The Evolution of Long‐Duration Cusp Spot Emission During Lobe Reconnection With Respect to Field‐Aligned Currents

Abstract

We track a remarkably bright and persistent auroral cusp spot emission in the high‐latitude Northern Hemisphere polar cap, well inside the main auroral oval, for approximately 11 hr on 16 and 17 June 2012. The auroral emissions are presented in both the Lyman‐α and Lyman‐Birge‐Hopfield bands, as observed by the Special Sensor Ultraviolet Spectrographic Imager on board two of the Defense Meteorological Satellite Programme spacecraft, and supported by detections of precipitating particles by the same spacecraft. The auroral observations are accompanied by patterns of field aligned currents, obtained from the Active Magnetosphere and Planetary Electrodynamics Response Experiment, along with ionospheric convection patterns from the Super Dual Auroral Radar Network. These data provide unprecedented coverage of a cusp spot, unusually seen in both electron and proton aurora. The location and movement of the auroral emissions, current systems, and ionospheric convection patterns are extremely distorted under the northward to Y‐component‐dominated interplanetary magnetic field. The cusp spot emission region is associated with the sunward flow region of the ionosphere. Ion dispersion signatures are detected on traversal of the region of brightest proton auroral emissions. Proton‐excited Lyman‐α emissions are most evident following impulses of high solar wind density. The auroral emissions, field‐aligned current patterns, and ionospheric convection are consistent with a model of a compressed magnetosphere under strongly northward interplanetary magnetic field, following an impact of an Interplanetary Coronal Mass Ejection and associated magnetic cloud at the magnetopause, inducing high‐latitude lobe reconnection that progresses increasingly tailward during the presented interval.

Dual‐Lobe Reconnection and Horse‐Collar Auroras

“…Auroras are known to be colocated with the upward NBZ FAC, known as high‐latitude detached auroras or HiLDAs (Carter et al, 2018; Frey, 2007). Figure 3i also shows where auroras can form (green) due to direct precipitation of magnetosheath plasma downstream (sunward) of the footprint of the lobe reconnection site, to form an auroral cusp spot (Carter et al, 2020; Frey et al, 2002; Milan et al, 2000), which would be accompanied by a reverse ion dispersion signature (Woch & Lundin, 1992). The FAC pattern also indicates that auroras might be expected along the poleward edge of the dawn horse‐collar region.…”

“…Then the combined action of the magnetosheath flow and B Y -associated tension forces causes the field lines to be carried around the dawn and dusk flanks of the magnetosphere; whether more flux is carried dawnward or duskward, and hence whether the dawn or dusk lobe convection cell dominates, depends on the polarity of B Y (e.g., Milan et al, 2005): Figure 2e shows the asymmetry expected in the northern hemisphere for B Y > 0. Two auroral phenomena are associated with lobe reconnection: Direct precipitation of magnetosheath plasma along the newly reconnected field lines can produce a "cusp spot" downstream of the ionospheric projection of the reconnection site (Carter et al, 2020;Frey et al, 2002;Milan et al, 2000); upward FAC associated with the vorticity of the dawn lobe cell can produce "high-latitude detached arcs" or HiLDAs (Carter et al, 2018;Frey, 2007). Figure 2b shows SLR occurring in the northern Journal of Geophysical Research: Space Physics 10.1029/2020JA028567 hemisphere: It can occur simultaneously in the southern hemisphere, though independently and possibly at a different rate.…”

“…Auroral features that appear when the north‐south component of the IMF is positive ( B Z > 0) include polar cap arcs, transpolar arcs, sun‐aligned arcs, or bending arcs (see reviews by Zhu et al, 1997, and Hosokawa et al, 2020), cusp spots (Carter et al, 2020; Frey et al, 2002; Milan et al, 2000), and high‐latitude detached arcs or HiLDAs (Carter et al, 2018; Frey, 2007). Many of these phenomena are understood to be a consequence of magnetic reconnection (Dungey, 1961) occurring in the magnetotail or between the IMF and the high‐ or low‐latitude dayside magnetopause.…”

Dual-lobe reconnection and horse-collar auroras

“…Auroral features that appear when the north-south component of the IMF is positive (B Z > 0) include polar cap arcs, transpolar arcs, sun-aligned arcs, or bending arcs (see reviews by Zhu et al, 1997, andHosokawa et al, 2020), cusp spots (Carter et al, 2020;Frey et al, 2002;Milan et al, 2000), and high-latitude detached arcs or HiLDAs (Carter et al, 2018;Frey, 2007). Many of these phenomena are understood to be a consequence of magnetic reconnection (Dungey, 1961) occurring in the magnetotail or between the IMF and the high-or low-latitude dayside magnetopause.…”

“…Then the combined action of the magnetosheath flow and B Y -associated tension forces causes the field lines to be carried around the dawn and dusk flanks of the magnetosphere; whether more flux is carried dawnward or duskward, and hence whether the dawn or dusk lobe convection cell dominates, depends on the polarity of B Y (e.g., Milan et al, 2005): Figure 2e shows the asymmetry expected in the northern hemisphere for B Y > 0. Two auroral phenomena are associated with lobe reconnection: Direct precipitation of magnetosheath plasma along the newly reconnected field lines can produce a "cusp spot" downstream of the ionospheric projection of the reconnection site (Carter et al, 2020;Frey et al, 2002;Milan et al, 2000); upward FAC associated with the vorticity of the dawn lobe cell can produce "high-latitude detached arcs" or HiLDAs (Carter et al, 2018;Frey, 2007). Figure 2b shows SLR occurring in the northern hemisphere: It can occur simultaneously in the southern hemisphere, though independently and possibly at a different rate.…”