Ionospheric irregularities exist in the low latitude ionosphere as large changes in the spatial distribution of the total ion density.
Seen here are signatures of the total ion concentration observed from an equartorial orbit. The scales show the concentration versus solar local time with the longitude regions covered shown above the traces.
Structure in the plasma density appears quite differently at different locations.
Sometimes it is embedded in a large scale reduction in the density. Other times it is embedded in rather smooth and large background densities.
The structure itself may take the form of one or a few large scale depletions or it may appear as many small depletions.
We do not know what controls the different appearance of structure or the relationships between these different structures and radio scintillation.
You can learn more about these structures and how the CINDI mission will address some of the key questions by checking out this slide show