Satellite Observations of Evolving Properties of Equatorial Ionospheric Plasma Structures
Smith, Jonathon M
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The Earth’s atmosphere contains a layer of plasma created primarily by photo-ionization of neutral species between 80 km and 1,000 km called the ionosphere. There are many complex processes near the equator where the Earth’s magnetic field is approximately horizontal. In this region large scale irregularities, sometimes referred to as equatorial plasma bubbles (EPBs), in the plasma density occur. Data recorded by the Ion Velocity Meter (IVM) as part of the Coupled Ion Neutral Dynamics Investigation (CINDI) aboard the Communication/Navigation Outage Forecasting System (C/NOFS) satellite are used to study EPBs during nightside local times at altitudes from 350 to 850 km. The data are taken during the seven years from 2008 to 2014, more than one half of a magnetic solar cycle, that include a deep solar minimum and a moderate solar maximum. EPB data are divided into four longitude sectors and two local time regions to determine seasonal and solar cycle variability. During solar minimum EPBs occur late in local time, primarily after midnight in all longitude sectors. Conversely, by the solar moderate conditions of 2014, 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 during all levels of solar activity. This suggests that seeding conditions are independent of solar activity. 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. Each EPB has a depth measured as the percent change between the background and minimum density (∆N/N). During solar moderate activity bubbles observed in the topside postsunset sector are more likely to have large depths compared to those observed in the topside postmidnight sector. Large bubble depths can be observed near 350 km in the bottomside F region in the postsunset period. Conversely at solar minimum the distribution of depths is similar in the postsunset and postmidnight sectors in all longitude sectors. Deep bubbles are rarely observed in the topside postsunset sector and never in the bottomside above 400 km in altitude. We suggest that these features result from the vertical drift of the plasma for these two solar activity levels. These drift conditions affect both the background density in which bubbles are embedded and the growth rate of perturbations in the bottomside where bubbles originate.