First detection of charged dust particles in the Earth's mesosphere

Abstract

Some theories for the observed anomalous radar backscatter during the summer (polar mesospheric summer echoes, or PMSE) and electron bite outs measured by rockets require the presence of charged dust. To investigate this, two dust probes have been launched in 1994 from Andøya Rocket Range and we here report the results from the dust and an electron probe on the two payloads. The dust probes were designed to block out the electron and ion components at the mesopause but to detect primary currents due to impacts of charged dust and also to detect secondary plasma production during dust impacts. The results indicate that both during PMSE and noctilucent cloud (NLC) conditions, large amounts of dust, with average sizes apparently of about 0.1 μm and less, were present. The number densities Nd can be up to many thousand per cubic centimeter, and the charge density NdZd likewise. Large local gradients in density and charge density of dust are detected. Dust carrying both positive and negative charges can apparently be present on different occasions. In some parts of the NLC/PMSE layers we find that the negative charge density locked in grains is so large that the number of free electrons is significantly reduced there because the dust acts like sinks for electrons, and an electron bite out results. We also find that in one case the presence of positive dust leads to an increase in the local electron density by photoionization. The main uncertainties in the data analysis are the structure of the dust and the secondary plasma production at the comparatively low dust impact velocities (1 km s−1) in the experiment.

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

“…The currents from G1, G2 and the gold-plated DC are all measured. The EDD probe is similar to the original DUSTY probe (Havnes et al, 1996) but the shape of the grids has been altered to change the production of secondary charges from the impacting dust particles. Also, the G1 wire thickness was reduced from 0.8 mm on DUSTY to 0.25 mm, resulting in a reduction of the ratio of the dust probe opening which is covered, from σ 1 =0.23 to 0.08.…”

“…It is now becoming increasingly clear that secondary charging effects, due to impacting dust particles on rocket payloads and their probes, can affect some measurements (Zadorozhny et al, 1993;Havnes et al, 1996;Vostrikov et al, 1997;Gumbel and Witt, 1998) and probably sometimes totally dominate them (Barjatya and Swenson, 2006;Havnes and Naesheim, 2007). In the last paper, it was demonstrated that secondary charging by dust particles impacting on surfaces at a high impact angle θ i (= angle with the normal to the surface), which fragmented and carried away negative charge, could give the appearance of incoming positive charges.…”

“…The interior of the dust probe is closed to the ambient thermal ions and electrons by grid 1, while the heavy dust particles pass through it (Havnes et al, 1996). 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.…”

“…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). This probe showed that large amounts of negatively charged dust of a charge density of up to ∼−4×10 9 e m −3 was present in two strong electron bite-outs.…”

“…We will, in the following, call these dust particles although they, for the most part, probably consist of water ice. They were first recognized as being present as visual particles in the noctilucent clouds (NLC) (Gadsden and Schröder, 1989;Thomas, 1991) and later it was suspected that non-visual small dust particles could cause the so-called electron bite-outs, strong local depletions of the electron density which often are measured by rocket probes in the summer mesosphere (Pedersen et al, 1969;Ulwick et al, 1988;Havnes et al, 1996). Lidar observations of NLC particles (von Cossart et al, 1999) indicate that visual NLC particles had an average radius of ∼50 nm and average density of ∼ 8×10 7 m −3 which is confirmed by satellite measurements (Eremenko et al, 2005).…”