Interplanetary magnetic field dependency of stable sun‐aligned polar cap arcs

Abstract

This is the first analysis, using a statistically significant data set, of the morphological dependence of the presence, orientation, and motion of stable sun‐aligned polar cap arcs upon the vector interplanetary magnetic field (IMF). For the one winter season analyzed we had 1392 all‐sky 630.0‐nm images of 2‐min resolution containing a total of 150 polar cap arcs, all with corresponding values of the IMF as measured by IMP 8 or ISEE 2. After demonstrating an unbiased data set with smooth normal distributions of events versus the dimensions of time, space, and IMF component, we examine IMF dependencies of the properties of the optical arcs. A well‐defined dependence for Bz is found for the presence/absence of stable Sun‐aligned polar cap arcs. Consistent with previous statistical studies, the probability of observing polar cap aurora steadily increases for larger positive values of Bz, and linearly decreases when Bz becomes more negative. The probability of observing Sun‐aligned arcs within the polar cap is determined to vary sharply as a function of the arc location; arcs were observed 40% of the time on the dawnside and only 10% on the duskside. This implies an overall probability of at least 40% for the whole polar cap. 20% of the arcs were observed during “southward IMF conditions”, but in fact under closer inspection were found to have been formed under northward IMF conditions; these “residual” positive Bz arcs had a delayed residence time in the polar cap of about what would be expected after a north to south transition of Bz. A firm dependence on By is also found for both the orientation and the dawn‐dusk direction of motion of the arcs. All the arcs are Sun‐aligned to a first approximation, but present deviations from this orientation, depending primarily upon the location of the arc in corrected geomagnetic (CG) coordinates. The arcs populating the 06‐12 and the 12‐18 quadrants of the CG coordinate system point toward the cusp. The By dependency of the arc alignment is consistent with a cusp displacement in local time according to the sign of By. We found that the arc direction of motion depended both on By and the arc location within the polar cap. For a given value of By, two well‐defined regions (or cells) exist. Within each cell the arcs move in the same direction toward the boundary between the cells. The arcs located in the duskside move dawnward; those in the dawnside move duskward. The relative size of these dusk and dawn regions (or cells) are controlled by the magnitude of By. This persistent dusk‐dawn motion of the polar cap arcs is interpreted in terms of newly open flux tubes entering the polar cap and exerting a displacement of the convective cells and the polar cap arcs that are embedded within them.