In the nighttime low latitude ionosphere, plasma depletions occur often and a climatological picture has been developed. After sunset, the conditions for instabilities are favorable due to a large vertical plasma density gradient, the nearly horizontal magnetic field with electric fields and gravity acting perpendicular to it. However, the mechanism generating equatorial plasma depletion is not fully understood and it is unclear why they tend to occur in clusters or quasi periodic wave trains.
Rodríguez‐Zuluaga et al.  use GOLD observations between October 2018 and June 2020 to examine the characteristics of wave like features in the latitudinal extends of the off-equatorial plasma enhancements at nighttime. Interestingly, they find that the identified wave-like features in the 95 nights appear to be stationary and the maximum latitudinal extend of plasma enhancement is clustered around 20o and 60o quasi-dipole longitude (53 o W and 14 o W, respectively).
These observations indicate a preferred zonal wavelength of 35o and 65o, which now needs to be explained. The authors did not find any correlation to geomagnetic activity, but a large day-to-day variability, indicating that the coupling to the lower atmosphere might play an important role. In addition, more than 80% of the observed wave-like features in the nighttime ionosphere are accompanied by plasma depletions, an important space weather phenomenon. This highlights the importance of understanding the origin of these wave-like features in the equatorial nighttime ionosphere.
Citation: Rodríguez‐Zuluaga, J., Stolle, C., Yamazaki, Y., Xiong, C., & England, S. L. . A synoptic‐scale wavelike structure in the nighttime equatorial ionization anomaly. Earth and Space Science, 8, e2020EA001529. https://doi.org/10.1029/2020EA001529
—Astrid Maute, Editor, Earth and Space Science