station in the southern hemisphere. It was found that, when these emissions showed large temporal intensity variations, there were also short period quasi-coherent temporal variations superposed on them, suggesting a possible passage of internal gravity waves in the emission layers. Cross-correlation analysis indicates that the time lag between the different emissions is smaller for short period variations compared with the long period variations. The wave parameters, namely a vertical wavelength of 12 km, a horizontal derived wavelength of 200 km with a period of 80 min, estimated from one of the observed short-period coherent oscillations, are typical of the internal gravity waves at the airglow emission height.

Investigation of OI 557.7 nm and OI 630.0 nm nightglow intensity ratios during the occurrence of equatorial F-region plasma bubbles

Abstract It is well-known that, at equatorial and low-latitudes, the OI 557.7 nm emission emanates both from the mesosphere and F-region. Earlier investigations using, in general, ground-based simultaneous observations indicate that the contribution to the OI 557.7 nm emission from the F-region is about 20% of the F-region OI 630.0 nm emission. Both of these emissions are excited by the dissociative recombination process (O 2 + + e → O ∗ ( 1 S, 1 D) + O) in the F-region. The nocturnal intensity variations of these emissions sometimes show large short-lived depletions, associated with the passage of equatorial spread-F plasma bubbles through the field of view of a photometer. Regular observations of these emissions have been carried out at Cachoeira Paulista (22.7°S, 45.0°W), Brazil, since 1973. In this paper, we present and discuss simultaneous observations of these emissions, during periods of strong equatorial spread-F conditions, when both emissions show large intensity drop-outs. Twenty-five nights of measurements from Cachoeira Paulista have been analysed for this study, which indicate that the average ratio of the intensity depletions ( ΔI557.7 nm ΔI630 nm ) is about 0.26±0.07 for F-region altitudes, and this component has a dependence on the height of the electron density profile.

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.

Relationship between generation of equatorial F-region plasma bubbles and thermospheric dynamics

Abstract Simultaneous zenith measurements of the OI 630.0 nm and 557.7 nm nightglow emissions, using a multi-channel tilting filter type photometer and nighttime thermospheric neutral wind velocities from the Doppler shifts of the OI 630.0 nm emission line, using a Fabry-Perot interferometer, have been carried out at Cachoeira Paulista (23°S, 45°W), Brazil, during 1988–1991. These measurements constitute a unique opportunity to study the relationship between the generation of F-region plasma bubbles (regions of depleted electron densities) and thermospheric dynamics. During the period of March 1988 to June 1991, we have 9 nights on which there were nearly full nights of simultaneous observations of thermospheric wind velocities (zonal and meridional) and the OI 630.0 nm and 557.7 nm emissions showing formation of plasma bubbles. On the night of March 18, 1991, the zonal wind was unusually strong in the beginning of the night (300 m/s) and this night shows very strong formation of plasma bubbles, indicating that a strong zonal wind in some way may be connected with plasma bubble formation through the electric field created by the interaction between the zonal wind and the Earths magnetic field. However, both the zonal and meridional winds on several nights with, and without plasma bubbles, are comparable. This feature possibly indicates only a secondary role of thermospheric winds in the generation of the equatorial ionospheric plasma depletions. It should be pointed out that an analysis of the nocturnal intensity variations of simultaneous mesospheric (E-region) emissions (OI 557.7 nm, O 2 atm. 864.5 nm, NaD 589.0 nm and OH (9-4) 775.0 nm) on all the nine nights studied, shows the presence of gravity waves, indicating possible relation between wave processes in the mesosphere and F-region irregularities.

Plasma drifts inferred from thermospheric neutral winds and temperature gradients observed at low latitudes

Abstract Low-latitude plasma drifts (zonal and meridional) in the F-region are inferred from the observed night-time thermospheric neutral wind velocities and temperature gradients, together with models for the neutral density (MSIS-86 model) and the electron density (IRI model). The 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), Brazil. The equations considered are the ideal gas law and the momentum equation for the thermosphere, which includes the time variation of the neutral wind, the pressure gradient which is related to the temperature and density gradients and the ion drag force. The present method to infer the night-time plasma drift using observed neutral parameters (time variation of neutral wind velocities and temperature gradients) showed results that are in reasonable agreement with our calculated plasma drifts and those observed in other low-latitude locations. On the other hand, it is surprising that sometimes the winds flow from the observed coldest sector to the hottest part of the thermosphere during many hours, suggesting that plasma drift can drive the neutral winds at low latitudes for a period of time.

Observations of thermospheric neutral winds and temperatures at Cachoeira Paulista (23°S, 45°W) during a geomagnetic storm

Abstract Observations of nighttime thermospheric neutral wind velocities and temperatures from measurements of Doppler shifts and widths, respectively, of the OI 630.0 nm airglow emission line, using a Fabry-Perot interferometer, have been carried out at Cachoeira Paulista (23°S, 45°W), Brazil, since 1988. In this paper we present observations of thermospheric neutral winds and temperatures during the period July 6–10, 1991, which included the strong magnetic disturbance of July 8–9, 1991 when the Kp value reached 8. During this storm the meridional and zonal neutral wind velocity patterns were strongly modified. On July 9–10, the observed meridional wind during the storm shows a behaviour opposite to that observed during winter and the observed zonal wind at the beginning of the night shows a behaviour similar to its quiet time behaviour (to the east), but at 01:15 LT the zonal wind changes its direction rapidly to the west and at 03:45 LT it reverses to east again. Also, from the observed thermospheric temperatures, inferred from the thermal broadening of the 630.0 nm line emission, we find that during this event there is thermospheric cooling over Cachoeira Paulista. This result is surprising, because we would normally expect thermospheric heating during geomagnetic storms, since a large amount of energy is deposited in the thermosphere at high latitudes.