The $F$ -Region Gravity and Pressure Gradient Current Systems: A Review.

The ionospheric gravity and pressure-gradient current systems are most prominent in the low-latitude $F$ -region due to the plasma density enhancement known as the equatorial ionization anomaly (EIA). This enhancement of plasma density which builds up during the day and lasts well into the evening supports a toroidal gravity current which flows eastward around the Earth in the $F$ -region during the daytime and evening, and eventually returns westward through the $E$ -region. The existence of pressure-gradients in the EIA region also gives rise to a poloidal diamagnetic current system, whose flow direction acts to reduce the ambient geomagnetic field inside the plasma. The gravity and pressure-gradient currents are among the weaker ionospheric sources, with current densities of a few $\mbox{nA/m}^{2}$ , however they produce clear signatures of about 5-7 nT in magnetic measurements made by low-Earth orbiting satellites. In this work, we review relevant observational and modeling studies of these two current systems and present new results from a 3D ionospheric electrodynamics model which allows us to visualize the entire flow pattern of these currents throughout the ionosphere as well as calculate their magnetic perturbations. [ABSTRACT FROM AUTHOR]