Optical observations of plasma bubble westward drifts over Brazilian tropical region

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Abstract

Previous studies based on experimental observations of plasma bubbles occurrences over the Brazilian region revealed that they often propagate eastward in the postsunset sector. Optical observations of the OI630 nm airglow made possible to carry out the present study based on the observed signatures on the OI630 nm airglow. From September 2000 to April 2007 at São João do Cariri (7.4S; 36.5W) 16 events were found to propagate westward presenting a defined seasonality, six of them occurring during June–July–August, seven of them during September–October–November, the others three occurring during March–April–May, and none during December–January–February. Ten observed cases occurred under quiet or weak magnetic periods showing that the intense magnetic activity is not the only cause of the westward direction propagation of the plasma bubbles.

Introduction

Large-scale ionospheric plasma depletions or bubbles are typical phenomena of the tropical F-region. The occurrence of plasma bubbles is related to the prereversal enhancement of the ionosphere and its generation mechanism is attributed to the Rayleigh–Taylor (RT) instability, but the starting process is not yet completely established (e.g., Batista et al., 1996, Bittencourt et al., 1997, Pimenta et al., 2001, Pimenta et al., 2003, Fejer et al., 2005, Chu et al., 2005, Gentile et al., 2006).

The observations of the large-scale ionospheric irregularities started in the 1960 decade using VHF radars. In those observations it was verified that those irregularities reached a width of 400km in the east–west direction and had zonal velocities of 100m/s (Clemesha, 1964). In the next decade, extended observations were carried out through the coherent scattering radar by Woodman (1970), revealing that the plasma vertical velocity is positive (upward) during daytime and negative (downward) during nighttime.

Studies of plasma bubbles morphology and dynamics have been done using optical techniques of nighttime airglow observation. The first optical irregularity observation was reported by VanZandt and Peterson (1968). Later, Weber et al. (1978) observed that bubbles and geomagnetic lines were quasi-aligned. In Brazil, the first optical observation was done by Sobral et al., 1980a, Sobral et al., 1980b, they verified that plasma bubble presence in airglow emissions vary the emission intensity and they characterized these bubbles as regions of abrupt decrease of plasma density. Taylor et al. (1997) were the first to use a CCD camera to determine bubbles drifts during the Guará campaign.

From then on many studies were done using optical technique to evaluate the airglow intensity in the presence of the plasma bubble (e.g., Fagundes et al., 1995b), to evaluate the thermospheric dynamics with the bubbles formations (e.g., Fagundes et al., 1995a), to study the seasonality of plasma bubbles occurrence (e.g., Sahai et al., 1998, Sahai et al., 1999, Sahai et al., 2000, Pimenta et al., 2001, Sobral et al., 2002, Paulino et al., 2007), and to study the plasma bubbles dynamics (e.g., Mendillo et al., 2001, Santana et al., 2001, Otsuka et al., 2002, Martinis et al., 2003, Pimenta et al., 2003, Abalde et al., 2004, Arruda et al., 2006, Makela et al., 2006, Sobral et al., 2009).

Most often the direction of propagation plasma bubbles over Brazil is eastward; however, during almost seven years of airglow observations (September 2000 to April 2007) at São João do Cariri (7.4S; 36.5W) 16 cases have been registered in which bubbles assumed the westward propagation direction. This work presents a statistical study of the frequency of occurrence of westward traveling bubbles with season, local time and geomagnetic activity. Abdu et al. (2003a) studied a case observed at Cachoeira Paulista (22.6S; 45W) with all sky imager and Pimenta (2002) also reported two cases at this site. These previous observations were related with intense magnetic activity, hence, and so it has been thought that occurrence of westward plasma drift was associated with intense magnetic storm. However, among the cases that will be presented here, 10 of them are related with weak magnetic storms and just one case is related with the intense magnetic storm.

Section snippets

Observations and instrumentation

The OI630 nm emission is produced in bottomside ionospheric F region between 250 and 300 km height. The responsible mechanism is the dissociative recombination process, which can be summarized inO2+eO*+O*(D1),with spontaneous photoemission by the excited Oxygen O* (1D), orO*(D1)O+hυ(630nm),as the emitted photon is in red wavelength range this emission is generally called red line of atomic oxygen and it represents one of the main tracer of ionospheric studies.

The airglow observation was carried

Results

Most frequently the plasma bubbles direction of propagation observed over São João do Cariri is eastward. We observed 334 nights with plasma bubble during almost seven years (September, 2000 to April, 2007), just 16 nights (less than five percent) of westward traveling bubbles were observed and are reported here. Fig. 2 shows an observed example at São João do Cariri on November 8th, 2004. From the beginning of the observations, before 22:00 Universal Time (UT) until near 01:00 UT the bubbles

Discussions

Each case that the bubbles velocity is westward should be analyzed separately, being considered, not just the seasonal effects, but the magnetic conditions and the F layer physical conditions. However, it can be seen clearly in Fig. 4 which exists a tendency for that phenomenon to occur during JJA and SON periods. Using a mass accelerometer, Liu et al. (2006) found a seasonal variation for the zonal thermospheric wind. Then, assuming the hypothesis that the F region dynamo controls the F region

Conclusions

From September 2000 to April 2007, 16 cases of ionospheric plasma bubbles inversion of direction of propagation from eastward to westward were observed by means of OI630 nm images.

Those events showed a very defined monthly distribution, six of them occurring during JJA and 10 cases in SON and MAM. The zonal motion of the plasma during nighttime after the E-region sunset at conjugate points is controlled by the neutral winds. As in the JJA period the zonal thermospheric mean wind velocity is less

Acknowledgements

The Cariri imager was financed by CNPq/PRONEX grant No. 76.97.1079.00, and the USU camera and operations were supported by NSF grant No. ATM-9525815. This work has also been supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP). We appreciate the effort by the science team WDC for Geomagnetism (Kyoto Dst index service) for making the data available.

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