Structured Storm-Time Polar Ionosphere and Its Drivers: A Review

This chapter provides a review of the most recent progress in understanding polar ionosphere (including both high- and subauroral latitudes in this chapter) variability and its drivers. It also aims to point out future directions for this topic. We first give a general introduction of polar physical processes of the coupled thermosphere and ionosphere system and the prominent polar ionosphere structures in the introduction. The morphology, characteristics, formation mechanisms, and numerical simulations of the midlatitude ionospheric F 2 region electron density trough and subauroral polarization streams are then reviewed. In the next section, we discuss the storm-time behavior of ionospheric electron density structures, such as storm-enhanced density (SED), the tongue of ionization (TOI), and polar cap patches. We further point out the different theories proposed for the generation mechanisms of SED and its behavior. Plasma irregularities at the edge of these polar ionosphere density structures are then summarized. In the last part, we discuss the possible future efforts, including both first-principles numerical investigations and observational studies that need to be done to better our understanding of the behavior and its variability of the coupled magnetosphere-ionosphere-thermosphere system at high latitudes.