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

Abstract

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.

Plasma Observations in the Distant Magnetotail During Intervals of Northward IMF

“…These observations of the more distant magnetotail support the interpretation of Milan et al (2023) that the plasma is trapped on closed magnetic field lines during periods where the magnetosphere is almost-entirely closed as a result of dual-lobe reconnection occurring under northward IMF conditions. Milan et al (2023) suggested that the trapped plasma population in the magnetotail observed by Cluster and Geotail could be the source plasma for cusp-aligned arc (CAA) signatures that were observed simultaneously in the polar region in their study. The CAA emission has been found to have inverted-V precipitation signatures suggesting that the plasma is accelerated down into the polar cap by field-aligned currents associated with flow shears in the convection pattern (Q.-H. Zhang et al, 2020;Milan et al, 2022).…”

“…The plasma in the magnetotail is coincident with simultaneous observations of similar plasma signatures observed in Cluster and Geotail data during the same period investigated by Milan et al (2023). Using pitch angle data available from the CIS and PEACE instruments onboard Cluster, Milan et al (2023) showed that the observed ion fluxes had pitch angles of 0°and 180°indicating two counter streaming ion populations whereas the electron population showed a double loss-cone distribution. Both of these pitch angle distributions indicate that the plasma is trapped on closed field lines.…”

“…In this study, we use ARTEMIS data to observe the structure and plasma characteristics further down the magnetotail at ∼60 R E during the same period of prolonged northward IMF. The turquoise boxes in Figures 2 and 3 indicate positive observations of CAAs in the polar region identified by Milan et al (2023) while the ARTEMIS spacecraft traversed the magnetotail for reference and are numbered 1-9. The estimated start and end times of the CAA observations are approximate due to the limitations of the DMSP SSUSI observations and the cadence of the DMSP orbits.…”

“…The observations are presented in the same format as Figure 2. Milan et al (2023) identified a period of prolonged northward IMF during which multiple instances of cuspaligned arcs (CAAs) were identified in the polar region using the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) instrument (Paxton et al, 1992) onboard the Defense Meteorological Spacecraft Program (DMSP) satellites. Milan et al (2023) observed high plasma fluxes in the near-Earth tail in Cluster data that coincided with the observations of cusp-aligned arcs.…”

“…Observations and simulations of the length of the closed magnetosphere under northward IMF have varying estimates between ∼28 R E (Wang et al, 2023) to 200 R E (Fairfield et al, 1996). Milan et al (2023) have studied a prolonged period of predominantly northward IMF during 15 days in October 2011 during which time several intervals of CAAs were observed. Milan et al (2023) examined high density plasma in both the high latitude (Z ± 13 R E ) northern and southern magnetotail observed simultaneously by both the Cluster and Geotail satellites coincident with the CAA observations.…”