Conjugate Observations of the Evolution of Polar Cap Arcs in Both Hemispheres

Abstract

We report results from the analysis of a case of conjugate polar cap arcs (PCAs) observed on 5 February 2006 in the Northern Hemisphere by the ground‐based Yellow River Station all‐sky imager (Svalbard) and in both hemispheres by the space‐based DMSP/SSUSI and TIMED/GUVI instruments. The PCA's motion in dawn‐dusk direction shows a clear dependence on the interplanetary magnetic field (IMF) By component and presents a clear asymmetry between Southern and Northern Hemispheres, that is, formed on the duskside and moving from dusk to dawn in the Northern Hemisphere and vice versa in the other hemisphere. The already existing PCAs' motion is influenced by the changes in the IMF By with a time delay of ~70 min. We also observed strong flow shears/reversals around the PCAs in both hemispheres. The precipitating particles observed in the ionosphere associated with PCAs showed properties of boundary layer plasma. Based on these observations, we might reasonably expect that the topological changes in the magnetotail can produce a strip of closed field lines and local processes would set up conditions for the formation and evolution of PCAs.

Interhemispheric Survey of Polar Cap Aurora

“…Using the particle data from the DMSP SSJ/4 spectrometer they showed the arcs to be associated with ion precipitation, consistent with formation on closed field lines. Xing et al () presented a similar observation with a conjugate PCA associated with ion precipitation in the DMSP particle data that was hence also consistent with a closed field line mechanism. DMSP particle data were also published in association with a large‐scale PCA in Fear et al ().…”

“…Data from the particle spectrometer on board the DMSP spacecraft were obtained for events when the spacecraft track crossed the arc and were used to classify whether each event corresponded to open or closed field lines. If arcs are associated with an ion signature and seen in both hemispheres, then they are consistent with a closed field line formation mechanism (e.g., Carter et al, ; Xing et al, ). If the arcs are associated with an electron‐only signature, then they are consistent with an open field line mechanism (Newell et al, ; Reidy et al, ), whether they are seen in one or both hemispheres.…”

Aurora in the Polar Cap: A Review

“…1.3 and 5.2). Carter et al (2017) and Xing et al (2018) presented separate SSUSI observations of polar cap arcs occurring simultaneously in both hemispheres that were found to be consistent with formation on closed field lines. In the case of Carter et al (2017), they showed two transpolar arcs occurring in both hemispheres simultaneously.…”

“…used these data to suggest that there are three different formation mechanisms for polar cap arcs: two which occur on closed fields and one on open. Fear et al (2014), Carter et al (2017) and Xing et al (2018) presented DMSP particle observations, as well as observations from multiple ground-based and spacecraft instruments, of transpolar arcs occurring on closed field lines. The DMSP particle detector measured ion and electron precipitation associated with each of these arcs.…”

“…Another indication for closed field lines is the observed TPA conjugacy. There exist many reports that TPAs are a conjugate phenomenon appearing simultaneously in the northern and southern hemispheres on opposite oval sides and moving in opposite directions (e.g., Mizera et al 1987;Obara et al 1988;Huang et al 1989;Craven et al 1991;Cumnock et al 2006;Carter et al 2017;Xing et al 2018). However, Østgaard et al (2003) showed that during certain IMF B x conditions, a clear TPA appears only in one hemisphere (see Sect.…”