Multi‐Instrument Analysis of the Formation and Segmentation of Tongue of Ionization Into Two Consecutive Polar Cap Patches.

This paper investigates the formation and segmentation of the tongue of ionization into two consecutive polar cap patches using multi‐instrument observations from 27 February 2014. We provide insights into how the interplanetary magnetic field (IMF) variations influence the formation and segmentation of these patches. Our findings reveal that the entry of dayside dense plasma into the polar cap is predominantly driven by the modified convection near the cusp region, which is controlled by the transition of IMF By or the sudden drop of IMF Bz. Furthermore, we observe a rapid north‐westward plasma flow within the patch segmentation region, accompanied by equatorward‐expanded and enhanced convection near the cusp region. This fast‐moving flow, approximately 1.5 km/s, is characterized by low density and high electron temperature and shows a signature of a Subauroral Polarization Stream. This suggests that the fast‐westward flow, in conjunction with the expansion and contraction of ionospheric convection, plays a crucial role in the segmentation of polar cap patches from the dayside plasma reservoir. This study provides a comprehensive observation of the evolution of polar cap patches, thereby advancing our understanding of the dynamic mechanisms governing patch formation and segmentation. Plain Language Summary: Polar cap patches are highly dense structures in the polar cap region, typically forming in the cusp region and moving toward the nightside due to ionospheric convections. To better understand the factors that govern the formation and segmentation of these patches, a combination of satellite and ground‐based observations is used to investigate two consecutive polar cap patches on 27 February 2014. We found that the configuration of IMF By and Bz significantly influences cusp region convection, causing dayside sunlit dense plasma to enter the polar cap when cusp convection expands equatorward and the flow shifts northward, a process that leads to patch formation. Additionally, we observe strong north‐westward flows of low‐density, high‐electron‐temperature plasma in the subauroral region during periods of geomagnetic disturbance, displaying characteristics of Subauroral Polarization Streams (SAPS). This flow is observed in the patch segmentation region along with the equatorward‐expanding, enhanced cusp convection. These observations indicate that SAPS, along with the expansion and contraction of ionospheric convection, play a key role in the segmentation of polar cap patches from their equatorward plasma reservoir. This study significantly advances our understanding of the dynamic mechanisms governing the formation and segmentation of polar cap patches. Key Points: The transient change of IMF By and Bz leads to the formation of the tongue of ionization and segmentation of polar cap patchesA tongue of ionization is segmented into two polar cap patches by low electron density, high electron temperature, fast westward flowsThe expansion/contraction of the ionospheric convection also plays a crucial role in the segmentation of dayside dense plasma [ABSTRACT FROM AUTHOR]