Observations of high energy jets in the plasma sheet of Earth's magnetosphere provide key insights into the processes that accelerate and heat the plasma. Rare in situ measurements made by the four Cluster spacecraft have made it possible to pinpoint the mechanism responsible for the acceleration of energetic particles.
Cluster observes jet braking and plasma heating
One of the key energy conversion processes in magnetised plasmas is magnetic reconnection, when the sudden rearrangement of magnetic fields leads to the explosive release (depicted as a white glow in this animation) of much of the stored magnetic energy. Most of this energy is transferred to the surrounding plasma, causing acceleration and heating of the particles in the plasma.
On 3 September 2006, the four Cluster spacecraft were flying in tetrahedral formation through the centre of the magnetotail, about 96 000 km from Earth (roughly one quarter of the Earth-Moon distance). Their particle, electric and magnetic field instruments detected an Earthward-moving plasma flow in the plasma sheet region that was travelling at a maximum speed of more than 800 km/s.
Cluster's detection of 'whistler waves' at this time provided the first confirmation that the magnetic field in the plasma was piling up, an effect which led to the heating and acceleration of electrons - a process known as betatron acceleration.
These Cluster observations confirmed that the acceleration of energetic particles in the Earth's magnetotail is caused by magnetic reconnection followed by betatron acceleration.