On the efficiency of rocket-borne particle detection in the mesosphere

Abstract

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 is known about these particles. Important questions concern the smoke number density and size distribution as a function of altitude as well as the fraction of charged particles. Sounding rockets are used to measure smoke in situ, but aerodynamics has remained a major challenge. Basically, the small smoke particles 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 meteoric smoke has been available from measurements of heavy charge carriers. Quantitative ways are needed that relate these measured particle population to the atmospheric particle population. This requires in particular knowledge about the size-dependent, altitude-dependent and charge-dependent detection efficiency for a given instrument. 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. Detection efficiencies are determined for three detector designs, including two with ventilation holes to allow airflow through the detector. 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 in order to significantly improve the detection efficiency.

Mesospheric dust and its secondary effects as observed by the ESPRIT payload

“…We will assume that the fraction of dust charge density carried by the very smallest dust particles, which are not detected by the dust probe, is small compared to the total dust charge density. Model calculations show that in the lower PMSE layer, dust of radius down to around 3 nm will probably enter the probe with an efficiency of around 0.7, while for the upper layer slightly smaller dust should also enter the probe with a high efficiency (Hedin et al, 2007). The comparatively small size limits detection, together with recent findings that the majority of dust particles smaller than 2-3 nm probably are neutral at sunlit conditions (Havnes and Kassa, 2009) due to the effect of photodetachment (Weingartner and Draine, 2001) support our assumption.…”

“…Very small particles of radius ≤4-5 nm can be seriously affected by drag from the airflow around the payload (Horanyi et al, 1999;Rapp et al, 2005;Hedin et al, 2007) and may be prevented from reaching the interior of the dust probe. We will assume that the fraction of dust charge density carried by the very smallest dust particles, which are not detected by the dust probe, is small compared to the total dust charge density.…”

“…Later observations by rocket mass spectrometers (Björn and Arnold, 1981;Kopp et al, 1985;Schulte and Arnold, 1992) indicated the presence of massive ions, or microclusters. It is, however, unclear how the effect of airflow could have affected these observations (Horanyi et al, 1999;Hedin et al, 2006). The first probe to unambiguously detect heavy charged mesospheric dust particles of sizes probably from a few nm and upwards, was the DUSTY probe flown in 1994 (Havnes et al, 1996).…”

Mass analysis of charged aerosol particles in NLC and PMSE during the ECOMA/MASS campaign

“…The first solves Laplaces's equation in three dimensions inside of the instrument housing to determine the electrostatic potential profile. The second finds the aerodynamic flow in three dimensions around and within the instrument by the direct simulation Monte-Carlo method (Bird, 1994;Gumbel, 2001;Hedin et al, 2007). The third finds the trajectories of the charged aerosol particles from the equations of motion using (1) the force of the electrostatic field assuming unit charge on the aerosol particle and (2) the impulsive changes to the particles' velocity vector caused by random collisions with air molecules.…”

“…The aerodynamic flow around a rocket payload may carry the smallest particles around the detector Hedin et al, 2007;Amyx et al, 2008). An alternate detector design, the Gerdien condenser, is open at the bottom and allows passage of air through the instrument.…”