| An investigation of the 2003 Cluster tail season has revealed small flux
ropes in the near-tail plasma sheet of Earth. These flux ropes manifest
themselves as a bipolar magnetic field signature (usually predominantly in
the Z-component) associated with a strong transient peak in one or more of
the other components (usually the Y-component). These signatures are
interpreted as the passage of a cylindrical magnetic structure with a strong
axial magnetic field over the spacecraft position. On the 2 October 2003
all four Cluster spacecraft observed a flux rope in the plasma
sheet at X (GSM) ~-17 RE. The flux rope was travelling
Earthward and duskward at ~160 kms-1, as determined from
multi-spacecraft timing. This is consistent with the observed
south-then-north bipolar BZ signature and corresponds to a size of
~0.3 RE (a lower estimate, measuring between the inflection
points of the bipolar signature). The axis direction, determined from
multi-spacecraft timing and the direction of the strong core field, was close
to the intermediate variance direction of the magnetic field. The current
inside the flux rope, determined from the curlometer technique, was
predominantly parallel to the magnetic field. However, throughout the flux
rope, but more significant in the outer sections, a non-zero component of
current perpendicular to the magnetic field existed. This shows that the flux
rope was not in a "constant α" force-free
configuration, i.e. the magnetic force, J×B
was also non-zero. In the variance frame of the magnetic
field, the components of J×B suggest that the magnetic pressure force was acting to expand the flux rope,
i.e. directed away from the centre of the flux rope, whereas the smaller
magnetic tension force was acting to compress the flux rope. The plasma
pressure is reduced inside the flux rope. A simple estimate of the total
force acting on the flux rope from the magnetic forces and surrounding plasma
suggests that the flux rope was experiencing an expansive total force.
On 13 August 2003 all four Cluster spacecraft observed a flux
rope at X (GSM) ~-18 RE. This flux rope was travelling tailward
at 200 kms-1, consistent with the observed north-then-south bipolar
BZ signature. The bipolar signature corresponds to a size of
~0.3 RE (lower estimate). In this case, the axis, determined from
multi-spacecraft timing and the direction of the strong core field, was
directed close to the maximum variance direction of the magnetic field. The
current had components both parallel and perpendicular to the magnetic field,
and J×B was again larger in the
outer sections of the flux rope than in the centre. This flux rope was also
under expansive magnetic pressure forces from J×B,
i.e. directed away from the centre of the flux rope,
and had a reduced plasma pressure inside the flux rope. A simple total force
calculation suggests that this flux rope was experiencing a large expansive
total force.
The observations of a larger J×B
signature in the outer sections of the flux ropes when compared to the centre
may be explained if the flux ropes are observed at an intermediate stage of
their evolution after creation by reconnection at multiple X lines near the
Cluster apogee. It is suggested that these flux ropes are in the process of
relaxing towards the force-free like configuration often observed further
down the tail. The centre of the flux ropes may contain older reconnected
flux at a later evolutionary stage and may therefore be more force-free. |