The Association of Cusp‐Aligned Arcs With Plasma in the Magnetotail Implies a Closed Magnetosphere.

We investigate a 15‐day period in October 2011. Auroral observations by the Special Sensor Ultraviolet Spectrographic Imager instrument onboard the Defense Meteorological Satellite Program F16, F17, and F18 spacecraft indicate that the polar regions were covered by weak cusp‐aligned arc (CAA) emissions whenever the interplanetary magnetic field (IMF) clock angle was small, |θ| < 45°, which amounted to 30% of the time. Simultaneous observations of ions and electrons in the tail by the Cluster C4 and Geotail spacecraft showed that during these intervals dense (≈1 cm−3) plasma was observed, even as far from the equatorial plane of the tail as |ZGSE| ≈ 13 RE. The ions had a pitch angle distribution peaking parallel and antiparallel to the magnetic field and the electrons had pitch angles that peaked perpendicular to the field. We interpret the counter‐streaming ions and double loss‐cone electrons as evidence that the plasma was trapped on closed field lines, and acted as a source for the CAA emission across the polar regions. This suggests that the magnetosphere was almost entirely closed during these periods. We further argue that the closure occurred as a consequence of dual‐lobe reconnection. Our finding forces a significant re‐evaluation of the magnetic topology of the magnetosphere during periods of northwards IMF. Plain Language Summary: The magnetosphere is usually assumed to contain both open and closed magnetic flux. Closed magnetic field lines have both ends connected to the Earth; open field lines connect to the Earth at one end and into the interplanetary medium at the other. There tends to be little plasma on open field lines as the particles escape down the magnetotail, whereas plasma on closed field lines is trapped. Open flux near the poles naturally explains the oval configuration of Earth's auroras, with a lack of auroras at very high latitudes where there is no plasma to cause emissions. Somewhat unexpectedly, we show that auroral emission near the poles is common and that at these times there is significant plasma in the magnetotail, indicating that the magnetosphere contains only closed flux. We propose that this magnetic configuration is formed by a process known as dual‐lobe magnetic reconnection which occurs when the interplanetary magnetic field (IMF) within the solar wind points northwards. We must re‐evaluate the standard picture of magnetospheric structure during these periods of northwards IMF. Key Points: Cusp‐aligned arcs (CAAs) observed by the Defense Meteorological Satellite Program spacecraft occur frequently for northward interplanetary magnetic fieldCluster and Geotail observations show that the arcs are accompanied by trapped plasma at high latitudes in the magnetotailWe interpret CAAs as a signature of a magnetosphere almost entirely closed by dual‐lobe reconnection [ABSTRACT FROM AUTHOR]