Probing the magnetic structure of a pair of transpolar arcs with a solar wind pressure step

We present observations of the Northern Hemisphere auroras taken with the far ultraviolet cameras onboard the Imager for Magnetopause‐to‐Aurora Global Exploration spacecraft during a compression of the magnetosphere by a solar wind pressure step on 30 December 2001. The compression occurs during a period of northward interplanetary magnetic field which has given rise to the presence of a pair of transpolar arcs (TPAs) near the dawnside oval. The compression causes a brightening of the oval, from dayside to nightside over the course of 10 min, followed by a brightening of the midnight sector oval and TPAs from nightside to dayside, again over 10 min. We suggest that the brightening is caused by pitch angle scattering of particles trapped on closed magnetic field lines and that the sequence of the brightening tracks the solar wind pressure step as it progresses along the length of the magnetotail. Traveling at 600 km s , the step reaches up to 90 down‐tail over the period of brightening, suggesting that the magnetic field lines which map to the TPAs are closed and stretch almost this length down‐tail

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

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