J. A. Bittencourt

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.

OI 630 nm imaging observations of equatorial plasma depletions at 16° S dip latitude

Abstract Equatorial ionospheric irregularities in the F-layer have been the subject of intensive experimental and theoretical investigations during recent years. The class or irregularities which continues to receive much attention is characterized by large scale plasma depletions, generally referred to as ionospheric plumes and bubbles. The OI 630.0 nm F-region night-glow emissions arising from recombination processes can be used to observe the dynamics of transequatorial ionospheric plasma bubbles and smaller scale plasma irregularities. In a collaborative project between the Center for Space Physics of Boston University and Brazils National Institute for Space Research (INPE), an all-sky imaging system was operated at Cachoeira Paulista (22.7° S, 45.0° W, dip latitude 15.8° S), between March 1987 and October 1991. In addition to the imager, photometer and VHP polarimeter observations were conducted at Cachoeira Paulista, with ionospheric soundings carried out at both C. Paulista and Fortaleza, the latter at 3.9° S, 38.4° W, 3.7° S dip latitude. For this longitude, the observed seasonal variation of the airglow depletions shows a maximum from October through March and a very low occurrence of airglow depletions from April through September. This long series of OI 630.0 nm imaging observations has permitted us to determine that when there are extended plumes, the altitudes affected over the magnetic equator often exceed 1500 km and probably exceed 2500 km at times, the maximum projection that can be seen from Cachoeira Paulista. This holds true even during years of low solar flux.

Observations of equatorialFregion plasma bubbles using simultaneous OI 777.4 nm and OI 630.0 nm imaging: New results

Simultaneous observations of the OI 630.0 nm and OI 777.4 nm nightglow emissions using all-sky imaging systems and ionospheric radio sounding using a Canadian Advanced Digital Ionosonde (CADI) digisonde have been recently carried out at Sao Jose dos Campos (23.21°S, 45.86°W), Brazil. The all-sky imaging systems use novel CCD devices, with high quantum efficiency and which provide an exceptional capacity for quantitative measurement of faint- and low-contrast emissions. On October 23–24, 2000 (high solar activity), the presence of large-scale F region plasma irregularities (plasma bubbles) was observed using both techniques (i.e., optical and radio). The high-resolution images, recorded using the OI 777.4 nm nightglow emission, show a new striated or ray-like pattern, which has not been detected before. These OI 777.4 nm optical observations show for the first time, in great detail, the field-aligned ionospheric plasma bubble structures, in contrast with the OI 630.0 nm images, which show a diffuse image of the bubbles. The optical signatures of the OI 777.4 nm emission are more closely related to the actual ionospheric bubble structure, owing to its prompt emission and dependence only on the electron density, with no F layer height dependence. On the other hand, the OI 630.0 nm emission comes from the bottomside of the F layer with a strong F layer height dependence and shows blurred images due to its 110-s lifetime. An additional advantage of using the OI 777.4 nm emission for ionospheric irregularity studies is that the plasma bubbles can be observed earlier on the OI 777.4 nm images than on the OI 630.0 nm images (by ∼ 15 min).

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.

On the IRI model predictions for the low-latitude ionosphere

Abstract Measurements of ionospheric electron density vertical profiles, carried out at a magnetic equatorial station located at Fortaleza (4°S, 38°W; dip latitude 2°S) in Brazil, are analyzed and compared with low-latitude electron density profiles predicted by the International Reference Ionosphere (IRI) model. The analysis performed here covers periods of high (1979/1980) and low (1986) solar activities, considering data obtained under magnetically quiet conditions representative of the summer, winter and equinox seasons. Some discrepancies are found to exist between the observed and the IRI model-predicted ionospheric electron density profiles. For high solar activity conditions the most remarkable one is the observed fast upward motion of the F-layer just after sunset, not considered in the IRI model and which precedes the occurrence of nighttime ionospheric plasma irregularities. These discrepancies are attributed mainly to dynamical effects associated with the low latitude E × B electromagnetic plasma drifts and the thermospheric neutral winds, which are not satisfactorily reproduced either in the CCIR numerical maps or in the IRI profile shapes. In particular, the pre-reversal enhancement in the vertical E × B plasma drifts around sunset hours has a great influence on the nighttime spatial distribution of the low-latitude ionospheric plasma. Also, the dynamical control exerted by the electromagnetic plasma drifts and by the thermospheric neutral winds on the low-latitude ionospheric plasma is strongly dependent on the magnetic declination angle at a given longitude. These important longitudinal and latitudinal dependences must be considered for improvement of IRI model predictions at low latitudes.

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.

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.