Cluster observations of whistler-mode waves.
(Image: Y. Khotyaintsev, Swedish Institute of Space Physics)
A team from the Swedish Institute of Space Physics (Institutet för rymdfysik, IRF) and Mullard Space Science Laboratory at University College London have made an important breakthrough with their study of high speed plasma flows, often referred to as jets, which are extremely common across the Universe. Such jets are observed in Earth's magnetosphere, in solar flares, and near various objects powered by black holes. Rare in-situ measurements of plasma flows made by the European Space Agency’s (ESA’s) Cluster spacecraft have provided new insights into the processes that modify these streams of ionised particles.
Most visible matter in the Universe exists as plasma, an extremely hot gas in which atoms have been stripped of electrons so that the particles become electrically charged (ionised) and magnetised. How this plasma is heated to create high-energy particles remains a fundamental question, since astrophysical plasmas are often so sparsely populated that collisions between particles are extremely rare.
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 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.
One place where such processes can be studied by spacecraft is on Earth's night side, where the planet's magnetic field is drawn out into a long tail. In the centre of the tail is a region known as the plasma sheet, which contains plasma with ion temperatures of about 50 million degrees Celsius, making it one of the hottest places in the Universe. When magnetic reconnection occurs in the magnetotail, the plasma sheet is energised and jets are created.
Observations of high energy jets in the plasma sheet provide key insights into the processes that accelerate and heat the plasma. In particular, in situ measurements made by the four Cluster spacecraft have made it possible to pinpoint the mechanism responsible for the acceleration of energetic particles.
The team from the Swedish Institute of Space Physics and Mullard Space Science Laboratory made their important breakthrough while analysing Cluster observations made on 3 September 2006.
At that time, the four satellites 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.
The plasma jet was produced by magnetic reconnection, which occurred when the magnetic field lines from the south and north sides of the plasma sheet suddenly joined together. This focused the particles along the field lines, creating a jet. However, prior to the flow maximum, the team noticed a sharp increase in the strength of the magnetic field and an accompanying increase in electron energy to hundreds of kiloelectronvolts (keV).
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, together with the enhanced magnetic field, caused the front of the jet to slow down. This led to a pile up of the magnetic field in the plasma, rather like snow piling up in front of a moving snow plough.Increasing magnetic field strength in this so-called "flux pileup region" led to further heating of the plasma and acceleration of the electrons - a process known as betatron acceleration. The Cluster data revealed the presence of so-called "whistler waves", providing the first confirmation that a persistent pile up process was driving the particle acceleration.
"In a more steady state situation, these waves would disperse rather quickly," said Arnaud Masson, ESA’s Deputy Project Scientist for the Cluster mission. "However, the action of the pile up sustains them."
"In line with previous studies, this new paper confirms that the betatron process takes place after magnetic reconnection, creating high-energy particles in the Earth's magnetotail," Masson added.
"What is remarkable here is how the high resolution time measurements made by the Cluster satellites have enabled the presence of electromagnetic waves and their role in the betatron process to be determined for the first time. The waves play a key role in this process by scattering the particles and effectively enhancing collisions between them, which are otherwise very rare."
The new measurements also provide new insights into plasma jets that occur far beyond our planet.
"In a general astrophysical context, our observations suggest that one can expect particle acceleration anywhere that plasma jets are interacting with the local environment and braking," said Yuri Khotyainsev of the Swedish Institute of Space Physics, lead author of the paper in Physical Review Letters.
"In addition to shocks, particle acceleration and heating will result from the pile up of the magnetic field at the jet fronts," he added. "Our data explain how jets can become more energised when they are far from the original site of magnetic reconnection."Reference publication
The Swedish Institute of Space Physics (IRF) is a governmental research institute which conducts research and postgraduate education in atmospheric physics, space physics and space technology. Measurements are made in the atmosphere, ionosphere, magnetosphere and around other planets with the help of ground-based equipment (including radar), stratospheric balloons and satellites. IRF was established (as Kiruna Geophysical Observatory) in 1957 and its first satellite instrument was launched in 1968. The head office is in Kiruna (geographic coordinates 67.84° N, 20.41° E) and IRF also has offices in Umeå, Uppsala and Lund.
Institutet för rymdfysik, IRF, är ett statligt forskningsinstitut under Utbildningsdepartementet. IRF bedriver grundforskning och forskarutbildning i rymdfysik, atmosfärsfysik och rymdteknik. Mätningar görs i atmosfären, jonosfären, magnetosfären och runt andra planeter med hjälp av ballonger, markbaserad utrustning (bl a radar) och satelliter. För närvarande har IRF instrument ombord på satelliter i bana runt tre planeter, jorden, Mars och Saturnus. IRF har ca 100 anställda och bedriver verksamhet i Kiruna (huvudkontoret), Umeå, Uppsala och Lund.