The characteristics and generation mechanism of small‐amplitude and large‐amplitude ESF irregularities observed by the C/NOFS satellite

Abstract

AbstractIt is well understood that equatorial plasma bubbles are initiated in the bottomside F region and grow into the topside F region, producing equatorial spread F (ESF) irregularities in both bottomside and topside F regions. However, it is not known whether small‐amplitude and large‐amplitude irregularities have the same occurrence pattern and how the generation of small‐amplitude and large‐amplitude irregularities is related to the postsunset vertical plasma drift and potential seeding processes. In this study, we use ion density and velocity data measured by the Communications/Navigation Outage Forecasting System satellite during May 2008 to July 2014 to derive the longitude‐month distributions of the occurrence probability of ESF irregularities at different amplitudes. It is found that the occurrence probability of large‐amplitude irregularities is well correlated with the postsunset vertical plasma drift and that small‐amplitude irregularities do not show a clear pattern at low solar activity but is anticorrelated with that of large‐amplitude irregularities at moderate solar activity. That is, the months and longitudes with high occurrence probability of large‐amplitude irregularities are exactly those with low occurrence probability of small‐amplitude irregularities, and vice versa. ESF irregularities are mostly limited below 500 km during low solar activity in 2008–2010, but large‐amplitude irregularities can reach 700–800 km in altitude during moderate solar activity in 2011–2014. The generation of large‐amplitude ESF irregularities is controlled by the postsunset vertical plasma drift at both low and moderate solar activities. In contrast, small‐amplitude ESF irregularities at moderate solar activity occur more frequently in the areas where the postsunset vertical plasma drift is small.