Investigating Temporal-Spatial Compatibility of Magnetic Data from Madagascar Repeat Stations

Accurate regional geomagnetic field modeling is crucial for navigation, resource exploration, and solid-Earth geophysics. In Madagascar, however, field model consistency has been hindered by severe spatio-temporal irregularities in the reoccupation of its twenty-five-station repeat network, including the pivotal Antananarivo observatory, between 1983 and 2001. To overcome these data limitations, this comprehensive study evaluates the spatial and temporal compatibility of the Malagasy repeat station network to establish robust regional geomagnetic field models. Assuming negligible altitude variance across the island region, the parameterization framework relies exclusively on geographic latitude and longitude coordinates. Gauss coefficients were derived using two complementary modeling methods specifically suited for sparse, non-uniformly distributed datasets: regional harmonic analysis and surface polynomial modeling. Both techniques produced highly consistent and robust field representations for periods characterized by stable network geometry, notably 1983 through 1985. In sharp contrast, the 1986 dataset exhibited pronounced anomalous coefficients and residuals stemming from observational inconsistencies, necessitating its complete exclusion from subsequent calculations. A rigorous comparative evaluation revealed that regional harmonic models maintain far greater mathematical stability under non-uniform station distributions. Conversely, polynomial models suffer from boundary artifacts and produce unreliable extrapolation across unmonitored regions lacking observational data. Consequently, while both approaches perform comparably under dense spatial sampling, regional harmonic analysis proves markedly superior within data-sparse regimes. These findings highlight the absolute necessity of maintaining broad, uniformly distributed monitoring networks across the island. Furthermore, they provide a methodological framework for annual field interpolation, guide future cartographic mapping efforts, and strongly advocate for continued, systematic geomagnetic surveys across Madagascar to continually refine these essential regional field representations.