. During the Spread F Experiment campaign, under NASA Living with a Star (LWS) program, carried out in the South American Magnetic Equator region from 22 September to 8 November 2005, two airglow CCD imagers, located at Cariri (7.4° S, 36.5° W, geomag. 11° S) and near Brasilia (14.8° S, 47.6° W, geomag. 10° S) were operated simultaneously and measured the equatorial ionospheric bubbles and their time evolution by monitoring the airglow OI 6300 intensity depletions. Simultaneous observation of the mesospheric OH wave structures made it possible to investigate the relationship between the bubble formation in the ionosphere and the gravity wave activity at around 90 km. On the evening of 30 September 2005, comb-like OI 6300 depletions with a distance of ~130 km between the adjacent ones were observed. During the same period, a mesospheric gravity wave with a horizontal wavelength of ~130 km was observed. From the 17 nights of observation during the campaign period, there was a good correlation between the OI 6300 depletion distances and the gravity wave horizontal wavelengths in the mesosphere with a statistically significant level, suggesting a direct contribution of the mesospheric gravity wave to plasma bubble seeding in the equatorial ionosphere.

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Simultaneous observation of ionospheric plasma bubbles and mesospheric gravity waves during the SpreadFEx Campaign

Equatorial ionosphere–thermosphere system: Electrodynamics and irregularities

[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 hmF2 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.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2008JA013749

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

[1] We present initial steps toward unifying our understanding of storm time equatorial spread F (ESF) by searching for the common elements in past case studies and statistical occurrence patterns. We show that the development (or inhibition) of equatorial irregularities during magnetically active periods can be understood using the AE-parameterized Fejer-Scherliess model for disturbance vertical drifts versus storm time and local time. This model takes into account the different sources of perturbation electric fields (magnetospheric and ionospheric dynamos) that ultimately drive the equatorial vertical drifts, showing prompt and delayed effects in the premidnight sector (where both generation and suppression can occur), as well as in the postmidnight period where generation dominates. The postsunset period exhibits the greatest variability for storm time ESF versus longitude, and thus we demonstrate the Fejer-Scherliess model's applicability in a test case (6 April 2000) that had an AE pattern compatible with their parameterization scheme. The model successfully accounts for the pronounced longitude confinement in the observed postsunset ESF patterns. Finally, we move beyond the empirically derived relationships between geomagnetic indices and the occurrence of ESF (Aarons, 1991) into a framework of true solar-terrestrial parameters that drive such effects. Additional case studies taken from the published literature are then used to show a consistent linkage between postsunset ESF onset and the interplanetary electric field (IEF) Esw. While AE, Dst, Kp and Dst/dt indices were used in earlier studies to determine the dusk-longitude sector of disturbance electric fields, here we attribute to the IEF the main role in the determination of this longitude sector.

/pdf/toward-a-synthesis-of-equatorial-spread-f-onset-and-2ekkq1gjxc.pdf

Toward a synthesis of equatorial spread F onset and suppression during geomagnetic storms

[1] The technique of radio occultation (RO) is demonstrated to be a powerful tool for studying equatorial F-region irregularities (EFIs) associated with equatorial plasma bubbles. The extensive 4.9 year RO dataset of the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) satellites was employed in this study and contains EFI observations under a wide variety of solar and geomagnetic conditions. From an analysis of the EFI occurrence dependence on season/longitude, it is found that the EFI occurrence statistics largely match those reported previously, with the exception of an equinoctial EFI occurrence maximum in the American sector that is absent from previous studies. It is revealed that this maximum is due to enhanced EFI occurrence near the South Atlantic anomaly, where EFIs are expected to be suppressed by particle precipitation. An investigation into the solar activity dependence of the EFI occurrence characteristics revealed significant increases in the range of local times and latitudes with solar activity for most longitude sectors and seasons. Finally, the EFI suppression and enhancement effects of storm-time electric fields are also investigated using the COSMIC data.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/jgra.50089

M. G. de Aquino

Papers

Ionospheric plasma bubble climatology over Brazil based on 22 years (1977–1998) of airglow observations

Horizontal gradients of the nocturnal OI 557.7 nm and OI 630 nm photoemission rates in the equatorial ionosphere based on rocket electron density data