Probing the large-scale magnetic field inside the Sun from three decades of observed surface magnetograms

Space-weather and disturbances in the heliosphere are manifestations of the solar magnetic field, which is solely driven by the interior dynamo, and constraining the solar interior magnetic field and its oscillatory behavior is one of the major challenges in solar physics. Observationally, none of the techniques, including helioseismology, are able to provide an estimation of the interior magnetic field. We reconstruct, for the first time, the dynamics of the interior large-scale magnetic fields by assimilating observed line-of-sight photospheric magnetogram data from MDI/SOHO & HMI/SDO along with helioseismic differential rotation data over three decades (1996-2025) into a 3D Babcock-Leighton dynamo model. The assimilation of observational magnetogram data allows us in realistic modelling of Babcock-Leighton mechanism as observed on the Sun without any simplified parameterization. As a result, our data-driven model successfully reproduces key observational features such as the surface butterfly diagram, accurate polar field evolution, and axial dipole moment. The reconstructed interior field dominated by toroidal component exhibits an equatorward migration and reproduces the realistic amplitude and modulation of cycles 23-25. We observe that the non-axisymmetric behaviour of the interior toroidal field becomes less prominent as we move deep towards the tachocline according to our model. A strong correlation between the simulated toroidal field and sunspot number establishes our 3D magnetogram-driven model as a robust predictive model of the solar cycle.