Interhemispheric Conjugacy of Multiple Transpolar Arcs

Multiple transpolar arcs appearing simultaneously in the polar cap have gained much interest in recent years. By analyzing Defense Meteorological Satellite Program Special Sensor Ultraviolet Spectrographic Imagers data, we report for the first time, that less than half of the multiple arc events occur simultaneously in both hemispheres. In 60% of the cases, multiple arcs appear in only one hemisphere. There is a clear difference in interplanetary magnetic field (IMF) conditions for those two groups. Conjugate multiple arcs appear on average during stronger northward IMF and smaller IMF clock angles than non-conjugate multiple arcs. Only non-conjugate multiple arcs show a dependence on IMF B-X. They form in the northern (southern) hemisphere during negative (positive) B-X. An IMF B-X induced interhemispheric asymmetry in the magnetospheric field line topology might explain why multiple arcs appear sometimes in only one hemisphere.