Spatio-temporal properties of density irregularities in the night-side polar cap are inferred from multi-point observations of auroral hiss at a radial distance of 5 Earth radii. In this case study, we use high resolution data of the wave instruments onboard the four Cluster spacecraft to estimate the local plasma density, obtaining values close to 1 cm-3 with density depletions decreasing down to 0.15 cm-3. Combined analysis in different points in space conclusively shows for the first time that these density structures are field-aligned and that they predominantly drift in the anti-sunward direction at speeds of a few km/s. This corresponds to a dawn-to-dusk convection electric field of approximately 1 mV/m. The transverse dimension of the observed density depletions, mapped down to the ionospheric heights, is of the order of a few tens of km. Their lifetime is probably longer than tens of seconds.