Thin current sheets in space: where the action is

August 1, 2012
The four Cluster satellites in orbit round the Earth. (Artist's impression: ESA)

Much of the exciting action is space is confined to thin boundaries. The Universe is filled with plasma, a charged gas consisting of ions and electrons. Thin sheets with currents separate large plasma regions in space. Scientists at the Swedish Institute of Space Physics (IRF) have now finally measured the fundamental properties of one of the waves mixing and accelerating plasmas within these sheets.

Around Earth, the processes accelerating electrons which hit the atmosphere and cause beautiful auroras are often initiated in thin current sheets. Similar processes, and thin current sheets are found around other planets such as Jupiter and Saturn. Plasma regions close to the hot are separated by thin current sheets, and similar boundaries should also be common around . In man-made plasmas, thin boundaries are found in the tokamak plasma employed in nuclear fusion research and may help us understand fusion plasmas.

The blows plasma at the Earth's magnetic field. This causes the so-called magnetotail, stretching several hundred thousand kilometres downstream from the Earth. There is a thin current sheet separating the northern and southern parts of the tail.

In large parts of space, the plasma is too tenuous for the particles to actually collide. However, since the particles are charged, electric fields caused by some particles will interact with other particles. Often rather specific waves in the electric field interchange energy between the plasma particles. These waves replace ordinary collisions.

Thin current sheets in space: where the action is
The electric field forms vortices propagating past two of the Cluster spacecraft. Since the electrons have much lower mass than the ions and can easily be moved around by the electric field, the electrons create a magnetic field, as shown in the bottom panel. (Fig. from Norgren et al., PRL, 2012)

The lower hybrid drift waves have been studied for 50 years and are thought to play an important role in these narrow current sheets. However, due to their relatively short wavelength, it has been impossible to observe their . IRF's scientists have now, for the first time, been able to make direct measurements of the wavelength and velocity of these waves.

It has not been possible to measure the wavelength with a single spacecraft, but this can be done with the European Space Agency's four Cluster spacecraft. Taking advantage of the short 40 km separation between two of the four spacecraft in the magnetotail during August 2007, the scientists could observe the same wave propagating past first one and then the other spacecraft. The wavelength could be determined to be about 60 km (comparable to the radius of the electron gyro-motion in the magnetic field) and the velocity to about 1000 km/s (comparable to the ion velocity). The results appeared in the scientific journal Physical Review Letters on 31 July.

"We see small vortices that propagate in this narrow current sheet. They are just big enough so that both of the spacecraft can see them at the same time and be sure it is the same structure," says Cecilia Norgren of the Swedish Institute of and a PhD student at Uppsala University. "The assumptions, used for several decades, have finally been verified by direct observations."

Explore further: Cluster opens a new window on 'magnetic reconnection' in the near-Earth space

More information: Article in Physical Review Letters, vol. 109, issue 5 (2012):

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not rated yet Aug 01, 2012
Could plasma physics reveal a way to detect extrasolar planets directly from their interaction with the stellar winds of the stars they orbit?
1 / 5 (7) Aug 01, 2012
What is astounding is that this author somehow managed to avoid using the plasma physics term for this phenomenon - the double layer.

From http://en.wikiped...(plasma)


A double layer is a structure in a plasma and consists of two parallel layers with opposite electrical charge. The sheets of charge cause a strong electric field and a correspondingly sharp change in voltage (electrical potential) across the double layer. Ions and electrons which enter the double layer are accelerated, decelerated, or reflected by the electric field. In general, double layers (which may be curved rather than flat) separate regions of plasma with quite different characteristics.


The ionosphere is widely acknowledged to be a double layer. Wal Thornhill contends that the heliopause is a stellar double layer, which if true, might suggest that the Sun exhibits a net electric charge since DL's in the lab are observed to act as a sheath for the plasma's electrical charge.
5 / 5 (5) Aug 01, 2012
Perhaps the author avoided using the phrase "double layer" because in fact these are not double layers. A neutral current sheet is not a double layer, and as proof I direct you to a paper on "Double layer formation during current sheet disruptions in a reconnection experiment," http://adsabs.har...c9c18563
4 / 5 (1) Aug 01, 2012
This is not the first time anyone has thought of the concept, it is the first time we have direct observational evidence of it. So if you want to call them out on something, stick to what the announcement is ("IRF's scientists have now, for the first time, been able to make direct measurements of the wavelength and velocity of these waves."), and the results they presented.

The rest is just fluff, not even very bad fluff at that. If you have a problem with fluff, your going to have a problem with every article and announcement that this site hosts.
5 / 5 (2) Aug 02, 2012
I worked on instrument systems there (not the Cluster crafts) and studied astrobiology in that department, so I know some of them personally. (Uppsala University is my alma mater.) And something on the science from listening to their seminars: you need to search and analyze massive data sets to get results, and as noted the geometry of the crafts vs the phenomena can be low likelihood events. Hence the "fluff".

The abstract cuts away some of the fluff, btw. The predictions from LHDW theory stands up. And these waves "can strongly affect the electron dynamics" (stands for " ~10% of the electron temperature"), so it didn't topple earlier research as much as tested an important piece.

On the EU belief, these sheets aren't a lot of things. They aren't cold process plasma sheath layers either.

Conversely, all EU belief is, is a pseudoscience pattern search. It is well tested _every time_ they comment. It is also "insanity", when it is sloppily defined as repeating futile behavior.

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