Structure Of Diffusion Region And Electron Acceleration In Collisionless Magnetic Reconnection

Magnetic reconnection is one important subject of space physics and laboratory plasma physics. It is a very difficult issue and hot topic also. Observation by spacecrafts is one important method to study magnetic reconnection. Using measurements of the Cluster spacecraft, we study structure of ion diffusion region, electron density cavity, energetic electron acceleration, electron pitch angle distribution, substorrn. and so on. The results are shown as follows.1. Structure of ion diffusion region of collisionless magnetic reconnection and secondary magnetic islandUsing the measurement of Cluster on 4th October 2003. a magnetic reconnection event is reported. One secondary island with a strong core magnetic field is found near the center of the ion diffusion region. The aspect ratio of the island is about 2:1. which means the island is squashed in z axis of GSM coordinates. The fluxes of energetic electron up to 200keV peak within the island and the parallel temperature is stronger than the perpendicular temperature. Furthermore, electron density peaks in the outer region while dips in the core region of the island. At the same time, a powerful electron beam along the magnetic field is found in the outer region of the island, which would form an electron current antiparallel to the magnetic field lines in the region. The electric current produces the strong core magnetic field observed in the core region. This core magnetic field will expel electrons out of the core region. Thus electrons will pile up in the outer region of the island. Electron energy spectrum in the ion diffusion region displays fiat-top distribution which can be observed in the outer region of the island also. However, the distribution shows exponential feature in the core region. Electrons with the lower energy are accelerated to the higher energy. Consequently, the PSD at the lower energy declines whereas that at the higher energy increases, which leads to the constant PSD over certain energy range. So. the flat-top distribution is formed. Both the reconnection electric field and lower hybrid waves are observed in the diffusion region and the outer region of the island. The electric field and lower hybrid waves might accelerate electrons to higher energy.2. Electron pitch angle distribution in ion diffusion region of collisionless magnetic reconnectionOn 10 September 2001. Cluster crossed a diffusion region of magnetic reconnection from tailward to earthward in the magnetotail. In the vicinity of the X line, at lower energies the distributions are field-aligned bidirectional anisotropic. while at higher energies, the electrons are observed to flow away from the X line along the magnetic field lines. The electron distributions change largely in the outflow region. At the edge of the outflow region, at lower energies, the electrons flow toward the X line, while the electrons at higher energies are directed away from the X line. When the spacecraft approaches the center of the current sheet, at lower energies, the electrons have field-aligned bidirectional distributions, while at higher energies, the electron distributions are isotropic. The generation mechanisms of such distributions are explained by following typical electron trajectories in the electric and magnetic fields of magnetic reconnection which are obtained in two-dimensional particle-in-cell simulations. It is shown that the observed high-energy electrons directed away from the X line both in the vicinity of the X line and in the outflow region are due to the acceleration by the reconnection electric field near the X line, and the field-aligned bidirectional distributions at lower energies are caused by the effects of the magnetic mirror in the reconnection site. The isotropic distributions at higher energies in the outflow region are the results of the electron stochastic motions when their gyroradii are comparable to the curvature radii of the magnetic field lines.3. Spatial distribution of energetic electrons in ion diffusion region of collisionless magnetic reconnectionBy the magnetic reconnection event observed by Cluster on 10 September 2001, we analyzed spatial distribution of energetic electrons in the ion diffusion region. Most of energetic electrons reside in the magnetic field line pileup region, and a depletion of energetic electrons can be found near the centre of the diffusion region. The temperature shows a similar distribution. Moreover, the energetic electron fluxes in the earthward of the active X-line are larger than those in the tailward. Therefore, we can conclude that the diffusion region is only region where electrons are accelerated, and the maximum energy reaches in the magnetic field line pileup region. There is no secondary acceleration in the magnetic field line pileup region; otherwise we should find that the peak of the energetic electrons will continue to increase when the satellites leave this region. The asymmetrical distribution of energetic electrons between the earthward and the tailward of the X line might be caused by the dipolar magnetic field of Earth.4. Single substorm eventSubstorm is closely correlated with magnetic reconnection in the magnetotail. With measurements from the Cluster spacecraft in the middle magnetotail, LANL-01 and LANL-97 at the synchronous orbit, POLAR in the near-earth magnetotail and IMAGE at polar region, a single substorm event is presented. The analysis indicates that magnetic reconnection is 3 minutes earlier than the current disruption in the near-earth magnetotail. Aurora brightening is observed by IMAGE 4 minutes after the current disruption. Simultaneously, the sharply increment of the AE index implies the substorm onset. The observational result is consistent with the Near Earth Neutral Line model (NENL).