Conjugate Observations of Field-Line Resonances in the Inner Magnetosphere from the MEASURE and SAMBA Magnetometer Chains

The newly installed SAMBA (South American Meridional B-field Array) chain is a low latitude meridional chain of 11 magnetometers at L=1.1 to L=2.5 along the coast of Chile and in the Antarctica peninsula. Five of the SAMBA magnetometers at L=1.7 to L=2.5 are directly conjugate to an equivalent number of northern hemisphere magnetometers of the MEASURE (Magnetometers Along the Eastern Atlantic Seaboard for Undergraduate Research and Education) chain. A unique characteristic is that both the SAMBA and the MEASURE magnetometers are set up as pairs of stations making it possible to determine the local Field-Line Resonance (FLR) from conjugate points and thus testing the accuracy of the technique and the models used. An initial study of the local FLR from conjugate pairs of stations at L=1.6 and L=1.7 was done during the moderate storm of July 11, 2003. The frequency of the local resonance can be used to determine the equatorial mass density of the resonating flux tube. From our two conjugate pairs we found, surprisingly, that the difference between the derived mass density between these two very close L values does not agree with densities predicted by any of the existing models or from past observations. If this result is general and not an isolated or rare occurrence it indicates that our understanding of the inversion of FLRs to determine the equatorial mass density is not complete and existing models could be constrained by our observations. We have now isolated a period of approximately 60 days during 2003 with complete coverage from conjugate pairs of stations and we are extending the study of conjugate FLRs to a large amount of days in order to determine the generality, or not, of the results from the July 11, 2003 storm. Initial analysis indicates quite a number of asymmetries in the ULF wave power between the two hemispheres, which might be a contributing factor to the difference in the mass density derived from the conjugate pairs. We plan to investigate these asymmetries more and determine their source and cause.