Cluster Examines Thin Current Sheets In The Magnetosphere
August 2, 2012

Cluster Takes A Closer Look At The Magnetosphere’s Thin Boundaries

April Flowers for - Your Universe Online

The Universe is filled with plasma, a charged gas consisting of ions and electrons. Thin sheets, or boundaries, with currents separate large plasma regions. These boundaries are where most of the exciting action in space happens.

Scientists at the Swedish Institute of Space Physics (IRF) have measured the fundamental properties of one of the waves mixing and accelerating plasmas within these sheets.

These thin current sheets are found in many surprising places. The processes accelerating electrons which hit the atmosphere and create beautiful auroras, like the Aurora Borealis, are often initiated in thin current sheets. Other planets, such as Jupiter and Saturn, have similar processes of auroras and thin current sheets. Thin current sheets separate plasma regions close to the hot solar surface, and similar boundaries should also be common around distant stars. In synthetic plasmas, thin boundaries are found in the tokamak plasma employed in nuclear fusion research and space observations may help us understand fusion plasmas.

The solar wind blows plasma at the Earth's magnetic field, causing the so-called magnetotail, stretching several hundred thousand kilometers downstream from the Earth. There is a thin current sheet separating the northern and southern part of the tail.

In large sections of space, the plasma is too tenuous for the particle to actually collide. Since these particles are charged, however, electric fields caused by some particles will interact with others. This forms rather specific waves in the electric field which exchange energy between the particles, replacing ordinary collisions.

Scientists have studied the lower hybrid drift waves for nearly 50 years. These drift waves are thought to play an important role in the formation of the narrow current sheets. The waves have a relatively short wavelength, making it nearly impossible before now to observe their fundamental properties. IRF's research team has recently been able to take direct measurements of the wavelength and velocity of these waves.

Previous attempts to measure the waves were made with a single spacecraft, which was found to be impossible. But, the European Space Agency's four Cluster spacecraft make it very possible.

Cluster is a group of four spacecraft flying in formation around Earth. They collect three-dimensional information about the relationship between Earth and the Sun, and the stream of perpetual subatomic particles that bombard the Earth.

In August 2007, while traveling through the Earth's magnetotail, two of the four spacecraft were only 40 kilometers apart. This allowed the research team to observe the same wave propagating past first one craft then the other. The wavelength could be determined to be about 60 kilometers and the velocity to about 1000 kilometers per second.

"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 Space Physics and a PhD student at Uppsala University. "The assumptions, used for several decades, have finally been verified by direct observations."

The results of this study were published in Physical Review Letters on July 31, 2012.

Image 2 (below): The four Cluster satellites in orbit round the Earth. Credit: ESA