Aurora in the Polar Cap: A Review

Abstract

This paper reviews our current understanding of auroral features that appear poleward of the main auroral oval within the polar cap, especially those that are known as Sunaligned arcs, transpolar arcs, or theta auroras. They tend to appear predominantly during periods of quiet geomagnetic activity or northwards directed interplanetary magnetic field (IMF). We also introduce polar rain aurora which has been considered as a phenomenon on open field lines. We describe the morphology of such auroras, their development and Auroral Physics Edited by 15 Page 2 of 44 K. Hosokawa et al.dynamics in response to solar wind-magnetosphere coupling processes, and the models that have been developed to explain them.

A New Type of Polar Cap Arc Observed in the ~1500 MLT Sector: 1. Northern Hemisphere Observations

“…The auroral arcs observed in the polar cap have been given different names, such as transpolar arc (Fear & Milan, 2012), sun‐aligned arc (Berkey et al, 1976), and theta aurora (Frank et al, 1982), which are generally called polar cap arcs (PCAs) in this paper. In the past half‐century, PCA studies have made tremendous progress in various aspects (Hosokawa et al, 2020; Kullen, 2013; Milan et al, 2005; Xing et al, 2018; Zhu et al, 1997). Based on comprehensive studies, PCA classification has been carried out in different ways (Gussenhoven, 1982; Meng , 1981).…”

Multiple transpolar auroral arcs reveal insight about coupling processes in the Earth’s magnetotail

“…aurora | solar-terrestrial interaction | magnetosphere | polar ionosphere | transpolar auroral arcs T ranspolar arcs (TPAs) are the large-scale auroral forms that stretch almost entirely across the polar cap and connect the dayside and nightside auroral oval (1)(2)(3)(4)(5). TPAs can appear as a single or multiple arcs (6)(7)(8).…”

“…Transpolar arcs (TPAs) are the large-scale auroral forms that stretch almost entirely across the polar cap and connect the dayside and nightside auroral oval ( 1 – 5 ). TPAs can appear as a single or multiple arcs ( 6 – 8 ).…”

“…TPAs can appear as a single or multiple arcs ( 6 – 8 ). Previous theories and observations indicated that TPAs are related to the magnetospheric cusp as their dayside ends join together in the cusp region (called “cusp-aligned” auroral arcs) ( 9 , 10 ), and are generated by the solar wind–magnetosphere–ionosphere coupling processes as a whole system during northward interplanetary magnetic field (IMF) and quiet geomagnetic conditions ( 2 – 5 ). TPAs are sometimes seen simultaneously in both polar caps ( 11 ) and have been inferred to be on both open and closed magnetic field lines ( 12 ).…”

“…TPA formation processes have long been debated, in particular whether they are driven by dayside or nightside processes ( 5 ). The main candidates for dayside processes have been dayside reconnection at the low-latitude magnetopause or high-latitude lobe regions ( 13 – 17 ) and plasma instabilities (e.g., Kelvin–Helmholtz instability, interchange instability) near the magnetospheric low and high-latitude boundary layers ( 8 , 18 – 20 ).…”