On the generation of cusp HF backscatter irregularities

HF radar observations, 630.0 nm cusp auroral emission, and an ionospheric tomography scan are compared to investigate the mechanism by which radar cusp echoes are generated. A collocation is reported between the equatorward boundaries of broad spectral width HF radar backscatter and 630.0 nm cusp auroral precipitation. In addition, the radio tomographic image reveals spatial structures in ionospheric electron density that map closely to the precipitation. However, the large‐scale horizontal gradient appears to be insufficiently steep to account alone for the rapid growth of irregularities by the gradient drift mechanism. It is proposed that the initial source of the decameter‐scale features responsible for the backscatter may result from fine structure within the precipitation itself, or alternatively, from the cascade down of unstable intermediate‐scale gradients created in the plasma convecting poleward from the source region. The latter process may explain the ongoing backscatter found poleward of the auroral band.