Interhemispheric conjugacy of transpolar arcs

Most models predict that transpolar arcs (TPAs) occur simultaneously in both hemispheres. Conjugate TPAs are expected to appear in the northern and southern hemisphere on opposite oval sides. However, several observational studies have shown that this is not always the cases. It has been suggested that IMF Bx and/or the Earth dipole tilt may be responsible for non-conjugate TPAs. During strongly negative IMF Bx and/or positive Earth dipole tilt a TPA is expected to occur only in the northern hemisphere (for positive Bx and/or negative dipole tilt only in the southern hemisphere).

In the present work we revisit this question by investigating three previously published and one new TPA dataset regarding the influence of IMF Bx and Earth dipole tilt on interhemispheric TPA occurrence. The results show, the Earth dipole tilt has no statistical effect on TPA conjugacy while IMF Bx may have a small influence. However, this influence is much smaller than previously reported, when normalizing the IMF Bx distribution during TPAs with the average IMF Bx distribution in the solar wind during the time period covered by the respective dataset.

In the second part of this study we present results from the new TPA dataset, which is based on three months of SUSSI DMSP images. Arc location and IMF conditions during conjugate and non-conjugate TPAs are discussed in detail and possible reasons for non-conjugate TPA events are discussed.