The postsunset vertical plasma drift and its effects on the generation of equatorial plasma bubbles observed by the C/NOFS satellite

Abstract

The prereversal enhancement (PRE) of the vertical plasma drift in the postsunset sector is an important factor that controls the generation of equatorial plasma bubbles. In this study, we use the measurements of the ion velocity meter on board the Communication/Navigation Outage Forecasting System satellite during 2008-2014 to identify the PRE and its effects on the occurrence of plasma bubbles. The seasonal and longitudinal distributions of the PRE are derived at different solar flux levels. Large PRE occurs at 240-360 longitudes in equinoctial months and December solstice, and small or downward PRE occurs around ±60 in June solstice. The seasonal and longitudinal distributions of large-amplitude equatorial spread F (ESF) (ΔN > 5 × 1010 m-3) are similar to that of the PRE, while the occurrence probability of ESF including smaller-amplitude perturbations (ΔN > 1 × 1010 m-3) can be quite high at any longitude in any season. A quantitative relationship between the PRE and the ESF occurrence probability is derived and well characterized by the cumulative distribution function of a continuous probability distribution. Such a distribution implies that the occurrence of ESF is a probability event. The ESF occurrence probability is small when the PRE is zero or downward and becomes larger than 80% when the PRE is greater than 40 m s-1. Both the ESF occurrence probability and the amplitude increase with the solar radio flux. Key Points Global distributions of the PRE at low and moderate solar activities are derived New distribution of the relationship between the PRE and ESF is identified Dependence of ESF occurrence rate and amplitude on solar flux is determined