Conjugate Point Equatorial Experiment (COPEX) campaign in Brazil: Electrodynamics highlights on spread F development conditions and day‐to‐day variability

Abstract

[1] A Conjugate Point Equatorial Experiment (COPEX) campaign was conducted during the October-December 2002 period in Brazil, with the objective to investigate the equatorial spread F/plasma bubble irregularity (ESF) development conditions in terms of the electrodynamical state of the ionosphere along the magnetic flux tubes in which they occur. A network of instruments, including Digisondes, optical imagers, and GPS receivers, was deployed at magnetic conjugate and dip equatorial locations in a geometry that permitted field line mapping of the conjugate E layers to dip equatorial F layer bottomside. We analyze in this paper the extensive Digisonde data from the COPEX stations, complemented by limited all-sky imager conjugate point observations. The Sheffield University Plasmasphere-Ionosphere Model (SUPIM) is used to assess the transequatorial winds (TEW) as inferred from the observed difference of h m F 2 at the conjugate sites. New results and evidence on the ESF development conditions and the related ambient electrodynamic processes from this study can be highlighted as follows: (1) large-scale bottomside wave structures/satellite traces at the equator followed by their simultaneous appearance at conjugate sites are shown to be indicative of the ESF instability initiation; (2) the evening prereversal electric field enhancement (PRE)/ vertical drift presents systematic control on the time delay in SF onset at off-equatorial sites indicative of the vertical bubble growth, under weak transequatorial wind; (3) the PRE presents a large latitude/height gradient in the Brazilian sector; (4) conjugate point symmetry/asymmetry of large-scale plasma depletions versus smaller-scale structures is revealed; and (5) while transequatorial winds seem to suppress ESF development in a case study, the medium-term trend in the ESF seems to be controlled more by the variation in the PRE than in the TEW during the COPEX period. Competing influences of the evening vertical plasma drift in favoring the ESF development and that of the TEW in suppressing its growth are discussed, presenting a perspective on the ESF day-to-day and medium-term variabilities.

Climatology of the scintillation onset over southern Brazil

“…The role of the PRVD on the onset of RTI leading to the formation of EPB structures and, consequently, to the presence of the irregularities producing scintillation is already known either in the form of preconditioning (e.g., Abdu, 2001;Abdu et al, 2009) or in the form of seeder (Woodman, 1994;Sousasantos et al, 2013). The PRVD is responsible for altering some critical parameters of the equatorial ionospheric plasma, such as the collision frequency between ions and neutrals and the density gradient.…”

“…The role of the PRVD in the development of irregularities has been widely discussed and the necessity of its existence as a requirement for subsequent EPB development is already well established. Abdu et al (2009), using measurements from a network of instruments during COPEX campaign (Conjugate Point Equatorial Experiment), argued that in the Brazilian longitudinal sector a threshold value around 22 m s −1 of prereversal vertical drift peak (V pk ) is needed for the development of the EPB structures. However, slightly smaller values of V pk may provide an ambient where spread F at the bottom side may occur, although it does not evolve into EPB structures.…”

Generation and characteristics of equatorial plasma bubbles detected by the C/NOFS satellite near the sunset terminator

“…Huang et al [2010a] presented the profile of the ion vertical drift with height measured by the Jicamarca incoherent scatter radar in a case and found that the variation of the ion drift was small over the altitude range of 200-700 km. On the other hand, Abdu et al [2009] showed that altitude variation of the zonal electric field in the postsunset equatorial ionosphere indeed occurred. In the cases presented in Figures 1 and 3, the altitude of C/NOFS varied in the range of 400-460 km between 17:00 and 23:00 LT. We assume that Figure 4 represents the variation of the ion drift with local time, rather than with altitude, because the change of altitude was not large.…”

“…A possible cause for this feature is the vertical gradient in the zonal electric field (the vertical prereversal enhancement). Abdu et al [2009] found that the zonal electric field in the equatorial ionosphere could vary with altitude and that the altitude variation could depend on longitude. During Orbit 19016 of Figure 1, C/NOFS was located at 13.8 magnetic latitude at 19:00 LT, and the altitude of C/NOFS was 432 km.…”

Longitudinal and seasonal structure of the ionospheric equatorial electric field

“…The best fit line nearly demonstrates a 1-to-1 correspondence, with a slope of 1.09 and intercept of À0.06 mV/m. Previous studies have shown the existence of altitude gradients in the zonal electric field [Abdu et al, 2009;Pingree and Fejer, 1987], which could explain some of the scatter in the figure, since the JULIA measurements are taken at 150 km, while we use C/NOFS data between 400 and 500 km altitude. But in general, this result indicates the C/NOFS IVM measurements are a good proxy for E-region electric fields.…”