Probing the Large-scale Magnetic Field inside the Sun from Three Decades of Observed Surface Magnetograms

Abstract Space weather and disturbances in the heliosphere are manifestations of the solar magnetic field, which is solely driven by the interior dynamo. 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 the Michelson Doppler Imager/Solar and Heliospheric Observatory and the Helioseismic and Magnetic Imager/Solar Dynamics Observatory, along with helioseismic differential rotation data over three decades (1996–2025), into a 3D Babcock–Leighton dynamo model. The assimilation of the observational magnetogram data allows for realistic modeling of the 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 the toroidal component exhibits an equatorward migration and reproduces realistic amplitudes and modulations of Cycles 23–25. We observe that the nonaxisymmetric behavior of the interior toroidal field becomes less prominent as we move deep toward 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.