Investigation of metal atom and ion number densities in the mesosphere and lower thermosphere by satellite remote sensing with SCIAMACHY/Envisat

Abstract

Extraterrestrial meteoroids continuously enter the Earth's atmosphere. The bulk of the meteoric mass comes from about 220 µm sized particles coming mostly from asteroids in the asteroid belt between Mars and Jupiter. The particle size ranges from atomic scale particles to macroscopic bodies with a mass of several tons. The input rate is very uncertain, ranging from 2 to 300 tons per day. The meteoroids collide with the Earth's atmosphere at very high velocities (on the order of 10 to several 10 km per second), which leads to frictional heating of the meteoroids. This leads to full or partial ablation of the meteoroids in the upper atmosphere. Since the meteoroids consist partly of metals, this ablation leads to the formation of metal atom layers in the mesosphere and lower thermosphere (MLT). The metals are partly ionized in the MLT and metal ion layers are also formed. The MLT cannot easily be probed by in situ measurements. The pressure is too low for balloons to fly in this region, and the drag on satellites is too high there for stable satellite orbits. Therefore, only rocket-borne in situ measurements can be performed in the MLT. Because of this, remote sensing techniques are preferred to investigate the MLT. The atmospheric metal atoms and ions can be observed by re- mote sensing, because they are strong emitters of resonance fluorescence. Therefore, they are valuable tracer species for wave propagation and other transport processes in the MLT. In addition to their role as tracers, meteoric metals are also important for the chemistry in the middle atmosphere. Information on their absolute number densities is therefore of interest. The metals are involved in the formation of cluster ions and meteoric smoke particles (MSP), which are important for the heterogeneous condensation and nucleation of aerosols and clouds in the middle atmosphere. To estimate the impact of meteoric metals on the chemistry of the middle atmosphere, global observations are necessary. These need to be combined with models, which also include the yet undetectable particle species like, e.g., the metal molecules. Only in the last decades have global long-term satellite observations of the Earth's atmosphere with good local, temporal, and vertical resolution and cover- age been available. One of these data sets comes from the observations of SCIAMACHY/Envisat, which provides daily limb and nadir observations from 2002 to 2012. Beginning in mid-2008, a special limb observation mode for the investigation of the MLT was performed. This data set was used to retrieve the number densities of Mg, Mg , and Na from 2008-2012. Therefore, an existing retrieval algorithm for Mg and Mg was significantly improved and also extended for application in the visible spectral region for Na. The improved retrieval algorithm and the results for Mg, Mg and Na are presented and discussed in this thesis, and the metal's interaction with polar mesospheric clouds is investigated. Furthermore, a survey of spectral signatures of other target species for the retrieval algorithm has been carried out. The measurements have been compared with model results, which include the total daily meteoric mass influx as well as other measurement results. The seasonal, altitudinal and latitudinal variation of metal species is in good agreement with the most recent model results.