Dual‐Lobe Reconnection and Horse‐Collar Auroras

Abstract

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). Auroral observations are provided by the Imager for Magnetopause-to-Aurora 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 antisunward 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 antisunward 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 northward 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. Plain Language Summary During quiet geomagnetic conditions, the global distribution of auroras can acquire a "horse-collar" configuration, in which regions of weak auroral emission appear at dawn and dusk poleward of the main auroral oval. We propose a new model to explain the formation of this configuration, which provides new insights into magnetospheric dynamics during periods of northward-directed interplanetary magnetic field. To support our proposal, we use observations of the auroras, ionospheric convection, and estimations of the pattern of electrical currents flowing between the ionosphere and magnetosphere from a suite of spacecraft. Our proposed model resolves a 40-year-old question regarding the nature of the horse-collar auroras and many other aspects of magnetospheric dynamics.

A Survey of 25 Years' Transpolar Voltage Data From the SuperDARN Radar Network and the Expanding‐Contracting Polar Cap Model

“…Examples include the effects of changes in the IMF B Y component (Cowley, Morelli, & Lockwood, 1991;Lockwood, 1991) or transient and propagating filamentary field aligned currents in traveling convection vortices induced by solar wind dynamic pressure pulses (e.g., Lühr et al, 1996). In addition, prolonged northward IMF can lead to open flux collecting toward midnight, giving a more triangular open flux region LOCKWOOD AND MCWILLIAMS characteristic of the "horse-collar" pattern in global auroral images (Birn et al, 1991;Hones et al, 1989): it has been proposed that this is enhanced by lobe reconnection taking place in both the hemispheres which removes dayside open flux (Milan et al, 2020). Hence, the use of a circular open polar cap in Figures 2 and 3 is illustrative only.…”

Quantifying the Lobe Reconnection Rate During Dominant IMF By Periods and Different Dipole Tilt Orientations

“…(2008) during IMF B y dominated periods in the Cluster EDI data, without mapping the measurements to the ionosphere. This behavior is expected from the lobe reconnection process, and is often referred to as stirring of lobe flux (e.g., Milan et al., 2020; Reiff & Burch, 1985). In the following discussion we interpret our quantified measure of plasma circulation within the polar cap, Φ lobe , as an estimate of an average lobe reconnection rate associated with the data selection conditions.…”

Climatological Statistics of Extreme Geomagnetic Fluctuations With Periods From 1 s to 60 min

“…Analysis of the R 1 distribution by Rogers et al. (2020) showed that these peaks near noon occur predominantly under northward IMF conditions during the summer months (i.e., under conditions of greatest dipole tilt angle), suggesting a possible relation to impulsive field line reconnection between the IMF and an “overdraped” tail lobe (Crooker, 1992; Milan et al., 2017, 2020; Wang et al., 2008; Watanabe et al., 2005). The MLT distribution of occurrence probability at dayside cusp latitudes does not match the distributions of MIEs observed by Lanzerotti et al.…”

“…(Note that the timestamps and MLTs associated with each cluster peak of / h E dB dt refers to the end of the n-minute period in question (from Equations 2 and 4) but this is not sufficient to account for the apparent shift of the maximum toward postnoon.) Analysis of the R 1 distribution by Rogers et al (2020) showed that these peaks near noon occur predominantly under northward IMF conditions during the summer months (i.e., under conditions of greatest dipole tilt angle), suggesting a possible relation to impulsive field line reconnection between the IMF and an "overdraped" tail lobe (Crooker, 1992;Milan et al, 2017Milan et al, , 2020Wang et al, 2008;Watanabe et al, 2005).…”