Velocity of E-region HF echoes under strongly-driven electrojet conditions

Data collected by the Stokkseyri SuperDARN HF radar simultaneously at short and far ranges are used to investigate the relationship between the velocity of E-region HF echoes, E x B electron drift and the isothermal ionacoustic speed CS. The work targets large E x B drifts of >1000 ms(-1) and observations predominantly along the flow. By considering the EISCAT temperature and electric field data, an empirical relationship between the E x B drift velocity and CS is established for a number of ionospheric heights. For the Stokkseyri HF radar beams oriented roughly along the E x B direction, the observed E-region HF velocities are consistent with the CS values at the bottom of the electrojet but not at its center. For a subset of the data with smooth and consistent velocity variation with the beam azimuth at both short and far radar ranges the velocity varies according to the cosine law. For the E-region echoes, the proportionality coefficient in the cosine law is consistent with the C-S values at the bottom of the electrojet. For these events, the E-region velocity maximum is shown to be between the ExB and electric field directions. The statistically average shift is similar to 20 degrees and it increases slightly with the E x B magnitude.