The statistical difference between bending arcs and regular polar arcs. Issue 12 (18th December 2015)

• Record Type: Journal Article
• Authors: Kullen, A.; Fear, R. C.; Milan, S. E.; Carter, J. A.; Karlsson, T.
• Is Part Of: Journal of geophysical research. Volume 120:Issue 12(2015:Dec.)
• Page Start: 10, 443
• Page End: 10, 465
• Publication Date: 2015-12-18
• Subjects: transpolar arc -- bending arc -- polar arc -- IMF -- Polar UV images -- SuperDARN; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1002/2015JA021298
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: In this work, the Polar UVI data set by Kullen et al. (2002) of 74 polar arcs is reinvestigated, focusing on bending arcs. Bending arcs are typically faint and form (depending on interplanetary magnetic field (IMF) By direction) on the dawnside or duskside oval with the tip of the arc splitting off the dayside oval. The tip subsequently moves into the polar cap in the antisunward direction, while the arc's nightside end remains attached to the oval, eventually becoming hook‐shaped. Our investigation shows that bending arcs appear on the opposite oval side from and farther sunward than most regular polar arcs. They form during By ‐dominated IMF conditions: typically, the IMF clock angle increases from 60 to 90° about 20 min before the arc forms. Antisunward plasma flows from the oval into the polar cap just poleward of bending arcs are seen in Super Dual Auroral Radar Network data, indicating dayside reconnection. For regular polar arcs, recently reported characteristics are confirmed in contrast to bending arcs. This includes plasma flows along the nightside oval that originate close to the initial arc location and a significant delay in the correlation between IMF By and initial arc location. In our data set, the highest correlations are found with IMF By appearing at least 1–2 h before arc formation. In summary, bending arcs are distinctly different from regular arcs and cannot be explained by existing polar arc models. Instead, these results are consistent with the formation mechanism described in Carter et al. (2015), suggesting that bending arcs are caused by dayside reconnection. Key Points: Bending arcs are polar arcs that split from the prenoon or postnoon oval and bend into the polar cap Bending arcs form during dayside reconnection The initial location of regular polar arcs is determined by IMF By conditions at least 1–2 h earlier