Measurements of the plasma density obtained during the interaction of the artificial plasma jet, fired into the ionosphere, with the body of the Porcupine main payload have been analyzed for times when there was a well-developed wake effect. Using wake theory, the maximum temperature of the quasineutral xenon ion beam has been determined for an intermediate distance from the ion beam source when the beam has left the diamagnetic region but is still much denser than the ionospheric background plasma. The beam temperature is found about 4 times less than the temperature at injection. This observation is very well explained by adiabatic cooling of the beam during its initial diamagnetic and current-buildup phases at distances r<10 m. Outside this region the beam conserves the temperature achieved. The observation proves that the artificial plasma jet passes through an initial gas-like diamagnetic phase restricted to the vincinity of the beam source where it expands adiabatically. Partial cooling also takes place outside the diamagnetic region where the beam current still builds up. The observations also support a recently developed current-closure model of the quasi-neutral ion beam.