Comparison of boundaries for HF auroral backscatter and VHF scintillation

A study is described of the equatorwards boundary of high-latitude ionospheric irregularities using measurements obtained by two different experimental techniques. Auroral traces observed on backscatter ionograms from an HF radar have been used to identify the boundary of the decametre scale-size irregularities responsible for the coherent backscatter. Simultaneous observations of scintillation on the 150 MHz signals from NNSS satellites have enabled the boundary for irregularities in the sub-kilometre scale regime to be located. Comparisons of the results from the two techniques indicate that reconciliation between boundary positions can be made in only about 50% of the cases considered. In general the scintillation boundary lies on average well polewards of that identified by the HF radar, suggesting that caution should be exercised in the mapping of irregularity boundary positions.