Frequency of hyperbolic and interstellar meteoroids     

Maria Hajduková Jr.  Leonard Kornoš  Juraj Tóth 《Meteoritics & planetary science》2014,49(1):63-68

Hyperbolic meteor orbits from the catalog of 64,650 meteors observed by the multistation video meteor network located in Japan (SonotaCo 2009) have been investigated with the aim of determining the relation between the frequency of hyperbolic and interstellar meteors. The proportion of hyperbolic meteors in the data decreased significantly (from 11.58% to 3.28%) after a selection of quality orbits, which shows its dependence on the quality of observations. Initially, the hyperbolic orbits were searched for meteors unbound due to planetary perturbation. It was determined that 22 meteors from the 7489 hyperbolic orbits in the catalog (and 2 from the selection of the orbits with the highest quality) had had a close encounter with a planet, none of which, however, produced essential changes in their orbits. Similarly, the fraction of hyperbolic orbits in the data, which could be hyperbolic by reason of a meteor's interstellar origin, was determined to be at most 3.9 × 10^?2. From the statistical point of view, the vast majority of hyperbolic meteors in the database have definitely been caused by inaccuracy in the velocity determination. This fact does not necessarily assume great measurement errors, since, especially near the parabolic limit, a small error in the value of the heliocentric velocity of a meteor can create an artificial hyperbolic orbit that does not really exist. The results show that the remaining 96% of meteoroids with apparent hyperbolic orbits belong to the solar system meteoroid population. This is also supported by their high abundance (about 50%) among the meteor showers.  相似文献   

The distance to the Galactic center determined by OB stars     

Richard L. Branham Jr. 《Astrophysics and Space Science》2014,353(1):179-190

The OB stars are concentrated near the Galactic plane and should permit a determination of the distance to the Galactic center. van Leeuwen’s new reduction of the Hipparcos catalog provides, after 824 Gould belt stars have been excluded, 6288 OB stars out to 1 kpc and Westin’s compilation an additional 112 stars between 1 kpc and 3 kpc. The reduction model involves 14 unknowns: the Oort A and B constants, the distance to the Galactic center R ₀, 2 second-order partial derivatives, the 3 components of solar motion, a K term, a first order partial derivative for motion perpendicular to the Galactic plane, a second-order partial for acceleration perpendicular to the plane, two terms for a possible expansion of the OB stars, and a C constant. The model is nonlinear, and the unknowns are calculated by use the simplex algorithm for nonlinear adjustment applied to 14313 equations of condition, 12694 in proper motion and 1619 in radial velocity. Various solutions were tried: an L₁ solution, a least squares solution with modest (2.7 %) trim of the data, and two robust least squares solutions (biweight and Welsch weighting) with more extreme trimming. The Welsch solution seems to give the best results and calculates a distance to the Galactic center 6.72±0.39 kpc. Statistical tests show that the data are homogeneous, that the reduction model seems adequate and conforms with the assumptions used in its derivation, and that the post-fit residuals are random. Inclusion of more terms, such as streaming motion induced by Galactic density waves, degrades the solution.  相似文献   

Additional Evidence Supporting a Model of Shallow,High-Speed Supergranulation     

T. L. Duvall Jr.  S. M. Hanasoge  S. Chakraborty 《Solar physics》2014,289(9):3421-3433

Recently, Duvall and Hanasoge (Solar Phys. 287, 71, 2013) found that large-distance separation [Δ] travel-time differences from a center to an annulus [δt _oi] implied a model of the average supergranular cell that has a peak upflow of 240 m?s^?1 at a depth of 2.3 Mm and a corresponding peak outward horizontal flow of 700 m?s^?1 at a depth of 1.6 Mm. In the present work, this effect is further studied by measuring and modeling center-to-quadrant travel-time differences [δt _qu], which roughly agree with this model. Simulations are analyzed that show that such a model flow would lead to the expected travel-time differences. As a check for possible systematic errors, the center-to-annulus travel-time differences [δt _oi] are found not to vary with heliocentric angle. A consistency check finds an increase of δt _oi with the temporal frequency [ν] by a factor of two, which is not predicted by the ray theory.  相似文献   

Results of fine‐scale studies of surface hailfall with some remarks on the calibration of hailpads: Recommendations for further research     

Griffith M. Morgan Jr 《大气与海洋》2013,51(1):49-60

Abstract

Recent studies on the patterns of surface hailfall based on measurements at several scales and by several techniques are described. This is used as a framework for indicating needs for further research. The work described was performed at the Illinois State Water Survey, and no attempt has been made to present a review of all significant research on surface hailfall characteristics.  相似文献   

Urban climate and clues of heat island events in the metropolitan area of Rio de Janeiro     

Andrews José de Lucena  Otto Corrêa Rotunno Filho  José Ricardo de Almeida França  Leonardo de Faria Peres  Luciano Nóbrega Rodrigues Xavier 《Theoretical and Applied Climatology》2013,111(3-4):497-511

This paper aims to map the thermal field in the metropolitan region of Rio de Janeiro (MARJ) considering the atmospheric characteristics and the land use that contribute to understanding the urban heat island. Three thermal maps are defined through the use of Landsat5-TM satellite images for three winter events chosen for the decades of 1980, 1990, and 2000, respectively. The results reveal a concentration of warmer cores in urban central areas as well as some local warmer areas in suburban region. Sites with lower temperatures correspond to vegetated areas which are away from the central part of the MARJ, including points of suburban areas. This work emphasizes the importance of the combined analysis of surface temperature with land use and atmospheric conditions, depicting a distinct pattern of heat islands for tropical climate.  相似文献   

A Hybrid Statistical-Dynamical Downscaling of Air Temperature over Scandinavia Using the WRF Model     

Jianfeng WANG  Ricardo M. FONSECA  Kendall RUTLEDGE  Javier MARTíN-TORRES  Jun YU 《大气科学进展》2020,37(1):57-74

An accurate simulation of air temperature at local scales is crucial for the vast majority of weather and climate applications.In this work,a hybrid statistical–dynamical downscaling method and a high-resolution dynamical-only downscaling method are applied to daily mean,minimum and maximum air temperatures to investigate the quality of localscale estimates produced by downscaling.These two downscaling approaches are evaluated using station observation data obtained from the Finnish Meteorological Institute over a near-coastal region of western Finland.The dynamical downscaling is performed with the Weather Research and Forecasting(WRF)model,and the statistical downscaling method implemented is the Cumulative Distribution Function-transform(CDF-t).The CDF-t is trained using 20 years of WRF-downscaled Climate Forecast System Reanalysis data over the region at a 3-km spatial resolution for the central month of each season.The performance of the two methods is assessed qualitatively,by inspection of quantile-quantile plots,and quantitatively,through the Cramer-von Mises,mean absolute error,and root-mean-square error diagnostics.The hybrid approach is found to provide significantly more skillful forecasts of the observed daily mean and maximum air temperatures than those of the dynamical-only downscaling(for all seasons).The hybrid method proves to be less computationally expensive,and also to give more skillful temperature forecasts(at least for the Finnish near-coastal region).  相似文献   

Long-term spatial and temporal trends in frost indices in Kansas, USA   总被引：2，自引：0，他引：2  

Aavudai Anandhi  Sriram Perumal  Prasanna H. Gowda  Mary Knapp  Stacy Hutchinson  John Harrington Jr.  Leigh Murray  Mary B. Kirkham  Charles W. Rice 《Climatic change》2013,120(1-2):169-181

Frost indices such as number of frost days (nFDs), number of frost-free days (nFFDs), last spring freeze (LSF), first fall freeze (FFF), and growing-season length (GSL) were calculated using daily minimum air temperature (T_min) from 23 centennial weather stations across Kansas during four time periods (through 1919, 1920–1949, 1950–1979, and 1980–2009). A frost day is defined as a day with T_min?<?0 °C. The long- and short-term trends in frost indices were analyzed at monthly, seasonal, and annual timescales. Probability of occurrence of the indices was analyzed at 5 %, 25 %, 50 %, 75 %, and 95 %. Results indicated a general increase in T_min from 1900 through 2009 causing a decrease in nFDs. LSF and FFF occurred earlier and later than normal in the year, respectively, thereby resulting in an increase in GSL. In general, northwest Kansas recorded the greatest nFD and lowest T_min, whereas southeast Kansas had the lowest nFD and highest T_min; however, the magnitude of the trends in these indices varied with location, time period, and time scales. Based on the long-term records in most stations, LSF occurred earlier by 0.1–1.9 days/decade, FFF occurred later by 0.2–0.9 day/decade, and GSL was longer by 0.1–2.5 day/decade. At the 50 % probability level, Independence in the south-eastern part of Kansas had the earliest LSF (6 April), latest FFF (29 October) and longest GSL (207 days). Oberlin (north-western Kansas) recorded the shortest GSL (156 days) and earliest FFF (7 October) had the latest LSF (2 May) at the 50 % probability level. A positive correlation was observed for combinations of indices (LSF and GSL) and elevation, whereas a negative correlation was found between FFF and elevation.  相似文献   

Metamorphism in the Martian crust          下载免费PDF全文

Harry Y. McSween Jr.  Theodore C. Labotka  Christina E. Viviano‐Beck 《Meteoritics & planetary science》2015,50(4):590-603

Compositions of basaltic and ultramafic rocks analyzed by Mars rovers and occurring as Martian meteorites allow predictions of metamorphic mineral assemblages that would form under various thermophysical conditions. Key minerals identified by remote sensing roughly constrain temperatures and pressures in the Martian crust. We use a traditional metamorphic approach (phase diagrams) to assess low‐grade/hydrothermal equilibrium assemblages. Basaltic rocks should produce chlorite + actinolite + albite + silica, accompanied by laumontite, pumpellyite, prehnite, or serpentine/talc. Only prehnite‐bearing assemblages have been spectrally identified on Mars, although laumontite and pumpellyite have spectra similar to other uncharacterized zeolites and phyllosilicates. Ultramafic rocks are predicted to produce serpentine, talc, and magnesite, all of which have been detected spectrally on Mars. Mineral assemblages in both basaltic and ultramafic rocks constrain fluid compositions to be H₂O‐rich and CO₂‐poor. We confirm the hypothesis that low‐grade/hydrothermal metamorphism affected the Noachian crust on Mars, which has been excavated in large craters. We estimate the geothermal gradient (>20 °C km^?1) required to produce the observed assemblages. This gradient is higher than that estimated from radiogenic heat‐producing elements in the crust, suggesting extra heating by regional hydrothermal activity.  相似文献   

Devolatilization or melting of carbonates at Meteor Crater,AZ?          下载免费PDF全文

F. Hörz  P. D. Archer Jr.  P. B. Niles  M. E. Zolensky  M. Evans 《Meteoritics & planetary science》2015,50(6):1050-1070

We have investigated the carbonates in the impact melts and in a monolithic clast of highly shocked Coconino sandstone of Meteor Crater, AZ to evaluate whether melting or devolatilization is the dominant response of carbonates during high‐speed meteorite impact. Both melt‐ and clast‐carbonates are calcites that have identical crystal habits and that contain anomalously high SiO₂ and Al₂O₃. Also, both calcite occurrences lack any meteoritic contamination, such as Fe or Ni, which is otherwise abundantly observed in all other impact melts and their crystallization products at Meteor Crater. The carbon and oxygen isotope systematics for both calcite deposits suggest a low temperature environment (<100 °C) for their precipitation from an aqueous solution, consistent with caliche. We furthermore subjected bulk melt beads to thermogravimetric analysis and monitored the evolving volatiles with a quadrupole mass spectrometer. CO₂ yields were <5 wt%, with typical values in the 2 wt% range; also total CO₂ loss is positively correlated with H₂O loss, an indication that most of these volatiles derive from the secondary calcite. Also, transparent glasses, considered the most pristine impact melts, yield 100 wt% element totals by EMPA, suggesting complete loss of CO₂. The target dolomite decomposed into MgO, CaO, and CO₂; the CO₂ escaped and the CaO and MgO combined with SiO₂ from coexisting quartz and FeO from the impactor to produce the dominant impact melt at Meteor Crater. Although confined to Meteor Crater, these findings are in stark contrast to Osinski et al. (2008) who proposed that melting of carbonates, rather than devolatilization, is the dominant process during hypervelocity impact into carbonate‐bearing targets, including Meteor Crater.  相似文献   

Minéralogie environnementale     

Georges Calas  Gordon E. Brown Jr. 《Comptes Rendus Geoscience》2011,343(2-3):83-89

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