Observations of closed magnetic flux embedded in the lobe during periods
of northward IMF
Abstract
The high latitude, lobe regions of the magnetosphere are often assumed
to contain cool, low energy plasma populations. However, during periods
of northward IMF, energetic plasma populations have occasionally been
observed. We present three cases when Cluster observed
uncharacteristically \say{hot} plasma populations in
the lobe. For two of the three events, we present simultaneous
observations of the plasma sheet observed by Double Star. The similarity
between the plasma in the lobe and the plasma sheet suggests that the
mechanism that produces plasma at high latitudes is likely to be tail
reconnection, resulting in a trapped \say{wedge} of
closed flux about the noon-midnight meridian. Complementary images from
IMAGE and DMSP/SSUSI show that transpolar arcs, which form in each event
in at least one hemisphere, directly intersect the footprint of the
Cluster spacecraft in all three events. The intersection of the Cluster
footprint with the transpolar arcs is synchronous with the observation
of the energetic plasma populations in the lobe. This further supports
the conclusion that it is likely this energetic plasma observed in the
high latitude lobe regions of magnetosphere is on closed field lines.