The semiannual variation of transpolar arc incidence and its relationship to the Russell–McPherron effect

Earth’s aurora is a luminescent phenomenon generated by the interaction between magnetospheric precipitating particles and the upper atmosphere; it plays an important role in magnetosphere–ionosphere (M-I) coupling. The transpolar arc (TPA) is a discrete auroral arc distributed in the noon-midnight direction poleward of the auroral oval and connects the dayside to the nightside sectors of the auroral oval. Studying the seasonal variation of TPA events can help us better understand the long-term variation of the interaction between the solar wind, the magnetosphere, and M-I coupling. However, a statistical study of the seasonal variation of TPA incidence has not previously been carried out. In this paper, we have identified 532 TPA events from the IMAGE database (2000–2005) and the Polar database (1996–2002), and calculated the incidence of TPA events for different months. We find a semiannual variation in TPA incidence. Clear peaks in the incidence of TPAs occur in March and September; a less pronounced peak appears in November. We also examine seasonal variation in the northward interplanetary magnetic field (IMF) over the same time period. The intensity and occurrence rate of the northward IMF exhibit patterns similar to that of the TPA incidence. Having studied IMF B_z before TPA onset, we find that strong and steady northward IMF conditions are favorable for TPA formation. We suggest that the semiannual variation observed in TPA incidence may be related to the Russell–McPherron (R-M) effect due to the projection effect of the IMF B_y under northward IMF conditions.