The initiation of the poleward boundary intensification of auroral emission by fast polar cap flows: A new interpretation based on ionospheric polarization

The auroral intensification at the poleward boundary of the auroral oval is often considered to be the ionospheric manifestation of the distant reconnection. In the present study, however, we propose that the poleward boundary intensifications (PBIs) are initiated by ionospheric polarization due to fast polar cap flows, which are known to be well correlated with PBIs. The current continuity at the ionosphere can be described in two different ways, that is, the reflection of an Alfvén wave and the closure of Pedersen and Hall currents with field-aligned currents (FACs). The required consistency between the two approaches sets a framework for modeling the ionospheric polarization, and we numerically test the aforementioned idea focusing on an induced upward FAC as indicative of PBIs. The results show that in case the polar cap flow channel approaches the auroral oval perpendicularly from poleward, (i) upward and downward FACs are induced at the poleward boundary to the west and east of the longitudinal center of the flow channel, respectively; (ii) those induced FACs extend much wider in longitude than the flow channel; (iii) the peak densities of those induced FACs are significantly larger than those of the incident FACs; (iv) those induced FACs are distributed almost symmetrically in longitude, indicating that the Pedersen polarization dominates the Hall polarization; and (v) if the polar cap flow inclined dawnward (duskward), an upward (downward) FAC is induced first. These results are consistent with the reported characteristics of PBIs, which are rather difficult to explain otherwise.