Abstract
The advent of missions comprised of phased arrays of spacecraft, with separation distances ranging down to at least mesoscales, provides the scientific community with an opportunity to accurately analyse the spatial and temporal dependencies of structures in space plasmas. Exploitation of the multi-point data sets, giving vastly more information than in previous missions, thereby allows unique study of their small-scale physics. It remains an outstanding problem, however, to understand in what way comparative information across spacecraft is best built into any analysis of the combined data. Different investigations appear to demand different methods of data co-ordination. Of the various multi-spacecraft data analysis techniques developed to affect this exploitation, the discontinuity analyser has been designed to investigate the macroscopic properties (topology and motion) of boundaries, revealed by multi-spacecraft magnetometer data, where the possibility of at least mesoscale structure is considered. It has been found that the analysis of planar structures is more straightforward than the analysis of non-planar boundaries, where the effects of topology and motion become interwoven in the data, and we argue here that it becomes necessary to customise the analysis for non-planar events to the type of structure at hand. One issue central to the discontinuity analyser, for instance, is the calculation of normal vectors to the structure. In the case of planar and `thin' non-planar structures, the method of normal determination is well-defined, although subject to uncertainties arising from unwanted signatures. In the case of `thick', non-planar structures, however, the method of determination becomes particularly sensitive to the type of physical sampling that is present. It is the purpose of this article to firstly review the discontinuity analyser technique and secondly, to discuss the analysis of the normals to thick non-planar structures detected in magnetometer data. Key words. Space plasma physics (discontinuities; instruments and techniques)
Highlights
Knowledge of the motion and size of structures in space plasmas provides vital clues as to their physics
Non-planar boundary surface topology, thickness of the boundary layer, and motion has been of interest to investigators in, for example, the solar wind where interplanetary shocks have been studied (e.g. Chao and Lepping, 1974; Lepping and Chao, 1976)
We have presented a summary of the discontinuity analyser (DA) technique, based on magnetometer data
Summary
Knowledge of the motion and size of structures in space plasmas provides vital clues as to their physics (such as composition, structure, relationship to the local and/or global plasma conditions). In terms of boundary analysis, there is an apparent distinction between macroscopic parameters (such as motion, orientation and form) and the boundary structure itself (as sampled individually by each spacecraft), it is not always a priori obvious how spacecraft dierences are to be combined with time series information, except where the boundary is planar orthin' (Dunlop and Woodward, 1998) Both sources of information may, in principle, provide independent knowledge of the boundary normals, for instance, but depend on these key boundary properties. Dunlop and Woodward (1998) describe in detail a multi-spacecraft magnetometer data analysis technique, which they call the discontinuity analyser (DA), designed to determine the structure, orientation and motion of boundaries using calculated normal vectors at each spacecraft to the structure as well as the associated times of encounter with the structure. We conclude with a discussion of the DA and the rami®cations on it of the thick boundary normal analysis
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