Locating Current Sheets in the Solar Corona

Current sheets are essential for energy dissipation in the solar corona, in particular by enabling magnetic reconnection. Unfortunately, sufficiently thin current sheets cannot be resolved observationally and the theory of their formation is an unresolved issue as well. We consider two predictors of coronal current concentrations, both based on geometrical or even topological properties of a force-free coronal magnetic field. First, there are separatrices related to magnetic nulls. Through separatrices the magnetic connectivity changes discontinuously. Coronal magnetic nulls are, however, very rare. Second, inspired by the concept of generalized magnetic reconnection without nulls, quasi-separatrix layers (QSL) were suggested. Through QSL the magnetic connectivity changes continuously, though strongly. The strength of the connectivity change can be quantified by measuring the squashing of the flux tubes which connect the magnetically conjugated photospheres. We verify the QSL and separatrix concepts by comparing the sites of magnetic nulls and enhanced squashing with the location of current concentrations in the corona. Due to the known difficulties of their direct observation, we simulated coronal current sheets by numerically calculating the response of the corona to energy input from the photosphere, heating a simultaneously observed Extreme Ultraviolet Bright Point. We did not find coronal current sheets at separatrices but at several QSL locations. The reason is that, although the geometrical properties of force-free extrapolated magnetic fields can indeed hint at possible current concentrations, a necessary condition for current sheet formation is the local energy input into the corona.     

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.     

Dust Environment Modelling of Comet 67P/Churyumov-Gerasimenko

Dust is an important constituent of cometary emission; its analysis is one of the major objectives of ESA’s Rosetta mission to comet 67P/Churyumov-Gerasimenko (C–G). Several instruments aboard Rosetta are dedicated to studying various aspects of dust in the cometary coma, all of which require a certain level of exposure to dust to achieve their goals. At the same time, impacts of dust particles can constitute a hazard to the spacecraft. To conciliate the demands of dust collection instruments and spacecraft safety, it is desirable to assess the dust environment in the coma even before the arrival of Rosetta. We describe the present status of modelling the dust coma of 67P/C–G and predict the speed and flux of dust in the coma, the dust fluence on a spacecraft along sample trajectories, and the radiation environment in the coma. The model will need to be refined when more details of the coma are revealed by observations. An overview of astronomical observations of 67P/C–G is given, because model parameters are derived from this data if possible. For quantities not yet measured for 67P/C–G, we use values obtained for other comets, e.g. concerning the optical and compositional properties of the dust grains. One of the most important and most controversial parameters is the dust mass distribution. We summarise the mass distribution functions derived from the in-situ measurements at comet 1P/Halley in 1986. For 67P/C–G, constraining the mass distribution is currently only possible by the analysis of astronomical images. We find that both the dust mass distribution and the time dependence of the dust production rate of 67P/C–G are those of a fairly typical comet.     

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.     

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.     

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.     

Medium energy neutral atom (MENA) imager for the IMAGE mission

The Medium Energy Neutral Atom (MENA) imager was developed in response to the Imaging from the Magnetopause to the Aurora for Global Exploration (IMAGE) requirement to produce images of energetic neutral atoms (ENAs) in the energy range from 1 to 30 keV. These images will be used to infer characteristics of magnetospheric ion distributions. The MENA imager is a slit camera that images incident ENAs in the polar angle (based on a conventional spherical coordinate system defined by the spacecraft spin axis) and utilizes the spacecraft spin to image in azimuth. The speed of incident ENAs is determined by measuring the time-of-flight (TOF) from the entrance aperture to the detector. A carbon foil in the entrance aperture yields secondary electrons, which are imaged using a position-sensitive Start detector segment. This provides both the one-dimensional (1D) position at which the ENA passed through the aperture and a Start time for the TOF system. Impact of the incident ENA on the 1D position-sensitive Stop detector segment provides both a Stop-timing signal and the location that the ENA impacts the detector. The ENA incident polar angle is derived from the measured Stop and Start positions. Species identification (H vs. O) is based on variation in secondary electron yield with mass for a fixed ENA speed. The MENA imager is designed to produce images with 8°×4° angular resolution over a field of view 140°×360°, over an energy range from 1 keV to 30 keV. Thus, the MENA imager is well suited to conduct measurements relevant to the Earth's ring current, plasma sheet, and (at times) magnetosheath and cusp.     

巨磁电阻材料La1—xZnxMnO3化合物的制备工艺及其结构研究

对巨磁电阻材料La1-xZnxMnO3(x=0.1,0.3,0.5,0.7,0.9,1.0)样品的制备工艺、晶体结构、微观结构进行研究。结果表明,样品ZnO与MnO2固相反应的烧结温度和时间分别大于1300℃和6h时,反应生成具有四面体结构的ZnMnO3化合物;压制成型的压力对结构具有一定影响;不同浓度的Zn2 (x=0.1,0.3,0.5,0.7,0.9)掺杂样品在1350℃烧结12h后,均具有钙钛矿结构,但掺杂浓度在大于x=0.5时有新的衍射峰,产生新的物相,从而影响巨磁电阻效应性能。     

ROMAP: Rosetta Magnetometer and Plasma Monitor

The scientific objectives, design and capabilities of the Rosetta Lander’s ROMAP instrument are presented. ROMAP’s main scientific goals are longterm magnetic field and plasma measurements of the surface of Comet 67P/Churyumov-Gerasimenko in order to study cometary activity as a function of heliocentric distance, and measurements during the Lander’s descent to investigate the structure of the comet’s remanent magnetisation. The ROMAP fluxgate magnetometer, electrostatic analyser and Faraday cup measure the magnetic field from 0 to 32 Hz, ions of up to 8000 keV and electrons of up to 4200 keV. Additional two types of pressure sensors – Penning and Minipirani – cover a pressure range from 10⁻⁸ to 10¹ mbar. ROMAP’s sensors and electronics are highly integrated, as required by a combined field/plasma instrument with less than 1 W power consumption and 1 kg mass.     

The Rosetta Mission: Flying Towards the Origin of the Solar System

The ROSETTA Mission, the Planetary Cornerstone Mission in the European Space Agency’s long-term programme Horizon 2000, will rendezvous in 2014 with comet 67P/Churyumov-Gerasimenko close to its aphelion and will study the physical and chemical properties of the nucleus, the evolution of the coma during the comet’s approach to the Sun, and the development of the interaction region of the solar wind and the comet, for more than one year until it reaches perihelion. In addition to the investigations performed by the scientific instruments on board the orbiter, the ROSETTA lander PHILAE will be deployed onto the surface of the nucleus. On its way to comet 67P/Churyumov-Gerasimenko, ROSETTA will fly by and study the two asteroids 2867 Steins and 21 Lutetia.