Equatorial Spread-F And Plasma Bubbles: A Step Towards Prediction

Equatorial spread F (ESF) and plasma bubbles are plasma irregularity phenomena that are known to occur in the<br>equatorial- and low-latitude ionospheric F region at evening and nighttime. Those irregularities cause<br>scintillation that disrupts trans ionospheric radio propagation, up to the GHz frequency range interfering on<br>communication systems including the Global Positioning System - GPS. Although the general characteristics of<br>spread F and bubbles, concerning their variation with season, solar cycle, and magnetic activity, are fairly well<br>understood, we do not know much about the cause of the day-to-day variability, a crucial point when prediction<br>is concerned. In this paper, we analyze ionospheric data from two low latitude stations in Brazil, in either<br>presence or absence of bubbles, in order to try to establish the conditions under which bubbles shall occur. We<br>conclude that the F layer critical frequency (foF2) in the afternoon-sunset hours (or equivalently the F layer<br>maximum density, NmF2) is a useful parameter to predict the occurrence of the bubble.