Received 2010-10-13; published 2010-10-22
We report detailed, in situ, observations by the four Cluster spacecraft of magnetic reconnection in near-Earth space. The space is filled with charged gas consisting of electrons and ions called plasma. Magnetic reconnection is one of the most important mechanisms for energy conversion in near-Earth space, laboratory and astrophysical plasmas. Solar flares, coronal mass ejections, geomagnetic storms and saw-tooth oscillations in tokamaks are a few examples that crucially depend on magnetic reconnection.
In the Earth's magnetotail, which is created behind the Earth by the solar wind and the Earth's magnetic field, reconnection occurs rather commonly. Magnetic reconnection heats the plasma, which can be 10 times hotter than in the centre of the Sun. We have explored in detail a magnetotail reconnection site and observed there a special type of plasma wave, an electron-scale electrostatic solitary wave, which indicates the presence of strong small-scale currents in the plasma. If the electric current becomes so intense that the plasma electrons carrying it start moving faster than the average (thermal) speed, then the plasma becomes unstable to wave generation. The energy contained in the current is then transferred to the waves, which dissipates the current.
We have discovered that the solitary waves are observed within a magnetic island - a huge plasma blob confined by the magnetic field. The waves are observed at the center of a magnetic island shortly after the reconnection process that produced the island itself. Numerical simulations suggest that collision and merging between islands should be rather common in different plasma environments and also rather violent in terms of energy release, causing efficient energization of both electrons and ions. For example production of anomalous cosmic rays can be caused by merging of magnetic islands in the vicinity of the heliopause.
The observed electrostatic waves serve as a "smoking gun" evidence for the presence of rather "extreme" small-scale currents. Our result also show that electrostatic solitary waves are one of the mechanisms responsible for dissipation of reconnecting currents at small scales and also provides an important background for future reconnection studies, e.g. by the coming NASA/MMS and the planned JAXA/SCOPE missions.
Khotyaintsev, Yu. V., A. Vaivads, M. André, M. Fujimoto, A. Retinó, and C. J. Owen, Observations of Slow Electron Holes at a Magnetic Reconnection Site, Phys. Rev. Lett. 105, 165002 (2010). As well as first author Yuri Khotyaintsev, contributions to the article were also made by Andris Vaivads and Mats André from IRF in Uppsala.
Yuri Khotyaintsev, scientist, IRF Uppsala, tel. +46-18-471 5929, firstname.lastname@example.org
Physical Review Letters: http://link.aps.org/doi/10.1103/PhysRevLett.105.165002
Press release: http://www.irf.se/link/press_smoking_gun_1010
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.