Equatorial plasma bubbles: Variations of occurrence and spatial scale in local time, longitude, season, and solar activity

Abstract

We examine the location and structure of equatorial topside depletions referred to here as equatorial plasma bubbles (EPBs). The Coupled Ion Neutral Dynamics Investigation Ion Velocity Meter instrument on board the Communication/Navigation Outage Forecasting System is used to measure the ion density from August 2008 to December 2014, a time period in which solar activity transitioned from solar minimum conditions to more moderate solar activity conditions. EPB data are divided into four longitude sectors to determine seasonal and solar cycle variability. In the early phase of the mission, during solar minimum, EPBs occur late in local time, primarily after midnight in all longitude sectors. In the later phase of the mission EPB occurrence after midnight diminishes in all seasons and longitude sectors with the exception of the sector extending from −15° to 60°. An examination of the widths of bubbles indicates that all longitudes show similar distributions between 115 km and 460 km with a prominent peak near 200 km. Among these widths is a distinct width that belongs to discrete individual bubbles with no substructure. We suggest that many bubbles are actually combinations of these individual bubbles; however, in the later phase of the mission there is a population of bubbles that do not conform to this description, perhaps due to the influence of large-scale plasma motions affecting the background density in which they are embedded.