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1.
The theory of magnetohydrodynamic (MHD) waves in solar coronal slabs in a zero-β configuration and for parallel propagation of waves does not allow the existence of surface waves. When oblique propagation
of perturbations is considered, both surface and body waves are able to propagate. When the perpendicular wavenumber is larger
than a certain value, the body kink mode becomes a surface wave. In addition, a sausage surface mode is found below the internal
cutoff frequency. When nonuniformity in the equilibrium is included, surface and body modes are damped by resonant absorption.
In this paper, first, a normal-mode analysis is performed and the period, the damping rate, and the spatial structure of the
eigenfunctions are obtained. Then, the time-dependent problem is solved, and the conditions under which one or the other type
of mode is excited are investigated. 相似文献
2.
The properties of slender isolated flux tubes, taking into account curvature effects, were investigated by Parker (1975, 1979) and Spruit (1981), and many studies have been made concerning the equilibrium of slender flux tubes in the solar corona. In this paper we use a different approach considering the coronal loop as a part of a circular torus and studying the position of its top when the loop is in equilibrium under toroidal forces. Toroidal forces were considered by Shafranov (1966) for toroidal pinches and the equilibrium can be studied for different values of the toroidal current intensity and external magnetic field. The results show that it is possible to have a coronal flux tube in equilibrium without considering gravity and external magnetic field. Furthermore, the total twist of the flux tube and its variation with the toroidal intensity has been studied. 相似文献
3.
The impact of global warming on the warmest and coldest days of the annual cycle is explored according to an A2 scenario simulated
by the CNRM-CM3 climate model in the framework of the IPCC AR4 intercomparison. Given the multi-model spread in IPCC projections,
a validation strategy is proposed using the NCEP/NCAR reanalysis. Validation of the late twentieth century model climatology
shows that warm and cold model events are slightly too long and infrequent. Although interannual trends in the warm (cold)
day occurrence were positive (negative) only for six (three) of the nine considered sub-continental regions, simulated model
trends are always positive (negative). This different behaviour suggests that simulated non-anthropogenic decadal variability
is small relative to anthropogenic trends. Large-scale synoptic processes associated with European regional warm and cold
peaks are also described and validated. Regional cold peaks are better reproduced than warm peaks, whose intensity accuracy
is limited by other physical variables. Positive (negative) winter anomalies of sea and land surface temperature lead to summers
with severe (weak) temperatures. These inter-annual anomalies are generated by a persistent pressure dipole over Europe. Regarding
climate change, warm (cold) events will become more (less) frequent and longer (shorter). The number of warm days will largely
rise and the number of cold days will dramatically decrease. The intensity of warm days will be particularly pronounced over
Europe, given the projected summer drying in this region. However, according to the limited skill of the CNRM model, these
results must be considered with caution. 相似文献
4.
5.
We have analyzed the daily record of sunspot areas during the current cycle 22 looking for the short-term periodicity of around 155 days which was present during some previous solar cycles. Two different methods have been used to compute the power spectra and the results indicate that such periodicity has been absent during the current solar cycle, which confirms the results obtained by other authors who used flares or flare-related data.However, we have found that, during some intervals of time, a periodicity close to 86 days is statistically significant. A similar periodicity was found by Landscheit (1986) in energetic X-ray flares, between 1970 and 1982 (second and first half of solar cycles 20 and 21, respectively), and by Bai (1992b) for important solar flares during solar cycle 20. 相似文献
6.
7.
Daily output from the PRUDENCE ensemble of regional climate simulations for the end of the twentieth and twenty-first centuries over Europe is used to show that the increasing intensity of the most damaging summer heat waves over Central Europe is mostly due to higher base summer temperatures. In this context, base temperature is defined as the mean of the seasonal cycle component for those calendar days when regional heat waves occur and is close, albeit not identical, to the mean temperature for July–August. Although 36–47% of future Central Europe July and August days at the end of the twenty-first century are projected to be extreme according to the present day climatology, specific changes in deseasonalized heat wave anomalies are projected to be relatively small. Instead, changes in summer base temperatures appear much larger, clearly identifiable and of the same order of magnitude as changes in the whole magnitude of heat waves. Our results bear important consequences for the predictability of central European heat wave intensity under global warming conditions. 相似文献
8.
On the estimation of long-wave radiation flux from clear skies 总被引：2，自引：0，他引：2
J. I. Jiménez L. Alados-Arboledas Y. Castro-Díez G. Ballester 《Theoretical and Applied Climatology》1987,38(1):37-42
Summary Data from measurements of long-wave radiation at Barcelona were used to test the validity of several methods for computing this component of the radiation balance in clear sky conditions.Two types of formulae, the analytically derived formulae of Swinbank and Brutsaert 1 and 2 and the empirically obtained formulae of Brunt, Idso and Jackson and Idso 1 and 2 were used in the test.Most of these methods are derived for a particular location and utilize local empirical coefficients. However, there are several which have been considered more universal.Estimates by the two methods give approximately the same value when the screen level air temperature is above 0°C.In addition, the test for temperatures below 0°C shows several discrepancies between the observed and estimated values of long-wave atmospheric radiation. This fact suggests that modifications may be needed for both analytical and empirical formulae. However, since we have few measurements in the temperature regime below 0°C, this statement cannot be justified as a conclusion of this study.
With 2 Figures 相似文献
Zusammenfassung Es werden Daten der langwelligen atmosphärischen Strahlung bei Barcelona herangezogen, um verschiedene Methoden zur Bestimmung dieser Komponente der Strahlungsbilanz bei klarem Himmel zu vergleichen. Dazu werden zwei unterschiedliche Formeltypen, die analytischen nach Swinbank und Brutsaert (1 und 2) und empirische nach Brunt, Idso und Jackson sowie Idso (1 und 2) benützt.Von diesen Methoden wurden die meisten für einen bestimmten Ort entwickelt und verwenden daher lokale empirische Koeffizienten. Allerdings gibt es einige, die als allgemein verwendbar angesehen werden.Für beide Typen der Abschätzung ergeben sich für Lufttemperaturen über 0°C im Hüttenniveau ähnliche Werte. Bei Lufttemperaturen unter 0°C zeigen sich jedoch einige Unterschiede zwischen den beobachteten und geschätzten Werten der langwelligen atmosphärischen Strahlung. Das legt nahe, die Koeffizienten sowohl für die analytischen als auch die empirischen Formeln anzupassen. Leider war das auf Grund des geringen Datenmaterials bei Temperaturen unter 0°C innerhalb dieser Untersuchung nicht möglich.
With 2 Figures 相似文献
9.
J. Davis A. Richichi P. Ballester Ph. Gitton A. Glindemann S. Morel M. Schoeller M. Wittkowski F. Paresce 《Astronomische Nachrichten》2005,326(1):25-30
An investigation of the stability of the transfer function of the European Southern Observatory's Very Large Telescope Interferometer has been carried out through observations of Fomalhaut, which was observed over a range in hour angle from 21:50–05:24 on 20 October 2002. No significant variation in the transfer function was found for the zenith angle range 5°–70°. The projected baseline varied between 139.7 m and 49.8 m during the observations and, as an integral part of the determination of the transfer function, a new accurate limb‐darkened angular diameter for Fomalhaut of 2.109 ± 0.013 mas has been established. This has led to improved values for the emergent flux = (3.43 ± 0.10)×108 Wm−2, effective temperature = 8819 ± 67 K and radius = (1.213 ± 0.011)×109 m (R/R⊙ = 1.744 ± 0.016). The luminosity has been found to be (6.34 ± 0.20)×1027 W (L/L⊙ = 16.5 ± 0.5). (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
10.
X-ray spectroscopy performed by different astronomical spacecrafts has shown that many active late-type stars possess coronae.
For such reason, the magnetic structure of stellar coronae has raised considerable interest and, by analogy with the Sun,
it is generally assumed that stellar coronae are structured by magnetic fields having the shape of arcades. Most of those
coronal magnetic field configurations assume translational symmetry and are based in planar source surfaces. However, as soon
as either the length or the width of such source surfaces become non negligible as compared to the stellar radius, the application
of the cylindrical geometry seems to be more appropriate. Then, one way of obtaining coronal magnetic configurations is to
deal with source domains extended over a cylindrical surface. In this paper we generate potential coronal arcades based on
cylindrical source surfaces with non negligible length or width compared to the stellar radius. The flux function, the magnetic
field components, the shape of magnetic field lines and other characteristic magnitudes have been obtained and analyzed for
both cases.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献