Lambda polarization in $^{108}$Ag +$^{108}$Ag and $^{197}$Au +$^{197}$Au collisions around a few GeV

Within the framework of Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model, we extract the global spin polarization of $\Lambda$ hyperon in $^{108}$Ag + $^{108}$Ag and $^{197}$Au + $^{197}$Au collisions at $\sqrt{s_{\rm NN}} = 2.42 - 7.7$ GeV. We use two different methods to calculate the $\Lambda$ polarization $P_y$: one is based on equilibrium assumption so that $P_y$ is determined by thermal vorticity and another assumes a proportional relation between $P_y$ and the system's angular momentum in $\Lambda$'s rest frame. We find that both methods can describe the experimental data suggesting that the global spin polarization is a result of global angular momentum and is thus insensitive to the intermediate prescription to extracting it.