ESA Science & Technology11-Jul-2005 14:59:58

Science Results

Multi-point observations of magnetic reconnection

Magnetic field component By versus Bx over 2 neutral sheet crossings and a sketch of reconnected current sheet structure with Hall currents. Purple line show the plasma flow observed on the ion instrument, the thin blue line are the current sheets, and the red dotted line shows the Cluster trajectory. (From Runov et al., Geophys. Res. Lett., 2003b)

Magnetic reconnection is a Universal phenomenon process in magnetized plasma where magnetic field lines are broken and then reconnected. This explosive process is converting magnetic energy into particle heating and acceleration. Reconnection is believed to occur is all magnetized bodies, from astronomical objects to the Sun and in the magnetosphere. Reconnection is occurring on the front of the magnetosphere and allows solar wind plasma to enter in the magnetosphere. It can also occurred in the magnetotail where it is believed to produce geomagnetic substorms.

The Cluster quartet allows, for the first time, to unambiguously determine the characteristics of the near-Earth's reconnection line on the ion scale. During a sequence of four traversals of the neutral sheet, a reversal in fast ion flow from tailward to Earthward and a concurrent change of field line curvature (the latter determined from the 4 s/c magnetometer measurements) gave clear evidence that the near-Earth neutral line was passing tailward over the Cluster tetrahedron.

During the current sheet crossings, the X-components of the magnetic field showed a very thin current sheet with a vertical extend of about 500 km near the neutral line, with bifurcated current sheets on either side. The changes in the magnetic Y-components during the first and last traversal gave clear evidence of the Hall effect, which is typically thought of being indicative of the decoupling of the ions from the magnetic field and the electron fluid.

Legend of reconnection_pointing flux figure: Poynting flux S measured on C1, C3 and C4, X component of the magnetic field measured on C1, and spacecraft potential (indicating the electron density) on 21 August 2002. The sketch on the right indicate the propagation of the reconnection line through the spacecraft.

Other evidence of tail reconnection is the observation that the poynting flux, which indicates the flow of energy in the plasma, is opposite on the different spacecraft. On 21 August 2002, the Cluster spacecraft were located in the tail and observed at the beginning a pointing flux directed tailward. This means that energy is flowing toward the tail and that magnetic reconnection was occurring between the spacecraft and the Earth. About 10 min. later while Cluster 1 and Cluster 3 were still observing the pointing flux tailward, SC4 observed it Earthward. That means that the reconnection point was in between the three spacecraft. About 30 min later, then all spacecraft were observing the pointing flux directed Earthward, indicating that the reconnection line would now be further down the tail.

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