We present a differential study of $\Lambda$ hyperon polarization in central Au+Au collisions at $\sqrt{s_{NN}} = 7.7$ GeV, employing the microscopic transport model UrQMD in conjunction with the statistical hadron-resonance gas model. The resulting thermal vorticity configuration effectively manifests as the formation of two vortex rings in the forward and backward rapidity regions. The polarization of $\Lambda$ hyperons exhibits oscillatory behaviour as a function of the azimuthal angle, offering a novel means to probe the structure of the fireball in central heavy-ion collisions.