RECONNECTION
A special kind of violation of the frozen field condition is the process of reconnection. In the classic review article by Vasyliunas (1975) reconnection was defined as “the process whereby plasma flows across a surface separating regions containing topologically different magnetic field lines” (cf. also Priest and Forbes, 2000). But change of connectivity between plasma elements is possible without a separator. Independently of topology, two elements of plasma that are at one instant of time on a common magnetic field line can be on different magnetic field line at another instant, if the condition given above is satisfied somewhere between the plasma elements. This more general definition of reconnection was proposed by Schindler et al. (1988) and Hesse and Schindler (1988) and further elaborated by Birn et al. (1997), but the term reconnection is commonly used for reconnection at a separatrix.
Reconnection is considered to be one of the most important phenomena in cosmic plasma, as a means of topology change and energy release. In the Earth’s magnetosphere, reconnection takes place both at the magnetopause and in the tail current sheet. In addition, local reconnection of limited strands of magnetic flux, so-called flux transfer events, are also common (Le et al. 2008). The reconnection events in the geomagnetic tail that are associated with magnetospheric substorms have many similarities to the fast energy release that takes place in solar flares (Lin et al., 2008). In the magnetosphere, the phenomenon can be studied empirically in great detail by means of in situ
measurements (Paschmann, 2008). The value of this for understanding solar flares and other kinds of energy release in cosmic plasmas can hardly be exaggerated. Reconnection is an extremely complicated phenomenon, and this makes it even more important to have actual measurements to guide theoretical work. One reason for complexity is that reconnection involves coupling between widely different spatial scales, from system-scale structure through ion scales and down to electron scales. Therefore, multipoint measurements are essential. Multipoint measurements are at present being made with the still operational Cluster satellites and the more recent five satellites of the THEMIS project. For example, Cluster observations showed that the extent of the electron disffusion region can far exceed what is expected from simulations (Phan et al., 2007) Substantial further progress can be expected from another four-spacecraft mission, Magnetospheric Multiprobes, which has recently been approved by NASA. Due to the widely different spatial scales involved in reconnection, a major advance would be simultaneous multipoint measurements on each of the three spatial scales. Such a project, called Cross Scale, is part of ESA’s long term plan Cosmic Vision. It involves 10 or 12 satellites forming tetrahedrons on each scale.
For the Cross scale mission, reconnection studies is only one of its purposes. It will, if it realized, be a formidable tool for studying many other space plasma phenomena. This is in particular true for two phenomena, namely collisionless shocks and turbulence. Both of these are of fundamental importance in cosmic plasmas and both are prominent in the magnetosphere and in accessible regions of the solar wind.