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1.
The rare phenomenon of the coalescence of two rotating sunspots of the same magnetic polarity during the emergence of the active region NOAA 11117 is investigated using data from the SDO space observatory. The coalescing spots rotated in opposite directions. The leading spot which formed from this process rotated counterclockwise with an angular velocity of 4°/h. A possible explanation is presented, based on a model of the emerging, twisted magnetic Ω flux tube that interacts with convective flows as it crosses the convective zone.  相似文献   

2.
The complex active region NOAA 9672 is studied when it was near the central meridian, from October 21–26, 2001. At that time, there was an emergence of new magnetic flux, with the ongoing formation of a filament. The dynamics of the magnetic field are studied in order to search for their possible manifestations in the filament structure, using SOHO MDI magnetograms, SOHO EIT and TRACE filtergrams in the 171 Å line, and Hα filtergrams available via the Internet. Our earlier conclusion that filaments form at the boundaries of supergranules near polarity-inversion lines is confirmed. The conclusion of Chae that sinistral filaments have positive magnetic helicity is also confirmed. New information about magnetic-field decay processes is obtained. The direction of motion of the magnetic poles and their relative positions suggest that the axial field of a filament forms as a result of either reconnection of cancelling magnetic poles, or emergence of horizontal magnetic-flux tubes.  相似文献   

3.
The emergence of new magnetic flux in the powerful active region NOAA 10488 on the Sun and the formation of a leading spot is studied using SOHO/MDI data. Magnetograms of the longitudinal magnetic field and radial-velocity data obtained with a temporal resolution of 1 min are analyzed. The analysis begins several hours before the appearance of the top of a rising buoyant loop-like tube of magnetic field in the photosphere and finishes two days later, when the leading spot has formed. The emerging arches of magnetic field had a complex, multi-layered structure. Their apparent concentration can be explained by the emergence of the leading base of an ascending ?? tube. The new magnetic flux emerged in the inner parts of the active region throughout the formation of the leading sunspot, and was accompanied by the development of a penumbra and the appearance of the Evershed effect in the southwest sector of the sunspot. Simultaneous with the development of Evershed flows, the outer parts of the longitudinal magnetic field were gradually separated from the sunspot in the radial direction. As a result, a moat and a quasi-annular structure were formed in the magnetic field. The formation of a ??moat?? cell is part of the unified large-scale formation of the sunspot and the entire active region. The formation of an active region and of its structures is a manifestation of large-scale processes taking place in subphotospheric layers.  相似文献   

4.
The emergence of photospheric magnetic fields and the dynamics of the associated pattern of vertical motions in a developing active region are studied based on SOHO/MDI data. Objects were selected for which complete time series of data were available, so as to make it possible to determine the onset time of the magnetic-field emergence at the surface and tracing the formation of the first pores. The active regions studied originated near the central meridian. The total area of sunspots in these regions exceeded 100 millionths of the hemisphere at the maximum of active region evolution. A generalized evolutionary scenario is constructed for the magnetic field and vertical motions in the emerging active region. An asymmetry in the Doppler velocities is noted at an early stage of the active-region development, which corresponds to a matter flow from the leading to the trailing end of the emerging Ω-shaped tube. A direct relationship is found between the matter-downflow velocity in the area of the pore development and the growth in the strength of the longitudinal magnetic field.  相似文献   

5.
We present the results of numerical simulations of the development of a current sheet in the solar corona over a bipolar region during the emergence of two new sunspots arranged collinearly with older spots. Two fronts of increased plasma density form at the boundary of the rising new magnetic flux. One of these is due to the generation of a current sheet, whose magnetic field accumulates energy for a flare. The other front is a branch of the density perturbation, and separates the old and new magnetic fluxes in a region where the magnetic field lines have the same direction on both sides of the boundary. The development of this perturbation is not associated with the energy accumulation in the corona, and hinders observation of the preflare state and complicates analysis of the results. This second front can be interpreted as the eruption of a filament before the onset of the flare. A scheme conservative with respect to magnetic flux was introduced in the Peresvet code that solves the MHD equations, in order to suppress numerical instabilities in regions of large magnetic-field gradients.  相似文献   

6.
Hydrothermal experiments combined with petrologic observations form the basis for a new two-stage model for the evolution of the pre-eruption Minoan magma chamber at Santorini, Greece. Ninety-nine percent of the erupted volume is two-pyroxene, rhyodacitic magma that had been stored at a temperature of ∼885 °C, based on magnetite-ilmenite and QUILF geothermometry. The rest of the volume is basaltic to andesitic magma, which occurs as <10 cm scoria clasts and as small inclusions in rhyodacite pumices. Petrologic observations show that these magmas mixed at different scales and at different times (i.e., multiple batches of mafic magma). Hydrothermal experiments were carried out on samples of rhyodacite and a mafic scoria in order to determine magma storage conditions and the mixing history of the two magmas. At 885 °C, the rhyodacite must have been stored at water-saturated pressures of ∼50 MPa, based on its phase assemblage, matrix-glass composition, and crystal content. However, glass inclusions inside rhyodacitic plagioclase phenocrysts contain more than 6 wt% H2O, indicating they formed at pressures >200 MPa. In addition, the composition of the plagioclase hosts (An56 ± 6) of the inclusions require temperatures of 825 ± 25 °C at pressures >200 MPa. This demonstrates that the Minoan rhyodacitic magma underwent a two-stage evolution, first crystallizing at ∼825∘C and >200 MPa, and then rinsing to a shallow ∼50 MPa storage region with a concomitant rise in temperature to ∼885 °C. We suggest that the episodic intrusion of mafic magmas provided the necessary heat and perhaps contributed to the ascent of the magma to shallow crustal depths where it reequilibrated before the cataclysmic eruption. Phase equilibria suggest that much of the heating of the rhyodacite occurred in the shallow storage region. Thermal budget calculations suggest that the rhyodacite magma could have been heated by intrusions of basalt rising at reasonable upwelling rates and injected into the storage zone over several hundred years. Preservation of amphibole in the mafic scoria indicate that injection of mafic magma continued up until days before the cataclysmic eruption, perhaps triggering the event. Received: 30 September 1997 / Accepted: 5 October 1998  相似文献   

7.
The standard equations of general relativity admit extension so that they can be supplemented, not only with Einsteinian cosmological repulsive forces described by the Λ term, but also with other forces. Accordingly, we suggest a model of a uniformly expanding Universe (an S model). In this model, the cosmological forces of attraction and repulsion precisely balance each other. This S model is a good approximation for describing the Universe’s evolution over a wide range of redshifts (up to z ∼ 1000). The S model can explain in a simple way observational data on the age of the Universe, the apparent magnitude-redshift relation for Type Ia supernovae, and the angular separation between the centers of neighboring bright spots against the uniform background of the cosmic microwave background radiation.  相似文献   

8.
The well-known model that attributes the formation of a bipolar sunspot group to the emergence of a flux tube disagrees sharply with the usual observed pattern of phenomena. At the same time, the observed patterns can be accounted for quite convincingly in terms of local magnetic-field amplification due to cellular convective motions of the solar plasma. In this study, magnetoconvection in a plane horizontal fluid layer is simulated numerically in the framework of the fully nonlinear, three-dimensional problem. A weak horizontal magnetic field and weak cellular flow are assumed to be present initially. Convection is shown to be capable of producing bipolar magnetic configurations of the strongly amplified magnetic field. Indications of magnetic freezing of the flow in the cell are found. The action of the amplification mechanism under study may be controlled by the large-scale toroidal magnetic field of the Sun.  相似文献   

9.
We have obtained new exact solutions describing the self-similar, uniform compression of twisted magnetic flux tubes (a magnetic flux rope) with a linear, force-free internal field with α = const in a resistive medium during the course of their dissipative evolution. Formally, the radius of the rope a decreases to zero over a finite time. This time turns out to be relatively short (several hours) for a rope with a radius of several hundred kilometers, if the helicity of the magnetic field in the rope is high: αa ≫ 1.  相似文献   

10.
 The combined information about the stratigraphies from the foreland basins surrounding the Swiss Alps, exhumation mechanisms and the structural evolution of the Alpine orogenic wedge allow an evaluation of the controls of erosion rates on large-scale Alpine tectonic evolution. Volumetric data from the Molasse Basin and fining-upward trends in the Gonfolite Lombarda indicate that at ∼20 Ma, average erosion rates in the Alps decreased by >50%. It appears that at that time, erosion rates decreased more rapidly than crustal uplift rates. As a result, surface uplift occurred. Because of surface uplift, the drainage pattern of the Alpine hinterland evolved from an across-strike to the present-day along-strike orientation. Furthermore, the decrease of average erosion rates at ∼20 Ma coincides with initiation of a phase of thrusting in the Jura Mountains and the Southern Alpine nappes at ∼50 km distance from the pre-20-Ma thrust front. Coupled erosion-mechanical models of orogens suggest that although rates of crustal convergence decreased between the Oligocene and the present, the reduction of average erosion rates at ∼20 Ma was high enough to have significantly influenced initiation of the state of growth of the Swiss Alps at that time. Received: 8 June 1998 / Accepted: 30 October 1998  相似文献   

11.
We present a detailed mineralogical, petrological and melt inclusion study of unusually fresh, primitive olivine + clinopyroxene phyric Lower Pillow Lavas (LPL) found near Analiondas village in the northeastern part of the Troodos ophiolite (Cyprus). Olivine phenocrysts in these primitive LPL show a wide compositional range (Fo82–92) and have higher CaO contents than those from the Upper Pillow Lavas (UPL). Cr-spinel inclusions in olivine are significantly less Cr-rich (Cr/Cr + Al = 28–67 mol%) compared to those from the UPL (Cr# = 70–80). These features reflect differences in melt compositions between primitive LPL and the UPL, namely higher CaO and Al2O3 and lower FeO* compared to the UPL at a given MgO. LPL parental melts (in equilibrium with Fo92) had ∼10.5 wt% MgO and crystallization temperatures ∼1210 °C, which are significantly lower than those previously published for the UPL (14–15 wt% MgO and ∼1300 °C for Fo92). The fractionation path of LPL parental melts is also different from that of the UPL. It is characterized initially by olivine + clinopyroxene cotectic crystallization joined by plagioclase at ∼9 wt% MgO, whereas UPL parental melts experienced a substantial interval of olivine-only crystallization. Primitive LPL melts were formed from a mantle source which was more fertile than that of tholeiites from well-developed intra-oceanic arcs, but broadly similar in its fertility to that of Mid-Ocean Ridge Basalt (MORB) and Back Arc Basin Basalts (BABB). The higher degrees of melting during formation of the LPL primary melts compared to average MORB were caused by the presence of subduction-related components (H2O). Our new data on the LPL coupled with existing data for the UPL support the existing idea that the LPL and UPL primary melts originated from distinct mantle sources, which cannot be related by progressive source depletion. Temperature differences between these sources (∼150 °C), their position in the mantle (∼10 kbar for the colder LPL source vs 15–18 kbar for the UPL source), and temporal succession of Troodos volcanism, all cannot be reconciled in the framework of existing models of mantle wedge processes, thermal structure and evolution, if a single mantle source is invoked. Possible tectonic settings for the origin of the Troodos ophiolite (forearc regions of intra-oceanic island arc, propagation of backarc spreading into arc lithosphere) are discussed. Received: 20 May 1996 / Accepted: 25 March 1997  相似文献   

12.
A 1355 m thick Bhuban sequence exposed along Tuirial section in the Aizawl district, Mizoram has been studied for magnetostratigraphic and rock magnetic attributes. The study reveals mono-mineral nature of the rocks with magnetite in the range of Stable Single Domain (SSD) showing the most favourable mineralogy to produce authentic (Natural Remanent Magnetization) NRM directions. Using routine demagnetization methods we reveal a total of 7 normal magneto-zones in the studied part of the Tuirial section. Correlation of the observed polarity with GPTS gives ages ∼12.5 Ma to ∼8 Ma. A notable increase in the rate of sedimentation at ∼9.5 Ma (∼750 m level in the section) indicates facies change from turbidite like sequence to pro-delta system. Detailed sedimentologic work is under progress to relate the magnetostratigraphy to understand some of the basin evolutionary processes.  相似文献   

13.
Recent sedimentary records from the Arabian Sea   总被引：1，自引：0，他引：1  
An attempt is made to understand the redox conditions that prevailed in the north eastern continental margins of the Arabian Sea and in the nearby deep water regions during the past few centuries using short undisturbed sediment cores. The geochronology is accomplished using210Pb excess method and the proxy indicators chosen for productivity and associated redox changes are CaCO3, organic matter (OM), Mn and U along with major elements Fe and Al. Such changes in principle are related to high productivity in the overlying waters which in turn depend on monsoonal intensity that causes upwelling responsible for increase in productivity. Alongwith the published data on gravity cores from the same region, our measurements suggest the following: At ∼ 300 m water depth, south of 21°N, the sediment-water interface at depths of ∼ 300 m had been anoxic during the time span represented by the presently studied cores for approximately ∼ 700y as evidenced by low Mn/Al (< 0.7 × 10−2) and high U/Al (> 10−4) weight ratios. In some adjacent deeper regions, however, the environment turned oxic around ∼ 200 y BP. Whereas both Mn and Ra were lost to the overlying waters in the anoxic regions (depth ∼340m), the Mn that diffused from deeper sections appears to have mineralized at the sediment-water-interface. Studies of this type on long undisturbed cores from the margins of the Arabian Sea and the Bay of Bengal, involving several proxies and geochronology by more than one method are needed to understand short term environmental (and monsoonal intensity) changes of the recent past with high resolution.  相似文献   

14.
The Legs Lake shear zone marks the southeastern boundary of an extensive region (>20,000 km2) of high-pressure (0.8–1.5+ GPa) granulite-facies rocks in the western Churchill Province, Canada. The shear zone is one of the largest exhumation-related structures in the Canadian Shield and coincides with the central segment of the ∼2,800 km long Snowbird tectonic zone. The movement history of this shear zone is critical for the development of models for the exhumation history of the high-pressure region. We used electron microprobe U–Th–Pb dating of monazite with supplemental ID-TIMS U–Pb geochronology to place constraints on the timing of shear zone activity. Combining these and other data, we suggest that regional exhumation occurred during at least three distinct phases over an ∼150 million year period. The first phase involved high temperature decompression from ∼1.0 to 0.8–0.7 GPa shortly following 1.9 Ga peak metamorphism, possibly under an extensional regime. The second phase involved rock uplift and decompression of the hanging wall to 0.5–0.4 GPa during east-vergent thrusting across the Legs Lake shear zone at ca. 1.85 Ga. This phase was likely driven by early collision-related convergence in the Trans-Hudson orogen. The final phase of regional exhumation, involving the removal of 15–20 km of overburden from both footwall and hanging wall, likely occurred after ∼1.78 Ga and may have been related to regional extensional faulting.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .  相似文献   

15.
O. M. Turkina 《Petrology》2010,18(2):158-176
Newly obtained U-Pb and Sm-Nd isotopic data on Early Precambrian metamorphic and granitoid complexes in the southwestern margin of the Siberian craton (Sharyzhalgai basement uplift) are synthe-sized in order to elucidate the crustal evolution starting at the Paleoarchean (∼3.6 Ga) to Late Paleoproterozoic (∼1.85 Ga), evaluate the lateral extent of the Paleo-Mesoarchean crust, and identify major stages in its growth and recycling. Two crustal growth stages were determined in the Onot and Bulun granite-greenstone terranes: at 3.6–3.3 and 2.8–2.9 Ga. The earliest recycling processes (at ∼3.4 and 3.2 Ga) involved partial melting, metamorphism, and migmatization and produced a stable continental crust. Crustal growth in the Mesoarchean (∼2.8–2.9 Ga) due to basaltoid magmatism was associated with the recycling of the Paleoarchean crust, which served as a source of felsic melts and of detrital material for terrigenous sediments. The Archean crust of the Irkut granulite-gneiss terrane was formed by two pulses of intermediate-felsic and basic volcanism at ∼3.6-3.4 and ∼2.7 Ga. In the terminal Archean (at ∼2.55 Ga), the preexisting crust was involved in metamorphic and magmatic processes. Traces of recycling of the Paleoproterozoic crust are identified in the isotopic parameters of the intermediate-felsic granulites. Two discrete stages in the influx of juvenile material are identified in the Paleoarchean: at ∼2.0 and 1.88–1.85 Ga, with the latter stage associated with the large-scale recycling of the Archean crust during the origin of granitoids.  相似文献   

16.
We present images of the star-forming regionG23.01–0.41 at 6.7GHz in the Class II methanol maser transition 51–60 A +, produced from archival observations on the European VLBI Network. Our map of the source and its maser spots contains 24 maser components. The data for each spot—absolute coordinates, coordinates relative to the calibration feature, peak flux and flux integrated over the spot, size, position angle, velocity along the line of sight, and line full width at half-maximum—are collected in tabular form. The spatial region occupied by the maser spots is approximated by a 200×130 milliarcsec ellipse in position angle PA = −0.40°, centered on the absolute coordinates α 0 = 18h34m40.282s, δ 0 = −09°00′38.27″ (J2000). If the source is a protoplanetary disk, then, for the distance estimate derived from trigonometric parallax, its diameter is 1800 AU, and the mass of the central protostar is 23.5M ⊙.  相似文献   

17.
Genesis of diamonds in the lower mantle   总被引：3，自引：0，他引：3  
The “forbidden” assemblage (ferropericlase + enstatite) as inclusions in diamonds has been taken as evidence to imply that these inclusions and their host diamonds formed initially in the lower mantle. Magnesite is probably the only stable carbonate at depths greater than ∼220 km. Like dehydration reactions, the reaction boundary for the decarbonation of magnesite has a positive dT/dP slope at lower pressures, which becomes negative at higher pressures, if no other phase intervenes. This reaction boundary probably intersects the geotherm between ∼900 and ∼1100 km, below which magnesite decomposes into an assemblage periclase + diamond + oxygen. Thus, ferropericlase is the most likely inclusion in diamond formed in the lower mantle. The high frequency of sole occurrence of ferropericlase in diamonds from Sao Luiz, Brazil seems to substantiate the present speculation. Received: 8 June 1998 / Accepted: 28 September 1998  相似文献   

18.
The Middle Cenozoic lava sequence of the Lake Kizi region was studied. It characterizes the activity of sources in the Northern zone of the eastern Sikhote Alin: a Middle Eocene pulse of slab-related magmatism and prolonged injection of magmas from the sublithospheric convecting mantle in the Late Oligocene. Low contents of high field strength elements (Nb and Ta) with low Nb/Ta, Ce/Pb, and Nb/La and high K/Nb ratios and a low (87Sr/86Sr)0 of 0.703399 were determined in a Middle Eocene dacite with an age of ∼43.5 Ma. Three phases of Late Oligocene volcanic eruptions were distinguished: (1) basaltic andesites (29–27 Ma), (2) basaltic trachyandesites and trachyandesites (27–24 Ma), and (3) andesites (∼23 Ma). The lavas of the first and third phases showed low Ce/Pb, Nb/La, and Ba/La and high K/Nb ratios, which are also characteristic of supraslab processes. The lavas of the second phase are shifted with respect to these ratios toward ocean island basalt compositions. The entire Late Oligocene volcanic sequence falls within a narrow range of the initial strontium isotope ratios, (87Sr/86Sr)0, from 0.703661 to 0.703853. Such ratios are characteristic of volcanic and subvolcanic rocks with ages of ∼37, 31–23, and ∼16 Ma over the whole region of the Tatar Strait coast.  相似文献   

19.
Data on small active regions on the Sun collected over three years (2007–2009) are analyzed. Under very quiescent conditions (a low X-ray background level), the shapes of the coronal loops of some active regions correspond fairly well to the shapes of magnetic-field lines calculated in a potential approximation. This is true of several active regions (e.g., the group AR 10999 in June 2008) in which no flares more powerful than B3 were observed. The radio emission of this active region detected by the RATAN-600 telescope was very weak and virtually no polarization was detected. Subflares were observed in most groups. It is demonstrated using AR 10933 (January 2007) as an example that a growth in the soft X-ray emission by up to factors of ten simultaneous with an increase in the radio flux is characteristic for such active regions. A source with the opposite polarization developed to the Northwest of the main spot in AR 10933. A series of SOHO/MDI (and also Hinode) magnetograms shows the emergence of new magnetic flux before the development of this polarized source, which continued for several hours on January 8, 2007. The current density at surfaces located at various heights is estimated based on observations of the total vector magnetic field (Hinode data) and a non-linear, force-free magnetic-field extrapolation. The height-integrated current becomes appreciably stronger at two nodes above a field neutral line, near the location of the main emerging flux. This supports the idea that the emergence of new magnetic flux is a key factor in the evolution of active regions at all stages of their existence. The development of this picture could help in elucidating the inter-relationship between current enhancements, plasma heating, and particle acceleration, in both weak active regions and strong activity complexes.  相似文献   

20.
Ancient crustal rocks provide the only direct evidence for the processes and products of early Earth differentiation. SHRIMP zircon U-Th-Pb dating has identified, amongst the Acasta gneisses of the western Slave Province, Canada, two metatonalites and a metagranodiorite that have igneous ages of 4002 ± 4, 4012 ± 6 and 4031 ± 3 Ga respectively. These are the first identified Priscoan terrestrial rocks. A record of metamorphic events at ∼3.75, ∼3.6 and ∼1.7 Ga also is preserved. These discoveries approximately double, to ∼40 km2, the area over which ∼4.0 Ga gneisses are known to occur. A single older zircon core in one sample suggests that rocks as old as 4.06 Ga might yet be found in the region. As early as 4.03 Ga, terrestrial differentiation was already producing tonalitic magmas, probably by partial melting of pre-existing, less differentiated crust. Received: 28 February 1997 / Accepted: 9 July 1998  相似文献   

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