EXPANSION OF THE NON-CONDENSED TRAPPED BOSE-GAS IN BOSE-EINSTEIN CONDENSATION

The anisotropic density distribution of Bose particles after expansion from a magnetic trap has been taken as an important signature of the formation of Bose-Einstein condensate. In this letter we show that eve n for a non-condensed trapped gas one can get an anisotropic distribut ion due to anisotropic collision rates along radial and axial directio ns. We demonstrate that the mean energies along those directions, diff erent from the energy in the trap, have opposite evolution tendencies with the expansion. This difference is particularly important for the case where a large number of bosons is trapped. We also show that the velocity distribution of the non-condensed gas in the trap, assumed in itially isotropic and Gaussian, becomes anisotropic and with linewidth dependent on the expansion direction.