In Situ Observations of Interaction Between the Closed Magnetic Field of Earth’s Transpolar Auroral Arcs and the Interplanetary Magnetic Field

Transpolar arcs (TPAs) are extensions of auroral emission poleward beyond the main ovals, forming partial or complete bisections known as ‘theta’ auroras. A prominent hypothesis suggests that a TPA occurs through stagnation of magneto-plasma returning from Earth’s magnetotail under northward interplanetary magnetic field (IMF), resulting in a ‘wedge’ of closed magnetic field lines in the open polar cap on which particles resemble those in the nightside equatorial plasma sheet. It has been proposed that a TPA’s closed-field lines may reach sufficiently high latitude to magnetically reconnect with the IMF at the lobe magnetopause, leading to observed coincidence of the TPA and a cusp spot. Using conjugate data from Cluster, Imager for Magnetopause-to-Aurora Global Exploration (IMAGE), Special Sensor Ultraviolet Spectrographic Imager (SSUSI), and other instruments, we demonstrate at least one case study of potential first in situ detection of TPA-IMF magnetic reconnection with magneto-plasma and visual aurora evidence, and several further instances of particles on closed magnetic field lines near the lobe magnetopause. Pending affirmative analysis, the existence of TPA-IMF reconnection events will not only further support the ‘wedge’ TPA formation hypothesis, but also indicate that lobe reconnection can open topologically closed nightside magnetic field lines, introducing new polar cap dynamics under northward IMF.