Open AccessMotion of polar cap arcs
Author(s) -
Hosokawa K.,
Moen J. I.,
Shiokawa K.,
Otsuka Y.
Publication year - 2011
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2010ja015906
Subject(s) - polar , motion (physics) , geology , geodesy , polar cap , geophysics , physics , ionosphere , classical mechanics , astronomy
A statistics of motion of polar cap arcs is conducted by using 5 years of optical data from an all‐sky imager at Resolute Bay, Canada (74.73°N, 265.07°E). We identified 743 arcs by using an automated arc detection algorithm and statistically examined their moving velocities as estimated by the method of Hosokawa et al. (2006). The number of the arcs studied is about 5 times larger than that in the previous statistics of polar cap arcs by Valladares et al. (1994); thus, we could expect to obtain more statistically significant results. Polar cap arcs are found to fall into two distinct categories: the B y ‐dependent and B y ‐independent arcs. The motion of the former arcs follows the rule reported by Valladares et al. (1994), who showed that stable polar cap arcs move in the direction of the interplanetary magnetic field (IMF) B y . About two thirds of the arcs during northward IMF conditions belong to this category. The latter arcs always move poleward irrespective of the sign of the IMF B y , which possibly correspond to the poleward moving arcs in the morning side reported by Shiokawa et al. (1997). At least one third of the arcs belong to this category. The B y ‐dependent arcs tend to move faster when the magnitude of the IMF B y is larger, suggesting that the transport of open flux by lobe reconnection from one polar cap compartment to the other controls their motion. In contrast, the speed of the B y ‐independent arcs does not correlate with the magnitude of the B y . The motions of both the B y ‐dependent and B y ‐independent arcs are most probably caused by the magnetospheric convection. Convection in the region of B y ‐dependent arcs is affected by the IMF B y , which indicates that their sources may be on open field lines or in the closed magnetosphere adjacent to the open‐closed boundary, whereas B y ‐independent arcs seem to be well on closed field lines. Hence, the magnetospheric source of the two types of arc may be different. This implies that the mechanisms causing the motion and generation of arcs could be different between the two types of polar cap arc.