PHOCUS

An estimated 10 to 100 tonnes of meteoric matter hits the Earth`s atmosphere daily; most of this material is too small to be seen or to reach the surface of the Earth. But this material evaporates and - presumably - provides the nuclei for ice particles in consequence causing other phenomena which occur in the mesosphere (70 to 90 km). Under special conditions NLC (= noctilucent clouds) can form from these ice particles which are visible from the ground. The probability and the altitude of the clouds have, within the accuracy of the triangulations and observations, not changed for more than 100 years. However, the brightness has detectably increased since the availability of satellite borne measurements, i.e. over only two decades. A connection of the variability of this phenomenon with man-made atmospheric emissions since the beginning of the industrial era is a hot topic of many investigations. For a sound understanding of the formation of NLC and the associated polar mesospheric radar echoes an investigation of all conceivable ingredients for the ice formation is essential. In Summer 2011 a sounding rocket will be launched from northern Sweden to investigate the relevant part of the atmosphere (the mesosphere) by using a novel combination of diagnostic instruments. Among these instruments is a new microwave radiometer to measure in situ water vapour in the mesosphere. Other diagnostic instruments will establish neutral temperatures with high resolution, the size of mesospheric ice particles, the densities of atomic oxygen and hydrogen, as well the densities of electrons and ions. This comprehensive set of measured parameters will provide insight into the atmospheric processes at the extremely cold polar mesosphere, and - in consequence - help to better interpret long term observations pertaining to global change

In the atmosphere above the height where the weather as we know it occurs, a new "player" in the chemistry and physics of the mesosphere has been identified, namely dust originating from meteor ablation and recondensation. These particles are too small to be optically detectable from the ground, but indications that they exist have long been discussed. One such clue is the occurrence of so-called Noctilucent Clouds (NLC) which can be seen in summer at high latitudes near the mesopause at about 83 km. The ingredients necessary for the formation of these NLC are extremely low temperatures, water vapour and condensation nuclei; the latter are thought to be the smoke particles from ablating meteors. The rocket payload coded PHOCUS (= Particles, Hydrogen and Oxygen Chemistry in the Upper Summer mesosphere) carried a very complex ensemble of diagnostic instruments to measure parameters expected to be relevant for the investigation of NLC. Most instruments had a flight heritage aboard sounding rockets, but notably the microwave radiometer to measure water vapour concentration has never been flown on a sounding rocket; it is based on a similar very successful instrument on the Swedish satellite ODIN, launched in 2002 and still providing data. Other experiments included particle impact detectors from three groups (and of three different designs), a high-resolution neutral densityemperatureurbulence detector, optical instruments to measure atomic oxygen and hydrogen, as well as the size of the ice particles. The measurements made possible by the FWF funding produced absolute electron densities with low height resolution and high resolution (relative) electron and ion densities. The launch of the PHOCUS payload took place on July 21st, 2011 at the rocket range ESRANGE near Kiruna in Swedish Lapland. Essentially all instruments aboard the very complex payload worked, but some details of the obtained data need to be inspected for unusual performance in parts of the flight. The apogee was - as predicted - at 108 km and the parachuted payload could safely be recovered within a few hours after the flight. For this payload recovery was a requirement since it also carried a sampler for post-flight analysis of the collected dust particles. The Austrian instruments are in working order and show no unusual performance; the reason for the in part unusual behaviour still needs to be investigated. The first get-together involving most of the experimenters was on the occasion of the MST-13 conference, an international workshop on MST radars, which took place in March 2012 in Kühlungsborn, Germany. Another dedicated data meeting was held at the University of Stockholm in September 2012; on this occasion three papers to be presented at the 2012 AGU Fall Meeting (= American Geophysical Union) in San Francisco were discussed and defined.