The Dynamics of Saturn's Ultraviolet Aurorae

Saturn’s aurorae are highly dynamic, controlled from within Saturn’s magnetosphere and by its interaction with the solar wind. This thesis investigates ultraviolet observations of these auroral emissions and corresponding in situ measurements of fields and particles, both mostly obtained by the Cassini mission, in order to separate different components of the aurorae and determine their origin. The brightest emissions are found to be generated by recurring magnetotail reconnection, the occurrence of which is controlled by solar wind conditions and the phasing of Saturn’s planetary period oscillation systems of rotating magnetic field perturbations and electric currents. The auroral signature resembles a series of bright patches emerging near local midnight and subcorotating with the planet’s rotation. Underlying these is a steady auroral band which may be driven by flow shears in the outer magnetosphere and is modulated in intensity and location by the rotating planetary period oscillation systems, accompanied by a dim equatorward outer emission which is suggested to be related to wave scattering of electrons in the inner ring current. Observations further show various small-scale transients such as short-lived ∼ 1 h quasiperiodic flashes possibly indicative of magnetodisc reconnection occurring predominantly near dusk, or numerous fine arcs only visible in the highest resolution imagery obtained by Cassini which may be related to interchange injection events. The relation between the source of auroral particles in the magnetosphere and the auroral emissions they generate upon impacting the atmosphere was investigated, with in situ measurements close above the aurorae revealing the presence of energetic field-aligned ion beams and conics as well as complex wave-particle interactions which may be responsible for their energization. While this thesis uncovers much unknown detail on the workings of Saturn’s aurorae, many questions remain to be answered in future research

Efficient coupling to an optical resonator by exploiting time-reversal symmetry

The interaction of a cavity with an external field is symmetric under time reversal. Thus, coupling to a resonator is most efficient when the incident light is the time reversed version of a free cavity decay, i.e. when it has a rising exponential shape matching the cavity lifetime. For light entering the cavity from only one side, the maximally achievable coupling efficiency is limited by the choice of the cavity mirrors' reflectivities. Such an empty-cavity experiment serves also as a model system for single-photon single-atom absorption dynamics. We present experiments coupling exponentially rising pulses to a cavity system which allows for high coupling efficiencies. The influence of the time constant of the rising exponential is investigated as well as the effect of a finite pulse duration. We demonstrate coupling 94% of the incident TEM00 mode into the resonator.Comment: 7 pages, 5 figure

Hardware-aware block size tailoring on adaptive spacetree grids for shallow water waves.

Spacetrees are a popular formalism to describe dynamically adaptive Cartesian grids. Though they directly yield an adaptive spatial discretisation, i.e. a mesh, it is often more efficient to augment them by regular Cartesian blocks embedded into the spacetree leaves. This facilitates stencil kernels working efficiently on homogeneous data chunks. The choice of a proper block size, however, is delicate. While large block sizes foster simple loop parallelism, vectorisation, and lead to branch-free compute kernels, they bring along disadvantages. Large blocks restrict the granularity of adaptivity and hence increase the memory footprint and lower the numerical-accuracy-per-byte efficiency. Large block sizes also reduce the block-level concurrency that can be used for dynamic load balancing. In the present paper, we therefore propose a spacetree-block coupling that can dynamically tailor the block size to the compute characteristics. For that purpose, we allow different block sizes per spacetree node. Groups of blocks of the same size are identied automatically throughout the simulation iterations, and a predictor function triggers the replacement of these blocks by one huge, regularly rened block. This predictor can pick up hardware characteristics while the dynamic adaptivity of the fine grid mesh is not constrained. We study such characteristics with a state-of-the-art shallow water solver and examine proper block size choices on AMD Bulldozer and Intel Sandy Bridge processors

Conceptual Approach of Robustness in Logistical Control

The logistical control and order release of structurally dynamic systems represent a great challenge. In contrast to classical shop floor production, these systems can involve transitions between the two different organizational forms job shop production and flow production. This results in a system that is susceptible to turbulence and malfunctions. This paper presents an approach that improves the robustness of these systems and achieves a stable system state even under external influences. Decentralized stock control loops control the WIP of the individual stations in a closed loop. When organizational transitions occur, interlinking losses arise, which reduce the productivity of the system. Similar effects can cause disturbances and blockages. The introduction of a robustness controller adapts the input parameters of the order release to the current system status, so that an adapted reaction takes place in the event of strong disturbances

Stability and instability of lattices in semisimple groups

Using cohomological methods, we show that lattices in semisimple groups are typically stable with respect to the Frobenius norm but not with respect to the operator norm.Comment: incorporated remarks and corrections of the reviewer

Conflicts between reindeer herding and an expanding caribou herd in Alaska

The reindeer industry has existed in Alaska since 1892. This industry has largely been concentrated on the Seward Peninsula, Alaska because suitable habitat has been available and caribou have been absent here for over 100 years. Until recently, reindeer meat and velvet antler production consistently generated millions of dollars in revenue critical to the economies of rural Alaskan communities. From 1976 to 1996 the Western Arctic Caribou Herd (WACH) increased from about 75 000 to 463 000 animals. Concurrently, seasonal range use of the WACH shifted westward onto traditional reindeer ranges of the Seward Peninsula. Reindeer herders lost 75-100% of their herds through commingling and out¬migration with wild caribou. This loss of over 12 000 reindeer represents a potential economic value of 13 million dollars. Sustainable meat and velvet antler production and the economies of western Alaskan are likely to be affected by these changes

Deformation of the triangular spin-12 lattice in Na2SrCo(PO4)2

Crystal structure and thermodynamic properties of Na2SrCo(PO4)2, the chemical sibling of the triangular quantum spin-liquid candidate Na2BaCo(PO4)2, are reported. From single crystal x-ray diffraction and high-resolution synchrotron x-ray powder diffraction, the compound was found to crystallize in the monoclinic space group P21/a at room temperature, in contrast to the trigonal Na2BaCo(PO4)2. Above 650 K, the symmetry of Na2SrCo(PO4)2 changes to C2/m, while around 1025 K a further transformation toward trigonal symmetry is observed. The monoclinic symmetry leads to a small deformation of the CoO6 octahedra beyond the trigonal distortion ubiquitous in this structure type and results in the stronger g-tensor anisotropy (gz/gxy=1.6) as well as the increased XXZ anisotropy (Jz/Jxy=2.1) compared to the Ba compound (gz/gxy=1.1, Jz/Jxy=1.5), while the average coupling strength, Jav/kB=(2Jxy+Jz)/3kB≃1.3K, remains unchanged. The Néel temperature increases from 140 mK (Ba) to 600 mK (Sr), and an uncompensated in-plane moment of 0.066(4)μB/f.u. appears. We show that the ordering temperature of a triangular antiferromagnet is capably controlled by its structural distortions