Problems related to macroscopic electric fields in the magnetosphere     

Carl-Gunne Fälthammar 《Astrophysics and Space Science》1978,55(1):179-201

The macroscopic electric fields in the magnetosphere originate from internal as well as external sources. The fields are intimately coupled with the dynamics of magnetospheric plasma convection. They also depend on the complicated electrical properties of the hot, collisionless plasma. Macroscopic electric fields are responsible for some important kinds of energization of charged particles that take place in the magnetosphere and affect not only particles of auroral energy but also, by multistep processes, trapped high-energy particles.A particularly interesting feature of magnetospheric electric fields is the fact that they can have substantial components along the geomagnetic field. Several physical mechanisms have been identified by which such electric fields can be supported even when collisions between particles are negligible. Comments are made on the magnetic-mirror effect, anomalous resistivity, collisionless thermoelectric effect and electric double layers, emphasizing key features and differences and their significance in the light of recent observational data.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May, 1978.Reprinted, with due permission fromRev. Geophys. Space Phys. 15, (1977), 457  相似文献   

Neutron capture cross-sections of stable xenon isotopes and their application in stellar nucleosynthesis     

H. Beer  F. Käppeler  G. Reffo  G. Venturini 《Astrophysics and Space Science》1983,97(1):95-119

The neutron capture cross-sections of^{124, 132, 134}Xe have been measured by the activation technique at 25 keV neutron energy. These data were supplemented by calculated capture cross-sections for^{128, 129, 130, 131}Xe via the statistical model. The complete set of capture cross-sections obtained in this way served to determine the solar xenon abundance throughs-process systematics and to study a variety of isotopic anomalies.  相似文献   

Dust in turbulent circumstellar discs     

M. Verronen  H. Vanhala  K. A. Hämeen-Anttila 《Astrophysics and Space Science》1993,200(1):75-89

The collisional model for turbulent discs (Hämeen-Anttila, 1991) is shown to be equivalent to the model if the same coefficient of viscosity is adopted in both cases and if an unknown numerical coefficient in the former model is chosen appropriately. A combination of both approximations is generalized to include the behaviour of dust in the turbulent disc. If the dust grains are not too large, evolution leads to a stable final state having a non-zero temperature. Physical interpretation of the collisional model leads to a new type of turbulence, in which, if a sufficiently large element of gas is randomly displaced even in a stable disc, the resulting oscillations around the equilibrium position are maintained by the orbital energy. This effect produces an irregular wind of 10 to 100 m s^–1 which is sufficient to prevent the sedimentation of dust grains on the equatorial plane.  相似文献   

Tertiary Andean volcanism in a caldera-graben setting     

R. Thiele  I. Beccar  B. Levi  J. O. Nyström  M. Vergara 《International Journal of Earth Sciences》1991,80(1):179-186

The Miocene Farellones Formation in central Chile (32°-35°S) is one of several up to 3000 mthick Tertiary volcanic sequences in the Central Andes with ash flows and intercalations of lacustrine sediments in their lower part, and intermediate to basic lavas and rhyolitic domes in their upper part. The Farellones rocks were probably deposited in a volcano-tectonic graben formed through a series of caldera collapses. This is suggested by (a) the fact that the formation frequently is delimited by normal faults towards which the subhorizontal strata pinch out and become upraised, indicating deposition during subsidence, (b) the huge volume of erupted acid magma and (c) a high paleothermal gradient of geothermal field type. Similar Tertiary volcanic sequences in northern Chile and central Peru probably also formed by eruptions in a caldera-graben setting. This setting and the geothermal field type of alteration make these sequences good prospecting targets for epithermal preciousmetal deposits.

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Zusammenfassung Die miozäne Farellones-Formation im zentralen Chile (32°–35°S) ist eine von mehreren bis zu 3000 m mächtigen, tertiären vulkanischen Abfolgen der zentralen Anden, mit Ignimbriten und Zwischenlagerungen von lakustrinen Sedimenten im unteren, und mit intermediären bis basischen Laven und rhyolitischen Staukuppen im oberen Teil der Abfolgen. Die Gesteine der Farellones-Formation wurden wahrscheinlich in einem vulkano-tektonischen, durch eine Reihe von Caldera-Absenkungen entstandenen Graben abgelagert. Für diese Tatsache sprechen (a) häufige Begrenzung der Formation durch Normalverwerfungen gegen welche die subhorizontalen Schichten ausdünnen und aufgebogen werden, was Ablagerung während der Absenkung andeutet, (b) das große Volumen der sauren Vulkanite und (c) der hohe paläothermale Gradient von geothermalem Charakter. Ähnlich ausgebildete, tertiäre vulkanische Abfolgen im nördlichen Chile und zentralen Peru sind vermutlich ebenfalls durch Eruptionen in Caldera-Gräben gebildet worden. Aufgrund der tektonischen Ausbildung und des geothermalen Umwandlungstyp dürften diese Abfolgen für die Prospektion nach epithermalen Vorkommen von Edelmetallen von besonderem Interesse sein.

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Resumen La Formación Farellones de Chile central (32°–35°S), deedadmiocena, es una de las muchas secuencias terciarias de más de 3000 m de espesor que existen en la alta cordillera de los Andes Centrales que contiene depósitos de flujos piroclásticos e intercalaciones de sedimentos lacustres en su parte inferior, y lavas intermedias a básicas y domos riolíticos en su parte superior. Las rocas de la Formación Farellones se depositaron, probablemente, en un graben volcano-tectónico formado a través de una serie de colapsos de calderas. Esta interpretación se basa en: (a) la frecuente delimitación de la formación por fallas normales contra las cuales los estratos subhorizontales se adelgazany se levantan, indicando depositación durante subsidencia, (b) el gran volumen de magma ácido extruido y (c) un gradiente paleotermal alto de tipo campo geotérmico. En el norte de Chile y en Perú central existen secuencias terciarias similares depositadas probablemente también en depresiones volcano — tectónicas tipo caldera — graben. Este marco tectónico y la alteración de tipo campo geotérmico, hace que éstas secuencias sean buenos blancos de prospección para depósitos epitermales de metales preciosos.

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The flare of December 17, 1980 observed with high time resolution by a digital CCD camera     

N. Kämpfer  W. Schöchlin 《Solar physics》1982,78(2):215-224

An electro-optical system for the digital registration of H-images with high time resolution (1.6 s) is described. The sensor is a Charge-Coupled-Device (CCD) with 100 × 100 elements, resulting in our case in a resolution of 3 × 4 on the Sun. Data are digitized with a resolution of 8 bit and written on magnetic tape by means of a minicomputer. Image analysis on a large computer eliminates instrumental effects, calibrates the data and reproduces them in graphical form. The analysis of a first flare (December 17, 1980; 12:09 UT) shows that the different flare kernels brighten at different times and pulsate in diameter and intensity. The decay of the flare is slower if fainter regions are included. This supports the idea that an impulsive flare confined in a magnetic loop spreads out over a larger area during the gradual phase. Comparison with microwave observations shows the same risetime for both spectral ranges which indicates that they are excited by the same agent. Furthermore the H-flare lags the microwave burst by about 8 s indicating that the source of primary energy release lies above the H emitting layers.  相似文献   

Phase relations in the greenschist-blueschist-amphibolite-eclogite facies in the system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O (NCFMASH), with application to metamorphic rocks from Samos, Greece     

Thomas Will  Martin Okrusch  Esther Schmädicke  Guoli Chen 《Contributions to Mineralogy and Petrology》1998,132(1):85-102

Calculated phase equilibria among the minerals sodic amphibole, calcic amphibole, garnet, chloritoid, talc, chlorite, paragonite, margarite, omphacite, plagioclase, carpholite, zoisite/clinozoisite, lawsonite, pyrophyllite, kyanite, sillimanite, quartz and H₂O are presented for the model system Na₂O-CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O (NCFMASH), which is relevant for many greenschist, blueschist, amphibolite and eclogite facies rocks. Using the activity-composition relationships for multicomponent amphiboles constrained by Will and Powell (1992), equilibria containing coexisting calcic and sodic amphiboles could be determined. The blueschist–greenschist transition reaction in the NCFMASH system, for example, is defined by the univariant reaction sodic amphibole + zoisite = calcic amphibole + chlorite + paragonite + plagioclase (+ quartz + H₂O) occurring between approximately 420 and 450 °C at 9.5 to 10 kbar. The calculated petrogenetic grid is a valuable tool for reconstructing the PT-evolution of metabasic rocks. This is shown for rocks from the island of Samos, Greece. On the basis of mineral and whole rock analyses, PT-pseudosections were calculated and, together with the observed mineral assemblages and reaction textures, are used to reconstruct PT-paths. For rocks from northern Samos, pseudomorphs after lawsonite preserved in garnet, the assemblage sodic amphibole-garnet-paragonite-chlorite-zoisite-quartz and the retrograde appearance of albitic plagioclase and the formation of calcic amphibole around sodic amphibole constrain a clockwise PT-path that reaches its thermal maximum at some 520 °C and 19 kbar. The derived PT-trajectory indicates cooling during exhumation of the rocks and is similar to paths for rocks from the western part of the Attic-Cycladic crystalline complex. Rocks from eastern Samos indicate lower pressures and are probably related to high-pressure rocks from the Menderes Massif in western Turkey. Received: 8 July 1997 / Accepted: 11 February 1998  相似文献   

Latent Heat Flux from Small Sheltered Lakes   总被引：1，自引：0，他引：1  

Ari Venäläinen  Martti Heikinheimo  Tapio Tourula 《Boundary-Layer Meteorology》1998,86(3):355-377

The dependency of the latent heat flux on the over-water fetch on lakes surrounded by tall, dense forest was studied by making use of measurements made on two different-sized lakes. The measurements were made during the NOPEX (Northern Hemisphere Climate-Processes Land Surface Experiment) field campaign. It was found that, in the case of a typical Scandinavian lake with a size of less than 10 km², the latent heat flux will increase as a function of over-water fetch due to the increase of wind speed and in spite of the increased air humidity. This also has implications on area-averaged fluxes: when two lakes having similar shorelines, lake water temperatures and solar radiation conditions are compared, then the evaporation per unit area is smaller from the smaller lake. When the lakes are large, with fetches of several kilometres, then the significance of sheltering is small. If point measurements are used for the estimation of area-averaged latent heat fluxes from lakes with short fetches and forested shorelines then the distance of the measuring site from the shoreline should be taken into account, otherwise errors of tens of percent may occur.  相似文献   

红粘土深基坑边坡的稳定性评价与治理     

钱沪 《岩土工程技术》1998,(1):14-15

土质边坡常因客观条件和地质因素的改变而失去原有的稳定性.通过对某一工程的红粘土土质边坡的稳定性评价,提出了在治理基坑边坡工作中,抗剪指标参数的取用原则和建议.  相似文献   

Heating Events in the Quiet Solar Corona     

Benz  Arnold O.  Krucker  Säm 《Solar physics》1998,182(2):349-363

Sensitive observations of the quiet Sun observed by EIT on the SOHO satellite in high-temperature iron-line emission originating in the corona are presented. The thermal radiation of the quiet corona is found to fluctutate significantly, even on the shortest time scale of 2 min and in the faintest pixels. The power spectrum of the emission measure time variations is approximately a power law with an exponent of 1.79±0.08 for the brightest pixels and 1.69±0.08 for the average and the faintest pixels. The more prominent enhancements are identified with previously reported X-ray network flares (Krucker et al., 1997) above the magnetic network of the quiet chromosphere. In coronal EUV iron lines they are amenable to detailed analysis suggesting that the brightenings are caused by additional plasma injected from below and heated to slightly higher temperature than the preexisting corona. Statistical investigations are consistent with the hypothesis that the weaker emission measure enhancements originate from the same parent population. The power input derived from the impulsive brightenings is linearly proportional to the radiative loss in the observed part of the corona. The absolute amount of impulsive input is model-dependent. It cannot be excluded that it can satisfy the total requirement for heating. These observations give strong evidence that a significant fraction of the heating in quiet coronal regions is impulsive.  相似文献   

Gamma-Ray Line Versus Continuum Emission of Electron-Dominated Episodes During Solar Flares     

E. Rieger  W.Q. Gan  H. Marschhäuser 《Solar physics》1998,183(1):123-132

Electron-dominated episodes or events during solar flares are characterized by a flattening of the electron bremsstrahlung continuum above about 1 MeV. This flattening leads to a dominance of the continuum at MeV energies over nuclear emissions. We analyzed events recorded by the gamma-ray spectrometer on SMM with the aim to determine the nuclear contribution in the energy range between 4 and 8 MeV. We find that for comparable continuum fluences it is less by about an order of magnitude than for other flares. The spectral index of the best-fit power law of the > 1 MeV continuum with a median at –1.84 turns out to be independent of the heliocentric angle of the events, implying that the degree of anisotropy of the radiating particles was low. It is of interest to note that a value of –1.5 seems to be a limit. The spectral index of the continuum between 0.3 and 1 MeV does not differ significantly from that of other flares. Only measurements with detectors sensitive up to at least 10 MeV can, therefore, sort out electron dominated episodes during solar flares.  相似文献