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It is now known that the corona is filled with a multitude of loop-like structures. The likelihood of these loops being in static equilibrium is small and so this paper explores the possibility of steady isothermal or adiabatic flows, driven by a pressure difference between the loop feet. For a symmetric loop the flow becomes supersonic at the summit and is then retarded by a shock-wave at some point on the downflowing leg. The effect of adiabatic flow is to lower both pressure and temperature by at least a factor of two and so provide a possible explanation for the cool cores that are sometimes observed in coronal loops.
Asymmetric loops, whose cross-sectional area increases or decreases in the flow direction, are found to possess a wide range of both subsonic and shocked flows. Converging loops have subsonic flows if the pressure difference between the footpoints is small, but shocked flows if the pressure difference is large enough. Diverging loops exhibit only shocked flows towards a low pressure footpoint, but can have either subsonic or shocked flow towards a high pressure footpoint. Flows in diverging loops can therefore be either accelerated or decelerated. 相似文献
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The cooling of plasmas in closed coronal loops by thermal conduction is important when considering their detectability at
X-ray and EUV wavelengths. A non-local formalism of thermal conduction originating in laboratory plasmas is used and it is
shown that while the effect is unlikely to be important for loops that are in a steady state, it does play a significant role
in loops that are impulsively heated (e.g., by nanoflares). Such loops are “under-dense”, and so hot electrons have a relatively long mean free path. Analytic and numerical
models are presented, and it is shown that conduction cooling times are lengthened quite considerably. A comparison of various
cooling times with ionisation times is also presented, and it is noted that this conductive physics may enhance the chances
of observing hot nanoflare-heated plasma. 相似文献
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Report on the activity of IGCP Project 98 总被引：1，自引：0，他引：1
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E. A. Lucek M. W. Dunlop A. Balogh P. Cargill W. Baumjohann E. Georgescu G. Haerendel K.-H. Fornacon 《Annales Geophysicae》1999,17(12):1560-1573
Between December 1997 and March 1998 Equator-S made a number of excursions into the dawn-side magnetosheath, over a range of local times between 6:00 and 10:40 LT. Clear mirror-like structures, characterised by compressive fluctuations in |B| on occasion lasting for up to 5 h, were observed during a significant fraction of these orbits. During most of these passes the satellite appeared to remain close to the magnetopause (within 1–2 Re), during sustained compressions of the magnetosphere, and so the characteristics of the mirror structures are used as a diagnostic of magnetosheath structure close to the magnetopause during these orbits. It is found that in the majority of cases mirror-like activity persists, undamped, to within a few minutes of the magnetopause, with no observable ramp in |B|, irrespective of the magnetic shear across the boundary. This suggests that any plasma depletion layer is typically of narrow extent or absent at the location of the satellite, at least during the subset of orbits containing strong magnetosheath mirror-mode signatures. Power spectra for the mirror signatures show predominately field aligned power, a well defined shoulder at around 3–10 × 10−2 Hz and decreasing power at higher frequencies. On occasions the fluctuations are more sinusoidal, leading to peaked spectra instead of a shoulder. In all cases mirror structures are found to lie approximately parallel to the observed magnetopause boundary. There is some indication that the amplitude of the compressional fluctuations tends to be greater closer to the magnetopause. This has not been previously reported in the Earth’s magnetosphere, but has been suggested in the case of other planets. 相似文献
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V. K. Sharma K. B. Rhudy J. C. Cargill M. E. Tacker F. G. Vazquez 《Environmental Geology》2000,39(6):698-704
This paper deals with the problem of increased heavy metal constituents in agricultural soils due to the expanded use of
fertilizers and elevated atmospheric deposition. It discusses the extent of contamination in soil and establishes an environmental
monitoring program in the chosen area of concern in the southern coastal region of Texas. Grain size, pH, and metals (Cu,
Cd, Zn, Pb, Ni, Ba, As, Cr, Mn, and Fe) were determined in soils of the middle Rio Grande basin. The soils were mainly of
sand texture and alkaline in character. Fine sand constituted the major proportion of the soil, and clay and silt ranged from
8–30% of the soil. Correlations of metal concentrations to grain size and iron contents were performed. Metals, except Cd
and Pb, gave positive to negative relationships with decreases in grain size. Silt gave no relationship with metal content
while clay and silt had a positive relationship. All these metals had a positive correlation with iron in the soil. The results
indicate metals are associated with coarse sand, clay, and iron hydroxides surfaces of the soil. The comparison of metal content
in soil of the middle Rio Grande basin with metals from other areas of the world suggests that it is relatively uncontaminated.
Received: 14 December 1998 · Accepted: 19 Jaunuary 1999 相似文献
20.
M. W. Dunlop B. Lavraud P. Cargill M. G. G. T. Taylor A. Balogh H. Réme P. Decreau K. -H. Glassmeier R. C. Elphic J. -M. Bosqued A. N. Fazakerley I. Dandouras C. P. Escoubet H. Laakso A. Marchaudon 《Surveys in Geophysics》2005,26(1-3):5-55
This paper reviews Cluster observations of the high altitude and exterior (outer) cusp, and adjacent regions in terms of new multi-spacecraft analysis and the geometry of the surrounding boundary layers. Several crossings are described in terms of the regions sampled, the boundary dynamics and the electric current signatures observed. A companion paper in this issue focuses on the detailed plasma distributions of the boundary layers. The polar Cluster orbits take the four spacecraft in a changing formation out of the magnetosphere, on the northern leg, and into the magnetosphere, on the southern leg, of the orbits. During February to April the orbits are centred on a few hours of local noon and, on the northern leg, generally pass consecutively through the northern lobe and the cusp at mid- to high-altitudes. Depending upon conditions, the spacecraft often sample the outer cusp region, near the magnetopause, and the dayside and tail boundary layer regions adjacent to the central cusp. On the southern, inbound leg the sequence is reversed. Cluster has therefore sampled the boundaries around the high altitude cusp and nearby magnetopause under a variety of conditions. The instruments onboard provide unprecedented resolution of the plasma and field properties of the region, and the simultaneous, four-spacecraft coverage achieved by Cluster is unique. The spacecraft array forms a nearly regular tetrahedral configuration in the cusp and already the mission has covered this region on multiple spatial scales (100–2000 km). This multi-spacecraft coverage allows spatial and temporal features to be distinguished to a large degree and, in particular, enables the macroscopic properties of the boundary layers to be identified: the orientation, motion and thickness, and the associated current layers. We review the results of this analysis for a number of selected crossings from both the North and South cusp regions. Several key results have been found or have confirmed earlier work: (1) evidence for magnetically defined boundaries at both the outer cusp/magnetosheath interface and the␣inner cusp/lobe or cusp/dayside magnetosphere interface, as would support the existence of a distinct exterior cusp region; (2) evidence for an associated indentation region on the magnetopause across the outer cusp; (3) well defined plasma boundaries at the edges of the mid- to high-altitude cusp “throat”, and well defined magnetic boundaries in the high-altitude “throat”, consistent with a funnel geometry; (4) direct control of the cusp position, and its extent, by the IMF, both in the dawn/dusk and North/South directions. The exterior cusp, in particular, is highly dependent on the external conditions prevailing. The magnetic field geometry is sometimes complex, but often the current layer has a well defined thickness ranging from a few hundred (for the inner cusp boundaries) to 1000 km. Motion of the inner cusp boundaries can occur at speeds up to 60 km/s, but typically 10–20 km/s. These speeds appear to represent global motion of the cusp in some cases, but also could arise from expansion or narrowing in others. The mid- to high-altitude cusp usually contains enhanced ULF wave activity, and the exterior cusp usually is associated with a substantial reduction in field magnitude. 相似文献