SPATIAL INTERFEROMETRY MEASUREMENTS OF POLAR MESOSPHERE SUMMER ECHOES WITH THE EISCAT VHF RADAR

Spatial domain interferometry is a useful technique for studying the dynamics of both the ionosphere and the neutral atmosphere. In the ionospheric case the scatterers are highly localized in space and can be tracked as they drift through the beam to determine the horizontal winds. However, in the atmospheric case the scatterers can fill most of the resolution volume of the radar. For the EISCAT VHF radar in Tromso, Norway, with its single E-W baseline, this volume scattering only allows determination of the trace velocity parallel to the baseline. With this limitation we have applied this technique at EISCAT to study the dynamics of the mesosphere. On August 11, 1992, a thin sheet of strong scatterers during a polar mesospheric summer echo (PMSE) event, initially located at 83 km, was lifted upward by 300 m within a couple of minutes. The vertical velocity at this time, determined from the measured Doppler shifts, jumped from 2 m/s downward to more than 4 m/s upward. From the cross-spectral analysis we also see a change in the direction of the zonal wind from westward to eastward during the discontinuity in the vertical velocity. The behavior of the two velocity components is qualitatively similar to the wind corner observations of Widdel and von Zahn (1990). We also consider the possibility that a solitary wave may cause this event, but conclude that there is no compelling evidence to support this explanation.