Polarization measurements using the Bragg crystal spectrometers on Hinotori

The two Bragg crystal spectrometers on board Hinotori provide a new technique for measuring linear polarization of the soft X-ray line emission from solar flares. The results of the measurements of large (X class) flares are given in this paper, and the polarization degree averaged over the soft X-ray maximum phases is estimated to be less than 4%.     

A three-dimensional numerical method for modelling weakly ionized plasmas

Astrophysical fluids under the influence of magnetic fields are often subjected to single- or two-fluid approximations. In the case of weakly ionized plasmas, however, this can be inappropriate due to distinct responses from the multiple constituent species to both collisional and non-collisional forces. As a result, in dense molecular clouds and protostellar accretion discs, for instance, the conductivity of the plasma may be highly anisotropic leading to phenomena such as Hall and ambipolar diffusion strongly influencing the dynamics.
Diffusive processes are known to restrict the stability of conventional numerical schemes which are not implicit in nature. Furthermore, recent work establishes that a large Hall term can impose an additional severe stability limit on standard explicit schemes. Following a previous paper, which presented the one-dimensional case, we describe a fully three-dimensional method which relaxes the normal restrictions on explicit schemes for multifluid processes. This is achieved by applying the little-known Super TimeStepping technique to the symmetric (ambipolar) component of the evolution operator for the magnetic field in the local plasma rest frame, and the new Hall Diffusion Scheme to the skew-symmetric (Hall) component.     

No sulfur flows on Io

Physical and chemical properties of elemental sulfur are incompatible with the suggestion that the colored flows associated with volcanoes on Io are quenched unstable allotropes of sulfur. Either the volcanic flows are not sulfur, or some mechanism other than quenching is required to produce colored forms of sulfur in them. The properties of sulfur are unsuited to the production and survival of colored unstable allotropes on Io. The color of this object is probably due to some other material, possibly iron compounds.     

Measurements of total ozone using Dobson spectrometers: Some comments on their history

The Dobson ozone spectrometer has been used for studies of the ozone layer for about 60 years. It remains indispensable for assessing the accuracy of satellite-borne instruments. The steps which led to the present understanding of global ozone distribution and variations are reviewed.     

A WENO algorithm for the radiative transfer and ionized sphere at reionization

We show that the algorithm based on the weighted essentially nonoscillatory (WENO) scheme with anti-diffusive flux corrections can be used as a solver of the radiative transfer equations. This algorithm is highly stable and robust for solving problems with both discontinuities and smooth solution structures. We test this code with the ionized sphere around point sources. It shows that the WENO scheme can reveal the discontinuity of the radiative or ionizing fronts as well as the evolution of photon frequency spectrum with high accuracy on coarse meshes and for a very wide parameter space. This method would be useful to study the details of the ionized patch given by individual source in the epoch of reionization. We demonstrate this method by calculating the evolution of the ionized sphere around point sources in physical and frequency spaces. It shows that the profile of the fraction of neutral hydrogen and the ionized radius are sensitively dependent on the intensity of the source.     

Hall effects on thermal hydromagnetic instability of a partially ionized medium

Thermal instability of a finitely conducting hydromagnetic composite medium is considered including the effects of Hall currents and the collisions with neutrals. The equilibrium magnetic field is assumed to be uniform and vertical. For stationary convection, the collissions have no effect, while the Hall currents are found to have a destabilizing effect on the thermal instability. It is further shown that whenM is finite andQ the asymptotic behaviours of the critical Rayleigh number, the critical wave number and the critical temperature gradient remain the same as those obtained by Chandrasekhar whereM is a nondimensional number which includes the Hall current effects andQ stands for the Chandrasekhar number.     

Generation and performance of automated jarosite mineral detectors for visible/near-infrared spectrometers at Mars

We have developed two automated detectors that can recognize the sulfate mineral jarosite in unknown visible to near-infrared spectra (350-2500 nm). The two detectors are optimized for use within the terrestrial and martian atmospheres. The detectors are built from Support Vector Machines trained using a generative model to create linear mixtures of library mineral spectra. Both detectors performed with an average ∼90% accuracy on laboratory spectra of single minerals and the laboratory and field spectra of rocks collected in a hydrothermal environment. This type of algorithm will contribute to the efficiency of onboard data analysis of landed and orbital visible/near-infrared spectrometers at Mars.     

Temporal filters for isolating steady photospheric flows

A variety of temporal filters are tested on artificial data with 60 and 75 s sampling intervals to determine their accuracy in separating the nearly-steady photospheric flows from the p-mode oscillations in Doppler velocity data. Longer temporal averages are better at reducing the residual signal due to p-modes but they introduce additional errors from the rotation of the supergranule pattern across the solar disk. Unweighted filters (boxcar averages) leave residual r.m.s. errors of about 6 m s^–1 from the p-modes after 60 min of averaging. Weighted filters, with nearly Gaussian shapes, leave similar residual errors after only 20 min of averaging and introduce smaller errors from the rotation of the supergranule pattern. The best filters found are weighted filters that use data separated by 150 or 120 s so that the p-modes are sampled at opposite phases. These filters achieve an optimum error level after about 20 min, with the r.m.s. errors due to the p-mode oscillations and the rotation of the supergranules both at a level of only 1.5 m s^–1.     

Numerical MHD codes for modeling astrophysical flows

We describe a Godunov-type magnetohydrodynamic (MHD) code based on the Miyoshi and Kusano (2005) solver which can be used to solve various astrophysical hydrodynamic and MHD problems. The energy equation is in the form of entropy conservation. The code has been implemented on several different coordinate systems: 2.5D axisymmetric cylindrical coordinates, 2D Cartesian coordinates, 2D plane polar coordinates, and fully 3D cylindrical coordinates. Viscosity and diffusivity are implemented in the code to control the accretion rate in the disk and the rate of penetration of the disk matter through the magnetic field lines. The code has been utilized for the numerical investigations of a number of different astrophysical problems, several examples of which are shown.     

Stability of zonal flows on Jupiter

Jupiter's troposphere is bounded below by a deep layer of fluid which is probably extremely close to adiabatic. This paper explores the effect of this new boundary condition on the stability of atmospheric jets. The three parameters of the problem are the Rossby number Ro = U/fl, the ratio of the deformation radius l to the horizontal scale of jets y₀, and the nondimensional value of $\text{β,}\text{β}\text{= βl}{}^2\text{/U}$. The tropospheric deformation radius is unknown; all other quantities are constrained by observation. If the deformation radius is small, then nongeostrophic instabilities are of interest, and the regime is given by $\text{Ro = O(1), l/y}{}_0\text{? 1,}\text{β}\text{? 1}$. Using an Eady model, it is found for this case that the new boundary condition suppresses baroclinic instabilities but that if Ro > 1 (corresponding to a Richardson number less than unity) then inertial instabilities exist as usual. If the deformation radius is somewhat larger, the regime of interest if $\text{Ro ? 1, l/y}{}_0\text{? 1,}\text{β}\text{? 1}$. Horizontal shear can be neglected but β cannot. In this case, it is found that Eady-type baroclinic modes $\text{(}\text{β}\text{= 0)}$ again do not grow, but that the Charney modes $\text{(}\text{β}\text{≠ 0)}$ can exist. Finally, if l/y₀ = O(1) it is found that mixed barotropic-baroclinic instability can exist. Its stability boundaries and energetics approach those of pure barotropic instability when l/y₀ exceeds unity. Observations are discussed: the l/y₀ ? 1 regime appears most relevant. The mixed barotropic-baroclinic modes in this regime display upgradient momentum fluxes.     

Evidence for an ionized disc in the narrow-line Seyfert 1 galaxy Ark 564

We present simultaneous ASCA and RXTE observations of Ark 564, the brightest known 'narrow-line' Seyfert 1 in the 2–10 keV band. The measured X-ray spectrum is dominated by a steep (Γ≈2.7) power-law continuum extending to at least 20 keV, with imprinted Fe K-line and edge features and an additional 'soft excess' below ∼1.5 keV. The energy of the iron K-edge indicates the presence of highly ionized material, which we identify in terms of reflection from a strongly irradiated accretion disc. The high reflectivity of this putative disc, together with its strong intrinsic O  viii Ly α and O  viii recombination emission, can also explain much of the observed soft excess flux. Furthermore, the same spectral model also provides a reasonable match to the very steep 0.1–2 keV spectrum deduced from ROSAT data. The source is much more rapidly variable than 'normal' Seyfert 1s of comparable luminosity, increasing by a factor of ∼50 per cent in 1.6 h, with no measurable lag between the 0.5–2 keV and 3–12 keV bands, consistent with much of the soft excess flux arising from reprocessing of the primary power-law component in the inner region of the accretion disc. We note, finally, that if the unusually steep power-law component is a result of Compton cooling of a disc corona by an intense soft photon flux, then the implication is that the bulk of these soft photons lie in the unobserved extreme ultraviolet.     

Loran-C系统的技术改造与发展方向

简要介绍了近两年来美国和北欧Loran-C系统的研究和技术改造,以及未来数年内的发展方向政策,美国制订了的改造计划,对Loran-C系统及设备进行结构性调整和更新改造,同时对采用新技术后的Loran-C系统的性能和长期需求作出测试与评估,北欧Loran-C系统的研究重心是如何将陆基和空基系统有效地结合起来,Loran-C系统的作为和潜在应用还有于进一步的研究和探索。     

Undercooled water in basaltic regoliths and implications for fluidized bebris flows on Mars

Experiments on the freezing of water in wet, sand-sized “basaltic” substrates confirmed that substantial degrees of undercooling can be achieved under conditions applicable to Mars when the substrate particles are relatively poor nucleators of ice (i.e., igneous minerals). This physical undercooling is independent of (but would be enhanced by) freezing-point depressions caused by soluble salts. Using differential scanning calorimetry, undercooling was studied in a carbon dioxide atmosphere as a function of soil particle size (silt- and sand-sized intervals), water/soil mass ratio (range of 0.1–1), and cooling rate (range of 0.5–10°K/min). Results for a clay-poor, glacial-outwash soil from Mauna Kea, Hawaii (with negligible salt content), showed that degree of undercooling is approximately independent of both soil particle size and water/soil mass ratio but varies with cooling rate. Undercoolings of 6–9°K were achieved for the Mauna Kea soil and undercoolings of 10–13°K were achieved for powdered whole-rock samples of peridotite, basalt, and a shergottite meteorite. Initial melting of the same icy materials occurred at temperatures that were 1–3°K lower than for melting of pure bulk ice. Undercooling of water below 273°K in debris flows composed of relatively unweathered “basaltic” sand on Mars should be expected to support fluid flow over greater distances than might otherwise be expected. Likewise, incipient melting and remobilization of the same icy debris might occur at temperatures below 273°K.     

Effect of magnetic resistivity on the stability of a partially ionized compressible Hall plasma

The dynamic stability of a partially ionized, compressible Hall plasma of finite electrical conductivity has been investigated when the plasma is immersed in a uniform, horizontal magnetic field. Based on the variational principle, which is shown to characterize the problem, the solution has been obtained for a semi-infinite plasma confined between two planes and having an exponential density stratification along the vertical. It is found that the effect of neutral gas friction is stabilizing while magnetic resistivity, Hall currents and compressibility all have destabilizing influence.On leave of absence from Department of Mathematics, University of Jodhpur, Jodhpur, India.