Abstract
Abstract. The ECOMA series of rocket payloads use a set of aerosol particle, plasma, and optical instruments to study the properties of aerosol particles and their interaction with the ambient plasma environment in the polar mesopause region. In August 2007 the ECOMA-3 payload was launched into a region with Polar Mesosphere Summer Echoes (PMSE) and noctilucent clouds (NLC). An electron depletion was detected in a broad region between 83 and 88 km, coincident with enhanced density of negatively charged aerosol particles. We also find evidence for positive ion depletion in the same region. Charge neutrality requires that a population of positively charged particles smaller than 2 nm and with a density of at least 2×108 m−3 must also have been present in the layer, undetected by the instruments. A numerical model for the charging of aerosol particles and their interaction with the ambient plasma is used to analyse the results, showing that high aerosol particle densities are required in order to explain the observed ion density depletion. The model also shows that a very high photoionisation rate is required for the particles smaller than 2 nm to become positively charged, indicating that these may have a lower work function than pure water ice.
Highlights
The Earth is continuously bombarded with sub-millimetre meteors at an average global flux estimated between 10 and 100 tons/day (Love and Brownlee, 1993; Gabrielli et al, 2004)
The instruments included in the 2007 campaign were: The ECOMA instrument, whose DC channel measured the net aerosol charge density for particles >2 nm; a positively biased (+2.5 V relative to the payload) electron probe (EP); a negatively biased (−2.5 V relative to the payload) positive ion probe (PIP) surrounded by a grid at payload potential; a combined sensor for neutrals and electrons (CONE) for measuring densities of neutrals and electrons; a swept Langmuir probe (Cold Plasma Probe; CPP) for determining the payload potential; two Pirani gauges, to roughly determine neutral density; an instrument for measuring electron density using Faraday rotation and differential absorption (Faraday); a particle sampler collecting aerosol and meteoritic smoke particles (MAGIC); and a photometer for measuring backscattered light from noctilucent cloud (NLC) particles
The photometer is located in a section below the front deck, and the CPP and CONE instruments are located in the aft of the payload, becoming exposed to the ambient plasma after payload separation
Summary
The Earth is continuously bombarded with sub-millimetre meteors at an average global flux estimated between 10 and 100 tons/day (Love and Brownlee, 1993; Gabrielli et al, 2004). The smaller meteors ablate in the mesosphere and create “smoke” particles (Hunten et al, 1980) with a radius of some nanometres through re-condensation and coagulation. These particles are believed to be essential for many middle atmosphere processes. The formation of ice particles in the middle atmosphere, which form noctilucent clouds (NLC) and cause polar mesosphere summer echoes (PMSE), requires condensation nuclei to be present on which water ice may form, because the water vapour pressure is too small to allow homogeneous nucleation. Smoke particles of meteoric origin are believed to be the most likely condensation nuclei. Ablation of meteors provides the source material for metal layers that sporadically form in the D-region of the ionosphere (Plane, 1991; McNeil et al, 1998)
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