An almost complete disappearance of open-flux polar cap for strongly northward interplanetary magnetic field

Abstract

The Earth’s magnetosphere is the region of space where plasma behavior is dominated by the geomagnetic field. It has a long tail typically extending hundreds of Earth radii (RE) with plentiful open magnetic fluxes threading the magnetopause associated with magnetic reconnection and momentum transfer from the solar wind. The open-flux is greatly reduced when the interplanetary magnetic field points northward, but the extent of the magnetotail remains unknown. Here we report direct observations of an almost complete disappearance of the open-flux polar cap characterized by merging poleward edges of a conjugate horse-collar aurora (HCA) in both hemispheres’ polar ionosphere. The conjugate HCA is generated by particle precipitation due to Kelvin-Helmholtz instability in the dawn and dusk cold dense plasma sheets (CDPS). These CDPS are consist of solar wind plasma captured by a continuous dual-lobe magnetic reconnections, which is further squeezed into the central magnetotail, resulting in a short “calabash-shaped” magnetotail.

Universal Time Variations in the Magnetosphere and the Effect of CME Arrival Time: Analysis of the February 2022 Event that Led to the Loss of Starlink Satellites

“…This shows the aurora as seen by the F-17 satellite during a pass over the quiet, contracted polar cap in the northern hemisphere at 07:18 UT on d-o-y 33: this is during the descent toward the deep minimum in F PC at UT = 9.5 hr noted above. This pass reveals a horse-collar aurora with the putative OCB marked by narrow arcs that are almost parallel to the noon-midnight meridian (Wang et al, 2022). The polar cap is very far from circular at this time.…”

“…This is certainly not valid all of the time, particularly for strongly and prolonged northward IMF when F PC is well below its average value of about 0.4 GWb. In such cases, a "horse-collar" auroral form is often seen, indicating a teardrop-shaped open flux region (Bower et al, 2022;Elphinstone et al, 1993;Hones et al, 1989;Imber et al, 2006;Milan et al, 2020;Wang et al, 2022). Nevertheless, the assumption of a circular polar cap has been successfully used many times with the ECPC model (e.g., Lockwood et al, 1990;Milan et al, 2017), including an analysis of a full year of data (Milan et al, 2021) and has two major advantages for the present study.…”