Field‐Aligned Current During an Interval of BY‐Dominated Interplanetary‐Field; Modeled‐to‐Observed Comparisons

We model an interval of remarkable interplanetary magnetic field (IMF), for which we have a comprehensive set of observational data. This interval is associated with the arrival of an interplanetary coronal mass ejection. The solar wind densities at the time are particularly high and the IMF is primarily northward over many hours. This results in strong auroral emissions within the polar cap in a cusp spot, which we associate with lobe reconnection at the high‐latitude magnetopause. We also observe areas of upwards field‐aligned current (FAC) within the summer Northern Hemisphere polar cap that exhibit large current magnitudes. The model can reproduce the spatial distribution of the FACs well, even under changing conditions in the incoming IMF. Discrepancies exist between the modeled and observed current magnitudes. Notably, the winter Southern Hemisphere exhibits much lower current magnitudes overall. We also model a sharp transition of the location of magnetopause reconnection at the beginning of the interval, before the IMF remained northward for many hours. The reconnection location changed rapidly from a subsolar location at the low‐latitude magnetopause under southward IMF conditions, to a high‐latitude lobe reconnection location when the field is northward. This occurs during a fast rotation of the IMF at the shock front of a magnetic cloud. Plain Language Summary Under extreme incoming interplanetary magnetic field conditions following the impact of an Interplanetary Coronal Mass Ejection (CME) on the Earth's system, we observe a range of phenomena in the Northern Hemisphere ionosphere. This includes auroral emissions in the form of a cusp spot and associated precipitating particles, ionospheric flows, and strong field‐aligned currents (FACs) in the high‐latitude polar cap. These phenomena change in orientation and strength following variations in the incoming solar wind. We model the state of the magnetosphere during these observations. The modeled currents correspond well spatially with the observed currents, however, the current magnitudes are very different. The modeled FACs indicate that the site of magnetic reconnection can change rapidly from a lower‐latitude dayside position to a high‐latitude location in the magnetospheric lobes, which is reflected in field orientation within the magnetic cloud associated with the passing CME. Key Points We model an interval of interplanetary BY‐dominated field and high solar wind densities during the impa