We use a database of magnetic conjunctions between Defense Meteorological Satellite Program (DMSP) and geosynchronous satellites to (1) evaluate the mapping of the Tsyganenko T89a magnetic field model between the ionosphere and geosynchronous orbit and (2) test the ability of different magnetospheric parameters to identify the T89a stretching level (parameterized by Kp=0--5) which comes closest to reproducing the observed conjunctions. The database, covering a range of magnetospheric activity, local times, and season, was compiled using 4 months of plasma electron data from two geosynchronous satellites and three low-altitude DMSP satellites. The conjunctions were determined using an automated spectral comparison and selection technique which identified intervals of close spectral matching between the distributions observed at the two satellites. Comparison of the observed conjunctions with the closest T89a mappings show that the observed range of field line stretch at geosynchronous orbit (in the dawn and dusk local time sectors) is generally greater than that allowed by T89a. In nearly half the cases, the observed mappings are less stretched than the least-stretched (Kp=0) version of T89a. We found that none of the magnetospheric parameters we evaluated (Kp,AE,Dst, the Auroral Boundary Index (ABI), or the inclination of the magnetic field at geosynchronous orbit) could provide a unique pointer to the most appropriate T89a stretching level for every conjunction. However, both the tilt of the field at geosynchronous orbit and the ABI show a strong correlation with the observed degree of field stretch and indices based on these parameters are thus promising candidates for incorporation into future models.¿ 1997 American Geophysical Union