Abstract

<p>We propose a mechanism for the formation of the horse-collar auroral configuration common during periods of strongly northwards interplanetary magnetic field, invoking the action of dual-lobe reconnection (DLR).  Auroral observations are provided by the Imager for Magnetopause-to-Auroras Global Exploration (IMAGE) satellite and spacecraft of the Defense Meteorological Satellite Program (DMSP).  We also use ionospheric flow measurements from DMSP and polar maps of field-aligned currents (FACs) derived from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE).  Sunward convection is observed within the dark polar cap, with antisunwards flows within the horse-collar auroral region, together with the NBZ FAC distribution expected to be associated with DLR.  We suggest that newly-closed flux is transported antisunwards and to dawn and dusk within the reverse lobe cell convection pattern associated with DLR, causing the polar cap to acquire a teardrop shape and weak auroras to form at high latitudes.  Horse-collar auroras are a common feature of the quiet magnetosphere, and this model provides a first understanding of their formation, resolving several outstanding questions regarding the nature of DLR and the magnetospheric structure and dynamics during northwards IMF.  The model can also provide insights into the trapping of solar wind plasma by the magnetosphere and the formation of a low-latitude boundary layer and cold, dense plasma sheet.  We speculate that prolonged DLR could lead to a fully closed magnetosphere, with the formation of horse-collar auroras being an intermediate step.</p>

Highlights

  • It has been known since space-based auroral imagery was first available that during prolonged periods of northward-directed interplanetary magnetic field (IMF) the auroral oval can resemble a “horse-collar” and the usually round polar cap becomes teardrop-shaped (e.g., Elphinstone et al, 1993; Hones et al, 1989; Murphree et al, 1982)

  • We propose a mechanism for the formation of the horse-collar auroral configuration during periods of strongly northward interplanetary magnetic field (IMF), invoking the action of dual-lobe reconnection (DLR)

  • Sunward convection is observed within the dark polar cap, with antisunward flows within the horse-collar auroral region, together with the NBZ field-aligned currents (FACs) distribution expected to be associated with DLR

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Summary

Introduction

It has been known since space-based auroral imagery was first available that during prolonged periods of northward-directed interplanetary magnetic field (IMF) the auroral oval can resemble a “horse-collar” and the usually round polar cap becomes teardrop-shaped (e.g., Elphinstone et al, 1993; Hones et al, 1989; Murphree et al, 1982). The accumulation of closed field lines at the nose of the magnetosphere, possibly mass-loaded with captured solar wind plasma, has been argued to progress tailward over time, maybe through a viscous interaction with the magnetosheath flow, and contribute to the formation of the northward IMF “low-latitude boundary layer” or LLBL (e.g., Sandholt et al, 1999; Song & Russell, 1992; Scholer & Treumann, 1997) and a “cold, dense plasma sheet” or CDPS in the magnetotail (e.g., Imber et al, 2006; Øieroset et al, 2005; Taylor et al, 2008; Terasawa et al, 1997; Wing et al, 2006) This aspect of the interaction has received significant attention, there are observational difficulties in its study (a notable exception being Fuselier et al, 2015).

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Data Availability Statement

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