排序方式： 共有62条查询结果，搜索用时 31 毫秒
51.
Andrew J. Luhmann Matthew D. Covington Andrew J. Peters Scott C. Alexander Cale T. Anger Jeffrey A. Green Anthony C. Runkel E. Calvin Alexander Jr. 《Ground water》2011,49(3):324-335
Thermal patterns of karst springs and cave streams provide potentially useful information concerning aquifer geometry and recharge. Temperature monitoring at 25 springs and cave streams in southeastern Minnesota has shown four distinct thermal patterns. These patterns can be divided into two types: those produced by flow paths with ineffective heat exchange, such as conduits, and those produced by flow paths with effective heat exchange, such as small fractures and pore space. Thermally ineffective patterns result when water flows through the aquifer before it can equilibrate to the rock temperature. Thermally ineffective patterns can be either event‐scale, as produced by rainfall or snowmelt events, or seasonal scale, as produced by input from a perennial surface stream. Thermally effective patterns result when water equilibrates to rock temperature, and the patterns displayed depend on whether the aquifer temperature is changing over time. Shallow aquifers with seasonally varying temperatures display a phase‐shifted seasonal signal, whereas deeper aquifers with constant temperatures display a stable temperature pattern. An individual aquifer may display more than one of these patterns. Since karst aquifers typically contain both thermally effective and ineffective routes, we argue that the thermal response is strongly influenced by recharge mode. 相似文献
52.
E. Dubinin R. Lundin J. Woch A. Fedorov N. Krupp M. Holmström M. Yamauchi J.-J. Thocaven J. Sharber A. Coates K.S. Hsieh H. Koskinen P. Riihelä T. Säles J. Luhmann R. Cerulli-Irelli M. Maggi D. Williams P. Wurz C. Dierker M. Carter 《Icarus》2006,182(2):337-342
Observations made by the ASPERA-3 experiment onboard the Mars Express spacecraft found within the martian magnetosphere beams of planetary ions. In the energy (E/q)-time spectrograms these beams are often displayed as dispersive-like, ascending or descending (whether the spacecraft moves away or approach the planet) structures. A linear dependence between energy gained by the beam ions and the altitude from the planet suggests their acceleration in the electric field. The values of the electric field evaluated from ion energization occur close to the typical values of the interplanetary motional electric field. This suggests an effective penetration of the solar wind electric field deep into the martian magnetosphere or generation of large fields within the magnetosphere. Two different classes of events are found. At the nominal solar wind conditions, a ‘penetration’ occurs near the terminator. At the extreme solar wind conditions, the boundary of the induced magnetosphere moves to a more dense upper atmosphere that leads to a strong scavenging of planetary ions from the dayside regions. 相似文献
53.
A stream interaction region (SIR) forms when a fast solar stream overtakes a slow stream, leading to structure that evolves
as an SIR moves away from the Sun. Based on Wind (1995 – 2004) and ACE (1998 – 2004) in situ observations, we have conducted a comprehensive survey of SIRs at one AU, including a separate assessment of the longer-lasting
corotating interaction regions (CIRs) that recur on more than one solar rotation. In all there are 196 CIRs, accounting for
about 54% of the 365 SIRs. The largest proportion of CIRs to SIRs (64%) appears in 1999, and the smallest proportion (49%)
is in 2002. Over the ten years, the annual number of SIR events varies little, from 32 up to 45. On average, the occurrence
rate of shocks at SIRs at one AU is about 24%. Seventy percent of the SIRs with shocks have only forward shocks, more than
twice the percentage of SIRs with only reverse shocks. This preponderance of forward shocks is consistent with the deflections
of forward and reverse shocks relative to the ecliptic plane. In order to help address the effect of SIRs and CIRs on geomagnetic
activity, we determine the solar-cycle variation of the event duration, scale size, the change in velocity from slow stream
to fast stream, and the solar-cycle variation of the maximum magnetic field, peak total perpendicular pressure, and other
properties. These statistics also provide a baseline for future studies at other heliocentric distances and for validating
heliospheric models.
Electronic Supplementary Material Supplementary material is available for this article at 相似文献
54.
Kavousi Alireza Reimann Thomas Wöhling Thomas Birk Steffen Luhmann Andrew J. Kordilla Jannes Noffz Torsten Sauter Martin Liedl Rudolf 《Hydrogeology Journal》2023,31(4):1005-1030
Characterization of karst systems and forecast of their state variables are essential for groundwater management and engineering in karst regions. These objectives can be met by the use of process-based discrete-continuum models (DCMs). However, results of DCMs may suffer from inversion nonuniqueness. It has been demonstrated that the joint inversion of observations regulated by different natural processes can tackle the nonuniqueness issue in groundwater modeling. However, this has not been tested for DCMs thus far. This research proposes a methodology for the joint inversion of hydro-thermo-chemo-graphs, applying to two small-scale sink-to-spring experiments at Freiheit Spring, Minnesota, USA. In order to address conceptual uncertainty, a multimodel approach was implemented, featuring seven mutually exclusive variants. Spring hydro-thermo-chemo-graphs, for all the variants simulated by MODFLOW-CFPv2, were jointly inverted using a weighted least squares algorithm. Subsequently, models were compared in terms of inversion and forecast performances, as well as parameter uncertainties. Results reveal the suitability of the DCM approach for simultaneous inversion and forecast of hydro-physico-chemical behavior of karst systems, even at a scale of meters and seconds. The estimated volume of the tracer conduit passage ranges from approximately 46–51 m3, which is comparable to the estimate from the flood-pulse method. Moreover, it was demonstrated that the thermograph and hydrograph contain more information about aquifer characteristics than the chemograph. However, this finding can be site-specific and should depend on the analysis scale, the considered conceptual models, and the hydrological state, which are potentially affected by minor unaccountable processes and features.
相似文献55.
Thomas Luhmann 《地球空间信息科学学报》2021,24(1):152-159
Since 2011 an intensive international cooperation has been in place between the Institute for Applied Photogrammetry and Geoinformatics(IAPG)at the Jade University of Applied Sciences in Oldenburg,Germany,and various Ukrainian universities.Following an initial contact by Prof.Gottfried Konecny,the first visit was organized,and was followed by many more.In subsequent years an intensive cooperation was established particularly with the National University for Construction and Architecture(KNUCA)in Kiev.In addition to architects and civil engineers,KNUCA also trains geodesists,geo-information scientists and landscape planners.The cooperation today includes the reciprocal exchange of scientists and students,research projects,courses and cooperation at many other levels.In addition,a commercial company has been established,SPM3D LLC,which now employs more than 14 engineers in the field of 3D acquisition,point cloud processing and modeling.This article summarizes the history of the cooperation and presents the results of associated student projects.In addition,results of joint work on the development of a virtual laser scanner are presented,part of a German-Ukrainian initiative to digitize teaching. 相似文献
56.
57.
H. Broers H. Hemken T. Luhmann P. Ritschl 《ISPRS Journal of Photogrammetry and Remote Sensing》2002,56(5-6)
For the total replacement of the knee joint, the precise reconstruction of the mechanical axis is significantly determined by the alignment of the cutting tool with respect to the rotation centre of the femur head. Operation techniques supported by navigation allow for the precise three-dimensional location of the hip centre by cinematic analysis.Recent results permit the reconstruction of the femur axis to be better than 0.7°. Therefore, conventional verification methods such as the post-operative recording of the complete leg are not suitable due to their limited system accuracy of about 2°.As the femur head cannot be accessed directly during the operation, an X-ray method has been used to verify alignment. The paper presents a method and the results achieved for the calibration of a C-arm system by introducing photogrammetric parameters. Since the method is used during operation, boundary conditions such as minimal invasive surgical intervention and sterility have been considered for practical applications of patients. 相似文献
58.
L. K. Jian C. T. Russell J. G. Luhmann R. M. Skoug J. T. Steinberg 《Solar physics》2008,250(2):375-402
We have performed a survey of the characteristics of two types of large spatial-scale solar-wind structures, stream interaction
regions (SIRs), and interplanetary coronal mass ejections (ICMEs), near 5.3 AU, using solar-wind observations from Ulysses. Our study is confined to the three aphelion passes of Ulysses, and also within ± 10° of the solar ecliptic plane, covering a part of 1992, 1997 – 1998, and 2003 – 2005, representing three
slices of different phases of the solar activity cycle. Overall, there are 54 SIRs and 60 ICMEs in the survey. Many are merged
in hybrid events, suggesting that they have undergone multiple interactions prior to reaching Jovian orbit. About 91% of SIRs
occur with shocks, with 47% of such shocks being forward – reverse shock pairs. The solar-wind velocity sometimes stays constant
or even decreases within the interaction region near 5.3 AU, in contrast with the gradual velocity increase during SIRs at
1 AU. Shocks are driven by 58% of ICMEs, with 94% of them being forward shocks. Some ICMEs seem to have multiple small flux
ropes with different scales and properties. We quantitatively compare various properties of SIRs and ICMEs at 5.3 AU, and
study their statistical distributions and variations with solar activity. The width, maximum dynamic pressure, and peak perpendicular
pressure of SIRs all become larger than ICMEs. Dynamic pressure (P
dyn) is expected to be important for Jovian magnetospheric activity. We have examined the distributions of P
dyn of SIRs, ICMEs, and general solar wind, but these cannot explain the observed bimodal distribution of the location of the
Jovian magnetopause. By comparing the properties of SIRs and ICMEs at 0.72, 1, and 5.3 AU, we find that the ICME expansion
slows down significantly between 1 and 5.3 AU. Some transient and small streams in the inner heliosphere have merged into
a single interaction region.
Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users. 相似文献
59.
C. O. Lee J. G. Luhmann J. T. Hoeksema X. Sun C. N. Arge I. de Pater 《Solar physics》2011,269(2):367-388
The solar cycle 23 minimum period has been characterized by a weaker solar and interplanetary magnetic field. This provides
an ideal time to study how the strength of the photospheric field affects the interplanetary magnetic flux and, in particular,
how much the observed interplanetary fields of different cycle minima can be understood simply from differences in the areas
of the coronal holes, as opposed to differences in the surface fields within them. In this study, we invoke smaller source
surface radii in the potential-field source-surface (PFSS) model to construct a consistent picture of the observed coronal
holes and the near-Earth interplanetary field strength as well as polarity measurements for the cycles 23 and 22 minimum periods.
Although the source surface value of 2.5 R
⊙ is typically used in PFSS applications, earlier studies have shown that using smaller source surface heights generates results
that better match observations during low solar activity periods. We use photospheric field synoptic maps from Mount Wilson
Observatory (MWO) and find that the values of ≈ 1.9 R
⊙ and ≈ 1.8 R
⊙ for the cycles 22 and 23 minimum periods, respectively, produce the best results. The larger coronal holes obtained for the
smaller source surface radius of cycle 23 somewhat offsets the interplanetary consequences of the lower magnetic field at
their photospheric footpoints. For comparison, we also use observations from the Michelson Doppler Imager (MDI) and find that
the source surface radius of ≈ 1.5 R
⊙ produces better results for cycle 23, rather than ≈ 1.8 R
⊙ as suggested from MWO observations. Despite this difference, our results obtained from MWO and MDI observations show a qualitative
consistency regarding the origins of the interplanetary field and suggest that users of PFSS models may want to consider using
these smaller values for their source surface heights as long as the solar activity is low. 相似文献
60.
P. Riley R. Lionello J. A. Linker Z. Mikic J. Luhmann J. Wijaya 《Solar physics》2011,274(1-2):361-377
In an effort to understand the three-dimensional structure of the solar corona and inner heliosphere during the Whole Heliosphere Interval (WHI), we have developed a global magnetohydrodynamics (MHD) solution for Carrington rotation (CR) 2068. Our model, which includes energy-transport processes, such as coronal heating, conduction of heat parallel to the magnetic field, radiative losses, and the effects of Alfvén waves, is capable of producing significantly better estimates of the plasma temperature and density in the corona than have been possible in the past. With such a model, we can compute emission in extreme ultraviolet (EUV) and X-ray wavelengths, as well as scattering in polarized white light. Additionally, from our heliospheric solutions, we can deduce magnetic-field and plasma parameters along specific spacecraft trajectories. In this paper, we present a general analysis of the large-scale structure of the solar corona and inner heliosphere during WHI, focusing, in particular, on i) helmet-streamer structure; ii) the location of the heliospheric current sheet; and iii) the geometry of corotating interaction regions. We also compare model results with i) EUV observations from the EIT instrument onboard SOHO; and ii) in-situ measurements made by the STEREO-A and B spacecraft. Finally, we contrast the global structure of the corona and inner heliosphere during WHI with its structure during the Whole Sun Month (WSM) interval. Overall, our model reproduces the essential features of the observations; however, many discrepancies are present. We discuss several likely causes for them and suggest how model predictions may be improved in the future. 相似文献