Spinor: Visible and Infrared Spectro-Polarimetry at the National Solar Observatory     

Hector Socas-Navarro  David Elmore  Anna Pietarila  Anthony Darnell  Bruce W. Lites  Steven Tomczyk  Steven Hegwer 《Solar physics》2006,235(1-2):55-73

The Spectro-Polarimeter for Infrared and Optical Regions (SPINOR) is a new spectro-polarimeter that will serve as a facility instrument for the Dunn Solar Telescope at the National Solar Observatory. This instrument is capable of achromatic polarimetry over a very broad range of wavelengths, from 430 to 1600 nm, allowing for the simultaneous observation of several visible and infrared spectral regions with full Stokes polarimetry. Another key feature of the design is its flexibility to observe virtually any combination of spectral lines, limited only by practical considerations (e.g., the number of detectors available, space on the optical bench, etc.). Visiting Astronomers, National Solar Observatory, operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation.  相似文献   

Excitation mechanism of atmospheric pressure waves from the 1980 Mount St Helens eruption     

Takeshi Mikumo  Bruce A. Bolt 《Geophysical Journal International》1985,81(2):445-461

Summary. Atmospheric pressure waves from the Mount St Helens eruption 1980 May 18 have been clearly recorded by a sensitive microbarograph at Berkeley, California. The record shows three types of waves with different group velocities. The pressure waves can be interpreted in terms of direct waves A1, antipodean travelling waves A2 and circumnavigating waves A3, all of which are composed of several acoustic-gravity modes propagated in the lower atmosphere. Synthetic barograms appropriate to the Berkeley station have been calculated on the basis of the dynamic response of the lower atmospheric structure, together with various assumptions of source properties. Comparisons between synthetic and observed barograms provide estimates for ranges of the time history of upward particle velocity at the source, source dimensions and the velocity of the source spreading over the blast zone, as well as for the average dissipation effects over the circumferential path. The results suggest that two major compression pulses on the A1 record correlate with the arrival of pressure waves from the first (lateral) blast and second (vertical) blast, although the inferred interblast time interval is not consistent with that estimated from seismic observations.  相似文献   

The dry dredgers of Cincinnati, Ohio, USA     

President  JACK W. KALLMEYER & Professor of Geology  DAVID L. MEYER 《Geology Today》1997,13(6):228-230

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Observations of SN1988e: A mature supernova     

Gillian Pearce  Bruce Patchett  Paul Murdin  Jeremy Allington-Smith 《Astrophysics and Space Science》1990,164(1):59-62

We present the spectrum of the supernova SN1988e over the wavelength range 4750–9000 Å as recorded on 11 February, 1988. The spectrum was taken in one 2000 s exposure using the Faint Object Spectrograph on the 4.2 m William Herschel Telescope (WHT) at the Observatorio del Roque de los Muchachos.We conclude that SN1988e was a type I supernova, and that at the time of observation it had faded 7.5 mag from its predicted magnitude at maximum light. Spectra taken at such late stages in the light curve are comparatively rare, and are made possible only with the application of modern instrumentation.  相似文献   

The “Shiva Hypothesis”: Impacts, mass extinctions, and the galaxy     

Michael R. Rampino  Bruce M. Haggerty 《Earth, Moon, and Planets》1996,72(1-3):441-460

The “Shiva Hypothesis”, in which recurrent, cyclical mass extinctions of life on Earth result from impacts of comets or asteroids, provides a possible unification of important processes in astrophysics, planetary geology, and the history of life. Collisions with Earth-crossing asteroids and comets ≥ a few km in diameter are calculated to produce widespread environmental disasters (dust clouds, wildfires), and occur with the proper frequency to account for the record of five major mass extinctions (from ≥ 10⁸ Mt TNT impacts) and ~ 20 minor mass extinctions (from 10⁷–10⁸ Mt impacts) recorded in the past 540 million years. Recent studies of a number of extinctions show evidence of severe environmental disturbances and mass mortality consistent with the expected after-effects (dust clouds, wildfires) of catastrophic impacts. At least six cases of features generally considered diagnostic of large impacts (e.g., large impact craters, layers with high platinum-group elements, shock-related minerals, and/or microtektites) are known at or close to extinction-event boundaries. Six additional cases of elevated iridium levels at or near extinction boundaries are of the amplitude that might be expected from collision of relatively low-Ir objects such as comets. The records of cratering and mass extinction show a correlation, and might be explained by a combination of periodic and stochastic impactors. The mass extinction record shows evidence for a periodic component of about 26 to 30 Myr, and an ~ 30 Myr periodic component has been detected in impact craters by some workers, with recent pulses of impacts in the last 2–3 million years, and at ~ 35, 65, and 95 million years ago. A cyclical astronomical pacemaker for such pulses of impacts may involve the motions of the Earth through the Milky Way Galaxy. As the Solar System revolves around the galactic center, it also oscillates up and down through the plane of the disk-shaped galaxy with a half-cycle ~ 30±3 Myr. This cycle should lead to quasi-periodic encounters with interstellar clouds, and periodic variations in the galactic tidal force with maxima at times of plane crossing. This “galactic carrousel” effect may provide a viable perturber of the Oort Cloud comets, producing periodic showers of comets in the inner Solar System. These impact pulses, along with stochastic impactors, may represent the major punctuations in earth history.  相似文献   

Illite K–Ar dating and crystal growth processes in diagenetic environments: a critical review     

Alain Meunier  Bruce Velde  Patricia Zalba 《地学学报》2004,16(5):296-304

K–Ar dating of illitic minerals is commonly used in studies of diagenetic series applied to oil prospecting. In spite of a great number of specialized papers, some problems remain unresolved. These are mostly due to a misunderstanding of the argon accumulation process during illitization. Criteria for identifying detrital–authigenic mineral mixtures, crystal ripening, fast precipitation or continuous nucleation‐growth processes are discussed using K–Ar data available in the literature. Using different parameters, such as Δ_age (age_K–Ar ? age_strati), Δ_cryst (diagenetic age_K–Ar ? age_strati) or Δ_frac (age_{K–Arfraction} ?age_K–Arfinest), it is shown that the K–Ar age significance depends on the illite nucleation–growth processes. A ‘diagenetic age’ is obtained when these processes are rapid (the K₂O accumulation period is shorter than 2σ). If lower than this value, the K–Ar ratio depends on the proportions of new and old particles, respectively, which are controlled by the relative rates of nucleation, crystal growth and ripening.  相似文献   

Longevity of fluorine-bearing tremolite on Venus     

Natasha M. Johnson  Bruce Fegley Jr. 《Icarus》2003,165(2):340-348

There is a general belief that hydrous minerals cannot exist on Venus under current surface conditions. This view was challenged when Johnson and Fegley (2000, Icarus 146, 301-306) showed that tremolite (Ca₂Mg₅Si₈O₂₂(OH)₂), a hydrous mineral, is stable against thermal decomposition at current Venus surface temperatures, e.g., 50% decomposition in 4 Ga at 740 K. To further explore hydrous mineral thermal stability on Venus, we experimentally determined the thermal decomposition kinetics of fluorine-bearing tremolite. Fluor-tremolite is thermodynamically more stable than OH-tremolite and should decompose more slowly. However how much slower was unknown. We measured the decomposition rate of fluorine-bearing tremolite and show that its decomposition is several times to greater than ten times slower than that of OH-tremolite. We also show that F-bearing tremolite is depleted in fluorine after decomposition and that fluorine is lost as a volatile species such as HF gas. If tremolite ever formed on Venus, it would probably also contain fluorine. The exceptional stability of F-bearing tremolite strengthens our conclusions that if hydrous minerals ever formed on Venus, they could still be there today.  相似文献   

Preface: Shunso Ishihara     

Bruce W. Chappell 《Resource Geology》2004,54(3):213-214

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Measurements of the north polar cap of Mars and the Earth's northern hemisphere ice and snow cover     

J. Foster  M. Owe  Charles Capen 《Earth, Moon, and Planets》1986,35(3):223-235

The boundaries of the polar caps of Mars have been measured on more than 3000 photographs since 1905 from the plate collection at the Lowell Observatory. For the Earth, the polar caps have been accurately mapped only since the mid 1960's when satellites were first available to synoptically view the polar regions. The polar caps of both planets wax and wane in response to changes in the seasons, and interannual differences in polar cap behavior on Mars as well as Earth are intimately linked to global energy balance. Data on the year to year variations in the extent of the north polar caps of Mars and Earth have been assembled and compared, although only 6 years of concurrent data were available for comparison.  相似文献   

The Si+4 content of natural phengites: A reply     

Bruce Velde 《Contributions to Mineralogy and Petrology》1967,17(1):82-84

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