Callisto's Atmosphere and Its Space Environment: Prospects for the Particle Environment Package on Board JUICE

The JUpiter ICy moons Explorer (JUICE) of the European Space Agency will investigate Jupiter and its icy moons Europa, Ganymede, and Callisto, with the aim to better understand the origin and evolution of our Solar System and the emergence of habitable worlds around gas giants. The Particle Environment Package (PEP) on board JUICE is designed to measure neutrals and ions and electrons at thermal, suprathermal, and radiation belt energies (eV to MeV). In the vicinity of Callisto, PEP will characterize the plasma environment, the outer parts of Callisto's atmosphere and ionosphere and their interaction with Jupiter's dynamic magnetosphere. Roughly 20 Callisto flybys with closest approaches between 200 and 5,000 km altitude are planned over the course of the JUICE mission. In this article, we review the state of the art regarding Callisto's ambient environment and magnetospheric interaction with recent modeling efforts for Callisto's atmosphere and ionosphere. Based on this review, we identify science opportunities for the PEP observations to optimize scientific insight gained from the foreseen JUICE flybys. These considerations will inform both science operation planning of PEP and JUICE and they will guide future model development for Callisto's atmosphere, ionosphere, and their interaction with the plasma environment

Micrometeorological and morphological observations of surface hoar dynamics on a mountain snow cover

The formation, growth, and destruction of surface hoar crystals is an important feature of mountain snow covers as buried surface hoar layers are a frequent weak layer leading to unstable snowpacks. The energy and mass exchange associated with surface hoar dynamics is further an important part of land-atmosphere interaction over snow. A quantitative prediction of surface hoar evolution based on local environmental conditions is, however, difficult. We carried out measurements of crystal hoar size and total surface mass changes in the period between January and March 2007 on the Weissfluhjoch study plot of the WSL Institute for Snow and Avalanche Research SLF, located above Davos, Switzerland, at 2540 m above sea level. For the first time, a direct comparison between eddy correlation measurements of latent heat flux and lysimeter-like measurements of surface mass change has been made. Results show that the growth of surface hoar crystals is very well correlated with deposition of water vapor during clear-sky nights as measured by two eddy correlation systems placed close to the ground. By analyzing local meteorological data, we confirm that low to moderate wind speed, humid air, and clear-sky nights are the necessary ingredients for the occurrence of significant vapor fluxes toward the surface and thus for the growth of surface hoar. We also confirm that surface hoar crystals tend to preserve during daytime, when strong sublimation occurs, although their size significantly reduces. Despite the complexities associated with mountain terrain and snow surfaces, such as nonequilibrium boundary layers and stratification effects, the hoar formation could be predicted by the snow cover model SNOWPACK, which uses a bulk Monin-Obukhov (MO) parameterization for the turbulent heat fluxes. On the basis of the comparison between direct observations and model predictions, we suggest that neutral stability conditions in the MO formulation provide the most stable and least flawed prediction for surface hoar formatio

50 Gbit/s real-time test environment for integrated photonic DQPSK receivers

In this paper an FPGA-based test system for high-speed transmission experiments with integrated photonic receivers is presented. Pseudorandom binary sequences are generated inside the FPGA and encoded as either differential quadrature phase shift keying (DQPSK) or quadrature phase shift keying (QPSK) signals. The DQPSK encoder uses a 64-fold parallel-prefix-layers architecture for real-time operation which allows for a maximum internal encoder data rate of 64 Gbit/s. Two-fold parallel data streams of I and Q signals suitable for driving an optical IQ-modulator can be transmitted and received by four 12.5 Gbit/s transceivers. Integrated bit error testers are used to determine bit error rates in real-time