Cluster observations of a transient signature in the magnetotail: implications for the mode of reconnection

Abstract

Abstract. 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 fluxrope-type structures. In this study we examine 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. The magnetic structure has a velocity of (99, 154, −31) km s −1 in GSM (|V | = 186 km s −1 ), an estimated size of 1.19 R E along the direction of travel and a size between 1.94 and 2.86 R E in the direction perpendicular to the current sheet. As the structure passes the spacecraft, Cluster 1 and Cluster 4 observed a bipolar signature in B Z , plasma-sheet-like plasma and fieldaligned electron flows. Cluster 3 passed closest to the centre of the structure and 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 1Correspondence to: S. Beyene (sb2@mssl.ucl.ac.uk) and Cluster 4 and the TCR. Our 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 plasmasheet-like between them and only one TCR is a situation expected in multiple X-line reconnection.

Evidence of transient reconnection in the outflow jet of primary reconnection site

“…The flux bulge resulting from time-dependent reconnection could be used also to explain the observed Fr2 (e.g. Beyene et al, 2011). In this scenario, the spacecraft away from (within) the central plasma sheet will observe an asymmetric bipolar B z signature (a unipolar enhancement of B z ), and no core magnetic field is expected inside the flux bulge.…”