Abstract
Abstract. From September to November 2005, the NASA Living with a Star program supported the Spread-F Experiment campaign (SpreadFEx) in Brazil to study the effects of convectively generated gravity waves on the ionosphere and their role in seeding Rayleigh-Taylor instabilities, and associated equatorial plasma bubbles. Several US and Brazilian institutes deployed a broad range of instruments (all-sky imagers, digisondes, photometers, meteor/VHF radars, GPS receivers) covering a large area of Brazil. The campaign was divided in two observational phases centered on the September and October new moon periods. During these periods, an Utah State University (USU) all-sky CCD imager operated at São João d'Aliança (14.8° S, 47.6° W), near Brasilia, and a Brazilian all-sky CCD imager located at Cariri (7.4° S, 36° W), observed simultaneously the evolution of the ionospheric bubbles in the OI (630 nm) emission and the mesospheric gravity wave field. The two sites had approximately the same magnetic latitude (9–10° S) but were separated in longitude by ~1500 km. Plasma bubbles were observed on every clear night (17 from Brasilia and 19 from Cariri, with 8 coincident nights). These joint datasets provided important information for characterizing the ionospheric depletions during the campaign and to perform a novel longitudinal investigation of their variability. Measurements of the drift velocities at both sites are in good agreement with previous studies, however, the overlapping fields of view revealed significant differences in the occurrence and structure of the plasma bubbles, providing new evidence for localized generation. This paper summarizes the observed bubble characteristics important for related investigations of their seeding mechanisms associated with gravity wave activity.
Highlights
Since their first observation by ionosonde in the 1920s (Booker and Wells, 1938), equatorial spread-F (ESF) irregularities have been studied intensively because of their importance in understanding thermosphere-ionosphere coupling and their detrimental influence on radio communications
The SpreadFEx campaign was conducted during the equinoctial transition period which is the time of the year when the solar terminator is aligned with the magnetic meridian; the magnetic eastward wind tends to a maximum value as well as the longitudinal gradient in the integrated conductivity
As part of the SpreadFEx campaign, optical observations of the ionospheric OI (630 nm) emission layer were conducted from two sites (Brasilia and Cariri) with similar magnetic latitudes and overlapping fields of view, in order to simultaneously study the generation, structure and evolution of the plasma depletions in close proximity of the magnetic equator
Summary
Since their first observation by ionosonde in the 1920s (Booker and Wells, 1938), equatorial spread-F (ESF) irregularities have been studied intensively because of their importance in understanding thermosphere-ionosphere coupling and their detrimental influence on radio communications. The main goal of the SpreadFEx campaign was to investigate the properties of gravity waves at ionospheric heights and their potential role in seeding Rayleigh-Taylor instabilities, strong equatorial spread-F and plasma bubble development. It is believed that ESF development depends on three main factors: (1) the linear growth rate for generalized RayleighTaylor instability process (Dungey, 1956), (2) the flux tube integrated Pedersen conductivity that controls the nonlinear development, and (3) density perturbations that are needed to act as a seed to trigger the instability growth These density perturbations may be produced by different seeding sources: local variations in the vertical winds, electric fields, and gravity waves propagating upward from the troposphere
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
