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What is the source of magnetotail flux-ropes?

12 December 2011

Artist impression of the Earth's bow shock. (c) UCL

Travelling compression regions (TCRs) are perturbations in the magnetotail lobe magnetic field caused by structures moving Earthward or tailward within the plasma sheet. Previous works have suggested that these structures are created by either time-dependant reconnection occurring at a single X-line, forming a flux-bulge-type structure, or space-variant reconnection at multiple X-lines, forming flux-rope-type structures. By analysing a TCR and its source structure using the Cluster spacecraft, Beyene et al. (2011) have endeavoured to determine which of these mechanisms creates TCRs.

Beyene et al. (2011) examined an event in which Cluster 2 observed a TCR while the 3 remaining Cluster spacecraft observed the underlying magnetic structure at a range of distances from the neutral sheet.  As the structure passed the spacecraft, Cluster 2 observed a compression in the lobe (a TCR) and Cluster 1 and Cluster 4 observed a bipolar signature in BZ, plasma-sheet-like plasma and field-aligned electron flows. Cluster 3 passed closest to the centre of the structure.

The figure shows data from Cluster 3 during the passage of the magnetic structure. The top panel shows the magnetic field. The next four panels show the energy and pitch angle distributions of the ions and electrons. The bottom four panels show the ion density, temperature, plasma beta and velocity (perpendicular to the magnetic field).

Observations from Cluster 3 during the passage of a magnetic structure causing a TCR. From Beyene et al., 2011 (Ann. Geophys.)

C3 observed two separate reductions in the plasma density (with field-aligned electron flows); these drop-outs in the plasma sheet were possibly created by the actions of X-lines. The second drop-out in the plasma sheet also includes a reversal of the ion flow, a signature consistent with the passage of a reconnecting X-line past the spacecraft. Between the X-lines, the plasma outflow from the X-lines caused an increase in pressure which led to a localised expansion of the plasma and also the observations at Cluster 1 and Cluster 4. 

The observations do not uniquely match either of the flux rope or the flux bulge predictions although the observation of two plasma sheet drop-outs (interpreted as X-lines, one active, one dormant) with plasma-sheet-like between them and only one TCR is a situation expected in multiple X-line reconnection.

For more information see:

Beyene, S., Owen, C. J., Walsh, A. P., Forsyth, C., Fazakerley, A. N., Kiehas, S., Dandouras, I., and Lucek, E.: Cluster observations of a transient signature in the magnetotail: implications for the mode of reconnection, Ann. Geophys., 29, 2131-2146, doi:10.5194/angeo-29-2131-2011, 2011.

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