There remains much uncertainty over how magnetospheric boundaries map to ionospheric altitudes. Provided that magnetic flux tubes act as equipotentials, satellite measurements of electric fields and energetic particles can be readily matched to ionospheric signatures observed with an instrument complement (riometers, magnetometers, optical instruments etc.) such as that of SESAME. However, there is evidence for the occurrence of field-aligned potential drops, though this has been questioned by Bryant et al. (1991), and there are still major uncertainties in equipotential mapping (e.g. Heelis and Vickrey, 1990).
At an early stage in the GGS mission, it will be necessary to identify the ionospheric signatures of magnetospheric boundaries as recorded by the SESAME instrumentation. This will involve all the instruments on the POLAR spacecraft whilst it is flying through the field of view (fov) of the ground-based experiments. This type of approach has already been used very successfully by Baker et al. (1991) to identify the signatures of the cusp and the low latitude boundary layer in PACE data and on the nightside (Pinnock private communication) to identify precipitation boundaries.
It is known that high latitude VLF/ELF hiss (auroral hiss) is essentially co-located with the auroral oval, whereas chorus occurs principally at lower L-shells (Tsurutani and Smith, 1977; Ondoh et al., 1981). Further, the character of VLF/ELF signals varies across the plasmapause (Bullough et al., 1969). Therefore, a network of ground-based ELF/VLF wave activity observatories can be used to identify the temporal variations of important geospace boundaries. However, GGS space-borne VLF/ELF observations will be essential to act as a reference frame into which to place the ground-based data. A similar approach can be adopted with a ground-based network of high time resolution magnetometers.
The three dimensional, time-dependent, global ionosphere and thermosphere models, such as those developed at Utah State University, NCAR and UCL/Sheffield, are essential tools for understanding geospace and the interactions between different parts of the system. These models are now sufficiently mature that they can be refined by comparison with the observations and then be used to quantify our understanding. At present, their greatest limitation is uncertainty in input parameters. In particular, the size and shape of the auroral oval, the location of the cusp, the magnitude and direction of the electric fields and currents, and the time variations of these parameters are still inadequately specified or modelled. The GGS mission, with its combination of ground-based and space-based observations, offers unique opportunities to improve the input parameters to such models, and thus critically test their predictive capabilities, and the physical processes upon which they are based.