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 f or phenomenological and correlative studies of the solar- terrestrial coupl ing. They also open new opportunities to test, verify or falsify basic phys ical concepts. For the use of multi-spacecraft measurements in this sense n umerical modelling and simulations are becoming more and more a necessary t ool for interpreting the observations correctly. We take the reconnection p rocess as an example. Reconnection is supposed to release magnetic field en ergy from the sun, to couple the Earth's magnetosphere to the solar wind an d to suddenly release previously stored magnetic energy in the course of ma gnetospheric substorms, Hence, reconnection is a key concept in the solar-t errestrial relationship. On the other hand it is difficult to theoretically grasp collisionless reconnection in space plasmas. This complicates its ex perimental verification. In this paper we present some selected essentially kinetic features of spon taneous reconnection through collisionless thin current sheets in two and t hree dimensions: These should be observable, e.g., at the onset of magnetos pheric substorm. The kinetic approach is necessary due to the cross-scale c oupling, inherent to collisionless reconnection. We demonstrate that micros copic 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 form ation starts with irreversible energy transfer from fields to plasma. We de monstrate the different routes to macroscopic merging in two and three dime nsions. In two dimensions Landau damping in the potential wells of magnetic islands causes the classical slow tearing mode instability. In three dimen sions a sausage mode bulk current instability accelerates the transition to macroscopic reconnection. As a result three-dimensional magnetic islands a re 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 recon nection by means of in situ measurements in the Earth's magnetosphere. (C) 1998 Elsevier Science Ltd. All rights reserved.