Irregular structures observed below 71 km in the night-time polar D-region

A new rocket range, SvalRak, was opened in November 1997 at Ny-Alesund (79 degrees N) in the Svalbard archipelago. The first instrumented rocket was l aunched on 20 November, 1997, at 1730 UT during geomagnetically quiet condi tions. The payload was instrumented to measure plasma parameters in the mes osphere and lower thermosphere, but the payload only reached an altitude of 71 km. This resulted in a very flat trajectory through the lower D-region. The positive ion concentrations were larger than expected, and some unexpe cted plasma irregularities were observed below 71 km. The irregularities we re typically 100 m in spatial extent, with plasma densities a factor of two to five above the ambient background. In the dark polar night the plasma b elow 71 km must consist mainly of positive and negative ions and the only c onceivable ionising radiation is a flux of energetic particles. Furthermore only relativistic electrons have the large energies and the small gyro rad ii required in order to explain the observed spatial structure. The source of these electrons is uncertain.