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Unusual structures characterized by a very high-velocity divergence have been observed in the high-latitude F-region with SuperDARN radars (André et al., 2000). These structures have been interpreted as due to local demagnetization of the plasma in the ionospheric F-region, during very specific geophysical conditions. In this study, the collective wave scattering theory is used to characterize the decameter-scale turbulence (<font face="Symbol">l</font> approx 15 m) inside the structures. The distribution function of the diffusion coefficient is modified when the structures are generated, suggesting that two regimes of turbulence coexist. A temporal analysis decorrelates the two regimes and gives access to the dynamics associated with the structures. It is shown that a high turbulent regime precedes the plasma demagnetization and should be related to an energy deposition. Then a second regime appears when the plasma is demagnetized and disappears simultaneously with the structures. This study is the first application of the collective wave scattering theory to a specific geophysical event.<br><br><b>Key words. </b>Ionosphere (auroral ionosphere; ionospheric irregularities) – Space plasma physics (turbulence)