Near-Earth Space

CLUSTER a collection of four spacecraft flying in formation around Earth, opens a new window on ‘magnetic reconnection’ in the near-Earth space

Magnetic reconnection is a process that can occur almost anywhere that a magnetic field is found. In a reconnection event, the magnetic field lines are squeezed together somehow and spontaneously reconfigure themselves, releasing energy.

When it occurs near the surface of the Sun, such an event powers giant solar flares that can release thousands of millions of tonnes of electrically charged particles into space.

The Earth's magnetic field creates a buffer zone, the magnetosphere, between our planet's atmosphere and the particles released during these eruptions. The Sun also releases a steadier flow of charged particles called the solar wind. On the large-scale, any heading this way buffet the magnetosphere, and are deflected by it. Plasma physicists describe this behaviour with a theory called 'magneto-hydrodynamics' (MHD).

On smaller scales, however, the picture becomes rather more complicated. The particles can actually flow across the magnetic field lines.This makes the mathematics of the behaviour more difficult. First to misbehave are the ions (positively charged particles). These break away from simple MHD on scales of less than a few hundred kilometres. On even smaller scales, less than 10 kilometres, the electrons (negatively charged particles) begin playing by other rules, too.

Magnetic reconnection within Earth's magnetosphere regularly takes place on the night-time side of our planet, where the flow of the solar wind stretches out the magnetic field into a long tail. When the field reconnects in this region, it triggers jets of energetic particles that can cause auroral lights but can also damage satellites.

Read more from ESA International
Cluster opens a new window on ‘magnetic reconnection’ in the near-Earth space
The original paper, "Cluster PEACE observations of electron pressure tensor divergence in the magnetotail," by P.D. Henderson et al., is published in Geophysical Research letters (Vol. 33, L22106, doi:10.1029/2006GL027868, 2006).
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