While astrophysical jets are often powered by black holes, high-speed plasma flows are also ejected by solar flares and can even arise in the Earth’s magnetosphere. The four Cluster spacecraft have been lucky to observe one of the latter events from inside the plasma flow and witness jet-braking and plasma-heating processes.

The story of the Cluster mission to study the Earth’s magnetosphere and its environment in three dimensions is long and tumultuous. First proposed to the European Space Agency (ESA) in November 1982, the four identical satellites, to be flown in a tetrahedral configuration, should have benefited from a "free" launch on the first test flight of the Ariane-5 rocket. Unfortunately, this flight lasted just 37 s and ended abruptly by breaking up during launch on 4 June 1996. To recover at least part of the 10-year development effort, ESA decided to build one additional Cluster satellite named Phoenix, named after the mythical bird reborn out of its ashes. It soon became apparent that the scientific objectives would not be met by Phoenix alone and that a second Ariane-5 launch would be too expensive. Eventually, in the summer of 2000, all of the obstacles had been overcome and four new Cluster satellites were successfully carried into space, two at a time by Russian Soyuz rockets.

Eleven years after the launch, the Cluster mission is still operating, providing insights into the physical processes involved in the interaction between the solar wind and the magnetosphere of the Earth. These interactions often send electrons and ions to the Earth’s magnetic poles, where they hit neutral gas in the atmosphere and produce aurorae. This occurs either by direct entry of solar-wind particles through the polar cusps or by plasma acceleration in the magnetotail during substorms. The magnetotail is located on the night side of the Earth, where the planet’s magnetic field is drawn out into a long tail by the solar wind. It hosts in its centre the plasma sheet, a large reservoir of particles with ion temperatures of about 50 million degrees. When magnetic reconnection occurs in the magnetotail, the plasma sheet is energized and jets are created.

On 3 September 2006, the four Cluster satellites happened to fly through the magnetotail at an altitude of roughly a quarter of the Earth–Moon distance, just in time to witness the sudden rearrangement of the magnetic field leading to the explosive release into the plasma of much of the stored magnetic energy. The instruments aboard the four Cluster satellites monitored the flux of energetic particles focused along the magnetic field lines into a jet pointing towards the Earth. These observations and their implications are now published in Physical Review Letters by a team from the Swedish Institute of Space Physics, Uppsala, and the Mullard Space Science Laboratory, University College London.

The data indicate that the original, fairly "cold" jet was subsequently heated by a separate mechanism similar to friction. At first, the flow’s interaction with other particles and the enhanced magnetic field closer to Earth caused the front of the jet to slow down. This led to a pile up of the magnetic field in the plasma and to further heating and acceleration of the electrons. The process is called betatron acceleration in reference to the particle accelerators developed in the early 1940s, which used a variable electromagnetic field to accelerate electrons circling in a toroidal vacuum chamber. As Yuri Khotyaintsev, the lead author of the study, points out, this process is likely to occur in other types of astrophysical jets whenever they are interacting with the local environment and braking. So, not only shocks but also the pile-up of the magnetic field at the jet front can result in particle acceleration and heating.