Abstract
AbstractThis paper presents observations of polar cap auroral features on 19 January 2008, evaluated using multiple instruments with near‐simultaneous observations in both hemispheres. Analysis of the features indicates that there are at least two formation mechanisms/types of polar cap aurora occurring simultaneously on different magnetic field topologies (one on open and the other on closed magnetospheric field lines). Two high‐latitude structures were observed on opposing sides of the northern hemisphere polar cap in the same time interval. The structure on the duskside was formed on closed field lines that protruded into the polar cap and was generated by the precipitation of electrons with energies varying between 2 and 11 keV consistent with an identified mechanism for the formation of transpolar arcs. However, the structure did not extend fully across to the dayside of the auroral oval but rather stayed at ∼80° magnetic latitude for a minimum duration of 40 min. Thus, this structure is an example of a “failed” transpolar arc. The structure on the dawnside of the polar cap was associated with low‐energy electron precipitation (less than 1 keV) and no associated ion signatures, which is consistent with it being a common low‐intensity arc formed by accelerated polar rain on open field lines. The two separate types of polar cap auroras formed during the same interval, demonstrating the complexity of the solar wind‐magnetosphere coupling during the interval.
Highlights
Polar cap auroras occur at high latitudes above the main auroral oval and are sometimes known as highlatitude or Sun-aligned arcs
The aim of this study is to combine ground-based and satellite observations of polar cap auroras to investigate the suggestion that some polar cap arcs form on open field lines, while others form on closed field lines and to investigate whether it is possible for both configurations to form at the same time
Almost simultaneous observations of the northern and southern auroral regions were provided by the Sensor Ultraviolet Spectrographic Imager (SSUSI) instruments on board DMSP F16 and F17
Summary
Polar cap auroras occur at high latitudes above the main auroral oval and are sometimes known as highlatitude or Sun-aligned arcs These arcs have been studied for a century [Mawson, 1916] and are correlated with periods of quiet magnetospheric activity [Davis, 1963] and a northward directed interplanetary magnetic field (IMF) [Berkey et al, 1976; Gussenhoven, 1982]. Their location in the polar cap and their motion are dependent on the IMF By component [Frank et al, 1985; Huang et al, 1989; Craven et al, 1991; Valladares et al, 1994; Kullen et al, 2002]; there are exceptions to this dependence [e.g., Hosokawa et al, 2011]. These configurations, along with multiple mechanisms that have been advocated for them are discussed below
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