Surface emission and elliptic flow

The partonic elliptic flow in $\mathrm{Au}+\mathrm{Au}$ collisions at energies currently available at the BNL Relativistic Heavy Ion Collider (RHIC) ($\sqrt{{s}_{\mathit{NN}}}=200$ GeV) is analyzed using a multiphase transport (AMPT) model. It is shown that the partons freezing out at the early stage are mainly emitted from the surface of the collision system. The integral elliptic flow of these partons is shown to decrease with time and to be consistent with the surface emission picture. The curve of the differential elliptic flow of partons freezing out at the early stage is flat relative to that of partons freezing out at the late stage. Both integral and differential elliptic flows of partons in the surface emission picture approach saturation for large cross sections.