In this paper we address the question of whether auroral echoes are dependent on the ionospheric electric field. This question naturally arises from the concept of a plasma instability as the source mechanism producing the electron density irregularities responsbile for the auroral echoes. Using the data from a backscatter radar located at Homer, Alaska, and the Chatanika incoherent scatter radar in a joint experiment, we show that there is a distinct dependence of the occurrence of diffuse auroral echoes on a threshold electric field strength. The threshold value for the 398-MHz radar aurora is shown to be approximately 30 mV/m. When the electric field strength is greater than 30 mV/m, the auroral echo strength is shown to be always positively correlated with the electric field strength, regardless of the mean electron density. On the other hand, the auroral echo strength is not always positively correlated with the electron density. The results are shown to be consistent with the quasi-linear plasma instability theory of Sudan et al.(1973) and Greenwald (1974).