Interpretation of the prominence differential emissions measure for 3 geometries

Researchers have used prominence extreme ultraviolet line intensities observed from Skylab to derive the differential emission measure Q(T) in the prominence-corona (PC) interface from 3 x 10,000 to 3 times 1 million K, including the effects of Lyman Continuum absorption. Using lines both shortward and longward of the Lyman limit, researchers have estimated the importance of absorption as function of temperature. The magnitude of the absorption, as well as its rate of increase as a function of temperature, place limits on the thread scales and the character of the interfilar medium. Researchers have calculated models based on three assumed geometries: (1) threads with hot sheaths and cool cores; (2) isothermal threads; and (3) threads with longitudinal temperature gradients along the magnetic field. Comparison of the absorption computed from these models with the observed absorption in prominences shows that none of the geometries is totally satisfactory

Quantitative characterization of the x-ray imaging capability of rotating modulation collimators with laser light

We developed a method for making quantitative characterizations of bi-grid rotating modulation collimators (RMC ’s) that are used in a Fourier transform x-ray imager. With appropriate choices of the collimator spacings, this technique can be implemented with a beam-expanded He -Ne laser to simulate the plane wave produced by a point source at infinity even though the RMC ’s are diffraction limited at the He -Ne wavelength of 632.8 nm. The expanded beam passes through the grid pairs at a small angle with respect to their axis of rotation, and the modulated transmission through the grids as the RMC ’s rotate is detected with a photomultiplier tube. In addition to providing a quantitative characterization of the RMC ’s, the method also produces a measured point response function and provides an end-to-end check of the imaging system. We applied our method to the RMC ’s on the high-energy imaging device (HEIDI) balloon payload in its preflight configuration. We computed the harmonic ratios of the modulation time profile from the laser measurements and compared them with theoretical calculations, including the diffraction effects on irregular grids. Our results indicate the 25-in. (64-cm) x-ray imaging optics on HEIDI are capable of achieving images near the theoretical limit and are not seriously compromised by imperfections in the grids

Patterns of Mesophotic Benthic Community Structure on Banks Off vs Inside the Continental Shelf Edge, Gulf of Mexico

Information on the biodiversity and geographic patterns of mesophotic, sessile, epibenthic communities on banks around and at the edge of the continental shelf, northern Gulf of Mexico, has been limited. These communities vary in their environments and are prone to disturbance from Outer Continental Shelf oil- and gas-related activities and fishing (trawling and long-lining). We surveyed these communities on the flanks of 13 banks to determine species richness, species composition, similarities between benthic communities, and geographic patterns in community structure. We sampled to ≤ 181 m in depth via a remotely operated vehicle using a vertically mounted digital camera bearing two lasers for scale and a flash (generally 10 drop-sites/bank, 5 transects/drop-site, and ≤11 photos/transect). Data analysis via PATN revealed three main Bank Groups: the on-shelf group containing 29 Fathom and Sonnier Banks; an anomalous bank—Geyer Bank; and the shelf edge group—Horseshoe, 28 Fathom, Bright, Alderdice, Bouma, Rankin, Rezak, Elvers, McGrail, and Sidner Banks. Most species-rich banks (Bank Group 3) occurred at the shelf edge. Two of the species-poor banks (Bank Group 1) occurred further north, inside the shelf. Geyer Bank (Bank Group 2) occurred at the shelf edge but was anomalously species-poor. Box-and-whisker analyses identified four Species Groups driving the Bank Groupings. Species Group 4 (the Elatopathes abientina/Nicella sp. group) was the largest (also containing Peysonellia sp.), primarily defining Bank Group 3. Species Groups 2 (the Antipathes sp./Gorgonian G04 group) and 3 (low species abundances) were also associated with Bank Group 3. Species Group 4 (the Elatopathes abientina/Nicella sp. group) was a major contributor to Bank Group 2 (Geyer Bank). Species Group 2 (the Antipathes sp./Gorgonian G04 group) was the primary constituent of the on-shelf Bank Group 1, also characterized by low species richness. Most species had a comparative abundance of ≤20%. The high species richness and affinities exhibited by Bank Group 3 are likely due to continual exposure to warm, low-turbidity Caribbean water at the shelf edge. Banks inside the shelf likely vary from the others as a result of exposure to cooler winter temperatures and higher turbidity due to wind-forced inshore currents. The reasons for the unique community structure on Geyer Bank are as yet unknown. Shelf-edge banks tend to be more species rich than on-shelf banks

Determination of the Acceleration Region Size in a Loop-structured Solar Flare

In order to study the acceleration and propagation of bremsstrahlung-producing electrons in solar flares, we analyze the evolution of the flare loop size with respect to energy at a variety of times. A GOES M3.7 loop-structured flare starting around 23:55 on 2002 April 14 is studied in detail using \textit{Ramaty High Energy Solar Spectroscopic Imager} (\textit{RHESSI}) observations. We construct photon and mean-electron-flux maps in 2-keV energy bins by processing observationally-deduced photon and electron visibilities, respectively, through several image-processing methods: a visibility-based forward-fit (FWD) algorithm, a maximum entropy (MEM) procedure and the uv-smooth (UVS) approach. We estimate the sizes of elongated flares (i.e., the length and width of flaring loops) by calculating the second normalized moments of the intensity in any given map. Employing a collisional model with an extended acceleration region, we fit the loop lengths as a function of energy in both the photon and electron domains. The resulting fitting parameters allow us to estimate the extent of the acceleration region which is between $\sim 13 \rm{arcsec}$ and $\sim 19 \rm{arcsec}$. Both forward-fit and uv-smooth algorithms provide substantially similar results with a systematically better fit in the electron domain.The consistency of the estimates from these methods provides strong support that the model can reliably determine geometric parameters of the acceleration region. The acceleration region is estimated to be a substantial fraction ($\sim 1/2$) of the loop extent, indicating that this dense flaring loop incorporates both acceleration and transport of electrons, with concurrent thick-target bremsstrahlung emission.Comment: 8 pages, 5 figures, accepted to Astronomy and Astrophysics journa

The Herschel effect and solar photography

A method of photographing the solar image in the UV part of the solar spectrum is described. The resulting images show the bright photospheric network of Sheeley and Chapman; these have been recorded at the small central distance of 20°. During a flare of importance 1b near a spot group, no detectable changes in the spots, their relative positions or the bright photospheric network could be observed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43708/1/11207_2005_Article_BF02276575.pd

Antiferromagnetic ordering in a 90 K copper oxide superconductor

Using elastic neutron scattering, we evidence a commensurate antiferromagnetic Cu(2) order (AF) in the superconducting (SC) high-$\rm T_c$ cuprate $\rm YBa_2(Cu_{1-y}Co_y)_3O_{7+\delta}$ (y=0.013, $\rm T_c$=93 K). As in the Co-free system, the spin excitation spectrum is dominated by a magnetic resonance peak at 41 meV but with a reduced spectral weight. The substitution of Co thus leads to a state where AF and SC cohabit showing that the CuO$_2$ plane is a highly antiferromagnetically polarizable medium even for a sample where T$_c$ remains optimum.Comment: 3 figure

Inhomogeneous magnetization in dipolar ferromagnetic liquids

At high densities fluids of strongly dipolar spherical particles exhibit spontaneous long-ranged orientational order. Typically, due to demagnetization effects induced by the long range of the dipolar interactions, the magnetization structure is spatially inhomogeneous and depends on the shape of the sample. We determine this structure for a cubic sample by the free minimization of an appropriate microscopic density functional using simulated annealing. We find a vortex structure resembling four domains separated by four domain walls whose thickness increases proportional to the system size L. There are indications that for large L the whole configuration scales with the system size. Near the axis of the mainly planar vortex structure the direction of the magnetization escapes into the third dimension or, at higher temperatures, the absolute value of the magnetization is strongly reduced. Thus the orientational order is characterized by two point defects at the top and the bottom of the sample, respectively. The equilibrium structure in an external field and the transition to a homogeneous magnetization for strong fields are analyzed, too.Comment: 17 postscript figures included, submitted to Phys. Rev.