Aeolian dust dynamics in agricultural land areas in Lower Saxony,Germany     

Dirk Goossens  Jens Gross  Wim Spaan 《地球表面变化过程与地形》2001,26(7):701-720

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Unrestricted second-order tensor virial equations for linear oscillations of magnetic configurations     

M. Goossens 《Astrophysics and Space Science》1979,60(2):401-421

This paper deals with the second-order tensor virial equations for the linear oscillations of a gaseous mass in the presence of a magnetic field. It is shown that the commonly used linearized versions of the tensor virial equations are restricted integral equations that incorporate the linearized equation of motion but not the boundary condition. These restricted equations only allow trial functions that fulfil the boundary condition and are of limited practical value.The unrestricted variational principle for the linear oscillations of a magnetic configuration is used to derive a more general formulation of the second-order tensor virial equations so that the linear trial function _i =X _ij x _j can be used to study the oscillations of a configuration with a magnetic field that extends in the exterior vacuum. The unrestricted virial equations have been applied to Ferraro's model and approximate results for the eigenfrequencies and eigenfunctions have been obtained for nine oscillation modes.  相似文献   

Regionalization of the Mediterranean climate     

Ch. Goossens 《Theoretical and Applied Climatology》1986,37(1-2):74-83

Summary Principal component analysis and cluster analysis applied to annual mean temperature records distributed all over the European Mediterranean countries have allowed the identification of climatic regions.The first three components are significant. They were selected by a dominant variance selection rule, called rule N. The first component corresponds to a fairly uniform field, the second marks the influence of continentality and the third the influence of Africa.A cluster analysis based on the major components shows the existence of four climatic regions. A subjective method, using the zero-line characterizing the significant components, shows similar results.

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Zusammenfassung Komponenten- und Clusteranalysen, angewandt auf Aufzeichnungen der Jahresmitteltemperaturen aus allen europäischen Mittelmeerländern, ermöglichten die Identifizierung von klimatischen Regionen.Die ersten drei Komponenten sind signifikant. Sie wurden mit Hilfe einer Regel, die nach vorherrschenden Varianzen auswählt, der sogenannten N-Regel, bestimmt. Die erste Komponente entspricht einem ziemlich einheitlichen Gebiet, die zweite zeigt den kontinentalen Einfluß und die dritte den Afrikas.Eine Clusteranalyse, die auf den wichtigsten Komponenten basiert, zeigt die Existenz von vier klimatischen Regionen. Eine subjektive Methode zeigt, unter Verwendung einer Nullinie, welche die signifikanten Komponenten kennzeichnet, ähnliche Resultate.

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With 6 Figures  相似文献   

Additional results for unstable stratified toroidal magnetic fields in stars     

M. Goossens  D. Biront  R. J. Tayler 《Astrophysics and Space Science》1981,75(2):521-526

General conditions for adiabatic instability of weak axisymmetric toroidal magnetic fields are obtained. In particular all fields which haveH =0 and H ² />0 simultaneously are unstable. This includes all fields which exist near to the axis of symmetry and, indeed, any without an infinite current density anywhere. A detailed discussion is given of fields with an angular dependenceH ² |P (cos )|. These fields are dynamically unstable with respect to both axisymmetric and non-axisymmetric perturbations for all values of the azimuthal wave number |m|. The maximum growth rates are independent ofm and they are largest for the fields that are defined in the regions closest to the polar axis.  相似文献   

Dry aeolian dust accumulation in rocky deserts: ­a medium‐term field experiment based on short‐term wind tunnel simulations     

Dirk Goossens 《地球表面变化过程与地形》2000,25(1):41-57

The spatial pattern of medium‐term (a few months) dry aeolian dust accumulation in rocky deserts is predicted using short‐term deposition and erosion experiments in a wind tunnel. The predictions are tested in a field experiment set up in the northern Negev Desert of Israel. The results show that superimposing wind tunnel deposition and erosion maps usually leads to correct predictions of medium‐term dust accumulation. The predictions are somewhat less confident near the inflection lines of windward hillslopes, where small‐scale irregularities in the local topography make it difficult to locate the exact position of the areas of little accumulation. Elsewhere in the topography predictions are good, and the method works satisfactorily. Highest accumulation occurs on concave windward slopes and, to a lesser extent, on slopes parallel to the wind. Little accumulation occurs on the convex windward slopes and in dust separation bubbles. The smallest accumulation rates are observed immediately upwind of the top of pronounced hills and on leeslopes. The rate of dry dust accumulation measured during the field experiment varied from 17 to 93 g m⁻² a⁻¹, depending on the topographic position of the accumulation plots. For most plots, it was of the order of 30–60 g m⁻² a⁻¹. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

Influence of a chromospheric magnetic field on solar acoustic modes     

Vanlommel  P.  Goossens  M. 《Solar physics》1999,187(2):357-387

This paper studies the effect of a magnetic atmosphere on the global solar acoustic oscillations in a simple Cartesian model. First, the influence of the ratio of the coronal and the photospheric temperature τ and the strength of the magnetic field at the base of the corona B_c on the oscillation modes is studied for a convection zone-corona model with a true discontinuity. The ratio τ seems to be an important parameter. Subsequently, the discontinuity is replaced by an intermediate chromospheric layer of thickness L and the effect of the thickness on the frequencies of the acoustic waves is studied. In addition, nonuniformity in the magnetic field, plasma density and temperature in the transition layer gives rise to continuous Alfvén and slow spectra. Modes with characteristic frequencies lying within the range of the continuum may resonantly couple to Alfvén and/or slow waves.  相似文献   

The effect of a poloidal magnetic field on the linear and adiabatic oscillations of a polytropen=3     

M. Goossens 《Astrophysics and Space Science》1977,52(1):3-10

The frequencies of the linear and adiabatic oscillations of a gaseous polytrope with a poloidal magnetic field are determined with the aid of a perturbation method. The influence of the poloidal magnetic field on the different types of spheroidal oscillation modes is discussed. The poloidal magnetic field generally strengthens the stability of the oscillation modes and this effect is the largest in the case of the non-radialp-modes.  相似文献   

A time-integrated MODIS burn severity assessment using the multi-temporal differenced normalized burn ratio (dNBR_MT)     

S. Veraverbeke  S. Lhermitte  W.W. Verstraeten  R. Goossens 《International Journal of Applied Earth Observation and Geoinformation》2011

Burn severity is an important parameter in post-fire management. It incorporates both the direct fire impact (vegetation depletion) and ecosystem responses (vegetation regeneration). From a remote sensing perspective, burn severity is traditionally estimated using Landsat's differenced normalized burn ratio (dNBR). In this case study of the large 2007 Peloponnese (Greece) wildfires, Landsat dNBR estimates correlated reasonably well with Geo composite burn index (GeoCBI) field data of severity (R² = 0.56). The usage of Landsat imagery is, however, restricted by cloud cover and image-to-image normalization constraints. Therefore a multi-temporal burn severity approach based on coarse spatial, high temporal resolution moderate resolution imaging spectroradiometer (MODIS) imagery is presented in this study. The multi-temporal dNBR (dNBR_MT) is defined as the 1-year integrated difference between burned pixels and their unique control pixels. These control pixels were selected based on time series similarity and spatial context and reflect how burned pixels would have behaved in the case no fire had occurred. Linear regression between downsampled Landsat dNBR and dNBR_MT estimates resulted in a moderate-high coefficient of determination R² = 0.54. dNBR_MT estimates are indicative for the change in vegetation productivity due to the fire. This change is considerably higher for forests than for more sparsely vegetated areas like shrub lands. Although Landsat dNBR is superior for spatial detail, MODIS-derived dNBR_MT estimates present a valuable alternative for burn severity mapping at continental to global scale without image availability constraints. This is beneficial to compare trends in burn severity across regions and time. Moreover, thanks to MODIS's repeated temporal sampling, the dNBR_MT accounts for both first- and second-order fire effects.  相似文献   

A method to determine regional lunar gravity fields from earth-based satellite tracking data     

S. Goossens  P.N.A.M. Visser  B.A.C. Ambrosius 《Planetary and Space Science》2005,53(13):1331-1340

A method to determine regional gravity fields of the Moon from Earth-based Doppler and range satellite tracking data residuals of a low Moon-orbiting satellite has been developed and thoroughly tested in a controlled simulation environment. A short-arc approach, where one arc consists of the time it takes the satellite to cross the grid of interest on the lunar surface, is used in order to filter out most long-wavelength signal that can still be present in the residuals. Simulation results where the data are contaminated with either typical systematic or stochastic noise show that recovery of the local gravity field down to the level of several mGal is possible. The inclusion of extremely low-altitude data also means that regularisation in the sense of including a priori information in the form of a regularisation matrix is not necessary in order to obtain a good solution at high resolution.  相似文献   

On the Excitation of Leaky Modes in Cylindrical Loops     

J. Terradas  J. Andries  M. Goossens 《Solar physics》2007,246(1):231-242

The role of leaky waves in the coronal loop oscillations observed by TRACE is not yet clearly understood. In this work, the excitation of fast waves in solar coronal loops modelled as dense plasma cylindrical tubes in a uniform straight magnetic field is investigated. We study the trapped and especially leaky modes (whose energy escapes from the tube) that result from an initial disturbance by solving the time-dependent problem numerically. We find that the stationary state of the tube motion is given by the trapped normal modes. By contrast, the transient behaviour between the initial and the stationary phase is dominated by wave leakage. The so-called trig leaky modes are clearly identified since the transient behaviour shows periods and damping times that are in agreement with the values calculated from the normal-mode analysis. Consequently, these radiating modes have physical significance. However, we have not found any evidence for the excitation of other types of modes, such as the principal leaky kink mode. J. Andries is postdoctoral Fellow of the National Fund for Scientific Research – Flanders (Belgium) (F.W.O.-Vlaanderen).  相似文献