Dependence of spectral width of polar cap HF echoes upon electric field

The EISCAT Svalbard Radar (ESR) monitors plasma parameters in the ionospheric region that is frequently located near the polar cap boundary. The SuperDARN radar at Hankasalmi, Finland detects coherent echoes from this region and these echoes typically show increased spectral width. We consider data of joint ESR and SuperDARN observations to show that the spectral width of HF echoes tends to increase with the electric field. This relationship is explained in terms of non-linear evolution of the ExB gradient drift instability with energy cascade from hundred of meters wavelengths to meter wavelengths. We assume that non-linearly generated, relatively strong decameter waves (seen by the Hankasalmi radar) decay through three-wave interaction with shorter wavelengths and estimate that the decameter waves/irregularities decay time is determined by the parameters of the shorter wavelength structures. We associate the decameter wave decay time with the correlation time, and thus the spectral width, of HF echoes.