The theoretical distributions over the polar cap of the growth rates of the gradient and the Farley-Buneman two-stream instabilities are compared to the measured distribution of the 'slant E condition' (SEC) on polar cap ionograms. The Farley instability seems to be the most likely source of the SEC, and this opens the possiblity of detecting by means of ground-based ionospheric Doppler radars whether the polar cap E field is above or below a critical field and, if above, of determining the direction of the E field and the magnitude of the critical field.