Koustov, A.^{1}, Fiori, R.^{1}, Gorin, J.^{1} and Uspensky, M.^{2}

* ^{1} Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, Canada^{2} Finnish Meteorological Institute, Helsinki, Finland *

The ion-acoustic speed Cs at electrojet heights is an important parameter for understanding physics of small-scale irregularity formation at high latitudes. It is known that the mean Doppler shift of VHF coherent echoes, observed roughly along the electrojet direction, is close to Cs, but how much close is less clear, especially in view that Cs changes drastically with the intensity of the ExB plasma drift. In this study EISCAT measurements of the electron and ion temperatures at a number of electrojet heights are considered to establish analytical equations describing the relationship between Cs and the velocity of plasma of ExB drift, V. A simple quadratic dependence of the form Cs=A+BV2 is accepted. Numeric values for the coefficients A and B are presented for a large number of EISCAT measurements spanning ExB magnitudes of 400-1500 m/s. Obtained expressions are applied to compare trends in Cs and the velocity of VHF and HF coherent echoes increase with ExB.