Observations of 50-MHz type-II coherent echoes from within the polar cap

Abstract. 50-MHz type-II coherent echoes at geometrical aspect angles of 11.5° have been observed in the northern polar cap during pre-noon hours. The echoes had an unusually large Doppler width of 1200-1400 m s^-1 and were well correlated with strong magnetic disturbances in the range 500-1000 nT. The dependence of intensity, spectral width and skewness versus radial velocity were similar to those known from previous experiments at lower latitudes and at small aspect angles. It is concluded that echo onset was due to the combination of several factors, including a highly conducting ionosphere, the presence of a very intense electric field, and strong radar wave refraction.