EDIA-EISCAT comparison between small scale F-region irregularities and large scale electron density structures at sub-auroral latitudes

Observations of small scale subauroral F-region irregularities on February 6-7, 1984 are presented. The observations were made by two HF radars of the EDIA experiment (Bourdillon et al. 1982, 1986) with simultaneous scanning of the ionosphere between 57 and 66 deg latitude at a slightly different longitude by the European incoherent scatter (EISCAT) UHF system. The morphology of the irregularity regions detected by the HF radars and of their production mechanisms are examined and compared with the horizontal and vertical electron density profiles measured by EISCAT. It is shown that decametric irregularities observed at about 360-430 km height are not associated with any large scale horizontal density gradients in the F-region. A strong north-south gradient observed at lower altitudes (150-200 km), which probably indicates the southern boundary of the high energy particle precipitation zone, is well correlated with the strong scattering regions observed by the HF radars.