Resistive instabilities in coronal conditions     

Paola Batistoni  Giorgio Einaudi  Claudio Chiuderi 《Solar physics》1985,97(2):309-320

Resistive instabilities in a context referring to the solar corona are rigorously investigated. Various equilibrium configurations are considered, differing, among other things, by their behaviour with respect to fast, ideal instabilities. The computations presented cover in a unified scheme all known regimes of resistive modes and allow one to determine the fastest timescale over which resistivity can play a role. Comparisons with previous work as well as possible extensions are presented.  相似文献   

Heavy metals in Ligurian Sea Sediments: distribution of Cr,Cu, Ni,and Mn in Superficial Sediments     

B. Cosma  M. Drago  M. Piccazzo  G. Scarponi  S. Tucci 《Marine Chemistry》1979,8(2):125-142

Fifty-six samples of superficial marine sediment collected in the continental shelf area between Arenzano and Capo Noli were examined, along with nineteen fluvial sediment samples from creeks flowing into the area under examination. The materials were subjected to textural and chemical analyses, the latter to determine the Cr, Cu, Ni, and Mn contents using two different procedures.The geology and physiography of the coastal belt were also studied. From the data obtained, maps were drafted showing the distribution of metals: these experimental values were compared graphically to sediment texture values. High values for Cr and Ni were found, justified by stream erosion on basic rock outcrops. Only for Cr was a significant input of artificial origin found. In the case of Cu and Mn, relations with the organic fraction and texture of sediments, respectively, have been evidenced.  相似文献   

Evidence of diverse depletion and metasomatic events in harzburgite–lherzolite mantle xenoliths from the Iberian plate (Olot, NE Spain): Implications for lithosphere accretionary processes     

Gianluca Bianchini  Luigi Beccaluva  Costanza Bonadiman  Geoff Nowell  Graham Pearson  Franca Siena  Marjorie Wilson 《Lithos》2007,94(1-4):25-45

Mantle xenoliths from the Olot volcanic district (NE Spain) comprise a bi-modal suite consisting of protogranular spinel lherzolites (cpx 12–14%) sometimes with pargasitic amphibole, and highly refractory spinel harzburgites (cpx ≤ 1%) with coarse-grained granular textures. The lherzolites range from slightly depleted to moderately LREE-enriched with flat HREE patterns between 1.5 and 2.7 × chondrite (Ch). In contrast, the harzburgites are extremely depleted in HREE (down to 0.2 × Ch) and strongly LREE-enriched (La_N/Yb_N = 12.3–17.2). LA-ICP-MS analyses of clinopyroxene and amphibole of the lherzolites highlight variable degrees of LREE depletion (HREE up to 13 × Ch, La_N/Yb_N down to 0.01), with the exception of a single sample in which both clinopyroxene and amphibole are LREE-enriched (La_N/Yb_N up to 19). In the harzburgites, clinopyroxenes display totally different REE distributions, characterized by extreme HREE depletion (down to 0.4 × Ch) and upward convex positively fractionated middle-light REE patterns (Nd_N/Yb_N up to 20.7 × Ch; La_N/Yb_N up to 12 × Ch). Sr–Nd–Hf isotopic data for both whole-rocks and cpx separates, coherently indicate depleted mantle (DM) compositions for the lherzolites (εSr = − 15 to − 26, εNd = + 9 to + 17, εHf = + 18 to + 68) and enriched mantle (EM) compositions for the harzburgites (εSr = − 10 to + 36, εNd = − 1 to − 6, εHf = + 3 to + 8). Modelling of the clinopyroxene REE data and isotopic systematics suggest that some lherzolites were affected by pre-Paleozoic (0.6–1 By) low-degree partial melting processes, while others probably reflect some extent of refertilization of the mantle protolith by metasomatizing melts similar to the Triassic rift-related tholeiites reported from several Pyrenean localities. The harzburgites represent extreme refractory residua, resulting from a complex depletion history due to multistage melt extraction as often observed in the cratonic mantle. The distinctive REE patterns and isotopic systematics of their clinopyroxenes suggest that the harzburgites were formed by the interaction of an ultra-depleted peridotite matrix with highly alkaline basic melts similar in composition to the Permo-Triassic alkaline lamprophyres which are widespread within the Iberian plate. Lherzolites possibly represent younger lithosphere (accreted asthenosphere?) up-lifted and juxtaposed to the older subcontinental lithospheric mantle (harzburgites) during the post-Variscan rifting of the Iberian margin. These two genetically different, but adjoining, mantle domains intimately mingled along the northern Iberian margin during the subsequent plate convergence processes, leading to the close association of harzburgites and lherzolites observed in the Olot mantle xenoliths and in some Pyrenean peridotite massifs.  相似文献   

The Prominence-Corona Transition Region in transverse magnetic fields     

Franca Chiuderi Drago  Oddbjørn Engvold  Eberhart Jensen 《Solar physics》1992,139(1):47-64

An emission measure analysis is performed for the Prominence-Corona Transition Region (PCTR) under the assumption that the cool matter of quiescent filaments is contained in long, thin magnetic flux loops imbedded in hot coronal cavity gas. Consequently, there is a transition region around each thread.Comparison of the model and observations implies that the temperature gradient is perpendicular to the magnetic lines of force in the lower part of the PCTR (T < 10⁵ K). It is shown that in this layer the heating given by the divergence of the transverse conduction fails to account for the observed UV and EUV emission by several orders of magnitude. It is, therefore, suggested that the heating of these layers could be due to dissipation of Alfvén waves.In the high-temperature layers (T 10⁵ K), where the plasma 1, the temperature gradient is governed by radiative cooling balancing conductive heating from the surrounding hot coronal gas. Also in these outer layers the presence of magnetic fields reduces notably the thermal conduction relative to the ideal field-free case. Numerical modelling gives good agreement with observed DEM; the inferred value of the flux carried by Alfvén waves, as well as that of the damping length, greatly support the suggested form of heating. The model assumes that about 1/3 of the volume is occupied by threads and the rest by hot coronal cavity matter.The brightness of the EUV emission will depend on the angle between the thread structure and the line of sight, which may lead to a difference in brightness from observations at the limb and on the disk.  相似文献   

Parameterization of the logarithmic layer of double-averaged streamwise velocity profiles in gravel-bed river flows     

Mário J. Franca  Rui M.L. Ferreira  Ulrich Lemmin 《Advances in water resources》2008

The logarithmic layer of double-averaged (in time and space) streamwise velocity profiles obtained from field measurements made in the Swiss rivers, Venoge and Chamberonne is parameterized and discussed. Velocity measurements were made using a 3D Acoustic Doppler Velocity Profiler. Both riverbeds are hydraulically rough, composed of coarse gravel, with relative submergences (h/D₅₀) of 5.25 and 5.96, respectively. From the observations, the flow may be divided into three different layers: a roughness layer near the bed, an equivalent logarithmic layer and a surface or outer layer. It was found that a logarithmic law can describe the double-averaged profiles in the layer 0.30 < z/h < 0.75. The parameterization of the logarithmic law is discussed. Special emphasis is given to the geometric parameters roughness and zero-displacement heights and to the equivalent von Karman constant.  相似文献   

The European Large Area ISO Survey – IV. The preliminary 90-μm luminosity function     

S. Serjeant  A. Efstathiou  S. Oliver  C. Surace  P. Héraudeau  M. J. D. Linden-Vørnle  C. Gruppioni  F. La Franca  D. Rigopoulou  T. Morel  H. Crockett  T. Sumner  M. Rowan-Robinson  M. Graham 《Monthly notices of the Royal Astronomical Society》2001,322(2):262-268

We present the luminosity function of 90-μm-selected galaxies from the European Large Area ISO Survey (ELAIS), extending to z =0.3. Their luminosities are in the range 10⁹65⁻² L /L_⊙<10¹², i.e. non-ultraluminous. From our sample of 37 reliably detected galaxies in the ELAIS S1 region from the Efstathiou et al. S ₉₀100 mJy data base, we have found optical, 15-μm or 1.4-GHz identifications for 24 (65 per cent). We have obtained 2dF and UK Schmidt FLAIR spectroscopy of 89 per cent of identifications to rigid multivariate flux limits. We construct a luminosity function assuming that (i) our spectroscopic subset is an unbiased sparse sample, and (ii) there are no galaxies that would not be represented in our spectroscopic sample at any redshift. We argue that we can be confident of both assumptions. We find that the luminosity function is well described by the local 100-μm luminosity function of Rowan-Robinson, Helou & Walker. Assuming this local normalization, we derive luminosity evolution of (1+ z )^2.45±0.85 (95 per cent confidence). We argue that star formation dominates the bolometric luminosities of these galaxies, and we derive comoving star formation rates in broad agreement with the Flores et al. and Rowan-Robinson et al. mid-infrared-based estimates.  相似文献   

A dynamical model for the chromosphere-corona transition region     

Claudio Chiuderi  Iacopo Riani 《Solar physics》1974,34(1):113-124

A dynamical, homogeneous model of the chromosphere-corona transition region and of the lower corona is presented, based on the hydrodynamical equations and on a semi-empirical relation deduced from radio observations. The model is shown to be in agreement with radio and UV observations and with the particle flux given by solar wind measurements. A comparison with the analogous static model shows that dynamical effects are very small. From the model it is possible to give an estimate of the energy dissipated at each level by the waves that propagate in the solar atmosphere. It is shown that this energy source cannot be neglected with comparison to the usual conductive, convective and radiative sources. The importance of the kinetic energy flux connected with the spicules is also discussed.  相似文献   

Radio spectrum of two active regions     

Franca Chiuderi Drago 《Solar physics》1970,13(2):357-371

The results of the total solar eclipse of November 12, 1966, observed at 8 different wave-lengths between 3 and 21 cm, are studied and the spectrum of two active regions present on the disk is deduced. It is shown that the observed increase of the flux of the most intense source in the range 3–10 cm is due to geometrical effects. Neglecting the influence of the magnetic field, the following quantities are deduced.

1. the mean and central temperature of the coronal condensation.
2. the ∫ _corona N ²dh (N = electron density).

Both these quantities are in good agreement with optical observations.  相似文献   

Radio maps of the Sun at λ = 1.95 cm     

Franca Chiuderi Drago  Marcello Felli 《Solar physics》1970,14(1):171-175

Two radio maps of the Sun at = 1.95 cm, taken at the N.R.A.O. (Green Bank) with a resolving power of 2.1 are studied and compared with H pictures: both bright and dark features appear to be strongly correlated. The emitted flux and the angular extent of the bright regions are given; from the observed radio-darkening of a region, the height of a prominence is derived.On leave from Osservatorio Astrofisico di Arcetri, Firenze.  相似文献   

The BeppoSAX High Energy Large Area Survey (HELLAS) – III. Testing synthesis models for the X-ray background     

A. Comastri  F. Fiore  C. Vignali  G. Matt  G.C. Perola  F. La Franca 《Monthly notices of the Royal Astronomical Society》2001,327(3):781-787

The BeppoSAX High Energy Large Area Survey (HELLAS) has surveyed several tens of deg² of the sky in the     band down to a flux of about     . The source surface density of     at the survey limit corresponds to a resolved fraction of the     X-ray background (XRB) of the order of     per cent. The extrapolation of the HELLAS     towards fainter fluxes with a Euclidean slope is consistent with the first XMM–Newton measurements, in the same energy band, which are a factor of 20 times more sensitive. The source counts in the hardest band so far surveyed by X-ray satellites are used to constrain XRB models. It is shown that in order to reproduce the     counts over the range of fluxes covered by BeppoSAX and XMM–Newton a large fraction of highly absorbed     , luminous     active galactic nuclei is needed. A sizeable number of more heavily obscured, Compton-thick, objects cannot be ruled out but they are not required by the present data. The model predicts an absorption distribution consistent with that found from the hardness ratios analysis of the so far identified HELLAS sources. Interestingly enough, there is evidence of a decoupling between X-ray absorption and optical reddening indicators, especially at high redshifts/luminosities where several broad-line quasars show hardness ratios typical of absorbed power-law models with     .  相似文献