Atmospheric pressure plasma jet (APPJ) producing guided ionization wave (IW) in helium (He) is investigated by optical emission spectroscopy (OES) with regard to the hydrodynamics, i.e. helium-air mixing and the buoyancy force. A non-invasive method based on the analysis of the resonant broadening line profile is introduced to diagnose the action of the IW on the He laminar flow. The total force acting upon the gas flow (summarizing the electohydrodynamic (EHD) force and the buoyancy force (fb) is investigated experimentally. Quantitative results are in agreement with published data obtained by numerical modeling. Furthermore, low content of air fraction diffusing into the He flow can be determined from the resonant broadening line profile. The latter is of high interest in biomedical and agriculture applications as well as material surface and liquid activation.