The association of cusp-aligned arcs with plasma in the magnetotail
implies a closed magnetosphere
Abstract
We investigate a fifteen-day period in October 2011. Auroral
observations by the SSUSI instrument onboard the DMSP F16, F17, and F18
spacecraft indicate that the polar regions were covered by weak
cusp-aligned arc emissions whenever the 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 cusp-aligned arc
emission across the polar regions. This suggests that the magnetosphere
was almost entirely closed during these periods. We further argue that
the closure occured as a consequence of dual-lobe reconnection at the
dayside magnetopause. Our finding forces a significant re-evaluation of
the magnetic topology of the magnetosphere during periods of northwards
IMF.