Absolute density measurements in the middle atmosphere

In the last ten years a total of 25 sounding rock: ets employing ionization gauges have been launched at high latitudes (similar to 70 degrees N) to m easure total atmospheric density and its small scale fluctuations in an alt itude range between 70 and 110 km. While the determination of small scale f luctuations is unambiguous, the total density analysis has been complicated in the past by aerodynamical disturbances leading to densities inside the sensor which are enhanced compared to atmospheric values. Here, we present the results of both Monte Carlo simulations and wind tunnel measurements to quantify this aerodynamical effect. The comparison of the resulting 'ram-f actor' profiles with empirically determined density ratios of ionization ga uge measurements and falling sphere measurements provides excellent agreeme nt. This demonstrates both the need, but also the possibility, to correct a erodynamical influences on measurements from sounding rockets. We have determined a total of 20 density profiles of the mesosphere-lower-t hermosphere (MLT) region. Grouping these profiles according to season, a li sting of mean density profiles is included in the paper. A comparison with density profiles taken from the reference atmospheres CIRA86 and MSIS90 res ults in differences of up to 40%. This reflects that current reference atmo spheres are a significant potential error source for the determination of m ixing ratios of, for example, trace gas constituents in the MLT region.