Y. Sahai

Transequatorial F-region ionospheric plasma bubbles: solar cycle effects ☆

Abstract During the recent past, wide-angle optical imaging observations of F-region nightglow emissions (e.g. OI 630nm) have provided excellent results related to the occurrence, evolution and dynamics of strong large-scale range spread-F irregularities, as they are characterized by large-scale ionospheric plasma depletions, generally known as transequatorial plasma bubbles, which result in quasi north–south aligned intensity depleted bands. The intensity depletions seen in the airglow images are the optical signature, at the height range of the emitting layer of transequatorial magnetic field-aligned plasma bubbles. An all-sky imaging system, observing the OI 630nm emission, was operational at Cachoeira Paulista (22.7°S, 45.0°W; ∼16°S dip latitude), Brazil, during the period March 1987 to October 1991. It was put back in operation again in September 1994 and observations are continuing. These observations have provided an extensive data-base of OI 630 nm images which permitted us to address several aspects related to the formation and development of large-scale spread-F plasma irregularities during both high- and low solar activity periods. An analysis of about 11,000 images from these investigations are presented and discussed in this paper. The seasonal occurrence characteristics are fairly similar for both low and high solar activities. However, the occurrences of intensity depleted bands are much less during low solar activity (33%) as compared with high solar activity (55%). Also, some of the intensity depleted bands in the images (which show the optical signatures at the height of the emitting layer around 250–300 km) indicating that plasma bubbles attaining very high altitudes ( >1500 km ) at the magnetic equator (by mapping the depletion bands along geomagnetic field lines to the equatorial plane (e.g., Mendillo, Tyler, J. Geophys. Res. 88 (1983) 5758), are much less during low solar activity (34% of the images with intensity depleted bands) as compared with high solar activity (66% of the images with intensity depleted bands). The average nocturnal variations of intensity depleted regions show different characteristics during the high and low solar activity periods.

O(1S) and O(1D) quantum yields from rocket measurements of electron densities and 557.7 and 630.0 nm emissions in the nocturnal F-region

Abstract 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.

Ionospheric plasma bubble zonal drift: a methodology using OI 630 nm all-sky imaging systems

Abstract With the recent advances in all-sky imaging technology for nightglow emission studies, the F-region OI 630 nm emission has become an important tool for ionospheric/thermospheric coupling studies. At equatorial and low latitude regions, the all-sky imaging observations of the OI 630 nm emission show quasi north-south aligned intensity depletion bands, which are the optical signatures of large scale F-region plasma irregularities. By observing the motion of the intensity depleted bands it is possible to infer the ionospheric plasma zonal velocity of the depletion. The north-south aligned structures seen in the field of view of the all-sky imaging system corotate with the ionospheric plasma, so that by calculating the spatial displacements occurring during successive OI 630 nm emission images we can infer the ionospheric plasma drift velocity. However, the plasma bubbles have their own internal space-time dynamics leading to changes in their shape and dimensions and this may induce some errors in the calculated drift velocities. In this paper we take into account the space-time changes in the plasma bubbles in order to calculate the ionospheric plasma zonal drift velocities using the OI 630 nm nightglow emission.

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.

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.

Nightglow OH (8, 3) band intensities and rotational temperature at 23°S

Abstract The OH (8, 3) band airglow emission has been observed over 1 year at a latitude of 23°S. The average band intensity observed was 385 Rayleighs with a nocturnal range typically less than 100 R. The nocturnal variation in rotational temperature was usually less than 10°K, and the mean temperature was 179°K. The nocturnal variation of intensity is usually uncorrelated with that of the rotational temperature. Time average values of these parameters do, however, show some correlation. On some occasions large post-twilight and pre-dawn intensity enhancements are observed.

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.

Relevant aspects of equatorial plasma bubbles under different solar activity conditions

Abstract Observations of the OI 630 nm nightglow emission using a wide-angle imaging system have been carried out at Cachoeira Paulista (22.7° S, 45° W, 15.8° S dip latitude), Brazil during the period 1987 to 1999. The OI 630 nm images obtained during this period show frequently the optical signature of the plasma bubble (quasi north-south aligned depleted intensity regions). During the period studied a strong seasonal variation was noticed in the plasma bubble formations. Also, it was observed that, during high solar activity, the plasma bubble bifurcation occurrences were higher than during low solar activity. Important features from this set of observations 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.

Occurrence of large scale equatorial F-region plasma depletions during geo-magnetic disturbances

Abstract During the period March 1987 to October 1991, a wide-angle imaging system to observe the OI 630 nm nightglow emission was operational at Cachoeira Paulista (22.7°S, 45.0°W; dip latitude 15.8°S), Brazil. The OI 630 nm wide-angle imaging observations detect optical signatures of large scale equatorial F-region ionospheric plasma depletions or bubbles and the large data-base (about 4.5 years) permitted studies of their occurrence characteristics in the Brazilian sector. It has been observed that between the months of May and August the occurrence of large scale F-region plasma bubbles is at its minimum. However, during this period on several occasions at times of magnetic disturbances, the presence of large scale plasma bubbles was noted. In this paper we present and discuss several cases of the generation (or absence of generation) of plasma depleted regions during these months with magnetic disturbances. The imaging observations are complemented with ionospheric parameters obtained at Fortaleza (3.9°S, 38.4°W; dip latitude 3.7°S), Brazil. The possible influence of magnetic disturbance effects on equatorial ionospheric fields during the events studied is analysed and presented. It has been observed that on no plasma bubble nights with magnetic disturbances, possibly the storm induced high latitude electric field could not penetrate to the equatorial region due to the shielding charges in the inner magnetosphere, whereas on the nights with plasma bubbles, disturbance drifts result from the prompt penetration of high latitude electric fields.