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4 Recent Advances in Polar Cap Density Structure Research
Shasha Zou1, Gareth W. Perry2,3, and John C. Foster4
1 Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, USA
2 Department of Physics and Astronomy, University of Calgary, Calgary, Canada
3 Center for Solar‐Terrestrial Research, New Jersey Institute of Technology, Newark, NJ, USA
4 Massachusetts Institute of Technology Haystack Observatory, Westford, MA, USA
ABSTRACT
In the polar cap region, high‐density ionospheric structures, such as the large‐scale tongue‐of‐ionization and mesoscale polar cap patches, are frequently observed, which are of significant space weather concerns. In this chapter, recent progress on these polar cap high‐density ionospheric structures is reviewed with an emphasis on the polar cap patch. Topics include the statistical occurrence rate of patches, plasma structure and characteristics inside patches, dynamic horizontal and vertical transport of high‐density ionospheric structures, as well as the mechanisms and variability of optical patch emissions. For each topic, future works are envisioned following the progress reviews.
4.1 INTRODUCTION TO POLAR CAP DENSITY STRUCTURES
Ionospheric density varies significantly and affects the propagation of radio signals that pass through the ionosphere or are reflected by it. One example of these effects is the loss of phase lock and range errors in Global Navigation Satellite Systems (GNSS) signals. Because our modern society increasingly relies on ground‐to‐ground and ground‐to‐space communications and navigation, understanding the sources of the ionospheric density variability and monitoring its dynamics during space weather events is of great practical importance (e.g., Jin et al., 2014, 2017; Moen et al., 2013; Zhang et al., 2018).
Due to the geometry of the Earth's magnetic field, the solar wind has direct access to the high‐latitude polar cap region because the magnetic field lines there are open. The convection electric fields in the solar wind map directly down to the polar cap along the equipotential magnetic field lines (Lyons & Williams, 1984). The shape and size of the polar cap change considerably with varying solar wind and the polarity and magnitude of interplanetary magnetic field (IMF). Typically, the open‐closed field line boundary (OCB) of the polar cap expands to lower latitudes during southward IMF condition and retreats back to higher latitudes during northward IMF condition. Particles in the solar wind and magnetosheath can move along the magnetic field line down to the polar cap ionosphere and form the polar rain (Fairfield &