Solar wind dependence of the occurrence and motion of polar auroral arcs: A statistical study

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Abstract

Polar UV images from a 3‐month period in winter 1998–1999 are used for a statistical study of polar arcs. The study covers all auroral arcs that are located poleward of the northern auroral oval, and which are detectable by the UV imager. The arcs are examined with respect to their spatial and temporal behavior as well as to a possible connection to solar wind parameters using ACE satellite data. It is found that the majority of polar arcs appear during northward IMF, strong IMF magnitude, and high solar wind speed. A modified Akasofu‐Perreault epsilon parameter with a cosine function instead of a sine function ( vB 2 cos 4 (θ/2)( l 0 2 /μ 0 )) combines these results. It correlates well with the occurrence frequency of polar arcs for long timescales. The location of polar arcs is strongly dependent on the sign of the IMF B y component. Static polar arcs occur in the Northern Hemisphere on the dawn (dusk) side of the oval for negative (positive) IMF B y , whereas poleward‐moving arcs separate from the opposite side of the oval, and then move in the direction of IMF B y . All polar arcs are sorted into five different categories according to their spatial structure and evolution: oval‐aligned, bending, moving, midnight, and multiple arcs. Each polar arc type occurs for a characteristic combination of solar wind parameters. IMF clock angle changes seem to have a strong influence on what type of arc occurs. Oval‐aligned arcs appear mainly during steady IMF, bending arcs after an IMF B z sign change, and moving arcs after an IMF B y sign change. For the rare midnight and multiple arc events, no characteristic IMF clock angle dependence has been found. The different types of clear polar arcs are discussed in the context of existing observational studies and transpolar arc models.

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