Magnetic field lines are known to reorganize themselves in plasmas, converting magnetic to particle energy. Evidence harvested from the solar wind implies that the scale of the effect is larger than was thought.
The reconnection of magnetic fields that occurs in the ionized gases known as plasmas is a fascinating and enigmatic phenomenon. It transforms magnetic field configurations, converting energy stored in those fields into kinetic energy of the electrically charged particles that make up the plasma. Direct and indirect proof for the existence of the effect comes from many different quarters, ranging from explosive energy releases in the Sun's atmosphere to catastrophic disruptions in nuclear fusion reactors. On page 175 of this issue, Phan et al.1 present the latest observations of magnetic reconnection in the solar wind — a stream of plasma that is continuously emitted by the Sun — and in doing so clarify the spatial and temporal scales that govern the process.
Plasmas, which in space consist mainly of protons and electrons, are commonly permeated by magnetic fields. Plasma and field tend to behave as if frozen together: the plasma's particles lend magnetic field lines physical form by gyrating around them; equally, when the particles move, the magnetic field lines move with them. This means that, rather like an individual strand in a bowl of spaghetti that is being stirred, the same field line — although constantly changing position and shape — always connects the same particles of the plasma (Fig. 1a).
But what happens if the plasma's motion brings together two magnetic field lines that point in opposite directions? The frozen-in picture assures us that all particles will remain on their respective field lines, regardless of how hard these are pushed together. But this picture is only an approximation, and in some circumstances — poorly understood at present — field lines slip relative to the plasma, and break and cross-link at an ‘X-point’ (Fig. 1b). The field lines, now sharply bent, act as a slingshot, imparting their stored energy to the particles and ejecting them at high speeds. This is the phenomenon known as magnetic reconnection2,3,4.
Field lines in neighbouring planes may also reconnect, resulting in an ‘X-line’ linking many X-points (Fig. 1c). This is essentially what happens when the solar wind encounters Earth's magnetic field. The solar wind transports the Sun's magnetic field into interplanetary space, so it could not penetrate Earth's field if the frozen-in theorem held. But direct and indirect observations5,6,7 show that the frozen-in condition breaks down on the magnetopause, the surface that separates the solar wind and Earth's magnetic field. This allows terrestrial magnetic field lines to become connected directly with the Sun, and so the solar-wind plasma flows along reconnected field lines into Earth's magnetosphere.
But can reconnection happen within the solar wind itself? Because of the complex spatial and temporal variations in conditions near the Sun's surface, there are abrupt changes in the density and velocity of the solar wind, and associated rotations in the direction of its magnetic field. Without the breakdown of the frozen-in theorem, the plasmas on the two sides of such a transition would never mix. If such a breakdown occurs, so, by necessity, does magnetic reconnection: a spacecraft positioned in the solar wind would see the passage of a transition not just through sudden changes in the properties of the plasma and magnetic field, but through high-speed plasma flows characteristic of reconnection. NASA's Advanced Composition Explorer, ACE, has recently observed exactly this8,9.
The ACE observations used a single spacecraft, so ACE could not measure the length of the X-line causing the plasma flows. It was also not clear whether reconnection was active for longer than the few minutes it took the reconnection layer to sweep over the spacecraft at solar-wind speed. Phan et al.1 address these open questions by taking advantage of a fortuitous configuration of three spacecraft — NASA's ACE and Wind, and one of the European Space Agency's four Cluster spacecraft — that gave the researchers a large baseline for their measurement (see Fig. 1 on page 175). On 2 February 2002, one after the other, all three spacecraft recorded the passage of a reconnection layer with essentially identical characteristics, in particular the same net plasma and magnetic field changes and the same plasma flows.
The observed plasma flows agreed quantitatively with theoretical predictions based on the change in magnetic field across the layer and the local plasma density. Once the authors had inferred the direction of the X-line from a simple geometrical argument, they could calculate that the X-line must have been at least 2.5 million kilometres long — almost 200 times the diameter of Earth. And from the spacing of the passage times over the three spacecraft, it was evident that reconnection was not explosive, but instead operated steadily for at least two-and-a-half hours.
Phan and colleagues' observations1 of the spatial and temporal characteristics of magnetic reconnection will fuel the sometimes heated debate over what the phenomenon is like and what it can do. With the launch of NASA's STEREO mission, expected later this year, larger baselines will become available, and there is hope that the study of solar-wind reconnection can be extended to much larger scales. At the smallest scales, NASA's Magnetospheric Multi-Scale (MMS) mission, to be launched in 2013, will investigate the kinetic plasma processes near the X-line that allow the frozen field condition to be broken and reconnection to occur. The prospects for finally understanding the nature of reconnection, its ability to couple small- to large-scale phenomena, and the crucial role it plays in various cosmic settings, are excellent.
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