A spectroscopic technique has been used, and very accurate measurements of the Doppler broadening of the H_beta line at 4862AA (H atoms being heated by charge exchange) give the ion temperature radial profiles for different experimental conditions in a low-density plasma (n=10⁹-10¹⁰ cm^-3, T_e=13 eV, T₁= 1 eV, and B₀=1-3 kG). The improved spectroscopic system used is sufficiently sensitive for the plasma rotation to be measured where the azimuthal velocity is 10 times lower than the ion thermal velocity. The observation confirms the existence of a radial electric field E(r) in the plasma. The measurement of the rotation enables the frequency shifts of the unstable oscillations occurring in the laboratory frame to be investigated and an observed flute-type instability to be attributed to the centrifugal force arising from this E*B drift motion.