Abstract
AbstractIn 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.
Highlights
Auroral arcs appearing poleward of the main oval are often grouped into small-scale Sun-aligned arcs that have a rather high occurrence frequency [e.g., Valladares et al, 1994, and references therein] and large-scale polar arcs, which appear during about 10–16% of the time [Kullen et al, 2002]
As the original definition is not precise enough to identify unambiguously bending arcs, we define bending arcs here more precisely as polar arcs with one end separating from the prenoon or postnoon dayside oval before moving poleward and antisunward into the polar cap, while the nightside end of the arc remains attached to the oval such that the arc becomes hook-shaped after a while
The polar arc data set by Kullen et al [2002] has been reinvestigated regarding interplanetary magnetic field (IMF) dependence, polar arc location, and ionospheric plasma flows during arc formation
Summary
Auroral arcs appearing poleward of the main oval are often grouped into small-scale Sun-aligned arcs that have a rather high occurrence frequency [e.g., Valladares et al, 1994, and references therein] and large-scale polar arcs, which appear during about 10–16% of the time [Kullen et al, 2002]. The present study is based on a list of polar arcs that have been identified with Polar UV images [Kullen et al, 2002]; the focus is on large-scale polar arcs. As these often extend from the nightside to the dayside oval, they are sometimes referred to as theta aurora [Frank et al, 1982] or more commonly as transpolar arcs. We refer to all large-scale polar arcs as “polar arcs” rather than transpolar arcs, as some of the arcs studied in this work are (at least during part of their lifetime) not connected to the dayside oval. We refer to these categories using the same definitions as in Kullen et al [2002]
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