Radio Emission of Flares and Coronal Mass Ejections     

A. Nindos  H. Aurass  K.-L. Klein  G. Trottet 《Solar physics》2008,253(1-2):3-41

We review recent progress on our understanding of radio emission from solar flares and coronal mass ejections (CMEs) with emphasis on those aspects of the subject that help us address questions about energy release and its properties, the configuration of flare?–?CME source regions, coronal shocks, particle acceleration and transport, and the origin of solar energetic particle (SEP) events. Radio emission from electron beams can provide information about the electron acceleration process, the location of injection of electrons in the corona, and the properties of the ambient coronal structures. Mildly relativistic electrons gyrating in the magnetic fields of flaring loops produce radio emission via the gyrosynchrotron mechanism, which provides constraints on the magnetic field and the properties of energetic electrons. CME detection at radio wavelengths tracks the eruption from its early phase and reveals the participation of a multitude of loops of widely differing scale. Both flares and CMEs can ignite shock waves and radio observations offer the most robust tool to study them. The incorporation of radio data into the study of SEP events reveals that a clear-cut distinction between flare-related and CME-related SEP events is difficult to establish.  相似文献   

Early Jurassic basal sauropodomorpha dominated tracks from Guizhou,China: Morphology,ethology, and paleoenvironment     

Lida Xing  Martin G. Lockley  Dongjie Tang  Hendrik Klein  Guangzhao Peng  Yong Ye  Baoqiao Hao 《地学前缘(英文版)》2019,10(1):229-240

The newly discovered large(350 m^2) Yantan dinosaur tracksite, in the Lower Jurassic Ziliujing Formation of Guizhou Province, China, reveals at least 250 footprints of which ~97 can be resolved into trackways of sauropodomorphs. All the trackways are sub parallel likely indicating gregarious behavior. One theropod track(cf. Grallator) was recorded. The sauropodomorph tracks predominantly represent quadrupedal progression(Morphotype A), and footprint morphology is similar to the ichnospecies Liujianpusshunan, characterized by outward pes rotation. Three trackways indicate bipedal progression, and two of these(Morphotype B) indicate inward pes rotation, accompanied by elongate pes digit scratch marks. For the latter phenomenon three possible scenarios are discussed:(1) significant rotation changes accompanying changes in gait,(2) swimming behavior,(3) formation of undertracks.Sedimentological evidence indicates the tracks were made on a linguloid rippled, muddy, immature sandstone substrate characterized by significant differences in substrate consistency across the trackbearing surface. Microbially induced sedimentary structures(MISS) characterized by distinctive wrinkle marks indicate a stressed, probably semi-arid, paleoenvironment that was not conducive to habitation by invertebrate organisms. This is consistent with other evidence that Lower Jurassic sauropodomorph tracks are often associated with semi-arid paleoenvironments.  相似文献   

Permafrost at the time of the Last Glacial Maximum (LGM) in North America     

Hugh M. French  Susan W. S. Millar 《Boreas: An International Journal of Quaternary Research》2014,43(3):667-677

This paper presents three maps that summarize current knowledge as to the extent of Past permafrost and Relict permafrost in North America at approximately the time of the Last Glacial Maximum (LGM; c. 25–17 ka BP) and during subsequent deglaciation until c. 10 ka BP. Analysis of the post‐1983 literature suggests that the extent of Past permafrost south of the LGM limit was broader in eastern North America and slightly narrower in the Interior Great Plains than previously mapped. The recognition and dating of Relict permafrost in the nonglaciated terrain of the northwestern Arctic suggests that permafrost may be of great antiquity and can persist under changing climatic conditions. The formation of permafrost features during deglaciation suggests that ice‐proximal climatic conditions remained cold at least long enough for short‐lived permafrost aggradation; a latitudinal gradient is evident in the timing of its development as the Laurentide Ice Sheet retreated.  相似文献   

Titanite dates crystallization: Slow Pb diffusion during super‐solidus re‐equilibration     

Michael I. H. Hartnady  Christopher L. Kirkland  Chris Clark  Catherine V. Spaggiari  R. Hugh Smithies  Noreen J. Evans  Brad J. McDonald 《Journal of Metamorphic Geology》2019,37(6):823-838

Titanite can be found in rocks of wide compositional range, is reactive, growing or regrowing during metamorphic and hydrothermal events, and is generally amenable to U–Pb geochronology. Experimental evidence suggest that titanite has a closure temperature for Pb ranging from 550 to 650°C, and thus titanite dates are commonly interpreted as cooling ages. However, this view has been challenged in recent years by evidence from natural titanite which suggests the closure temperature may be significantly higher (up to 800°C). Here, we investigate titanite in an enclave of migmatitic gneiss included within a granite intrusion. The titanite crystals exhibit textural features characteristic of fluid‐mediated mass transfer processes on length scales of <100 µm. These textural features are associated with variation in both Pb concentrations and distinct U–Pb isotopic compositions. Zr‐in‐titanite thermometry indicates that modification of the titanite occurred at temperatures in excess of 840°C, in the presence of a high‐T silicate melt. The Pb concentration gradients preserved in these titanite crystals are used to determine the diffusivity of Pb in titanite under high‐T conditions. We estimate diffusivities ranging from 2 × 10^?22 to 5 × 10^?25 m²/s. These results are significantly lower than experimental data predict yet are consistent with other empirical data on natural titanites, suggesting that Pb diffusivity is similar to that of Sr. Thus our data challenge the wide‐held assumption that U–Pb titanite dates only reflect cooling ages.  相似文献   

Post‐orogenic sediment drape in the Northern Pyrenees explained using a box model     

Thomas Bernard  Hugh D. Sinclair  Mark Naylor  Frdric Christophoul  Mary Ford 《Basin Research》2021,33(1):118-137

The transition to a post‐orogenic state in mountain ranges has been identified by a change from active subsidence to isostatic rebound of the foreland basin. However, the nature of the interplay between isostatic rebound and sediment supply, and their impact on the topographic evolution of a range and foreland basin during this transition, has not been fully investigated. Here, we use a box model to explore the syn‐ to post‐orogenic evolution of foreland basin/thrust wedge systems. Using a set of parameter values that approximate the northern Pyrenees and the neighbouring Aquitaine foreland basin, we evaluate the controls on sediment drape over the frontal parts of the retro‐wedge following cessation of crustal thickening. Conglomerates preserved at approximately 600‐m elevation, which is ~ 300 m above the present mountain front in the northern Pyrenees are ca. 12 Ma, approximately 10 Myrs younger than the last evidence of crustal thickening in the wedge. Using the model, this post‐orogenic sediment drape is explained by the combination of a sustained, high sediment influx from the range into the basin relative to the efflux out of the basin, combined with cessation of the generation of accommodation space through basin subsidence. Post‐orogenic sediment drape is considered a generic process that is likely to be responsible for elevated low‐gradient surfaces and preserved remnants of continental sedimentation draping the outer margins of the northern Pyrenean thrust wedge.  相似文献   

Photometric Stability of the Lunar Surface   总被引：1，自引：0，他引：1  

Hugh H. Kieffer 《Icarus》1997,130(2):323-327

The rate at which cratering events currently occur on the Moon is considered in light of their influence on the use of the Moon as a radiometric standard. The radiometric effect of small impact events is determined empirically from the study of Clementine images. Events that would change the integral brightness of the moon by 1% are expected once per 1.4 Gyr. Events that cause a 1% shift in one pixel for low Earth-orbiting instruments with a 1-km nadir field of view are expected approximately once each 43 Myr. Events discernible at 1% radiometric resolution with a 5 arc-sec telescope resolution correspond to crater diameters of approximately 210 m and are expected once every 200 years. These rates are uncertain by a factor of two. For a fixed illumination and observation geometry, the Moon can be considered photometrically stable to 1 × 10⁻⁸per annum for irradiance, and 1 × 10⁻⁷per annum for radiance at a resolution common for spacecraft imaging instruments, exceeding reasonable instrument goals by six orders of magnitude.  相似文献   

Frost spectra: Comparison with Jupiter's Satellites     

Hugh H. Kieffer  W.D. Smythe 《Icarus》1974,21(4):506-512

The near infrared spectral reflectance of pure CH₄, CO₂, H₂O, H₂S, NH₃ and NH₄SH frosts has been measured. Comparison with recent Galilean satellite spectra indicates that H₂O at approximately 150°K and with about 0.01 cm grain size is the major component on the surface of JII (Europa) and JIII (Ganymede); upper limits for the remaining frosts range from 5 to 28%. Materials other than these frosts must be present on the surface of JI (Io) and JIV (Callisto).  相似文献   

Morphometric and geological conditions for sediment accumulation in the Udava River,Outer Carpathians,Slovakia     

Dušan Barabas  Ján Bóna  Daniel Klein  Lenka Balážovičová 《地理学报(英文版)》2017,27(8):981-998

The accumulation of sediment in river channels is a phenomenon that is not only influenced by the channel morphology, but also by the physical and geographical characteristics and the endogenous and exogenous processes taking place in the catchment. This paper presents an analysis of the impact the changes in lithological conditions have on the morphological and morphometric parameters of the Udava River channel and their relation to the channel accumulation forms representative of the river’s longitudinal profile as well as of its planform. Results document when accumulation forms occur and what is their spatial distribution within the longitudinal and cross-sectional river profiles. More resistant structures created sections with a lower degree of sedimentation, while in depression segments the degree of sedimentation was higher. With the increase in longitudinal slope, the impact of channel width on the average channel bar size increased. Also a difference in the accumulation was observed between the left and right bank which could be possibly explained by the impact of Coriolis force.  相似文献   

The Global Dynamics of the High-Temperature Corona     

Hudson  Hugh 《Solar physics》1999,190(1-2):91-106

This paper surveys coronal motions detected by the Yohkoh soft X-ray telescope SXT, emphasizing `global restructuring'. Large-scale structures in the solar corona can persist for time scales much longer than those of the supergranulation, and may have larger spatial scales. Flares and coronal mass ejections (CMEs) can disrupt these structures. A grazing-incidence telescope such as SXT provides a view of the corona biased in temperature towards the hotter components, but in a thick `observing slice' (spatial contribution function). This helps in seeing excitation (energy release) but may conceal some of the structural changes. The observations of restructuring largely appear to consist of expanding or outwards magnetic motions, which are endoergic. This suggests a conjecture regarding the existence of magnetic implosions on scales not yet detected, as a source of free energy.  相似文献   

Pilot Experiments on Planting Seedlings and Small Seagrass Propagules in Western Australia     

Hugh Kirkman 《Marine pollution bulletin》1999,37(8-12):460-467

The aim of this study was to investigate characteristics of some south-western Australian seagrasses that might be useful for further investigation with respect to restoration. Although seeds and fruits of seagrass species may be useful for producing propagules for planting, their collection is based on knowing when they become available. Posidonia australis, P. angustifolia and P. coriacea flower profusely annually, but P. sinuosa does not. Once collected, the fruits dehisce in a few days and are ready for planting. It was found that the best way to plant Posidonia seedlings is in clusters in Growool^® blocks but the rhizomes do not spread quickly. Single Posidonia seedlings, when planted out, were not successful and neither were naturally colonizing seedlings of Posidonia sinuosa in unexposed unvegetated areas. Successful planting of seedlings and sprigs of Amphibolis in matting needs months of calm weather for them to spread out from their attaching matting which must be very securely attached to the substratum. Little success was achieved in growing Amphibolis. In a search for Halophila ovalis seeds, few were found because of the patchy nature of their fruiting and small size. Germination was slow but 63% germinated within 6 months.  相似文献