Reconnection:: Space plasma simulations for multi-spacecraft satellite observations in the ISTP era

In the ISTP era multi-spacecraft measurements have not only become a tool for phenomenological and correlative studies of the solar- terrestrial coupling. They also open new opportunities to test, verify or falsify basic physical concepts. For the use of multi-spacecraft measurements in this sense numerical modelling and simulations are becoming more and more a necessary tool for interpreting the observations correctly. We take the reconnection process as an example. Reconnection is supposed to release magnetic field energy from the sun, to couple the Earth's magnetosphere to the solar wind and to suddenly release previously stored magnetic energy in the course of magnetospheric substorms, Hence, reconnection is a key concept in the solar-terrestrial relationship. On the other hand it is difficult to theoretically grasp collisionless reconnection in space plasmas. This complicates its experimental verification. In this paper we present some selected essentially kinetic features of spontaneous reconnection through collisionless thin current sheets in two and three dimensions: These should be observable, e.g., at the onset of magnetospheric substorm. The kinetic approach is necessary due to the cross-scale coupling, inherent to collisionless reconnection. We demonstrate that microscopic magnetic islands of the Debye length scale are always present through thin current sheets. They are, however, reversible and occur at the level of thermal fluctuations. Macroscopic reconnection and island structure formation starts with irreversible energy transfer from fields to plasma. We demonstrate the different routes to macroscopic merging in two and three dimensions. In two dimensions Landau damping in the potential wells of magnetic islands causes the classical slow tearing mode instability. In three dimensions a sausage mode bulk current instability accelerates the transition to macroscopic reconnection. As a result three-dimensional magnetic islands are formed, which are limited in all three spatial directions. We, finally, demonstrate how multi-spacecraft observations of the ISTP era together with well posed computer simulations can help to understand collisionless reconnection by means of in situ measurements in the Earth's magnetosphere. (C) 1998 Elsevier Science Ltd. All rights reserved.