On auroral dynamics observed by HF radar: 1. Equatorward edge of the afternoon-evening diffuse luminosity belt

Observations and modelling are presented which illustrate the ability of th e Finland CUTLASS HF radar to monitor the afternoon-evening equatorward aur oral boundary during weak geomagnetic activity. The subsequent substorm gro wth phase development was also observed in the late evening sector as a nat ural continuation of the preceding auroral oval dynamics. Over an 8 h perio d the CUTLASS Finland radar observed a narrow (in range) and persistent reg ion of auroral F- and (later) E-layer echoes which gradually moved equatorw ard, consistent with the auroral oval diurnal rotation. This echo region co rresponds to the subvisual equatorward edge of the diffuse luminosity belt (SEEL) and the ionospheric footprint of the inner boundary of the electron plasma sheet. The capability of the Finland CUTLASS radar to monitor the E- layer SEEL-echoes is a consequence of the nearly zero E-layer rectilinear a spect angles in a region 5-10 degrees poleward of the radar site. The F-lay er echoes are probably the boundary blob echoes. The UHF EISCAT radar was i n operation and observed a similar subvisual auroral are and an F-layer ele ctron density enhancement when it appeared in its antenna beam.