Cluster Observations of
Earthward Flowing Plasmoid in the Tail
The energetic electrons and ions embedded in
Earthward-moving plasmoid structures have been observed. These
plasmoids are associated with a rotational local Bz component
(bi-polar) signature. Energetic electrons are found to be
confined in a smaller spatial region than ions inside the
plasmoid. Energetic ions and electrons seem to be a good
indicator for the structure boundary. The fleet of Cluster spacecraft
cross the plasmoid structure in a "first entry, last out" order
(Note: when spacecraft cross a planar discontinuity, e.g.
magnetopause, they will be in "first entry, first out" order).
This documents the fact that the plasmoid has a non-planar nested
structure. The large separation distance (around 1 Re) of the Cluster
satellites in October 2002 is an advantage to provide constraints on
the size and shape of the plasmoid structure of interest. In addition,
the plasmoid (with closed field lines) should preserve the ion
composition information where it is formed. The ion
composition observed in the plasmoid shows significantly lower O and He
than in the ambient plasma. This implies few heavy ions are
involved in the reconnection process where the plasmoid is formed.
Multiple flux ropes/plasmoids observation presented in this paper can
be interpreted
as strong evidence for multiple X-lines.