On the efficiency of rocket-borne particle detection in the mesosphere
1Department of Meteorology, Stockholm University, 10691 Stockholm, Sweden
2Leibniz Institute of Atmospheric Physics, Schloss-Str. 6, 18225 Kühlungsborn, Germany
Abstract. Meteoric smoke particles have been proposed as a key player in the formation and evolution of mesospheric phenomena. Despite their apparent importance still very little are known about these particles. Sounding rockets are used to measure smoke in situ, but aerodynamics has remained a major challenge. Basically, smoke particles are so small that they tend to follow the gas flow around the payload rather than reaching the detector if aerodynamics is not considered carefully in the detector design. So far only indirect evidence for the existence of these smoke particles has been available in the form of measurements of heavy charge carriers. Important questions concern the smoke number density and size distribution as a function of altitude as well as the fraction of charged particles. Therefore, quantitative ways are needed that relate the measured particle population to the atmospheric particle population. In particular, we need to determine the size-dependent, altitude-dependent and charge-dependent detection efficiency for a given instrument design.
In this paper, we investigate the aerodynamics for a typical electrostatic detector design. We first quantify the flow field of the background gas, then introduce particles in the flow field and determine their trajectories around the payload structure. We use two different models to trace particles in the flow field, a Continuous motion model and a Brownian motion model. Brownian motion is shown to be of basic importance for the smallest particles. By defining an effective relative cross section we compare different model runs and quantitatively investigate the difference between the two particle motion models. Detection efficiencies are determined for three detector designs, two with ventilation holes to allow airflow through the detector, and one without such ventilation holes. Results from this investigation show that rocket-borne smoke detection with conventional detectors is largely limited to altitudes above 75 km. The flow through a ventilated detector has to be relatively large for there to be an increase in the detection efficiency.