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Scientists Find a Magnetic Null in Nature

An international team led by Chinese scientists has found the first in situ evidence for theexistance of a null point in the central area of megantic reconnection.

The discovery was reported in the July issue of Nature Physics.

Magnetic reconnection, a process whereby the lines of a complex magnetic field break andreconnect to alter its structure drastically, is of fundemental importance to the studies ofastrophysical, space and laboratory plasmas, says XIAO Chijie, lead author of the NaturePhysics article and an astrophysicst from the National Astronomical Obervatories at CAS(NAOC).

For instance, solar flares, the largest explosions in the solar system, are caused by reconnectionof large systems of magnetic flux on the Sun, releasing in minutes energy that is stored in themagnetic field over a period of weeks to years, which is sometimes billion times more powerful

than an atomic bomb. Magnetic reconnection in Earth's magnetosphere is responsible for theaurora, and it is important to the science of controlled nuclear fusion, which provides scientificbasis for the so-called "man-made sun," because it is one mechanism preventing magneticconfinement of the fusion fuel.

Identifying the structure around the point at which the magnetic field lines break andsubsequently reconnect, known as the magnetic null point, is crucial to improving ourunderstanding of the reconnection. But owing to the inherently three-dimensional nature of thisprocess, magnetic nulls are only detectable through measurements obtained simultaneouslyfrom at least four points in space. Until recently, there was no direct observation evidence fortheir existance.

A schematic diagram of the spiral structureof the magnetic field around the null.

Using data collected by the four spacecraft of the

Cluster constellation and Double Star Program (DSP)

and the method of Differential Topology developed byZHAO Hui, a Ph. D student at NAOC, Xiao and his co-

workers reported the first in situ evidence for the

structure of an isolated magnetic null. The resultsindicate that it has a positive-spiral structure whose

spatial extent is of the same order as the local ioninertial length scale, suggesting that the Hall effect

could play an important role in 3D reconnection

dynamics.

At the proposal of CAS scientists, the two DSP satellies join hands with four Cluster ones tocarry out the unprecedented six-point simultaneous observation network in space. It also

provides the state-of-the-art approach for space exploration in terms of terrestrial spaceenvironment, and three-dimensional characteristics of geomagnetic field. Magnetic

reconnection is one of the major issues of the DSP-Cluster mission.

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