AN EXAMINATION OF THE TSYGANENKO (T89A) FIELD MODEL USING A DATABASE OF 2-SATELLITE MAGNETIC CONJUNCTIONS

We use a database of magnetic conjunctions between Defense Meteorologi cal Satellite Program (DMSP) and geosynchronous satellites to (1) eval uate the mapping of the Tsyganenko T89a magnetic field model between t he ionosphere and geosynchronous orbit and (2) test the ability of dif ferent magnetospheric parameters to identify the T89a stretching level (parameterized by Kp = 0-5) which comes closest to reproducing the ob served conjunctions. The database, covering a range of magnetospheric activity, local times, and season, was compiled using 4 months of plas ma electron data from two geosynchronous satellites and three low-alti tude DMSP satellites. The conjunctions were determined using an automa ted spectral comparison and selection technique which identified inter vals of close spectral matching between the distributions observed at the two satellites. Comparison of the observed conjunctions with the c losest T89a mappings show that the observed range of field line stretc h 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 (K p = 0) Version of T89a. We found that none of the magnetospheric param eters we evaluated (Kp, AE, Dst, the Auroral Boundary Index (ABI), or the inclination of the magnetic field at geosynchronous orbit) could p rovide a unique pointer to the most appropriate T89a stretching level for every conjunction. However, both the tilt of the field at geosynch ronous orbit and the ABI show a strong correlation with the observed d egree of field stretch and indices based on these parameters are thus promising candidates for incorporation into future models.