Responses of the open–closed field line boundary in the evening sector to IMF changes: A source mechanism for Sun‐aligned arcs

Abstract

Simultaneous measurements from the Polar satellite and by ground‐based optical sensors suggest that brief variations of the poleward auroral boundary on the nightside correlate with changes in the interplanetary magnetic field (IMF) about an hour after a structure has propagated in the solar wind past the Earth. Short‐lived Sun‐aligned arcs may emerge along the open–closed magnetic field line boundary (OCB) and then disappear after ∼10 min. The arcs are fueled by energetic particles whose spectral characteristics are similar to those of the of boundary plasma sheet (BPS). Polar measurements confirm that these auroral protrusions into the polar cap occur on nearly isolated closed magnetic flux. Optical emissions from these arcs appear strongest at their intersection with the poleward boundary of the auroral oval. Detailed magnetohydrodynamic (MHD) simulations of dayside interactions, when the dominant IMF component BY changes sign, indicate that near the polarity reversal merging can occur between interplanetary field line segments within the magnetosheath [Maynard et al., 2001b]. Newly formed loops of interplanetary flux sweep past the Earth without interacting with the magnetosphere. Here, we consider some consequences within the distant magnetotail as the loops of disconnected flux propagate to XGSM ≈ −200 RE. The MHD simulations indicate that about an hour after intra‐IMF merging events fingers of closed field lines protrude from the OCB into the polar cap. Like the observed Sun‐aligned arc, these simulated auroral features grow and decay on scales of ∼10 min and have ionospheric footprints that are nearly surrounded by open magnetic flux. The simulated auroral fingers are conjugate to high‐pressure channels in the distant plasma sheet. We suggest that the short‐lived Sun‐aligned arcs are created via an interchange process similar to that proposed by Kan and Burke [1985] to explain one class of theta auroral forms. The continuity of magnetotail currents across a high‐pressure channel requires the development of field‐aligned currents carried by obliquely propagating Alfvén waves. Plasma drifts associated with the dusk‐to‐dawn electric fields of the Alfvén waves are away from the Earth in the magnetotail and poleward in the nightside ionosphere. The correlation of phenomena at the nightside OCB with variations in the IMF indicate that processes other than substorms can influence boundary dynamics. Effects of self‐interactions of the IMF within the dayside magnetosheath may be felt along the OCB as much as 1 hour later.