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1.
A weak active region (NOAA 11158) appeared on the solar disk near the eastern limb. This region increased rapidly and, having reached the magnetic flux higher than 1022 Mx, produced an X-class flare. Only weak field variations at individual points were observed during the flare. An analysis of data with a resolution of 45 s did not indicate any characteristic features in the photospheric field dynamics during the flare. When the flux became higher than 3 × 1022 Mx, active region NOAA 10720 produced six X-class flares. The field remained quiet during these flares. An increase in the magnetic flux above ～1022 Mx is a necessary, but not sufficient, condition for the appearance of powerful flares. Simple active regions do not produce flares. A flare originates only when the field distribution in an active region is complex and lines of polarity inversion have a complex shape. Singular lines of the magnetic field can exist only above such active regions. The current sheets, in the magnetic field of which the solar flare energy is accumulated, originate in the vicinity of these lines. 相似文献
2.
Abstract The Lortz dynamo with helical symmetry is re-examined. It is shown that by imposing appropriate boundary conditions the set of possible solutions can be broken down into various classes characterized by the behavior of the mean magnetic field. It is found that, as the cylindrical radius, s, tends to zero, <BΦ> ～ 0(sj), <Bz> ～ const + 0(sj?i), where j>5. It is proved that the azimuthal wavenumber associated with the j=5 class is necessarily equal to 2. The existence of at least one cylindrical surface inside which the dynamo is self-sustained is demonstrated. A new simple explicit solution is obtained. The topology the magnetic field is studied and three-dimensional pictures of the magnetic field lines are exhibited. Finally, a criterion for reversal of the magnetic field as a function of radius is ohtained and is applied to our solution. 相似文献
3.
Abstract A spherical αω-dynamo is studied for small values of the viscous coupling parameter ε ～ v1/2, paying attention particularly to large dynamo numbers. The present study is a follow-up of the work by Hollerbach et al. (1992) with their choice of α-effect and Archimedean wind including also the constraint of magnetic field symmetry (or antisymmetry) due to equatorial plane. The magnetic field scaled by ε1/2 is independent of ε in the solutions for dynamo numbers smaller than a certain value of D b (the Ekman state) which are represented by dynamo waves running from pole to equator or vice-versa. However, for dynamo numbers larger than D b the solution bifurcates and subsequently becomes dependent on ε. The bifurcation is a consequence of a crucial role of the meridional convection in the mechanism of magnetic field generation. Calculations suggest that the bifurcation appears near dynamo number about 33500 and the solutions for larger dynamo numbers and ε = 0 become unstable and fail, while the solutions for small but non-zero ε are characterized by cylindrical layers of local maximum of magnetic field and sharp changes of geostrophic velocity. Our theoretical analysis allows us to conclude that our solution does not take the form of the usual Taylor state, where the Taylor constraint should be satisfied due to the special structure of magnetic field. We rather obtained the solution in the form of a “weak” Taylor state, where the Taylor constraint is satisfied partly due to the amplitude of the magnetic field and partly due to its structure. Calculations suggest that the roles of amplitude and structure are roughly fifty-fifty in our “weak” Taylor state solution and thus they can be called a Semi-Taylor state. Simple estimates show that also Ekman state solutions can be applicable in the geodynamo context. 相似文献
4.
A summary of experiments and analyses concerning electromagnetic induction in the Moon and other extraterrestrial bodies is presented. Magnetic step-transient measurements made on the lunar dark side show the eddy current response to be the dominant induction mode of the Moon. Analysis of the poloidal field decay of the eddy currents has yielded a range of monotonic conductivity profiles for the lunar interior: the conductivity rises from 3·10?4 mho/m at a depth of 170 km to 10?2 mho/m at 1000 km depth. The static magnetization field induction has been measured and the whole-Moon relative magnetic permeability has been calculated to be . The remanent magnetic fields, measured at Apollo landing sites, range from 3 to 327 γ. Simultaneous magnetometer and solar wind spectrometer measurements show that the 38-γ remanent field at the Apollo 12 site is compressed to 54 γ by a solar wind pressure increase of 7·10?8 dyn/cm2. The solar wind confines the induced lunar poloidal field; the field is compressed to the surface on the lunar subsolar side and extends out into a cylindrical cavity on the lunar antisolar side. This solar wind confinement is modeled in the laboratory by a magnetic dipole enclosed in a superconducting lead cylinder; results show that the induced poloidal field geometry is modified in a manner similar to that measured on the Moon. Induction concepts developed for the Moon are extended to estimate the electromagnetic response of other bodies in the solar system. 相似文献
5.
The KLY-4S Kappabridge and KLF-4A Magnetic Susceptibility Meter enable automated measurement of susceptibility variation with
field in the ranges of 2–450 A/m and 5–300 A/m (in effective values), respectively. Unfortunately, the measurement accuracy
decreases with decreasing field and it is not easy to decide whether the susceptibility variation at the lowest fields is
natural phenomenon or results from measuring errors. To overcome this problem, the accuracies of both the above instruments
were investigated experimentally using artificial specimens (mixture of pure magnetite and plaster of Paris) with variable
susceptibilities ranging from 1 × 10−5 to 5 × 10−2. The complete curve of the field variation of susceptibility of each specimen was measured 10 times and the relative error
was calculated for each field.
In the KLY-4S Kappabridge, in specimens with susceptibilities higher than 100 × 10−6, the relative errors are lower than 3% in all fields and lower than 1% in the fields stronger than 10 A/m. In the KLF-4A
Magnetic Susceptibility Meter, in relatively strongly magnetic specimens with susceptibilities 5 × 10−4 to 5 × 10−2, the relative error is less than 1.5% in the entire field range. While the former instrument is convenient for investigating
almost all rock types, the latter instrument is convenient for measuring moderately and strongly magnetic specimens.
To facilitate work with field variation of susceptibility curves, showing variable accuracies with field, the programme FieldVar
was written. One of its options is plotting the measured data with corresponding field-variable error bars. In this way, a
tool is offered for interpreting such susceptibility changes that are sound and reasonable from the point of view of measuring
accuracy. 相似文献
6.
D.J. Stevenson 《Earth and Planetary Science Letters》1987,82(1-2)
Permanent magnetism and conventional dynamo theory are possible but problematic explanations for the magnitude of the Mercurian magnetic field. A new model is proposed in which thermoelectric currents driven by temperature differences at a bumpy core-mantle boundary are responsible for the (unobserved) toroidal field, and the helicity of convective motions in a thin outer core (thickness 102 km) induces the observed poloidal field from the toroidal field. The observed field of 3 × 10−7 T can be reproduced provided the electrical conductivity of Mercury's semiconducting mantle approaches 103 Ω−1 m−1. This model may be testable by future missions to Mercury because it predicts a more complicated field geometry than conventional dynamo theories. However, it is argued that polar wander may cause the core-mantle topography to migrate so that some aspects of the rotational symmetry may be reflected in the observed field. 相似文献
7.
Abstract This paper presents the first attempt to examine the stability of a poloidal magnetic field in a rapidly rotating spherical shell of electrically conducting fluid. We find that a steady axisymmetric poloidal magnetic field loses its stability to a non-axisymmetric perturbation when the Elsasser number A based on the maximum strength of the field exceeds a value about 20. Comparing this with observed fields, we find that, for any reasonable estimates of the appropriate parameters in planetary interiors, our theory predicts that all planetary poloidal fields are stable, with the possible exception of Jupiter. The present study therefore provides strong support for the physical relevance of magnetic stability analysis to planetary dynamos. We find that the fluid motions driven by magnetic instabilities are characterized by a nearly two-dimensional columnar structure attempting to satisfy the Proudman-Taylor theorm. This suggests that the most rapidly growing perturbation arranges itself in such a way that the geostrophic condition is satisfied to leading order. A particularly interesting feature is that, for the most unstable mode, contours of the non-axisymmetric azimuthal flow are closely aligned with the basic axisymmetric poloidal magnetic field lines. As a result, the amplitude of the azimuthal component of the instability is smaller than or comparable with that of the poloidal component, in contrast with the instabilities generated by toroidal decay modes (Zhang and Fearn, 1994). It is shown, by examining the same system with and without fluid inertia, that fluid inertia plays a secondary role when the magnetic Taylor number Tm ? 105. We find that the direction of propagation of hydromagnetic waves driven by the instability is influenced strongly by the size of the inner core. 相似文献
8.
A key non-linear mechanism in a strong-field geodynamo is that a finite amplitude magnetic field drives a flow through the Lorentz force in the momentum equation and this flow feeds back on the field-generation process in the magnetic induction equation, equilibrating the field. We make use of a simpler non-linear?α?2-dynamo to investigate this mechanism in a rapidly rotating fluid spherical shell. Neglecting inertia, we use a pseudo-spectral time-stepping procedure to solve the induction equation and the momentum equation with no-slip velocity boundary conditions for a finitely conducting inner core and an insulating mantle. We present calculations for Ekman numbers (E) in the range 2.5× 10?3 to 5.0× 10?5, for?α?=α 0cos?θ?sin?π?(r?ri ) (which vanishes on both inner and outer boundaries). Solutions are steady except at lower E and higher values of?α?0. Then they are periodic with a reversing field and a characteristic rapid increase then equally rapid decrease in magnetic energy. We have investigated the mechanism for this and shown the influence of Taylor's constraint. We comment on the application of our findings to numerical hydrodynamic dynamos. 相似文献
9.
Abstract The weak-field Benard-type dynamo treated by Soward is considered here at higher levels of the induced magnetic field. Two sources of instability are found to occur in the intermediate field regime M ～ T 1/12, where M and T are the Hartmann and Taylor numbers. On the time scale of magnetic diffusion, solutions may blow up in finite time owing to destabilization of the convection by the magnetic field. On a faster time scale a dynamic instability related to MAC-wave instability can also occur. It is therefore concluded that the asymptotic structure of this dynamo is unstable to virtual increases in the magnetic field energy. In an attempt to model stabilization of the dynamo in a strong-field regime we consider two approximations. In the first, a truncated expansion in three-dimensional plane waves is studied numerically. A second approach utilizes an ad hoc set of ordinary differential equations which contains many of the features of convection dynamos at all field energies. Both of these models exhibit temporal intermittency of the dynamo effect. 相似文献
10.
Deep and surface magnetic measurements, gravity and subbottom seismic profiling data have been gathered across a part of the northwest Atlantic smooth-rough magnetic border. These data indicate that the transition involves only a change in the magnetic field, without associated gravity or subbottom topographic signature. Model studies suggest that a change in basement magnetization from 0.010 emu/cm3 to 0.005 emu/cm3 could account for the magnetic field change across the boundary from the rough to the smooth sectors. Various theories previously proposed to explain this magnetic boundary are discussed with respect to these newer data; reduction of basement magnetization by weathering of the proto-Atlantic Ocean floor as described by Drake et al. (1968) is the preferred explanation. 相似文献
11.
Steven C. Cande 《Earth and Planetary Science Letters》1978,40(2):275-286
Marine magnetic anomalies 33 and 34, corresponding to the first two reversals following the long normal polarity interval in the Cretaceous, are anomalously skewed by 30° to 40° throughout the North and South Atlantic. This phenomenon is most likely related to some aspect of the dipole paleomagnetic field. Specifically the magnetic field at the time of anomalies 33 and 34 appears to be characterized by the following: the dipole field gradually decreases in average intensity between reversals and/or there is an increase in the frequency or duration of undetected short polarity events toward the end of long periods (>106 years) of predominantly one polarity. Such long-period trends in the field are in conflict with the popular model for the generation of the earth's magnetic field that treats reversals as a Poisson process and assumes that the core has no memory greater than about 104 years. 相似文献
12.
《Journal of Atmospheric and Solar》1999,61(17):1249-1257
Analysis of two strong sun-aligned arcs over the Canadian Eureka Observatory (89° CGM) near the north magnetic pole and accompanying related ground magnetometer measurements has identified an electrojet current of ∼1×104 A flowing within the arcs in a sunward direction. The electrojet current was carried by low energy electrons created by impacting precipitation drifting at E×B/B2 velocity within the arcs, where E is the dawn-to-dusk electric field. One of the arcs moved rapidly in a dawn to dusk direction. The measured arc velocity was 365 m/s. This agrees well with the velocity of 380 m/s inferred from the magnetic field signature of the electrojet current. This study suggests that such an electrojet is present whenever a polar arc is set up. However, a few conditions are required to observe clear ground magnetic signatures of the electrojet: (1) quiet local magnetic conditions; (2) a single sun aligned arc near or moving across zenith; and (3) arc excitation by ≥1 keV electrons. 相似文献
13.
C. A. Reddy Sudha Ravindran K. S. Viswanathan B. V. Krishna Murthy D. R. K. Rao T. Araki 《Annales Geophysicae》1994,12(6):565-573
A 54.95-MHz coherent backscatter radar, an ionosonde and the magnetometer located at Trivandrum in India (8.5○N, 77○E, 0.5○N dip angle) recorded large-amplitude ionospheric fluctuations and magnetic field fluctuations associated with a Pc5 micropulsation event, which occurred during an intense magnetic storm on 24 March 1991 (Ap=161). Simultaneous 100-nT-level fluctuations are also observed in the H-component at Brorfelde, Denmark (55.6○N gm) and at Narsarsuaq, Greenland (70.6○N gm). Our study of the above observations shows that the E-W electric field fluctuations in the E- and F-regions and the magnetic field fluctuations at Thumba are dominated by a near-sinusoidal oscillation of 10 min during 1730–1900 IST (1200-1330 UT), the amplitude of the electric field oscillation in the equatorial electrojet (EEJ) is 0.1-0.25 mV m−1 and it increases with height, while it is about 1.0 mV m−1 in the F-region, the ground-level H-component oscillation can be accounted for by the ionospheric current oscillation generated by the observed electric field oscillation in the EEJ and the H-component oscillations at Trivandrum and Brorfelde are in phase with each other. The observations are interpreted in terms of a compressional cavity mode resonance in the inner magnetosphere and the associated ionospheric electric field penetrating from high latitudes to the magnetic equator. 相似文献
14.
M. G. Mindubaev 《Geomagnetism and Aeronomy》2010,50(6):763-769
An axisymmetric model of convection in a rotating cylinder in an external uniform magnetic field has been considered. In the
considered model, the meridional circulation is created by a nonuniform rotation of the lower boundary relative to the other
boundaries. In the considered model, the time of formation of the stationary regime in the magnetic field considerably increases
if the vertical density (compressibility) inhomogeneity is taken into account for Ekman numbers of E = E
M
= 3 × 10−3. This example shows that the compressibility of a medium should be taken into account in the convection and dynamics of the
magnetic field when the magnetohydrodynamics of the Earth is analyzed. 相似文献
15.
S. Ghoshal 《Pure and Applied Geophysics》1970,81(1):223-229
Summary
Stewartson [1]2) has considered the inviscid flow past a sphere in the presence of a uniform magnetic field andMurray andLudford [2] have investigated a similar problem in which the magnetic field originates from an axially symmetric dipole field situated at the centre of the sphere. In connection with the study of earth's magnetic field, the toroidal part of this field plays a dominant part. This gives rise to the importance of studying the effect of a toroidal magnetic field on flows past different bodies of revolution; specially past spheres and spheroids. In the present note inviscid flows past a sphere, and a spheroid, are considered, for the case of a toroidal magnetic field originating in the fluid. In the case of the sphere the field inside the sphere consists of an electric dipole directed along the axis of symmetry together with a uniform electric field which produces a uniform current along the axis. In the case of the spheroid, the field inside it is due to an electric dipole and quadrupole directed along the axis of symmetry, together with a uniform electric field which produces a uniform current along this axis. 相似文献
16.
B. Mendoza 《Annales Geophysicae》1997,15(4):397-402
Following a given classification of geomagnetic activity, we obtained aa index values for the Maunder minimum (1645–1715). It is found that the recurrent and fluctuating activities were not appreciable and that the shock activity levels were very low. The aa index level was due almost entirely to the quiet days. Calculated average solar-wind velocities were 194.3 km s–1 from 1657 to 1700 and 218.7 km s–1 from 1700 onwards. Also, the coronal magnetic field magnitude and southward interplanetary magnetic field component Bz were lower. It is concluded that the nearly absent levels of geomagnetic activity during this period were due to lower coronal and Bz magnetic field magnitudes as well as to the continuous impinging on the Earth of a slow wind. 相似文献
17.
Peter Olson 《Physics of the Earth and Planetary Interiors》1983,33(4):260-274
The behavior of the main magnetic field components during a polarity transition is investigated using the α2-dynamo model for magnetic field generation in a turbulent core. It is shown that rapid reversals of the dipole field occur when the helicity, a measure of correlation between turbulent velocity and vorticity, changes sign. Two classes of polarity transitions are possible. Within the first class, termed component reversals, the dipole field reverses but the toroidal field does not. Within the second class, termed full reversals, both dipole and toroidal fields reverse. Component reversals result from long term fluctuations in core helicity; full reversals result from short term fluctuations. A set of time-evolution equations are derived which govern the dipole field behavior during an idealized transition. Solutions to these equations exhibit transitions in which the dipole remains axial while its intensity decays rapidly toward zero, and is regenerated with reversed polarity. Assuming an electrical conductivity of 3 × 105 mho m?1 for the fluid core, the time interval required to complete the reversal process can be as short as 7500 years. This time scale is consistent with paleomagnetic observations of the duration of reversals. A possible explanation of the cause of reversals is proposed, in which the core's net helicity fluctuates in response to fluctuations in the level of turbulence produced by two competing energy sources—thermal convection and segregation of the inner core. Symmetry considerations indicate that, in each hemisphere, helicity generated by heat loss at the core-mantle boundary may have the opposite sign of helicity generated by energy release at the inner core boundary. Random variations in rates of energy release can cause the net helicity and the α-effect to change sign occasionally, provoking a field reversal. In this model, energy release by inner core formation tends to destabilize stationary dynamo action, causing polarity reversals. 相似文献
18.
The results of a three-dimensional MHD simulation and data obtained using specialized spacecraft made it possible to construct
an electrodynamic model of solar flares. A flare results from explosive magnetic reconnection in a current sheet above an
active region, and electrons accelerated in field-aligned currents cause hard X rays on the solar surface. In this review,
we considered works where the boundary and initial conditions on the photosphere were specified directly from the magnetic
maps, obtained by SOHO MDI in the preflare state, in order to simulate the formation of a current sheet. A numerical solution
of the complete set of MHD equations, performed using the new-generation PERESVET program, demonstrated the formation of several
current sheets before a series of flares. A comparison of the observed relativistic proton spectra and the simulated proton
acceleration along a magnetic field singular line made it possible to estimate the magnetic reconnection rate during a flare
(∼107 cm s−1). Great flares (of the X class) originate after an increase in the active region magnetic flux up to 1022 Mx. 相似文献
19.
Based on ion distribution function found from the dynamic equation, the density distribution of He+ ions originating from the polar ionosphere and up-flowing along the magnetic field line is studied during quiet and weakly disturbed geomagnetic conditions. The results show the following. (1) The ionospheric up-flowing He+ ions mainly reside in the inner magnetosphere and their density has a negative radial gradient. (2) The ionospheric up-flowing He+ ion distributions along the magnetic field line are mainly controlled by gravity and the geomagnetic field configuration. Larger the gravity, larger is the ion density. Smaller the intensity of magnetic field, smaller is the ion density. (3) If the geomagnetic activity index Kp is high, more up-flowing He+ ions will enter the magnetosphere and the region where the up-flowing ions are dominant will grow. This is consistent with observations of ionospheric up-flowing ions. Some features of the geopause can be understood based on our theoretical results. 相似文献
20.
The electrodynamic flare model is based on numerical 3D simulations with the real magnetic field of an active region. An energy
of ∼1032 erg necessary for a solar flare is shown to accumulate in the magnetic field of a coronal current sheet. The thermal X-ray
source in the corona results from plasma heating in the current sheet upon reconnection. The hard X-ray sources are located
on the solar surface at the loop foot-points. They are produced by the precipitation of electron beams accelerated in field-aligned
currents. Solar cosmic rays appear upon acceleration in the electric field along a singular magnetic X-type line. The generation
mechanism of the delayed cosmic-ray component is also discussed. 相似文献