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We present in situ observations of mesospheric ice particles with a new particle detector\udwhich combines a classical Faraday cup with the active photoionization of particles and subsequent detection of\udphotoelectrons. Our observations of charged particles and free electrons within a decaying PMSE-layer\udreveal that the presence of charged particles is a necessary but not sufficient condition for\udthe presence of PMSE. That is, additional requirements like a sufficiently large electron density\ud– which we here estimate to be on the order of ~100 cm−3 –\udand the presence of small scale structures (commonly assumed to be caused by turbulence) need to be satisfied.\udOur photoelectron measurements reveal a very strong horizontal structuring\udof the investigated ice layer, i.e., a very broad layer (82–88 km) seen on the upleg\udis replaced by a narrow layer from 84.5–86 km only 50 km apart on the downleg of the rocket flight.\udImportantly, the qualitative structure of these photoelectron profiles is in remarkable qualitative agreement with\udphotometer measurements on the same rocket thus demonstrating the reliability of this new technique.\udWe then show that the photoelectron currents are a unique function of the ice particle volume density (and hence ice mass)\udwithin an uncertainty of only 15% and we derive corresponding altitude profiles of ice volume densities.\udDerived values are in the range ~2–8×10−14 cm3/cm3 (corresponding to mass densities\udof ~20–80 ng/m3, and water vapor mixing ratios of 3–12 ppm) and are the first such estimates with\udthe unique spatial resolution of an in situ measurement

First in situ measurement of the vertical distribution of ice volume in a mesospheric ice cloud during the ECOMA/MASS rocket-campaign