Plasma instabilities in the lower E-region observed during the DYANA campaign

During the DYANA campaign a number of instrumented sounding rockets were launched from Andøya Rocket Range (ARR) in Norway (69°N) to study the dynamics of the mesosphere and lower thermosphere during winter conditions. The EISCAT UHF radar in Tromsø, located 130 km to the east of ARR, was used to map the electric field and the wind field in the same height region. During some of the launches, plasma irregularities with scales in the range from a few metres to tens of metres were observed. We focus our attention on an event which took place on 8/9 March 1990. Two instrumented payloads code-named DAT-73 and DAT-76 were flown 92 min apart during different levels of disturbance in the E-region. We have studied the plasma fluctuations observed during these flights in terms of the modified two-stream and gradient-drift instabilities using linear fluid theory. The observed height extent of the disturbed region is in excellent agreement with theoretical predictions based upon measurements of background plasma parameters. We also find that the instability is much stronger during the first part of the event as a consequence of larger drift velocities. This is evident by the large plasma fluctuations measured in situ as well as the electron temperature measured by EISCAT. An adequate description of the observations must include effects of electron density gradients and recombinational damping. During the flight of DAT-73, the plasma waves must propagate in a direction different from 90° with respect to the magnetic field, while the DAT-76 observations can be explained by plasma waves propagating normal to the magnetic field.