Multi-Instrument Observations of the Relationships Between Polar Cap Patches and Arcs for Changing IMF Bz Orientation

Polar cap patches and polar cap arcs are two polar phenomena that typically occur under a southward and a northward interplanetary magnetic field (IMF), respectively. However, their morphology and plasma characteristics are significantly different due to completely different generation mechanisms. Here, we present multi-instrument observations of coexisting polar cap patches and arcs (distance ∼500 km) in an all-sky imager field of view as IMF Bz turns northward. When IMF Bz turns northward, the resulting sunward flow will prolong the evolution time of patches from the dayside to the nightside. A polar cap arc can form 1–2 hr after the northward turning, and it is driven by electron precipitation near the nightside poleward auroral boundary. As a consequence, both slow-moving polar cap patches and polar cap arcs may appear at close proximity in the nightside polar cap. Statistical results show that such events generally occur under specific IMF conditions, such as the IMF turning from southward to northward, the northward to southward direction constantly jumping back and forth, etc. The polar cap arc is sometimes accompanied by increased auroral brightness and GNSS total electron content, although weaker than that when the patches exist alone. Simultaneous echoes from the Hankasalmi SuperDARN radar indicate the formation of ionospheric irregularities, where polar cap arcs may be a potential source for these irregularities (like patches).