Three-dimensional graphic of the total electron content (TEC) showing the impact of equatorial spread F bubbles, the large meridional structures in the equatorial region
Three-dimensional graphic of the total electron content (TEC) showing the impact of equatorial spread F bubbles, the large meridional structures in the equatorial region

The Naval Research Laboratory has developed a comprehensive three-dimensional ionosphere model SAMI3 to study the evolution of equatorial spread F (ESF). The model follows the dynamical and chemical evolution of seven ion species (H+ , He+ , N+, O+ , N+2 , NO+, and O+2). The complete ion temperature equation is solved for three ion species (H+ , He+ and O+) as well as the electron temperature equation. The SAMI3 code has been modified to capture the onset and evolution of small-scale equatorial bubbles (L ~ 10 km) within the framework of a self-consistent global model (L ~ 1000s km) which is unprecedented.

Background: The earth's ionosphere is a partially ionized gas that surrounds the earth in the altitude range 90 - 1000's km. Electromagnetic waves propagating through the ionosphere can suffer a number of adverse effects such as phase and amplitude fluctuations, absorption, scattering, frequency shifts, etc. The degradation of electromagnetic radiation can adversely impact communication and navigation systems. The equatorial ionosphere can become extremely disturbed after sunset because of a phenomenon known as equatorial spread F (ESF). During ESF the equatorial ionosphere becomes unstable and large-scale 10's km electron density depletions (or 'bubbles') develop and rise to high altitudes (± 1000 km at times) which disrupt communication and navigation signals. Thus, it is important to understand and characterize ESF to improve operational systems.

Accomplishment: SAMI3 is a comprehensive ionosphere model that has been modified to self-consistently solve for the global neutral wind-driven dynamo electric field as well as the gravity-driven electric field associated with plasma bubbles. The latter is achieved with a high resolution longitudinal grid in the pre- to post-sunset sector (i.e., 1630 MLT - 2230 MLT). ). It is shown that ESF can be triggered by pre-sunset ionospheric density perturbations, and that an existing ESF bubble can trigger a new bubble. The impact on the total electron content (TEC) is shown in Figure 1. The capability of self-consistently modeling small-scale plasma bubbles (L ± 10 km) within the context of a global ionosphere model (L ~1000s km) is unprecedented; this is a major achievement for NRL and solidifies NRL's position as the world leader in modeling ionospheric irregularities. This research was published in Geo-physical Research Letters [Huba, J.D. and G. Joyce, Global modeling of equatorial plasma bubbles, Geophys. Res. Lett. 37, L17104, doi:10.1029/2010GL044281, 2010] and received an NRL research publication award. The paper was featured as an “AGU Journal Highlight” in EOS, Transactions, American Geophysical Union, Vol. 91 No. 44 2010. EOS highlights only 4 – 5 articles in each publication, selecting those articles that are judged to be of high interest and importance to the geophysical science community. The paper was also selected as an “Editor’s Choice” in the Space Weather Quarterly (Vol. 7, Issue 4, 2010) as an important contribution to understanding and predicting equatorial irregularities.

Significance The development of a self-consistent 3D model of ESF is a major step forward in our ability to both characterize and eventually predict the onset and evolution of equatorial spread F.

Application: The long term application of this effort is to develop models for the National Space Weather Program.