Mesoscale linear streaks on Mars: environments of dust entrainment

Variable surface albedo features on Mars are likely caused by the entrainment and deposition of dust by the wind. Most discrete markings are associated with topographic forms or with regional slopes that serve to alter the effective wind shear stress on the surface. Some of the largest variable features, here termed mesoscale linear streaks, are up to 400 km in length and repeatedly occur in one of the smoothest regions of Mars: Amazonis Planitia. Their orientations and apparent season of variability as observed by Viking and Mars Orbiter cameras indicate linear streak formation by enhanced surface wind stresses during regional or local dust storms and during the initial stages of global dust storms. They provide an example of the ability of large-scale winds, without significant local enhancement, to initiate dust motion on Mars. The sizes and spacing of the linear streaks may be controlled by boundary layer rolls. The repetitive formation of these streaks, over a span of more than 11 Mars years, gives one measure of the stability of Mars’ eolian processes.     

Comet Hale-Bopp (C/1995 O1): UVSTAR-FUV Spectroscopy from the Space Shuttle

We report here on unique post-perihelion (2.3 AU) measurements of Comet Hale-Bopp in the FUV-range (950–1250 Å) by means of the UVSTAR spectrometer from the space shuttle with the main purpose of searching for argon and other FUV emitters. New methods for separating the strong airglow emission at shuttle altitudes are here discussed in detail. Due to our low resolution (15 Å) and S/N ratio the possible rocket-borne detection of argon near perihelion (0.9 AU) could not be confirmed. New species as N₂ are suspected but difficult to separate from the strong airglow emission at shuttle altitudes. From the Lyα brightness (1.30± 0.08 kRy) a water production rate Q = 5.9 ± 0.4 × 10²⁹ molecules s^?1 could be derived and compared with other post-perihelion observations.     

The shape of the rotation curves of edge-on galaxies

We consider the effects of projection, internal absorption, and gas-or stellar-velocity dispersion on the measured rotation curves of galaxies with edge-on disks. Axisymmetric disk models clearly show that the rotational velocity in the inner galaxy is highly underestimated. As a result, an extended portion that imitates nearly rigid rotation appears. At galactocentric distances where the absorption is low (i.e., it does not exceed 0.3–0.5^m kpc^?1), the line profiles can have two peaks, and a rotation curve with minimum distortions can be obtained by estimating the position of the peak that corresponds to a higher rotational velocity. However, the high-velocity peak disappears in high-absorption regions and the actual shape of the rotation curve cannot be reproduced from line-of-sight velocity estimates. In general, the optical rotation curves for edge-on galaxies are of little use in reconstructing the mass distribution in the inner regions, particularly for galaxies with a steep velocity gradient in the central region. In this case, estimating the rotation velocities for outer (transparent) disk regions yields correct results.     

In-Orbit Vignetting Calibrations of XMM-Newton Telescopes

We describe measurements of the mirror vignetting in the XMM-Newton Observatory made in-orbit, using observations of SNR G21.5-09 and SNR 3C58 with the EPIC imaging cameras. The instrument features that complicate these measurements are briefly described. We show the spatial and energy dependences of measured vignetting, outlining assumptions made in deriving the eventual agreement between simulation and measurement. Alternate methods to confirm these are described, including an assessment of source elongation with off-axis angle, the surface brightness distribution of the diffuse X-ray background, and the consistency of Coma cluster emission at different position angles. A synthesis of these measurements leads to a change in the XMM calibration data base, for the optical axis of two of the three telescopes, by in excess of 1 arcmin. This has a small but measureable effect on the assumed spectral responses of the cameras for on-axis targets. This revised version was published online in July 2006 with corrections to the Cover Date.     

Observations of SN1988A with the faint object spectrograph on the 4.2 m William Herschel Telescope

We present two spectra of the supernova SN1988A in M58 (NGC4579) over the wavelength range 4000–9700 Å, as recorded by the Faint Object Spectrograph on the 4.2 m William Herschel Telescope at the Observatorio del Roque de los Muchachos. We conclude that SN1988A was a type II supernova.     

Flexure of Europa's lithosphere due to ridge-loading

Using photoclinometry, topographic profiles across europan ridges have been produced. These profiles allow the identification of bulges in the terrain adjacent to the ridges. The bulges are assumed to have been produced by flexure of the elastic lithosphere due to the load of the ridges, which lie along cracks in the crust. The distance from the crack to these “fore-bulges” depends on the thickness of the elastic plate being flexed. Based on a survey of ridges in Galileo images with resolution <300 m/pixel, the thickness of the elastic lithosphere has been determined by this method at a wide variety of sites along the leading and trailing hemispheres of Europa. The average thickness is about 200 m. The elastic lithosphere underneath smooth dilational bands tends to be thicker than plains morphology, an effect that is pronounced at Thynia Linea and Astypalaea Linea. Among the ridges investigated here, more recent loading correlates with a thicker elastic lithosphere, which may either reflect an intrinsically thicker layer, or less viscous relaxation over the shorter time period.     

Seasonal variations in prey selection by estuarine black-headed gulls (Larus ridibundus)

The number of black-headed gulls (Larus ridibundus) in the Clyde Estuary is large. In summer the average density has reached 1350 gulls km^?2 and in winter 180 gulls km^?2. This paper compares prey selection and feeding efficiency in gulls during summer and winter on tidal flats, and considers how seasonal differences may be adaptations to cope with seasonal changes in prey availability.Gross and net rates of energy intake were highest in summer because gulls captured more of the polychaete N. diversicolor than the amphipod C. volutator. In winter, gulls selected for C. volutator and therefore an energetically less profitable diet. Throughout the year gulls selected more C. volutator relative to N. diversicolor than expected on energetic grounds and so apparently did not maximize potential net rate of energy intake.Gulls used three techniques to capture prey and made most intensive use of the ‘crouch’ technique. Crouching gulls attained a much higher net rate of energy intake than ‘upright’ or ‘paddling’ gulls.A log-linear model showed that (a) season, water depth and gull density determined feeding technique and (b) feeding technique and season independently determined foraging success and prey selection. Thus gull density and water depth acted on prey selection through imposed variations in feeding technique.Reasons for gulls selecting energetically unprofitable C. volutator and for the use of several distinct feeding techniques are discussed.     

Rigidly rotating modes of the solar magnetic field

Discrete rigidly rotating components (modes) of the large-scale solar magnetic field have been investigated. We have used a specially calculated basic set of functions to resolve the observed magnetic field into discrete components. This adaptive set of functions, as well as the expansion coefficients, have been found by processing a series of digitized synoptic maps of the background magnetic field over a 20-year period. As a result, dependences have been obtained which describe the spatial structure and the temporal evolution of the 27-day and 28-day rigidly rotating modes of the Sun's magnetic field.The spatial structure of the modes has been compared with simulations based on the known flux-transport equation. In the simulations, the rigidly rotating modes were regarded as stationary states of the magnetic field whose rigid rotation and stability were maintained by a balance between the emergence of magnetic flux from stationary sources located at low latitudes and the horizontal transport of flux by turbulent diffusion and poleward directed meridional flow. Under these assumptions, the structure of the modes is determined solely by the horizontal velocity field of the plasma, except for the low-latitude zone where sources of magnetic flux concentrate. We have found a detailed agreement between the simulations and the results of the data analysis, provided that the amplitude of the meridional flow velocity and the diffusion constant are equal to 9.5 m s^–1 and 600 km² s^–1, respectively.The analysis of the expansion coefficients has shown that the rigidly rotating modes undergo rapid step-like variations which occur quasi-periodically with a period of about two years. These variations are caused by separate surges of magnetic flux in the photosphere, so that each new surge gives rise to a rapid replacement of old large-scale magnetic structures by newly arisen ones.     

Is dark matter created in the gravitation field of galaxies?

Using the heuristic arguments of quantum physics we describe a new mechanism of the creation of short-living particles from the virtual ones in a stationary gravitation field. The mass of these particles is a function of the intensity of gravitation field. We suppose that the particles created in the gravitation field form a part of the non-baryonic dark matter. Having the intensity of gravitation field in a galaxy we can calculate the density of dark matter created in it by the vacuum quantum fluctuation. We calculate the distribution of this dark matter in a model galaxy and show that its total mass is comparable with the visible mass of the galaxy.