Earth’s aurora appears as a ring of high conductance around each pole, shifting and undulating as the hours go by. The poleward edge of this auroral oval also marks a conductance boundary, often coinciding with the boundary of the polar cap, where the topology of Earth’s magnetic field lines changes from closed loops at lower latitudes to open to the solar wind at higher latitudes.
Many times every day, the edge of the aurora brightens in a poleward boundary intensification (PBI). PBIs have been studied extensively for decades, but researchers have not yet conclusively determined their source. Here Ohtani and Yoshikawa present an explanation of how PBIs may be caused by ionospheric convection, rather than the popular idea that they could be caused by magnetic reconnection far from Earth in the magnetosphere.
Fast polar cap flows, which transport ionospheric plasma from the dayside to nightside auroral oval across the polar cap, are strongly correlated to PBIs: The two occur together about 90% of the time. When these flows make contact with the auroral oval, the polarization of the ionosphere at the edge of the auroral oval causes a field-aligned current (FAC), a current of electrons that stream along Earth’s magnetic field lines. The authors hypothesize that FACs, if directed upward, are accompanied by enhanced electron precipitation, which would cause auroral intensification, in other words, a PBI.
This idea is supported by a number of characteristics of PBIs. For one, PBIs start immediately when a polar cap flow touches the auroral oval and have the same duration as the polar cap flow, which can be explained by the instantaneous nature of ionospheric polarization. For another, PBIs last as long as the polar cap flow is touching the auroral oval, a correlation that is difficult to explain if they are caused by distant reconnection.
PBIs are also wider than polar cap flows: If PBIs were caused by distant reconnection, the reconnection would have to be inexplicably triggered simultaneously inside and outside the flow channel. Finally, PBIs are often followed by the movement toward the equator of the open-close boundary marking the edge of the polar cap, connecting PBIs to the enhanced polar cap convection.
These statistical characteristics, when considered together, suggest that the vast majority of PBIs are caused by ionospheric polarization. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1002/2016JA023143, 2016)
—Leah Crane, Freelance Writer