Low-temperature magnetic properties of iron-bearing sulfides and their contribution to magnetism of cometary bodies     

Tomáš Kohout  Andrei Kosterov  Patricie Týcová 《Icarus》2010,208(2):955-962

In this study we present a review of low-temperature magnetic properties of alabandite (Fe, Mn)S, daubreelite FeCr₂S₄, pyrrhotite Fe_1−xS and troilite FeS updated with new experimental data. The results indicate that besides FeNi alloys mainly daubreelite with its Curie temperature T_C ∼ 150 K and strong induced and remanent magnetizations may be a significant magnetic mineral in cold environments and may complement that of FeNi or even dominate magnetic properties of sulfide rich bodies at temperatures below T_C.Comets are known to contain iron-bearing sulfides within dusty fraction and their surfaces are subject to temperature variations in the range of 100-200 K down to the depth of several meters while the cometary interior is thermally stable at several tens of Kelvin which is within the temperature range where alabandite, daubreelite or troilite are “magnetic”. Thus not only FeNi alloys, but also sulfides have to be considered in the interpretation of magnetic data from cometary objects such as will be delivered by Rosetta mission. Modeling indicates that magnetic interactions between cometary nucleus containing iron-bearing sulfides and interplanetary magnetic field would be difficult, but not impossible, to detect from orbit. Rosetta’s Philae lander present on the surface would provide more reliable signal.  相似文献   

Lithospheric drift on early Mars: Evidence in the magnetic field     

Daisuke Kobayashi 《Icarus》2010,210(1):37-42

The crustal magnetic anomalies on Mars may represent hot spot tracks resulting from lithospheric drift on ancient Mars. As evidence, an analysis of lineation patterns derived from the ΔBr magnetic map is presented. The ΔBr map, largely free of external magnetic field effects, allows excellent detail of the magnetic anomaly pattern, particularly in areas of Mars where the field is relatively weak. Using cluster analysis, we show that the elongated anomalies in the martian magnetic field form concentric small circles (parallels of latitude) about two distinct north pole locations. If these pole locations represent ancient spin axes, then tidal force on the early lithosphere by former satellites in retrograde orbits may have pulled the lithosphere in an east-west direction over hot mantle plumes. With an active martian core dynamo, this may have resulted in the observed magnetic anomaly pattern of concentric small circles. As further evidence, we observe that, of the 15 martian giant impact basins that were possibly formed while the core dynamo was active, seven lie along the equators of our two proposed paleopoles. We also find that four other re-magnetized giant impact basins lie along a great circle about the mean magnetic paleopole of Mars. These 11 impact basins, likely the result of fallen retrograde satellite fragments, indicate that Mars once had moons large enough to cause tidal drag on the early martian lithosphere. The results of this study suggest that the magnetic signatures of this tidal interaction have been preserved to the present day.  相似文献   

Alfven Wave Reflection model of field-aligned currents at Mercury     

Wladislaw Lyatsky  George V. Khazanov  James A. Slavin 《Icarus》2010,209(1):40-45

An Alfven Wave Reflection (AWR) model is proposed that provides closure for strong field-aligned currents (FACs) driven by the magnetopause reconnection in the magnetospheres of planets having no significant ionospheric and surface electrical conductance. The model is based on properties of the Alfven waves, generated at high altitudes and reflected from the low-conductivity surface of the planet. When magnetospheric convection is very slow, the incident and reflected Alfven waves propagate along approximately the same path. In this case, the net field-aligned currents will be small. However, as the convection speed increases, the reflected wave is displaced relatively to the incident wave so that the incident and reflected waves no longer compensate each other. In this case, the net field-aligned current may be large despite the lack of significant ionospheric and surface conductivity. Our estimate shows that for typical solar wind conditions at Mercury, the magnitude of Region 1-type FACs in Mercury’s magnetosphere may reach hundreds of kilo-Amperes. This AWR model of field-aligned currents may provide a solution to the long-standing problem of the closure of FACs in the Mercury’s magnetosphere.  相似文献   

Mercury’s magnetospheric magnetic field after the first two MESSENGER flybys     

Igor I. Alexeev  Elena S. Belenkaya  Haje Korth  Daniel N. Baker  Catherine L. Johnson  Menelaos Sarantos 《Icarus》2010,209(1):23-1862

The “paraboloid” model of Mercury’s magnetospheric magnetic field is used to determine the best-fit magnetospheric current system and internal dipole parameters from magnetic field measurements taken during the first and second MESSENGER flybys of Mercury on 14 January and 6 October 2008. Together with magnetic field measurements taken during the Mariner 10 flybys on 29 March 1974 and 16 March 1975, there exist three low-latitude traversals separated in longitude and one high-latitude encounter. From our model formulation and fitting procedure a Mercury dipole moment of 196 nT ·  (where R_M is Mercury’s radius) was determined. The dipole is offset from Mercury’s center by 405 km in the northward direction. The dipole inclination to Mercury’s rotation axis is relatively small, ∼4°, with an eastern longitude of 193° for the dipole northern pole. Our model is based on the a priori assumption that the dipole position and the moment orientation and strength do not change in time. The root mean square (rms) deviation between the Mariner 10 and MESSENGER magnetic field measurements and the predictions of our model for all four flybys is 10.7 nT. For each magnetic field component the rms residual is ∼6 nT or about 1.5% of the maximum measured magnetic field, ∼400 nT. This level of agreement is possible only because the magnetospheric current system parameters have been determined separately for each flyby. The magnetospheric stand-off distance, the distance from the planet’s center to the inner edge of the tail current sheet, the tail lobe magnetic flux, and the displacement of the tail current sheet relative to the Mercury solar-magnetospheric equatorial plane have been determined independently for each flyby. The magnetic flux in the tail lobes varied from 3.8 to 5.9 MWb; the subsolar magnetopause stand-off distance from 1.28 to 1.43 R_M; and the distance to the inner edge of the current sheet from 1.23 to 1.32 R_M. The differences in the current systems between the first and second MESSENGER flybys are attributed to the effects of strong magnetic reconnection driven by southward interplanetary magnetic field during the latter flyby.  相似文献   

The analytic properties of the flapping current sheets in the earth magnetotail     

Z.J. Rong  C. Shen  W.X. Wan 《Planetary and Space Science》2010,58(10):1215-1229

The current sheet in Earth’s magnetotail often flaps, and the flapping waves could be induced propagating towards the dawn and dusk flanks, which could make the current sheet dynamic. To explore the dynamic characteristics of current sheet associated with the flapping motion holistically and provide reasonable physical interpretations, detailed direct calculation and analysis have been applied to one approximate analytic model of magnetic field in the flapping current sheet. The main results from the model demonstrate: (1) the magnetic fluctuation amplitude is attenuated from the center of current sheet to the lobe regions; The larger wave amplitude would induce the larger magnetic amplitude; (2) the curvature of magnetic field lines (MFLs), with maximum at the center of current sheet, is only dependent on the displacement Z along the south-north direction from the center of current sheet, regardless of the tilt of current sheet; (3) the half-thickness of neutral sheet, h, the minimum curvature radius of MFLs, R_cmin, and the tilt angle of current sheet, δ, satisfies h=R_cmin cos δ; (4) the gradient of magnetic strength forms a double-peak profile, and the peak value would be more intense if the local current sheet is more tilted; (5) current density j and its j_y, j_z components reach the extremum at the center of CS. j and j_z would be more intense if the local current sheet is more tilted, but it is not the case for j_y; and (6) the field-aligned component of current density mainly appears in the neutral sheet, and the sign of it would change alternatively as the flapping waves passing by. To check the validity of the model, one simulation on the virtual measurements has been made, and the results are in well consistence with actual observations of Cluster.  相似文献   

Comparison of the Characteristics of Magnetic Clouds and Magnetic Cloud-Like Structures for the Events of 1995 – 2003     

Chin-Chun Wu  R. P. Lepping 《Solar physics》2007,242(1-2):159-165

Using nine years of solar wind plasma and magnetic field data from the Wind mission, we investigated the characteristics of both magnetic clouds (MCs) and magnetic cloud-like structures (MCLs) during 1995 – 2003. A MCL structure is an event that is identified by an automatic scheme (Lepping, Wu, and Berdichevsky, Ann. Geophys. 23, 2687, 2005) with the same criteria as for a MC, but it is not usually identifiable as a flux rope by using the MC (Burlaga et al., J. Geophys. Res. 86, 6673, 1981) fitting model developed by Lepping, Jones, and Burlaga (Geophys. Res. Lett. 95(11), 957, 1990). The average occurrence rate is 9.5 for MCs and 13.6 for MCLs per year for the overall period of interest, and there were 82 MCs and 122 MCLs identified during this period. The characteristics of MCs and MCL structures are as follows: (1) The average duration, Δt, of MCs is 21.1 h, which is 40% longer than that for MCLs (Δt=15 h); (2) the average (minimum B _z found in MC/MCL measured in geocentric solar ecliptic coordinates) is −10.2 nT for MCs and −6 nT for MCLs; (3) the average Dst_min (minimum Dst caused by MCs/MCLs) is −82 nT for MCs and −37 nT for MCLs; (4) the average solar wind velocity is 453 km s⁻¹ for MCs and 413 km s⁻¹ for MCLs; (5) the average thermal speed is 24.6 km s⁻¹ for MCs and 27.7 km s⁻¹ for MCLs; (6) the average magnetic field intensity is 12.7 nT for MCs and 9.8 nT for MCLs; (7) the average solar wind density is 9.4 cm⁻³ for MCs and 6.3 cm⁻³ for MCLs; and (8) a MC is one of the most important interplanetary structures capable of causing severe geomagnetic storms. The longer duration, more intense magnetic field and higher solar wind speed of MCs, compared to those properties of the MCLs, are very likely the major reasons for MCs generally causing more severe geomagnetic storms than MCLs. But the fact that a MC is an important interplanetary structure with respect to geomagnetic storms is not new (e.g., Zhang and Burlaga, J. Geophys. Res. 93, 2511, 1988; Bothmer, ESA SP-535, 419, 2003).  相似文献   

The evolution of rifting on the volcanic margin of the Pelotas Basin and the contextualization of the Paraná–Etendeka LIP in the separation of Gondwana in the South Atlantic     

《Marine and Petroleum Geology》2014

The Pelotas Basin is the classical example of a volcanic passive margin displaying large wedges of seaward-dipping reflectors (SDR). The SDR fill entirely its rifts throughout the basin, characterizing the abundant syn-rift magmatism (133–113 Ma). The Paraná–Etendeka Large Igneous Province (LIP), adjacent to west, constituted the pre-rift magmatism (134–132 Ma). The interpretation of ultra-deep seismic lines showed a very different geology from the adjacent Santos, Campos and Espírito Santo Basins, which constitute examples of magma-poor passive margins. Besides displaying rifts totally filled by volcanic rocks, diverse continental crustal domains were defined in the Pelotas Basin, such as an outer domain, probably constituted by highly stretched and permeated continental igneous crust, and a highly reflective lower crust probably reflecting underplating.The analysis of rifting in this portion of the South Atlantic is based on seismic interpretation and on the distribution of regional linear magnetic anomalies. The lateral accretion of SDR to the east towards the future site of the breakup and the temporal relationship between their rift and sag geometries allows the reconstitution of the evolution of rifting in the basin. Breakup propagated from south to north in three stages (130–127.5; 127.5–125; 125–113 Ma) physically separated by oceanic fracture zones (FZ). The width of the stretched, thinned and heavily intruded continental crust also showed a three-stage increase in the same direction and at the same FZ. Consequently, the Continental-Oceanic Boundary (COB) shows three marked shifts, from west to east, from south to north, resulting into rift to margin segmentation. Rifting also propagated from west to east, in the direction of the final breakup, in each of the three segments defined. The importance of the Paraná–Etendeka LIP upon the overall history of rupturing and breakup of Western Gondwanaland seems to have been restricted in time and in space only to the Pelotas Basin.  相似文献   

Experimental evaluation of reservoir quality in Mesozoic formations of the Perth Basin (Western Australia) by using a laboratory low field Nuclear Magnetic Resonance     

《Marine and Petroleum Geology》2014

Accurate porosity and permeability evaluation of rock formations is critical to estimate the quality and resource potential of a reservoir. In addition to directly measure the porosity and pore size distribution, low field Nuclear Magnetic Resonance (NMR) is able to measure the effective porosity and estimate the in-situ formation permeability, though its robustness is arguable and requires calibrations on cores with specific lithologies.The Mesozoic formations of the central Perth Basin (Western Australia) host hot sedimentary aquifers and recently became key targets for geothermal heat extraction. A collection of cores was retrieved from three wells intersecting these units. The characterisation of their flow properties complements the current evaluation of the Perth Basin by adding new data on effective porosity, pore size distribution, pore geometry and calibration of predictive models for the permeability according to a comprehensive facies classification scheme.This study highlights the consistency of the NMR approach when compared to conventional helium injection method. Most favourable lithologies for well production correspond to very coarse to fine sandstones of fluvial channel fill with porosities >15% and permeabilities >>1 mD. Similarly, these facies exhibit (i) the highest effective porosities, (ii) the highest pore space to pore throat ratio, and (iii) the lowest contribution of clay bound water. These aspects confirm the importance of clay occurrence in the assessment of the flow efficiency of a formation.The Yarragadee Formation presents the best reservoir quality regarding its porosity and permeability, even though high discrepancies occur locally owing to the great variability of lithofacies encountered. The scattered values observed for the Lesueur Sandstone are likely to be due to the basin architecture and fault system which generate different mechanical compaction and secondary cementation. Given an adequate facies analysis, the NMR method represents a powerful tool to estimate the flow efficiency of a reservoir.  相似文献   

物探软件MAGS2.0在峨腊厂高精度磁测中的应用     

焦紫晗 《云南地质》2014,(1):91-93,97

划分出3个磁异常场区,Ⅲ区规模较大.出露中元古界昆阳群大龙口组,是勘查区内铁矿床的赋矿层位,受断裂构造和岩体控制.  相似文献   

Understanding the Mesozoic kinematic evolution in the Cameros basin (Iberian Range,NE Spain) from magnetic subfabrics and mesostructures     

《Journal of Structural Geology》2014

Analysis of anisotropy of magnetic susceptibility (AMS) and brittle mesostructures (hydroplastic synsedimentary faults and tension gashes) is applied in this study in order to characterize the Mesozoic tectonic events in the Cameros basin (NW Iberian Range), formed between Tithonian and Albian times. Low-field AMS at room and low temperature (LF-AMS at RT and LF-AMS at LT, respectively) together with high-field AMS (HF-AMS) measurements allow separating ferro- and paramagnetic fabrics. The combination of LF-AMS at LT and HF-AMS torque measurements confirms the reliability of both procedures in terms of isolating the paramagnetic contribution to the AMS. Magnetic fabric results combined with the analyses of synsedimentary faults indicate a NW–SE extension direction during Aptian (and probably Barremian) times. This extension direction is perpendicular to the main extension direction (NE–SW) prevailing during early and late stages of basin evolution. It is also consistent with extension direction deduced from large-scale bending folds and tension gashes, developed after partial lithification. Cleavage development during Albian enhanced the orientation of the magnetic fabric in lithologies where the previous extensional magnetic lineation is coaxial with the expected one for compression.  相似文献