Characteristic size and diffusion of quiet sun magnetic patterns     

R. W. Komm  R. F. Howard  J. W. Harvey 《Solar physics》1995,158(2):213-225

We have previously studied large-scale motions using high-resolution magnetograms taken from 1978 to 1990 with the NSO Vacuum Telescope on Kitt Peak. Latitudinal and longitudinal motions were determined by a two-dimensional crosscorrelation analysis of pairs of consecutive daily observations using small magnetic features as tracers. Here we examine the shape and amplitude of the crosscorrelation functions. We find a characteristic length scale as indicated by the FWHM of the crosscorrelation functions of 16.6 ± 0.2 Mm. The length scale is constant within ±45° latitude and decreases by about 5% at 52.5° latitude; i.e., the characteristic size is almost latitude independent. The characteristic scale is within 3% of the average value during most times of the solar cycle, but it increases during cycle maximum at latitudes where active regions are present. For the time period 1978–1981 (solar cycle maximum), the length scale increases up to 1.7 Mm or 10% at 30° latitude. In addition, we derive the average amplitude of the crosscorrelation functions, which reflects the diffusion of magnetic elements and their evolutionary changes (including formation and decay). We find an average value of 0.091 ± 0.003 for the crosscorrelation amplitude at a time lag of one day, which we interpret as being caused by the combined effect of the lifetime of magnetic features and a diffusion process. Assuming a lifetime of one day, we find a value of 120 km² s^–1 for the diffusion constant, while a lifetime of two days leads to 230 km² s^–1.Operated by the Association of Universities for Research in Astronomy, Inc. under cooperative agreement with the National Science Foundation.  相似文献   

White-light and radio sounding observations of coronal transients     

M. K. Bird  H. Volland  R. A. Howard  M. J. Koomen  D. J. Michels  N. R. Sheeley Jr.  J. W. Amstrong  B. L. Seidel  C. T. Stelzried  R. Woo 《Solar physics》1985,98(2):341-368

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Solar surface velocity fields determined from small magnetic features     

R. F. Howard  J. W. Harvey  S. Forgach 《Solar physics》1990,130(1-2):295-311

We describe a method for the analysis of magnetic data taken daily at the Vacuum Telescope at Kitt Peak. In this technique, accurate position differences of very small magnetic features on the solar surface outside active regions are determined from one day to the next by a cross-correlation analysis. In order to minimize systematic errors, a number of corrections are applied to the data for effects originating in the instrument and in the Earth's atmosphere. The resulting maps of solar latitude vs central meridian distance are cross-correlated from one day to the next to determine daily motions in longitude and latitude. Some examples of rotation and meridional motion results are presented. For the months of May 1988 and October–November 1987, we find rotation coefficients A = 2.894 ± 0.011, B = - 0.428 ± 0.070, and C = -0.370 ± 0.077 in rad s^–1 from the expansion = A + B sin² + C sin⁴, where is the latitude. The differential rotation curve for this interval is essentially flat within 20 deg of the equator in these intervals. For the same intervals we find a poleward meridional motion a = 16.0 ± 2.8 m sec ^-1 from the relation v = a sin, where v is the line-of-sight velocity.Operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.  相似文献   

Platinum-group minerals from the Jinbaoshan Pd–Pt deposit,SW China: evidence for magmatic origin and hydrothermal alteration     

Christina Yan Wang  Hazel M. Prichard  Mei-Fu Zhou  Peter C. Fisher 《Mineralium Deposita》2008,43(7):791-803

The Jinbaoshan Pt–Pd deposit in Yunnan, SW China, is hosted in a wehrlite body, which is a member of the Permian (∼260 Ma) Emeishan Large Igneous Province (ELIP). The deposit is reported to contain one million tonnes of Pt–Pd ore grading 0.21% Ni and 0.16% Cu with 3.0 g/t (Pd + Pt). Platinum-group minerals (PGM) mostly are ∼10 μm in diameter, and are commonly Te-, Sn- and As-bearing, including moncheite (PtTe₂), atokite (Pd₃Sn), kotulskite (PdTe), sperrylite (PtAs₂), irarsite (IrAsS), cooperite (PtS), sudburyite (PdSb), and Pt–Fe alloy. Primary rock-forming minerals are olivine and clinopyroxene, with clinopyroxene forming anhedral poikilitic crystals surrounding olivine. Primary chromite occurs either as euhedral grains enclosed within olivine or as an interstitial phase to the olivine. However, the intrusion has undergone extensive hydrothermal alteration. Most olivine grains have been altered to serpentine, and interstitial clinopyroxene is often altered to actinolite/tremolite and locally biotite. Interstitial chromite grains are either partially or totally replaced by secondary magnetite. Base-metal sulfides (BMS), such as pentlandite and chalcopyrite, are usually interstitial to the altered olivine. PGM are located with the BMS and are therefore also interstitial to the serpentinized olivine grains, occurring within altered interstitial clinopyroxene and chromite, or along the edges of these minerals, which predominantly altered to actinolite/tremolite, serpentine and magnetite. Hydrothermal fluids were responsible for the release of the platinum-group elements (PGE) from the BMS to precipitate the PGM at low temperature during pervasive alteration. A sequence of alteration of the PGM has been recognized. Initially moncheite and atokite have been corroded and recrystallized during the formation of actinolite/tremolite, and then, cooperite and moncheite were altered to Pt–Fe alloy where they are in contact with serpentine. Sudburyite occurs in veins indicating late Pd mobility. However, textural evidence shows that the PGM are still in close proximity to the BMS. They occur in PGE-rich layers located at specific igneous horizons in the intrusion, suggesting that PGE were originally magmatic concentrations that, within a PGE-rich horizon, crystallized with BMS late in the olivine/clinopyroxene crystallization sequence and have not been significantly transported during serpentinization and alteration.  相似文献   

Summertime diurnal variations of atmospheric peroxides and formaldehyde in Sweden     

Howard B. Ross  Christer Johansson  Claes de Serves  John Lind 《Journal of Atmospheric Chemistry》1992,14(1-4):411-423

Atmospheric peroxides and formaldehyde were measured at two sites in Sweden; inside a Scots pine stand (Jädraås) and on top of Mt. Åreskutan (1250 msl). Peroxide levels at Jädraås were highest during the day and lowest during the night. Mid-day concentrations of H₂O₂ varied between 0.05 and 2 ppbv. Isentropic trajectories together with local O₃ measurements indicated the importance of long range transport on surface H₂O₂ lévels. Large diurnal variations and vertical profiles showed the importance of turbulent mixing processes and dry deposition. A comparison of H₂O₂ and O₃ diurnal variations indicated a more rapid dry deposition of H₂O₂ to the forest. It would appear that terpenes emitted from the forest play a minor role in controlling the H₂O₂ levels. Formaldehyde at Jädraås had a different diurnal variation than peroxides; highest levels were observed in the early evening indicating chemical production of CH₂O. Diurnal variations of peroxides on Mt Åreskutan were opposite to those at Jädraås, highest concentrations were observed during the night. This result is to be expected if during the day air from inside the valley, with lower peroxide levels relative to the free troposphere, rises to the mountain top. In the evening, subsidence brings free tropospheric air with higher peroxides levels to the mountain.  相似文献   

Rainwater acidity and ion concentration correlations in a midwest storm system     

Howard A. Bridgman 《Journal of Atmospheric Chemistry》1991,12(4):299-317

The pH and the concentrations of sulfate, nitrate, ammonia, and calcium in rainwater were measured for two periods of a single midwest rainstorm which occurred over a mesometeorological network in central Illinois on 24–25 July 1979. Regression analysis was used to compare ion concentrations with rainfall amount, and ion balance was used to compare cation and anion concentrations at individual sites. Only the ions SO₄ ^2- and NO₃ ^- show any significant relationship to rainfall amount, decreasing as rainwater amounts increase (r=–0.57 and –0.60, respectively). During the first period of the rainstorm, a sequential sampler measurements allowed the calculation of detailed temporal variations in SO₄ ^2-, pH, and rain rate. SO₄ ^2- decreased, and pH increased as the rate increased and the opposite temporal pattern occurred as the rain decreased at the end of the period. Reasons for these variations are discussed.Research done while a visiting scientist at the Illinois State Water Survey, Champaign, Illinois, U.S.A.  相似文献   

EIT: Extreme-ultraviolet Imaging Telescope for the SOHO mission   总被引：10，自引：0，他引：10  

J. -P. Delaboudinière  G. E. Artzner  J. Brunaud  A. H. Gabriel  J. F. Hochedez  F. Millier  X. Y. Song  B. Au  K. P. Dere  R. A. Howard  R. Kreplin  D. J. Michels  J. D. Moses  J. M. Defise  C. Jamar  P. Rochus  J. P. Chauvineau  J. P. Marioge  R. C. Catura  J. R. Lemen  L. Shing  R. A. Stern  J. B. Gurman  W. M. Neupert  A. Maucherat  F. Clette  P. Cugnon  E. L. Van Dessel 《Solar physics》1995,162(1-2):291-312

The Extreme-ultraviolet Imaging Telescope (EIT) will provide wide-field images of the corona and transition region on the solar disc and up to 1.5 R above the solar limb. Its normal incidence multilayer-coated optics will select spectral emission lines from Fe IX (171 ), Fe XII (195 ), Fe XV (284 ), and He II (304 ) to provide sensitive temperature diagnostics in the range from 6 × 10⁴ K to 3 × 10⁶ K. The telescope has a 45 x 45 arcmin field of view and 2.6 arcsec pixels which will provide approximately 5-arcsec spatial resolution. The EIT will probe the coronal plasma on a global scale, as well as the underlying cooler and turbulent atmosphere, providing the basis for comparative analyses with observations from both the ground and other SOHO instruments. This paper presents details of the EIT instrumentation, its performance and operating modes.  相似文献   

The Large Angle Spectroscopic Coronagraph (LASCO)   总被引：12，自引：0，他引：12  

G. E. Brueckner  R. A. Howard  M. J. Koomen  C. M. Korendyke  D. J. Michels  J. D. Moses  D. G. Socker  K. P. Dere  P. L. Lamy  A. Llebaria  M. V. Bout  R. Schwenn  G. M. Simnett  D. K. Bedford  C. J. Eyles 《Solar physics》1995,162(1-2):357-402

The Large Angle Spectroscopic Coronagraph (LASCO) is a three coronagraph package which has been jointly developed for the Solar and Heliospheric Observatory (SOHO) mission by the Naval Research Laboratory (USA), the Laboratoire d'Astronomie Spatiale (France), the Max-Planck-Institut für Aeronomie (Germany), and the University of Birmingham (UK). LASCO comprises three coronagraphs, C1, C2, and C3, that together image the solar corona from 1.1 to 30 R (C1: 1.1 – 3 R, C2: 1.5 – 6 R, and C3: 3.7 – 30 R). The C1 coronagraph is a newly developed mirror version of the classic internally-occulted Lyot coronagraph, while the C2 and C3 coronagraphs are externally occulted instruments. High-resolution imaging spectroscopy of the corona from 1.1 to 3 R can be performed with the Fabry-Perot interferometer in C1. High-volume memories and a high-speed microprocessor enable extensive on-board image processing. Image compression by a factor of about 10 will result in the transmission of 10 full images per hour.  相似文献   

Primary cumulus platinum minerals in the Monts de Cristal Complex,Gabon: magmatic microenvironments inferred from high-definition X-ray fluorescence microscopy     

Stephen J. Barnes  Louise A. Fisher  Bélinda Godel  Mark A. Pearce  Wolfgang D. Maier  David Paterson  Daryl L. Howard  Christopher G. Ryan  Jamie S. Laird 《Contributions to Mineralogy and Petrology》2016,171(3):23

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Fertilization Changes Seagrass Community Structure but not Blue Carbon Storage: Results from a 30-Year Field Experiment     

Jason L. Howard  Alex Perez  Christian C. Lopes  James W. Fourqurean 《Estuaries and Coasts》2016,39(5):1422-1434

Seagrass ecosystems are attracting attention as potentially important tools for carbon (C) sequestration, comparable to those terrestrial and aquatic ecosystems already incorporated into climate change mitigation frameworks. Despite the relatively low C stocks in living biomass, the soil organic carbon pools beneath seagrass meadows can be substantial. We tested the relationship between soil C storage and seagrass community biomass, productivity, and species composition by revisiting meadows experimentally altered by 30 years of consistent nutrient fertilization provided by roosting birds. While the benthos beneath experimental perches has maintained dense, Halodule wrightii-dominated communities compared to the sparse Thalassia testudinum-dominated communities at control sites, there were no significant differences in soil organic carbon stocks in the top 15 cm. Although there were differences in δ¹³C of the dominant seagrass species at control and treatment sites, there was no difference in soil δ¹³C between treatments. Averages for soil organic carbon content (2.57?±?0.08 %) and δ¹³C (?12.0?±?0.3?‰) were comparable to global averages for seagrass ecosystems; however, our findings question the relevance of local-scale seagrass species composition or density to soil organic carbon pools in some environmental contexts.  相似文献