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We identify coronal holes using a histogram-based intensity thresholding technique and compare their properties to fast solar wind streams at three different points in the heliosphere. The thresholding technique was tested on EUV and X-ray images obtained using instruments onboard STEREO, SOHO and Hinode. The full-disk images were transformed into Lambert equal-area projection maps and partitioned into a series of overlapping sub-images from which local histograms were extracted. The histograms were used to determine the threshold for the low intensity regions, which were then classified as coronal holes or filaments using magnetograms from the SOHO/MDI. For all three instruments, the local thresholding algorithm was found to successfully determine coronal hole boundaries in a consistent manner. Coronal hole properties extracted using the segmentation algorithm were then compared with in situ measurements of the solar wind at ~?1 AU from ACE and STEREO. Our results indicate that flux tubes rooted in coronal holes expand super-radially within 1 AU and that larger (smaller) coronal holes result in longer (shorter) duration high-speed solar wind streams. 相似文献
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W. Alan Delamere Livio L. Tornabene Kris Becker Nathan T. Bridges Dennis Gallagher Laszlo Keszthelyi Guy K. McArthur Moses Milazzo Nicolas Thomas 《Icarus》2010,205(1):38-52
HiRISE has been producing a large number of scientifically useful color products of Mars and other planetary objects. The three broad spectral bands, coupled with the highly sensitive 14 bit detectors and time delay integration, enable detection of subtle color differences. The very high spatial resolution of HiRISE can augment the mineralogic interpretations based on multispectral (THEMIS) and hyperspectral datasets (TES, OMEGA and CRISM) and thereby enable detailed geologic and stratigraphic interpretations at meter scales. In addition to providing some examples of color images and their interpretation, we describe the processing techniques used to produce them and note some of the minor artifacts in the output. We also provide an example of how HiRISE color products can be effectively used to expand mineral and lithologic mapping provided by CRISM data products that are backed by other spectral datasets. The utility of high quality color data for understanding geologic processes on Mars has been one of the major successes of HiRISE. 相似文献
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A fundamental understanding of the relation between stress concentrations at grain contacts and microfractures in granular aggregates is obtained through two-dimensional photomechanical model studies and is tested through observational studies of experimentally deformed sandstone discs, glass beads, and quartz sand.In uncemented aggregates, the state of stress in each grain is controlled by the manner in which the applied load is transmitted across grain contacts. The angles between lines connecting pairs of contacts and the axis of the principal load acting at the boundaries of the aggregate determine which of all contacts will be most highly stressed or “critical”. Microfractures follow that maximum principal stress trajectory which connects critical contacts, and they propagate through those points where the magnitude of the local maximum stress difference is the greatest. Microfractures, therefore, are extension fractures. It then follows that both the locations and orientations of fractures can be predicted if the state of stress in the grains is known.Positioning of critical contacts depends primarily on sorting, packing, grain shapes, and the boundary load conditions applied to the aggregate. Some critical contacts and, therefore, microfractures tend to join together in a series or “chain”. Orientations of chains are most strongly influenced by the direction of the maximum compressive load at the boundary of the aggregate. A hydrostatic load applied on the boundaries of an aggregate can cause microfracturing within grains. Orientations for microfractures and contact lines are random in poorly sorted aggregates, but they are influenced by packing in well sorted aggregates.Grains of cemented aggregates are more highly stressed at their centers than at contacts. By analogy, microfracture orientations depend strongly on the position of the greatest load axis and only slightly on the low-magnitude stress concentrations at contacts. These microfractures parallel the greatest principal stress trajectory in regions where the magnitude of the maximum stress difference is greatest. These observations lead to the conclusion that fractures in grains of cemented aggregates are also extension fractures and should exhibit a higher degree of preferred orientation than in uncemented counterparts.These conclusions hold when cementing materials have about the same elastic moduli as the grains. Cements may be so weak that the aggregate behaves as if it were uncemented in terms of microfracture fabric, or so stiff that the major part of the load is transmitted by the cement, and the composite is no longer an aggregate in the mechanical sense. 相似文献
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Aisling Gallagher 《New Zealand geographer》2017,73(1):15-24
There has been a significant growth in childcare in New Zealand (NZ) since 2006. Shaped by debates around the marketisation of childcare, this paper will make some key observations about the NZ childcare market. I will argue that the landscape of childcare has changed in favour of a burgeoning private sector and consider the recent impetus for corporatisation. Finally, I will examine the discourse of parental ‘choice’ which pervades policy discussions around ECE in New Zealand and how this plays out in the face of an expanding private for‐profit sector. 相似文献
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An examination of some uncertainties associated with estimates of sedimentation rates and tectonic subsidence 总被引:1,自引:0,他引:1
KERRY Gallagher 《Basin Research》1989,2(2):97-114
Abstract The sensitivity of backstripping calculations (sedimentation rates and tectonic subsidence) to uncertainties regarding porosity reduction is examined. Models simulating compaction and externally sourced cementation are considered to provide first-order bounds on the thickness and mass changes for individual sedimentary units. These bounds can be used to estimate uncertainties in sedimentation rate and subsidence estimates. With these models, the timing of cement development can be regarded as unimportant for backstripping calculations. Calculations have been made to evaluate the effect on backstripping calculations of uncertainties in sediment porosity, density and the mechanisms of porosity reduction. Departures from theoretically predicted subsidence curves of the order of 100 m or so have been variously interpreted as the result of fluctuations or uncertainties in sea-level, palaeobathymetry, tectonic stress, sedimentation rates and stratigraphic age. Two examples are given to illustrate that such departures may occur in some subsidence curves merely as a result of imprecise assumptions regarding porosity reduction. Consideration should be given to the uncertainties in models for porosity reduction when using subsidence curves to infer second order tectonic influence during basin evolution. 相似文献
50.
D.R. Williams K.J.H. Phillips P. Rudawy M. Mathioudakis P.T. Gallagher E. O'Shea F.P. Keenan P. Read B. Rompolt 《Monthly notices of the Royal Astronomical Society》2001,326(2):428-436
The Solar Eclipse Corona Imaging System (SECIS) was used to record high-cadence observations of the solar corona during the total solar eclipse of 1999 August 11. During the 2 min 23.5 s of totality, 6364 images were recorded simultaneously in each of the two channels: a white light channel, and the Fe xiv (5303 Å) 'green line' channel ( T ∼2 MK) . Here we report initial results from the SECIS experiment, including the discovery of a 6-s intensity oscillation in an active region coronal loop. 相似文献