Turbulence in the Magnetospheres of the Outer Planets

The magnetospheres of the outer planets exhibit turbulent phenomena in an environment which is qualitatively different compared to the solar wind or the interstellar medium. The key differences are the finite sizes of the magnetospheres limited by their physical boundaries, the presence of a strong planetary background magnetic field and spatially very inhomogeneous plasma properties within the magnetospheres. Typical turbulent fluctuations possess amplitudes much smaller than the background field and are characterized by Alfvén times, which can be smaller than the non-linear interaction time scales. The magnetospheres of the outer planets are thus interesting laboratories of plasma turbulence. In Jupiter's and Saturn's magnetospheres, turbulence is well-established thanks to the in-situ measurements by several spacecraft, in particular the Galileo and Cassini orbiter. In contrast, the fluctuations in Uranus' and Neptune's magnetospheres are poorly understood due to the lack of sufficient data. Turbulence in the outer planets' magnetospheres have important effects on the systems as a whole. The dissipation of the turbulent fluctuations through wave-particle interaction is a significant heat source, which can explain the large magnetospheric plasma temperatures. Similarly, turbulent wave fluctuations strongly contribute to the acceleration of particles responsible for the planet's auroras.

Intra-annual variations of spectrally resolved gravity wave activity and observations of turbulence in the UMLT region

<p>Multi-year temperature time series from OH-airglow infrared (IR) spectrometers deployed at different sites in Europe as part of the Network for the Detection of Mesospheric Change (NDMC) are used to estimate the gravity wave activity in the upper mesosphere / lower thermosphere (UMLT) region.</p><p>The seasonal course of gravity wave activity is found to be strongly dependent on the wave period. While there is almost no clear variability of gravity wave activity for periods lower than about 60 minutes, we find strong evidence for an increasing variation throughout the year for periods longer than ca. 60 min. A dominant semi-annual structure with maxima at the solstices is found up to a periodicity of about 200 minutes, where a gradual transition to an annual cycle with maximum activity during winter and minimum activity during summer is observed.</p><p>The energy and momentum carried by gravity waves is dissipated in terms of turbulent wave breaking. Using observations of airglow imagers with high spatial and temporal resolution which were operated at the same time as the abovementioned IR-spectrometers we performed an investigation of turbulent gravity wave dynamics. The estimations of the turbulent eddy diffusion coefficient and the energy dissipation rate from the image series of a turbulent wave front agree quite well with the few available values in literature. A machine learning approach for the systematic extraction of turbulent episodes from the very large data set is presented.</p><p>This work received funding from the Bavarian State Ministry of the Environment and Consumer Protection.</p>

TURBULENCE-RESOLVING NUMERICAL SIMULATION OF FINE SEDIMENT TRANSPORT OVER BEDFORMS

Wave-supported gravity currents in turbulent wave bottom boundary layer (WBBL) are one of the most important processes causing cross-continental shelf sediment transport. The high numerical accuracy 3D numerical model has been used to investigate the fine sediment transport in the WBBL and several different transport modes have been found due to sedimentinduced density stratification (Ozdemir et al., 2010; Cheng et al., 2015). However, laboratory experiments suggest the presence of a small amount of sand fraction and the formation of ripple bed alter the structure of WBBL significantly (Hooshmand, et al., 2015). The purpose of this study is to understand the interplay of fine sand, bedforms, and sediment-induced density stratification in determining the transport modes of fine sediments in WBBL through turbulence-resolving numerical simulations.