Vertical ion motion observed with incoherent scatter radars in the polar lower ionosphere

Vertical ion velocities in excess of a few tens m s−1 have been found in the lower ionosphere (95–105 km) at high latitudes. Statistics using data from the European Incoherent Scatter (EISCAT) Tromsø UHF radar are performed to understand general character of vertical motions in the lower ionosphere for geomagnetically quiet conditions. We estimate the probability distribution function for the real vertical ion velocity, taking into account the measurement uncertainty. This calculation suggests that while most vertical ion velocities have amplitudes smaller than 20 m s−1 at 100 km level, the magnitudes larger than 20 m s−1 are observed as statistical exception. To understand if these large vertical motions in the lower ionosphere are associated with thermospheric motions, differences between ion and neutral wind velocities are calculated using data from experiments with the Sondrestrom incoherent scatter radar on 5 and 12 September 2003. The vertical ion velocities show magnitudes larger than 20 m s−1 below 103 km where the ion velocity is considered to be equal to the neutral wind velocity during the experiments according to calculations of the velocity difference.