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

Abstract. 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×B electron drift and the isothermal ion-acoustic speed C_S. The work targets large E×B drifts of >1000 m s⁻¹ and observations predominantly along the flow. By considering the EISCAT temperature and electric field data, an empirical relationship between the E×B drift velocity and C_S is established for a number of ionospheric heights. For the Stokkseyri HF radar beams oriented roughly along the E×B direction, the observed E-region HF velocities are consistent with the C_S 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 E×B and electric field directions. The statistically average shift is ~20° and it increases slightly with the E×B magnitude.