Theodor Meyer—Lost pioneer of gas dynamics

Theodor Meyer's 1908 doctoral dissertation, with Ludwig Prandtl (1875–1953) as his advisor, introduced much of what has now become basic gas dynamics: not only the Prandtl–Meyer expansion but also the oblique-shock-wave theory as well. It is arguably the most influential dissertation in all of fluid dynamics. Yet no biography or even a photograph of Meyer has been available in the intervening century. This biography provides some insight into his character and covers his education, dissertation, World War I combat service and long career as an engineer and a teacher of math and physics.     

A Pre-Landing Assessment of Regolith Properties at the InSight Landing Site

This article discusses relevant physical properties of the regolith at the Mars InSight landing site as understood prior to landing of the spacecraft. InSight will land in the northern lowland plains of Mars, close to the equator, where the regolith is estimated to be \(\geq3\mbox{--}5~\mbox{m}\) thick. These investigations of physical properties have relied on data collected from Mars orbital measurements, previously collected lander and rover data, results of studies of data and samples from Apollo lunar missions, laboratory measurements on regolith simulants, and theoretical studies. The investigations include changes in properties with depth and temperature. Mechanical properties investigated include density, grain-size distribution, cohesion, and angle of internal friction. Thermophysical properties include thermal inertia, surface emissivity and albedo, thermal conductivity and diffusivity, and specific heat. Regolith elastic properties not only include parameters that control seismic wave velocities in the immediate vicinity of the Insight lander but also coupling of the lander and other potential noise sources to the InSight broadband seismometer. The related properties include Poisson’s ratio, P- and S-wave velocities, Young’s modulus, and seismic attenuation. Finally, mass diffusivity was investigated to estimate gas movements in the regolith driven by atmospheric pressure changes. Physical properties presented here are all to some degree speculative. However, they form a basis for interpretation of the early data to be returned from the InSight mission.     

Theory and Simulation of Reconnection

Reconnection is a major commonality of solar and magnetospheric physics. It was conjectured by Giovanelli in 1946 to explain particle acceleration in solar flares near magnetic neutral points. Since than it has been broadly applied in space physics including magnetospheric physics. In a special way this is due to Harry Petschek, who in 1994 published his ground breaking solution for a 2D magnetized plasma flow in regions containing singularities of vanishing magnetic field. Petschek’s reconnection theory was questioned in endless disputes and arguments, but his work stimulated the further investigation of this phenomenon like no other. However, there are questions left open. We consider two of them – “anomalous” resistivity in collisionless space plasma and the nature of reconnection in three dimensions. The CLUSTER and SOHO missions address these two aspects of reconnection in a complementary way -- the resistivity problem in situ in the magnetosphere and the 3D aspect by remote sensing of the Sun. We demonstrate that the search for answers to both questions leads beyond the applicability of analytical theories and that appropriate numerical approaches are necessary to investigate the essentially nonlinear and nonlocal processes involved. Necessary are both micro-physical, kinetic Vlasov-equation based methods of investigation as well as large scale (MHD) simulations to obtain the geometry and topology of the acting fields and flows.     

Size and cell number of the utricle in kinetotically swimming fish: a parabolic aircraft flight study.

Humans taking part in parabolic aircraft flights (PAFs) may suffer from space motion sickness (SMS, a kinetosis). Since it has been repeatedly shown earlier that some fish of a given batch also reveal a kinetotic behavior during PAFs (especially so-called spinning movements and looping responses) and due to the homology of the vestibular apparatus among all vertebrates, fish can be used as model systems to investigate the origin of susceptibility to motion sickness. Therefore, we examined the utricular maculae (they are responsible for the internalization of gravity in teleosteans) of fish swimming kinetotically at microgravity in comparison with animals from the same batch who swam normally. On the histological level, it was found that the total number of both sensory and supporting cells of the utricular maculae did not differ between kinetotic animals as compared to normally swimming fish. Cell density (sensory and supporting cells/100 micrometers2), however, was reduced in kinetotic animals (p<0.0001), which seemed to be due to malformed epithelial cells (increase in cell size) of the kinetotic specimens. Susceptibility to kinetoses may therefore originate in malformed sensory epithelia.     

35 Day cycle and the rotation period of Her X-1

Sampling observations of a complete 35^d cycle of Her X-1 during 1 March to 5 April 1984 with the low and medium energy X-ray detectors of EXOSAT are presented. The intensities measured in the Argon detectors are compared with the UHURU 35^d light curve to obtain the turn-on times of two consecutive cycles as JD 2445753.0 ± 1.0, and JD 2445788.0 ± 0.5. The low energy data is used to determine the rotation period and its derivative as: 1.23779200 s ± 50 ns, and (-2 ± 1) × 10^-13 respectively, at the epoch JD 2445778.56. Analysis of the pulse arrival phases indicate that during mid-on phase of the 35^d cycle there is shift of about 180°. Evidence for the long term correlated changes of P₃₅ and P_1.24s is presented, confirming the low quality of the 35^d clock and suggesting that the basic clock mechanism must lie in the disk structure itself.     

Physical Causes of Solar Variability – Discussion Session 1b

This report is divided into three parts: Section 1 gives a short introduction and a summary of the topics discussed. Section 2 is a position statement by J. Kuhn on the interpretation of the irradiance measurements, while Section 3 gives a position statement by M. Schüssler discussing observations of stars that could be useful for understanding solar variability.     

Evaluation of mixed mode-I/II criteria for fatigue crack propagation using experiments and modeling

In this study, in-plane mixed mode-I/II fatigue crack growth simulations and experiments are performed for the Al 7075-T651 aluminum alloy which is widely used in the aerospace industry. Tests are carried out under different mode mixity ratios to evaluate the applicability of a fracture criterion developed in a previous study to mixed mode-I/II fatigue crack growth tests. Results obtained from the analyses and experiments are compared with existing and developed criteria in terms of crack growth lives. Compact Tension Shear (CTS) specimens, which enable mixed mode loading with loading devices under different loading angles, are used in the simulations and experiments. In an effort to model and simulate the actual conditions in the experiments, crack surfaces of fractured specimens are scanned, crack paths are modeled exactly, and contacts are defined between the contact surfaces of a specimen and the loading device for each crack propagation step in the analyses. Having computed the mixed mode stress intensity factors from the numerical analyses, propagation life cycles are predicted by existing and the developed mixed mode-I/II criteria and then compared with experimental results.     

Search for millisecond rotational periods in some low-mass X-ray binaries observed by EXOSAT

We present the results of a search for millisecond rotational pulsations in the X-ray emission of five low-mass X-ray binaries: MXB1728-34, 4U1702-42, 4U1705-44, 4U1755-33 and GX17+2. The data were obtained by the EXOSAT Medium Energy Experiment Argon Counters (1–20 keV) with a time resolution of 7.8125 milliseconds. We searched for periods in the range from 16 msec to 8 sec. Upper limits of 1.5 – 5% were found at a 3 sigma level.     

Mesopheric structure and the d-region winter anomaly

Model mechanisms to explain the D-region winter anomaly have so far not really been successful, considering the many different aspects of this atmospheric/ionospheric phenomenon. A new model is therefore proposed relating the winter anomaly essentially to atmospheric temperature and wind variations. The new mechanism is checked by a set of data obtained during the Western European Winter Anomaly Campaign 1975/76, and additionally by a second set of data taken in earlier years.