The effects of large- and small-scale density structures on the radio emission from coronal streamers     

G. Thejappa  M. R. Kundu 《Solar physics》1994,149(1):31-49

The radio observations of the coronal streamers obtained using Clark Lake radioheliograph at 73.8, 50.0, and 38.5 MHz during a period of minimum activity in September 1986 are presented. Streamers appear to correlate with two prominent disk sources whose intensities fluctuated randomly. The variations in half-power diameter of the radio Sun are found to correspond with the variations in the white-light extents of the coronal streamers. It appears that the shape of the radio Sun is not a function of the phase of the solar cycle; instead it depends on the relative positions of the streamers in the corona. The observed peak brightness temperatures,T _B , of the streamers are found to be very low, being 6 × 10⁴ K.We compute the brightness temperature distribution along the equator by tracing the rays in the coronal plasma. The rays are deflected away by the streamers before reaching the critical density level, whereas they penetrate deeper into the coronal hole for small angles between the line of sight and the streamer axis. As a consequence, it is found that the streamers and coronal holes appear in the calculated equatorial brightness distribution as irregular brightness depressions and enhancements, respectively. The fine structures are found to disappear when the scattering due to small-scale density inhomogeneities is included in the ray-tracing calculations. The required relative level of density fluctuations, ₁ = N/N, is found to be greater than 12% to reduce the peak brightness temperature from 10⁶ K to 6 × 10⁴ K for all the three frequencies.On leave from Indian Institute of Astrophysics, Bangalore 560034, India.  相似文献   

Location of type I radio continuum and bursts on Yohkoh soft X-ray maps     

S. Krucker  A. O. Benz  M. J. Aschwanden  T. S. Bastian 《Solar physics》1995,160(1):151-169

A solar type I noise storm was observed on 30 July, 1992 with the radio spectrometer Phoenix of ETH Zürich, the Very Large Array (VLA) and the soft X-ray (SXR) telescope on board theYohkoh satellite. The spectrogram was used to identify the type I noise storm. In the VLA images at 333 MHz a fully left circular polarized (100% LCP) continuum source and several highly polarized (70% to 100% LCP) burst sources have been located. The continuum and the bursts are spatially separated by about 100 and apparently lie on different loops as outlined by the SXR. Continuum and bursts are separated in the perpendicular direction to the magnetic field configuration. Between the periods of strong burst activities, burst-like emissions are also superimposed on the continuum source. There is no obvious correlation between the flux density of the continuum and the bursts. The burst sources have no systematic motion, whereas the the continuum source shows a small drift of 0.2 min^–1 along the X-ray loop in the long-time evolution. The VLA maps at higher frequency (1446 MHz) show no source corresponding to the type I event. The soft X-ray emission measure and temperature were calculated. The type I continuum source is located (in projection) in a region with enhanced SXR emission, a loop having a mean density of n _e = (1.5 ± 0.4) × 10⁹ cm^–3 and a temperature ofT = (2.1 ± 0.1) × 10⁶ K. The centroid positions of the left and right circularly polarized components of the burst sources are separated by 15–50 and seem to be on different loops. These observations contradict the predictions of existing type I theories.Presented at the CESRA-Workshop on Coronal Magnetic Energy Release at Caputh near Potsdam in May 1994.  相似文献   

Origin of the Earth's ocean basins     

Herbert Frey 《Icarus》1977,32(2):235-250

The Earth's original ocean basins are proposed to be mare-type basins produced 4 billion y.a. by the flux of asteroid-sized objects responsible for the lunar mare basins. Scaling upward from the observed number of lunar basins for the greater capture cross-section and impact velocity of the Earth indicates that at least 50% of an original global crust would have been converted to basin topography. These basins were flooded by basaltic liquids in times short compared to the isostatic adjustment time for the basin. The modern crustal dichotomy (60% oceanic, 40% continental crust) was established early in the history of the Earth, making possible the later onset of plate tectonic processes. These later processes have subsequently reworked, in several cycles, principally the oceanic parts of the Earth's crust, changing the configuration of the continents in the process. Ocean basins (and oceans themselves) may be rare occurences on planets in other star systems.  相似文献   

New observations of solar noise storm radiation at decameter wavelengths     

G. Thejappa  M. R. Kundu 《Solar physics》1991,132(1):155-171

We report multifrequency observations of storm continuum and other radio bursts. Based on their positional study and their correlation with other coronal and photospheric features, we deduce that the storm source is located in the magnetic field lines lying above a single bipolar active region. Energetic electrons trapped in the magnetic structures above the spots must be responsible for the storm radiation. We show that spontaneous emission of Langmuir waves by anisotropic distributions can explain both storm continuum and bursts self-consistently. Whenever the collisional damping ( _c ) is more than the growth (- _A ), there is a steady emission responsible for the continuum, and whenever _c = - _A (which may be satisfied randomly) there is a sudden jump in T _b giving rise to bursts. The number density of energetic particles required to explain the storm continuum at 73.8, 50, and 30.9 MHz frequencies is estimated to lie in the limits n _b /n _e 10^–10–10^–9 in the context of the present observations. The brightness spectrum of the storm continuum is computed and compared with observations.On leave from Indian Institute of Astrophysics, Bangalore 560034, India.  相似文献   

First principles quantum mechanical calculation of the electric dipole polarizabilities of the borate,carbonate, nitrate and related ions     

J. A. Tossell 《Physics and Chemistry of Minerals》1990,17(2):133-141

Electric dipole polarizabilities have been calculated from first principles of quantum mechanics for the BO ₃ ^3? , CO ₃ ^2? , NO ₃ ^? series and for NO ₂ ^? and LiNO₃(g). Calculated trends in average polarizability and polarizability anisotropy in the BO ₃ ^3? -NO ₃ ^? series are in agreement with experiment and can be qualitatively interpreted in terms of the varying energies of the a₁′, a₂″ and e′ symmetry unoccupied MO's of the oxyanions. Embedding a CO ₃ ^2? ion in a D_3h symmetry array of divalent cations reduces both the average polarizability and its anisotropy, particularly when diffuse s and p functions are included in the calculation. Calculations on the gas phase LiNO₃ molecule and on the free NO ₃ ^? ion in the distorted geometry found in LiNO₃(g) allow us to separate polarizability contributions internal to the NO ₃ ^? and Li⁺ ions from those which arise from the Li⁺-NO ₃ ^? interaction. The Li⁺-NO ₃ ^? interaction term so obtained is much smaller than the NO ₃ ^? contribution but is in turn larger than the Li⁺ contribution, suggesting that the inclusion of this interaction term is essential for obtaining accurate results for ion pairs. Although static polarizabilities are in reasonable agreement with experiment for NO ₃ ^? the wavelength dispersion of the polarizability is underestimated by about a factor of two, apparently as a result of inadequacies in the quantum mechanical method. Calculated values are also presented for ¹⁴N NMR shieldings in the nitrogen oxyanions but these are in only qualitative agreement with the experimental values. Similarly, calculated values of magnetic susceptibility are in only qualitative agreement with experiment although trends along the BO ₃ ^3? -NO ₃ ^? series are properly reproduced.  相似文献   

Observation of temporal and spatial variations in the Fe/O charge composition of the solar particle event of 4 July, 1974     

T. P. Armstrong  S. M. Krimigis  D. Hovestadt  B. Klecker  G. Gloeckler 《Solar physics》1976,49(2):395-407

Comprehensive measurements of the temporal variations of the Z 6 charge composition in the 4 July 1974 solar event made with experiments aboard IMP-7 and 8 show that large variations in the charge composition occur in three-hour-averaged intensities. Hourly averaged fluxes show variations as large as factors of 3 to 4 from one hour to the next and 3 hour averages show nearly a factor of 10 peak to minimum over the event. The precision of the measurements are limited by counting statistics of the Fe-group channel. Iron to oxygen variations are established by both rate channel and pulse height analysis techniques. Comparison of measurements from IMP-7 and 8, separated by about 70 R _E shows that, while significant differences in composition and intensity exist for brief periods, the gross compositional variations are reproduced well at both spacecraft. These observations provide particularly stringent conditions for theories of the acceleration, release, and propagation of solar energetic particles.  相似文献   

Mass-dependent fractionation of quadruple stable sulfur isotope system as a new tracer of sulfur biogeochemical cycles     

Shuhei Ono  Boswell Wing  James Farquhar 《Geochimica et cosmochimica acta》2006,70(9):2238-2252

Sulfur isotope studies of post-Archean terrestrial materials have focused on the ratio ³⁴S/³²S because additional isotopes, ³³S and ³⁶S, were thought to carry little information beyond the well-known mass-dependent relationship among multiple-isotope ratios. We report high-precision analyses of Δ³³S and Δ³⁶S values, defined as deviations of ³³S and ³⁶S from ideal mass-dependent relationships, for international reference materials and sedimentary sulfides of Phanerozoic age by using a fluorination technique with a dual-inlet isotope ratio mass spectrometer. Measured variations in Δ³³S and Δ³⁶S are explained as resulting from processes involve branching reactions (two or more reservoirs formed) or mixing. Irreversible processes in closed systems (Rayleigh distillation) amplify the isotope effect. We outline how this new isotope proxy can be used to gain new insights into fundamental aspects of the sulfur biogeochemical cycle, including additional constraints on seawater sulfate budget and processes in sedimentary sulfide formation. The isotope systematics discussed here cannot explain the much larger variation of Δ³³S and Δ³⁶S observed in Archean rock records. Furthermore, Phanerozoic samples we have studied show a characteristic Δ³³S and Δ³⁶S relationship that differs from those measured in Archean rocks and laboratory photolysis experiments. Thus, high precision analysis of Δ³³S and Δ³⁶S can be used to distinguish small non-zero Δ³³S and Δ³⁶S produced by mass-dependent processes from those produced by mass-independent processes in Archean rocks and extraterrestrial materials.  相似文献   

High precision analysis of all four stable isotopes of sulfur (32S, 33S, 34S and 36S) at nanomole levels using a laser fluorination isotope-ratio-monitoring gas chromatography–mass spectrometry     

《Chemical Geology》2006,225(1-2):30-39

The discovery of mass-independent isotope effects observed in Archean rocks, certain classes of meteorites, and atmospheric aerosols has had profound implications to our understanding of ancient and present atmospheric sulfur chemistry. We present a new technique that takes advantage of continuous He flow isotope-ratio-monitoring gas chromatography–mass spectrometry to achieve precise analysis of all four stable sulfur isotopes (³²S, ³³S, ³⁴S, and ³⁶S) at nanomole level samples. The technique involves fluorination of sulfide (silver sulfide or pyrite), and separation of product gas by gas chromatography and the removal of mass-131 interference by a liquid-nitrogen ethanol slush at − 110 °C. This technique works with an optimum sample size of 100 to 200 nmol with precision for Δ³³S and Δ³⁶S at 0.1 and 0.5‰ (2σ). Samples, as small as tens of nanomole, can be analyzed using this new method. One of the major sources of error in irm-GCMS is found to be tailing of the major ion beam (³²SF₅⁺) onto minor beams (³³SF₅⁺ and ³⁶SF₅⁺), which results in contraction of the measured δ³³S and δ³⁶S scales. This effect is corrected by measuring a series of reference sulfide samples with mass-dependent sulfur isotope compositions. This methodology increases the spatial resolution of the laser ablation in situ analysis and considerably reduces the analysis time as compared with conventional dual inlet methods.  相似文献   

Using Remote Sensing to Assess Russian Forest Fire Carbon Emissions   总被引：7，自引：0，他引：7  

A. S. Isaev  G. N. Korovin  S. A. Bartalev  D. V. Ershov  A. Janetos  E. S. Kasischke  H. H. Shugart  N. H. F. French  B. E. Orlick  T. L. Murphy 《Climatic change》2002,55(1-2):235-249

Russian boreal forests are subject to frequent wildfires. The resulting combustion of large amounts of biomass not only transforms forest vegetation, but it also creates significant carbon emissions that total, according to some authors, from 35–94 Mt C per year. These carbon emissions from forest fires should be considered an important part of the forest ecosystem carbon balance and a significant influence on atmospheric trace gases. In this paper we discuss a new method to assess forest fire damage. This method is based on using multi-spectral high-resolution satellite images, large-scale aerial photography, and declassified images obtained from the space-borne national security systems. A normalized difference vegetation index (NDVI) difference image was produced from pre- and post-fire satellite images from SPOT/HRVIR and RESURS-O/MSU-E images. A close relationship was found between values of the NDVI difference image and forest damage level. High-resolution satellite data and large-scale aerial-photos were used to calibrate the NDVI-derived forest damage map. The method was used for mapping of forest fire extent and damage and for estimating carbon emissions from burned forest areas.  相似文献   

Air–sea interaction over the Indian Ocean due to variations in the Indonesian throughflow   总被引：1，自引：0，他引：1  

R. Wajsowicz 《Climate Dynamics》2002,18(5):437-453

The effects of the Indonesian throughflow on the upper thermocline circulation and surface heat flux over the Indian Ocean are presented for a 3-D ocean model forced by two different monthly wind-stress climatologies, as they show interesting differences, which could have implications for long-term variability in the Indian and Australasian monsoons. The effects are determined by contrasting a control run with a run in which the throughflow is blocked by an artificial land-bridge across the exit channels into the Indian Ocean. In the model forced by ECMWF wind stresses, there is little impact on the annual mean surface heat flux in the region surrounding the throughflow exit straits, whereas in the model forced by SSM/I-based wind stresses, a modest throughflow of less than 5 ᎒⁶ m³s^-1 over the upper 300 m induces an extra 10-50 Wm^-2 output. In the SSM/I-forced model, there is insignificant penetration of the throughflow into the northern Indian Ocean. However, in the ECMWF-forced model, the throughflow induces a 5-10 Wm^-2 reduction in heat input into the ocean, i.e., an effective output, over the Somali Current in the annual mean. These differences are attributed to differences in the strength and direction of the Ekman transport of the ambient flow, and the vertical structure of the transport and temperature anomalies associated with the throughflow. In both models, the throughflow induces a 5-30 Wm^-2 increase in net output over a broad swathe of the southern Indian Ocean, and a reduction in heat output of 10-60 Wm^-2 in a large L-shaped band around Tasmania. Effective increases in throughflow-induced net output reach up to 40 (60) Wm^-2 over the Agulhas Current retroflection in the ECMWF (SSM/I)-forced model. Seasonal variations in the throughflow's effect on the net surface heat flux are attributed to seasonal variations in the ambient circulation of the Indian Ocean, specifically in coastal upwelling along the south Javan, west Australian, and Somalian coasts, and in the depth of convective overturning between 40°S to 50°S, and its sensing of the mean throughflow's thermal anomaly. The seasonal anomalies plus annual mean yield maximum values for the throughflow-induced net surface heat output in boreal summer. Values may exceed 40 Wm^-2 in the southern Indian Ocean interior in both models, exceed 60 Wm^-2 over the Agulhas retroflection and immediate vicinity of the exit channels in the SSM/I-forced model, and reach 30 Wm^-2 over the Somali jet in the ECMWF-forced model.  相似文献