Lobe Reconnection and Cusp‐Aligned Auroral Arcs

Abstract

Following the St. Patrick's Day (17 March) geomagnetic storm of 2013, the interplanetary magnetic field had near‐zero clock angle for almost two days. Throughout this period multiple cusp‐aligned auroral arcs formed in the polar regions; we present observations of, and provide a new explanation for, this poorly understood phenomenon. The arcs were observed by auroral imagers onboard satellites of the Defense Meteorological Satellite Program. Ionospheric flow measurements and observations of energetic particles from the same satellites show that the arcs were produced by inverted‐V precipitation associated with upward field‐aligned currents (FACs) at shears in the convection pattern. The large‐scale convection pattern revealed by the Super Dual Auroral Radar Network and the corresponding FAC pattern observed by the Active Magnetosphere and Planetary Electrodynamics Response Experiment suggest that dual‐lobe reconnection was ongoing to produce significant closure of the magnetosphere. However, we propose that once the magnetosphere became nearly closed complicated lobe reconnection geometries arose that produced interleaving of regions of open and closed magnetic flux and spatial and temporal structure in the convection pattern that evolved on timescales shorter than the orbital period of the DMSP spacecraft. This new model naturally explains many features of cusp‐aligned arcs, including why they focus in from the nightside toward the cusp region.

The Question of Transpolar Arc Conjugacy: New Results From Comparing Solar Wind Data and Dipole Tilt Distribution of Five Different Datasets

“…Zhang et al (2016) proposed multiple TPAs to be caused by field-aligned current (FAC) filaments mapping into the polar cap that have been caused by local reconnection during Kevin-Helmholtz instabilities at the tail flanks. Milan et al (2022) suggested that temporal dual-lobe reconnection during strongly northward IMF would cause alternating strips of open and closed field lines in the polar cap, ultimately leading to a closure of the polar in form of multiple cusp-aligned TPAs.…”

“…They are typically cusp‐aligned (Zhang et al., 2016) and fill both the dawn and dusk oval sides simultaneously, resulting in a small teardrop‐shaped polar cap area devoid of aurora (in e.g., Murphree et al., 1982, Hones et al., 1989, Tanaka et al., 2017, and Milan et al., 2020 described as horse‐collar aurora). Occasionally, multiple TPAs fill the entire polar cap (for examples, see Zhang et al., 2016 their Figure 2 or Milan et al., 2022, their Figure 3).…”

“…Milan et al. (2022) suggested that temporal dual‐lobe reconnection during strongly northward IMF would cause alternating strips of open and closed field lines in the polar cap, ultimately leading to a closure of the polar in form of multiple cusp‐aligned TPAs.…”

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

“…Assuming that the HCA is located on closed field lines, the polar cap became very small and teardrop-shaped 24,31,32 when the HCA formed. After about 1 hour (around 3 hours after IMF turn north), the two poleward edges of the HCA merged together around the noon-midnight meridian with sunward flows around dayside cusp region in both hemispheres indicating ongoing dual-lobe reconnections (DLRs) (blue curve in Figure 2 a and g and Supplementary Figure S1) 23,24,33 . This suggests that the polar cap nearly disappeared (normally the polar cap is empty without discrete auroral emissions and linked to the open lobe field line region) and that a nearly fully closed magnetosphere was generated by dual-lobe reconnection 16,28 .…”

A statistical study of space hurricanes in the Northern Hemisphere

“…Furthermore, we argued that the weak tendency for the duskward movement of space hurricanes with the increasing IMF clock angle (see Figure 4C) is related to the extension of the reconnection site in the dayside magnetopause under the nonzero IMF By. Under positive/negative IMF By, the reconnection site extends toward dusk/dawn in the Northern Hemisphere (Luhmann et al, 1984;Park et al, 2006), and the open flux tubes are added asymmetrically to the magnetosphere (Cowley, 1981;Tenfjord et al, 2015;Milan et al, 2022). The evolution of polar cap aurora-like transpolar arcs is also modulated by IMF By, which is related to the night-side magnetotail process (Fear and Milan, 2012;Xing et al, 2018).…”