The thermal continuum in coronal loops: Instability criteria and the influence of perpendicular thermal conduction     

R. A. M. Van Der Linden  M. Goossens 《Solar physics》1991,134(2):247-273

The linear MHD spectrum is investigated for cylindrical equilibrium models under typical coronal conditions. Non-ideal effects are included and attention is focussed on the thermal instability and the influence of perpendicular thermal conduction. It is shown that, when thermal conduction across magnetic field lines is neglected, the classic Alfvén and slow continua are supplemented by a new thermal continuum. Surprisingly, the existence of this non-ideal continuous spectrum appears to have been overlooked for a very long time. Unlike the (still purely oscillatory) Alfvén continuum modes and the slow continuum modes (overstable or damped), the thermal continuum modes are exponentially growing or decaying in time. As with the Alfvén and slow continua, discrete modes may be present above or below the thermal continuum, depending upon the choice of equilibrium parameters. These modes are localized using a simple WKB approach. The knowledge of the thermal subspectrum is then exploited to find necessary and sufficient conditions for instability.The inclusion of perpendicular thermal conduction eliminates this thermal continuum, but replaces it by a dense set of eigenmodes located at the same values of the growth rate. It is shown that perpendicular thermal conduction has no significant influence on the thermal stability of the equilibrium, though individual modes may be strongly influenced. The corresponding eigenfunctions still have a nearly-singular behaviour, but in addition they may contain a rapid spatial oscillation on a scale proportional to some power of the perpendicular thermal conduction coefficient. This rapid oscillation is confined to the most unstable part of the equilibrium and may be of relevance for the formation of fine-scale structure (threads) in prominences.Research Assistant of the National Fund for Scientific Research (Belgium).  相似文献   

Aeolian deposition of dust over hills: the effect of dust grain size on the deposition pattern     

Dirk Goossens 《地球表面变化过程与地形》2006,31(6):762-776

Wind tunnel experiments were conducted to investigate the effects of topography on the grain size characteristics of aeolian dust deposits. Experiments were performed on three isolated hills having various size and aspect ratios. The longitudinal profile of the median grain diameter was investigated for each hill. The longitudinal dust deposition profile was also studied for nine grain size classes of between 10 and 104 µm, as were wind and dust concentration profiles in the atmosphere upwind of, over and downwind of a hill. The wind tunnel experiments show that the grain size characteristics of aeolian dust deposits are affected by topography. Most apparent is the occurrence of a zone of reduced grain size on the leeside of hills, which extends from just upwind of the summit to a distance of several times the height of the hill. Slightly coarser than normal dust is deposited on the concave windward hill slope and in a zone downwind of the area of reduced grain size, but the increase in grain size in these zones remains very small. Although the normalized dust deposition profile for a hill does not vary substantially as a function of grain size, systematic trends are observed. The most important tendencies are: (1) a progressive extension, in the downwind direction, of a zone of decreased dust deposition on the leeside of a hill (the coarser the grains, the further downwind the zone of reduced deposition extends); (2) a progressive increase in dust deposition immediately upwind of a hill (the finer the grains, the higher the deposition value upwind of a hill becomes). Both tendencies are explained by the difference in inertia of the grains, which is controlled by grain size. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

Comments on the publication by S. Janković     

P. J. Goossens 《Mineralium Deposita》1978,13(2):275-277

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Temporal cycles of karst denudation in northwest Georgia,U.S.A.     

Rudi H. Kiefer 《地球表面变化过程与地形》1994,19(3):213-232

Time patterns of karst denudation in northwest Georgia (U.S.A.) were investigated at three spring sites for 12 months and at five stream sites for 10 years. Rainfall was evenly distributed and showed no significant seasonality. At the springs, as well as the streams, water hardness was largely controlled by discharge. At the springs, soil pCO₂ and water pH were strongly correlated (r + -0·69 to -0·83). Solute transport in spring waters was highly seasonal, with two conduit flow springs removing more limestone in the winter, and the diffuse flow spring removing more during the growing season. At the stream sites, most denudation occurred during the winter and spring seasons, and least during the summer. Fourier analysis showed that variations in denudation occur on deterministic (long-wave) as well as stochastic (shortwave) time scales. As contributing variables, discharge varied in short-wave and long-wave cycles, whereas soil pCO₂ showed only a long-wave cycle. The 12 month deterministic cycles were the most important, with changes in discharge taking precedence over soil pCO₂. Time series regression explains up to 69 per cent of changes in denudation through rain and soil pCO₂. Time cycles in available water are the key controlling factor of denudation, and amounts of available soil CO₂ may not be as important in the temporal patterns of karst downwearing as has been believed previously.  相似文献   

The groundwater vulnerability map for the Flemish region: its principles and uses   总被引：1，自引：0，他引：1  

Bart Fobe  Marc Goossens 《Engineering Geology》1990,29(4):355-363

Fobe, B. and Goossens, M., 1990. The groundwater vulnerability map for the Flemish region: its principles and uses. Eng. Geol., 29: 355–363.

The vulnerability map of the groundwater for the Flemish region demonstrates the possible risk for contamination of the groundwater in the upper aquifer of economical value. The map was ordered by the Flemish government and distributed among the people working in the environmental sector. The vulnerability map, in scale 1/100,000, is based on static factors, like the lithology of the aquifer and its possible coverlayers and the depth of the water table. The map will serve as a tool for proper management of the groundwater. Because of its smaller scale, the document will be used to determine areas where particular regulations and actions for the protection of groundwater are necessary.

Some of the data presented by the map suggest a safer situation than actually present. This is because the compilation followed strictly the principles of the legend. In the future, care should be taken to avoid such ambiguous situations on a map that is available for the public.

Plans exist to start with the compilation of dynamic vulnerability maps. One experimental project is already finished. This study and other recent scientific research gave experience about the influence of topography on the recharge of groundwater. It will be necessary to review the risk of groundwater contamination in some of the areas on the vulnerability map, especially in sandy hill ridges.  相似文献   

2009年度大气科学领域项目评审与研究成果分析   总被引：2，自引：2，他引：0  

张朝林  丹利  夏茹娣 《地球科学进展》2009,24(12)

1 2009年项目申请、送审与资助情况 2009年地球科学五处共受理各类项目768项(表1),其中,面上项目382项,青年科学基金254项,地区科学基金37项,上述3类项目共673项;"天气与气候系统变化过程与机制"优先发展领域重点项目18项(均为大气学科),"全球变化及其区域响应"优先发展领域重点项目41项(其中大气学科11项);国家杰出青年科学基金24项,海外及港澳学者合作研究基金5项,共计29项;创新研究群体科学基金3项;科学仪器专项基金3项;联合资助基金1项.  相似文献   

Techniques to measure the dry aeolian deposition of dust in arid and semi‐arid landscapes: a comparative study in West Niger     

Dirk Goossens  Jean Louis Rajot 《地球表面变化过程与地形》2008,33(2):178-195

Seven techniques designed to measure the dry aeolian deposition of dust on a desert surface were tested during field experiments in Niger, central‐west Africa. Deposition fluxes were measured during eight periods of 3–4 days each. Experimental techniques tested were the MDCO (marble dust collector) method, the Frisbee method, the glass plate method (optical analysis of dust deposited on glass surfaces using particle imaging software), the soil surface method (deposition on a simulated desert floor) and the CAPYR (capteur pyramidal) method. Theoretical techniques tested were the inferential method and the combination method (gradient method extended with a deposition term for coarse dust particles). The results obtained by the MDCO, Frisbee, inferential and combination methods could be directly compared by converting the data to identical standard conditions (deposition on a water surface producing no resuspension). The results obtained by the other methods (glass plate, soil surface, CAPYR) were compared relatively. The study shows that the crude (unconverted) deposition fluxes of the five experimental techniques were similar, while the crude deposition fluxes calculated by the two theoretical techniques were substantially higher, of the order of four to five times as high as for the experimental techniques. Recalculation of the data to identical environmental conditions (the standard water surface) resulted in nearly identical deposition fluxes for the MDCO, Frisbee, inferential and combination techniques, although the latter two still had slightly higher values (but the differences remained small). The measurements illustrate the need to include a grain shape factor in theoretical dust deposition models. Without such a factor, theoretical models overestimate the deposition. The paper also discusses the advantages and disadvantages of the techniques tested. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

Resonant behaviour of MHD waves on magnetic flux tubes     

Takashi Sakurai  Marcel Goossens  Joseph V. Hollweg 《Solar physics》1991,133(2):227-245

The absorption of solar five-min oscillations by sunspots is interpreted as the resonant absorption of sound waves by a magnetic cylinder. The absorption coefficient is calculated both analytically under certain simplifying assumptions, and numerically under more general conditions. The observed magnitude of the absorption coefficient, which is up to 0.5 or even more, can be explained for suitable ranges of parameters. Limitations in the present model are also discussed.  相似文献   

Non-adiabatic discrete Alfvén waves in coronal loops and prominences     

Rony Keppens  Ronald A. M. Van Der Linden  Marcel Goossens 《Solar physics》1993,144(2):267-281

Discrete Alfvén waves in coronal loops and prominences are investigated in non-ideal magnetohydrodynamics. The non-ideal effects included are anisotropic, thermal conduction, and optically thin radiation. The classic ideal Alfvén continuum is not altered by these non-ideal effects, but the discrete Alfvén modes, which exist under certain conditions above or below the Alfvén continuum in ideal MHD, are shown to be influenced by non-adiabatic effects.The existence of discrete, non-adiabatic Alfvén waves, and their damping and overstability are examined for 1D cylindrical equilibrium states with twisted magnetic fields. First, analytic results are obtained for modes of high radial order by means of a WKB-analysis. The subspectrum of discrete Alfvén modes is computed with a numerical code, with particular emphasis on the modes of low radial order. The results show that discrete Alfvén waves are of potential importance for solar applications and also that the information obtained with the WKB-analysis is of limited use in this context.Research Assistant of the Belgian National for Scientific Research.  相似文献   

About the paper “metallogenic provinces and epochs in southern central America” Ferenčić mineral. Deposita 6, 77–89 (1971)     

P. J. Goossens 《Mineralium Deposita》1971,6(3):258-259

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