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We study supersonic flow of rarefied weakly ionized plasma at diverging magnetic field. While the flows of ions and neutral are treated by particle-based direct simulation Monte Carlo method, the electron is treated as a fluid, i.e., we carried out hybrid simulation. We calculate number density, velocity, temperature, and electric potential of charged particles and neutrals when the arc plasma flows out of the uniform magnetic field into lower pressure region in a steady state. We find the velocity increase just after passing the open magnetic field line, followed by deceleration due to collisions with residual molecules. We also find the temperature increase during the deceleration. In this acceleration/deceleration phenomena, the velocity difference between the neutrals and charged species are found, which also changes the electric potential. We discuss the mechanisms of potential formation along the plasma flow mainly by the pressure difference and the friction force between the charged particles and neutral species. The numerical results are, at least qualitatively, consistent with our previous experimental results.