共查询到20条相似文献，搜索用时 31 毫秒
1.
SOHO/MDI magnetograms have been used to analyze the longitude distribution of the squared solar magnetic field 〈B
2〉 in the activity cycle no. 23. The energy of the magnetic field (〈B
2〉) is shown to change with longitude. However, these variations hardly fit the concept of active longitudes. In the epochs
of high solar activity, one can readily see a relationship between longitude variations of the medium-strong ((|B| > 50 G or |B| > 100 G) and relatively weak (|B| ≤ 50 G or |B| ≤ 100 G) fields at all latitudes. In other periods, this relationship is revealed mainly at the latitudes not higher than
30°. The background fields (|B| ≤ 25 G) also display longitude variations, which are, however, not related to those of the strong fields. This makes us
think that the fields of solar activity are rather inclusions to the general field than the source of the latter. 相似文献
2.
Magnetic field extrapolation is an alternative method to study chromospheric and coronal magnetic fields. In this paper, two
semi-analytical solutions of force-free fields (Low and Lou in Astrophys. J. 352:343, 1990) have been used to study the errors of nonlinear force-free (NLFF) fields based on force-free factor α. Three NLFF fields are extrapolated by approximate vertical integration (AVI) Song et al. (Astrophys. J. 649:1084, 2006), boundary integral equation (BIE) Yan and Sakurai (Sol. Phys. 195:89, 2000) and optimization (Opt.) Wiegelmann (Sol. Phys. 219:87, 2004) methods. Compared with the first semi-analytical field, it is found that the mean values of absolute relative standard deviations
(RSD) of α along field lines are about 0.96–1.19, 0.63–1.07 and 0.43–0.72 for AVI, BIE and Opt. fields, respectively. While for the
second semi-analytical field, they are about 0.80–1.02, 0.67–1.34 and 0.33–0.55 for AVI, BIE and Opt. fields, respectively.
As for the analytical field, the calculation error of 〈|RSD|〉 is about 0.1∼0.2. It is also found that RSD does not apparently depend on the length of field line. These provide the basic
estimation on the deviation of extrapolated field obtained by proposed methods from the real force-free field. 相似文献
3.
Digitized synoptic charts of photospheric magnetic fields were analyzed for the past 4 incomplete solar activity cycles (1969–2000).
The zonal structure and cyclic evolution of large-scale solar magnetic fields were investigated using the calculated values
of the radial B
r, |B
r|, meridional B
θ, |B
θ|, and azimuthal B
φ, |B
φ| components of the solar magnetic field averaged over a Carrington rotation (CR). The time–latitude diagrams of all 6 parameters
and their correlation analysis clearly reveal a zonal structure and two types of the meridional poleward drift of magnetic
fields with the characteristic times of travel from the equator to the poles equal to ∼16–18 and ∼2–3 years. A conclusion
is made that we observe two different processes of reorganization of magnetic fields in the Sun that are related to generation
of magnetic fields and their subsequent redistribution in the process of emergence from the field generation region to the
solar surface. Redistribution is supposed to be caused by some external forces (presumably, by sub-surface plasma flows in
the convection zone). 相似文献
4.
In this paper, a potential field extrapolation and three nonlinear force-free (NLFF) field extrapolations (optimization, direct
boundary integral (DBIE), and approximate vertical integration (AVI) methods) are used to study the spatial configuration
of magnetic field in the quiet Sun. It is found that differences in the computed field strengths among the three NLFF and
potential fields exist in the low layers. However, they tend to disappear as the height increases, and the differences are
of the order of 0.1 gauss when the height exceeds ≈ 2000 km above the photosphere. The difference in azimuth angles between
each NLFF field model and the potential field is as follows: for the optimization field, it decreases evidently as the height
increases; for the DBIE field, it almost stays constant and shows no significant change as the height increases; for the AVI
field, it increases slowly as the height increases. Our analysis shows that the reconstructed NLFF fields deviate significantly
from the potential field in the quiet Sun. 相似文献
5.
In this study, photospheric vector magnetograms obtained with the Synoptic Optical Long-term Investigations of the Sun (SOLIS) survey are used as boundary conditions to model three-dimensional nonlinear force-free (NLFF) coronal magnetic fields as a sequence of NLFF equilibria in spherical geometry. We study the coronal magnetic field structure inside an active region and its temporal evolution. We compare the magnetic field configuration obtained from NLFF extrapolation before and after the flaring event in active region (AR) 11117 and its surroundings observed on 27 October 2010, and we also compare the magnetic field topologies and the magnetic energy densities and study the connectivities between AR 11117 and its surroundings. During the investigated time period, we estimate the change in free magnetic energy from before to after the flare to be 1.74×1032?erg, which represents about 13.5?% of the NLFF magnetic energy before the flare. In this study, we find that electric currents from AR 11117 to its surroundings were disrupted after the flare. 相似文献
6.
The 1968–2000 data on the mean magnetic field (MMF, longitudinal component) of the Sun are analysed to study long-time trends of the Sun's magnetic field and to check MMF calibration. It is found that, within the error limits, the mean intensity of photospheric magnetic field (the MMF strength, |H|), did not change over the last 33 years. It clearly shows, however, the presence of an 11-year periodicity caused by the solar activity cycle. Time variations of |H| correlate well with those of the radial component, |B
r|, of the interplanetary magnetic field (IMF). This correlation (r=0.69) appears to be significantly higher than that between |B
r| and the results of a potential source-surface extrapolation, to the Earth's orbit, of synoptic magnetic charts of the photosphere (using the so-called `saturation' factor –1 for magnetograph measurements performed in the line Fei 525.0 nm; Wang and Sheeley, 1995). It seems therefore that the true source surface of IMF is the `quiet' photosphere – background fields and coronal holes, like those for MMF. The average `effective' magnetic strength of the photospheric field is determined to be about 1.9 G. It is also shown that there is an approximate linear relation between |B
r| and MMF intensity |H| (in gauss)|B
r|(H
0)min×(1+C|H|)where =1.5×10–5 normalizes the photospheric field strength to 1 AU distance from the Sun, (H
0)min=1.2 G is some minimal `effective' intensity of photospheric background fields and C=1.3 G–1 an empirical constant. It is noted that good correlation between time variations of |H| and |B
r| makes suspicious a correction of the photospheric magnetic fields with the use of saturation factor –1. 相似文献
7.
Experiments based on multi-source radio occultation measurements of the circumsolar plasma at R∼4.0−70R
S were carried out during 1997 – 2008 to locate the inner boundary of the solar-wind transonic transition region, R
in. The data obtained were used to correlate the solar-wind stream structure and magnetic fields on the source surface (R=2.5R
S) in the solar corona. The method of the investigation is based on the analysis of the dependence R
in=F(|B
R|) in the correlation diagrams, where R
in is the inner boundary of the solar-wind transition region and |B
R| is the intensity of the magnetic field at the source surface. On such diagrams, the solar wind is resolved into discrete
branches, streams of different types. The analysis of the stream types using a continuous series of data from 1997 to 2008
allowed us to propose a physical criterion for delimiting the epochs in the current activity cycle. 相似文献
8.
An analysis of a two-dimensional unsteady free convective flow of an incompressible viscous fluid past an infinite vertical porous plate has been carried out under the following conditions: (i) constant suction, (ii) the plate temperature oscillating in time about a constant non-zero mean, (iii) presence of the temperature-dependent sources in the fluid. Approximate solutions have been derived for the mean velocity and temperature fields, the transient velocity and temperature fields, the amplitude and the phase of the skin-friction and the rate of heat transfer. It is shown that an increase inS (the source-strength), leads to an increase in the value of |B| (the amplitude of the skin-friction) and |Q| (the amplitude of the rate of heat transfer), in case of air, but in case of water |B| and |Q| decrease. 相似文献
9.
This contribution is a follow-up to the recent paper of Kuznetsov et al. (Contrib. Astron. Obs. Skalnaté Pleso
36, 85, 2006) on the ground level enhancement (GLE) on 20 January 2005. We focused on a study of Forbush decrease (FD) of 17 – 18 and
21 – 22 January 2005, respectively. The data from the neutron monitor at Lomnicky Štít (1 min counts) and from the Geomagnetic
Observatory in Hurbanovo, both in Slovakia, were used as the basis for our investigation. The data on magnetic field and solar
wind from GOES 10 and 12, SOHO-CELIAS, ACE and WIND satellites were used for better understanding of the global evolution
of the event. The magnetic field is transformed to the RTN (Radial – Tangential – Normal) system where only the disturbed
part of the field is compared, i.e., daily variations and a constant part are subtracted. The field reduction method is described. Our results are temporal vector
diagrams of variation of all parameters at all positions from where we used the data. The amplitudes of |B| exceed 100 nT and variations during the arrival of the wavefront of CME take place simultaneously at the ground-based station
and at GOES satellites. The character of the variations is as if there would be regions with the dominant electric charge
of opposite signs, or electric currents with different orientations in the CME. On the basis of the values v
p and n
p and using certain assumptions we determined the mass of CME on 17 January and 21 January, respectively, of 1012 kg. A decrease of the cosmic ray level runs suddenly (during 10 minutes), starting, however, about two hours after a sudden
change of the magnetic field. 相似文献
10.
Over the last 15 to 20 years several high quality, high resolution data have been taken with the very large array (VLA). These data exhibit a wide range of ring opening angles (|B|=0 to 26°) and wavelengths (λ=0.7 to 20 cm). At these wavelengths the primary flux from the rings is scattered saturnian thermal emission, with a small contribution coming from the ring particles' own thermal emission. Much of the data do show signs of asymmetries due to wakes either on the ansae or the portion of the rings which occult the planet. As in previous work, we use our Monte Carlo radiative transfer code including idealized wakes [Dunn, D.E., Molnar, L.A., Fix, J.D., 2002. Icarus 160, 132-160; Dunn, D.E., Molnar, L.A., Niehof, J.T., de Pater, I., Lissauer, J.L., 2004. Icarus 171, 183-198] to model the relative contributions of the scattered and thermal radiation emanating from the rings and compare the results to that seen in the data. Although the models do give satisfactory fits to all of our data, we find that no single model can simulate the data at all different |B| and λ. We find that one model works best for moderate and low |B| and another one at higher |B|. The main difference between these models is the ratio of the wake width to their separation. We similarly find that the 2 cm data require higher density wakes than the longer wavelength data, perhaps caused by a preponderance of somewhat smaller ring material in the wakes. We further find evidence for an increase in the physical temperature of the rings with increasing |B|. Continuous observations are required to determine whether the above results regarding variations in wake parameters with |B| and λ are indeed caused by these parameters, or instead by changes over time. 相似文献
11.
We present a careful investigation of the magnetofrictional relaxation and extrapolation technique applied to the reconstruction
of two test fields. These fields are taken from the family of nonlinear force-free magnetic equilibria constructed by Low
and Lou (Astrophys. J.
352, 343, 1990), which have emerged as standard tests for extrapolation techniques in recent years. For the practically relevant case that
only the field values in the bottom plane of the considered volume (vector magnetogram) are used as input information (i.e., not including the knowledge about the test field at the side and top boundaries), the test field is reconstructed to a higher
accuracy than obtained previously. Detailed diagnostics of the reconstruction accuracy show that the implementation of fourth-order
spatial discretization was essential to reach this accuracy for the given test fields and to achieve near machine precision
in satisfying the solenoidal condition. Different variants of boundary conditions are tested, which all yield comparable accuracy.
In its present implementation, the technique yields a scaling of computing time with total number of grid points only slightly
below N
5/3, which is too steep for applications to large (≥10242) magnetograms, except on supercomputers. Directions for improvement are outlined. 相似文献
12.
We present the results of a study of solar wind velocity and magnetic field correlation lengths over the last 35 years. The
correlation length of the magnetic field magnitude λ
|B| increases on average by a factor of two at solar maxima compared to solar minima. The correlation lengths of the components
of the magnetic field lBXYZ\lambda_{B_{XYZ}} and of the velocity lVYZ\lambda_{V_{YZ}} do not show this change and have similar values, indicating a continual turbulent correlation length of around 1.4×106 km. We conclude that a linear relation between λ
|B|, VB
2, and Kp suggests that the former is related to the total magnetic energy in the solar wind and an estimate of the average
size of geoeffective structures, which is, in turn, proportional to VB
2. By looking at the distribution of daily correlation lengths we show that the solar minimum values of λ
|B| correspond to the turbulent outer scale. A tail of larger λ
|B| values is present at solar maximum causing the increase in mean value. 相似文献
13.
Using a perturbated (noised) dipole model of a sunspot magnetic field structure we simulated the influence of background noise or apparent noise (unresolved small-scale magnetic field structure) on sunspot magnetic field parameters. We evaluated mean values of the vertical and horizontal electric current densities |j|∥ and |j|⊥, respectively, of the force-free parameter α and of the Lorentz force |F|. For comparison we estimated |j⊥| and |F| of a standard sunspot magnetic field model (return-flux model, OSHEROVICH 1982). Furthermore, we compared our results with those from observations resulting in estimated values of |j|∥ for quiet sunspots. Our investigation led to the following results: the estimated values of 〈|F|〉 show clearly that due to the noise the axisymmetric magnetic dipole model is clustered into several subsystems of fluxbundles. The latter are connected with a system of electric current densities of the order of |j|∥ ∼ 10−3 Am−2 and |j|⊥ = 10−1 Am−2, i.e., this system is a noise-generated nonaxisymmetric magnetohydrostatic model. 相似文献
14.
Pablo Fosalba & Enrique Gaztañaga 《Monthly notices of the Royal Astronomical Society》1998,301(2):535-546
Cosmological perturbation theory (PT) is a useful tool to study the cumulants of the density and velocity fields in the large-scale structure of the Universe. In Papers I and II of this series we saw that the spherical collapse (SC) model provides the exact solution to PT at tree-level and gives a good approximation to the loop corrections (next-to-leading orders), indicating negligible tidal effects. Here, we derive predictions for the (smoothed) cumulants of the velocity divergence field θ ≡ ▽ ⊙ v for an irrotational fluid in the SC model. By comparing these with the exact analytic results of Scoccimarro &38; Frieman, it is shown that, at least for the unsmoothed case, the loop corrections to the cumulants of θ are dominated by tidal effects. However, most of the tidal contribution seems to cancel out when computing the hierarchical ratios, T J = 〈θ J 〉 / 〈θ2 〉 J −1 . We also extend the work presented in Papers I and II to give predictions for the cumulants of the density and velocity divergence fields in non-flat spaces. In particular, we show the equivalence between the spherically symmetric solution to the equations of motion in the SC model (given in terms of the density) and that of the Lagrangian PT approach (given in terms of the displacement field). It is shown that the Ω dependence is very weak for both cosmic fields even at one loop (a 10 per cent effect at most), except for the overall factor f (Ω) that couples to the velocity divergence. 相似文献
15.
Under the assumption that the photospheric quiet Sun magnetic field is turbulent, the cancellation function has previously been used to estimate the true, resolution-independent mean, unsigned vertical flux 〈|B z |〉true. We show that the presence of network elements, noise, and seeing complicate the measurement of accurate cancellation functions and their power law exponents κ. Failure to exclude network elements previously led to estimates that were too low for both the cancellation exponent κ and 〈|B z |〉true. However, both κ and 〈|B z |〉true are overestimated due to noise in magnetograms. While no conclusive value can be derived with data from current instruments, our Hinode/SP results of κ?0.38 and 〈|B z |〉true?270 gauss can be taken as upper bounds. 相似文献
16.
G. Allen Gary 《Solar physics》2009,257(2):271-286
The minimum dissipative rate (MDR) method for deriving a coronal non-force-free magnetic field solution is partially evaluated.
These magnetic field solutions employ a combination of three linear (constant-α) force-free-field solutions with one being a potential field (i.e., α=0). The particular case of the solutions where the other two α’s are of equal magnitude but of opposite sign is examined. This is motivated by studying the SOLIS (Synoptic Optical Long-term
Investigation of the Sun (SOLIS), a National Solar Observatory facility) vector magnetograms of AR 10987, which show a global
α value consistent with an α=0 value as evaluated by (∇×B)
z
/B
z
over the region. Typical of the current state of the observing technology, there is no definitive twist for input into the
general MDR method. This suggests that the special α case, of two α’s with equal magnitudes and opposite signs, is appropriate given the data. Only for an extensively twisted active region
does a dominant, nonzero α normally emerge from a distribution of local values. For a special set of conditions, is it found that (i) the resulting
magnetic field is a vertically inflated magnetic field resulting from the electric currents being parallel to the photosphere,
similar to the results of Gary and Alexander (Solar Phys. 186:123, 1999), and (ii) for α≈(α
max /2), the Lorentz force per unit volume normalized by the square of the magnetic field is on the order of 1.4×10−10 cm−1. The Lorentz force (F
L) is a factor of ten higher than that of the magnetic force d(B
2/8π)/dz, a component of F
L. The calculated photospheric electric current densities are an order of magnitude smaller than the maximum observed in all
active regions. Hence both the Lorentz force density and the generated electric current density seem to be physically consistent
with possible solar dynamics. The results imply that the field could be inflated with an overpressure along the neutral line.
However, the implementation of this or any other extrapolation method using the electric current density as a lower boundary
condition must be done cautiously, with the current magnetography. 相似文献
17.
A. F. Kholtygin S. N. Fabrika N. A. Drake V. D. Bychkov L. V. Bychkova G. A. Chountonov T. E. Burlakova G. G. Valyavin 《Astronomy Letters》2010,36(5):370-379
Based on an analysis of the catalog of magnetic fields, we have investigated the statistical properties of the mean magnetic
fields for OB stars. We show that the mean effective magnetic field B of a star can be used as a statistically significant characteristic of its magnetic field. No correlation has been found
between the mean magnetic field strength B and projected rotational velocity of OB stars, which is consistent with the hypothesis about a fossil origin of the magnetic
field. We have constructed the magnetic field distribution function for B stars, F(B), that has a power-law dependence on B with an exponent of ≈−1.82. We have found a sharp decrease in the function F(B) for B ⩽ 400 G that may be related to rapid dissipation of weak stellar surface magnetic fields. 相似文献
18.
We develop an approach to deriving the three-dimensional non-force-free coronal magnetic field from vector magnetograms. Based
on the principle of minimum dissipation rate, a general non-force-free magnetic field is expressed as the superposition of
one potential field and two constant-α (linear) force-free fields. Each is extrapolated from its bottom boundary data, providing the normal component only. The
constant-α parameters are distinct and determined by minimizing the deviations between the numerically computed and measured transverse
magnetic field at the bottom boundary. The boundary conditions required are at least two layers of vector magnetograms, one
at the photospheric level and the other at the chromospheric level, presumably. We apply our approach to a few analytic test
cases, especially to two nonlinear force-free cases examined by Schrijver et al. (Solar Phys.
235, 161, 2006). We find that for one case with small α parameters, the quantitative measures of the quality of our result are better than the median values of those from a set
of nonlinear force-free methods. The reconstructed magnetic-field configuration is valid up to a vertical height of the transverse
scale. For the other cases, the results remain valid to a lower vertical height owing to the limitations of the linear force-free-field
solver. Because our method is based on the fast-Fourier-transform algorithm, it is much faster and easy to implement. We discuss
the potential usefulness of our method and its limitations. 相似文献
19.
Models of the magnetic field in the solar chromosphere and corona are still mainly based on theoretical extrapolations of photospheric measurements. For the practical calculation of the global field, the so-called source-surface model has been introduced, in which the influence of the solar wind is described by the requirement that the field be radial at some exterior (source) surface. Then the assumption that the field is current-free in the volume between the photosphere and this surface allows for its determination from the photospheric measurement. In the present paper a generalization of the source-surface model to force-free fields is proposed. In the generalized model the parameter( = ×B·B/B
2)must be non-constant (or vanish identically) and currents are restricted to regions with closed field lines. A mathematical algorithm for computing the field from boundary data is devised. 相似文献
20.
The question about the interpretation of numerical experiments on magnetic reconnection in solar flares is considered. A correspondence
between the standard classification of magnetohydrodynamic discontinuities and the parameters characterizing the mass flux
through a discontinuity and the magnetic field configuration has been established within a classical formulation of the problem
on discontinuous magnetohydrodynamic flows. A pictorial graphical representation of the relationship between the angles of
the magnetic field vector relative to the normal to the discontinuity plane on both its sides has also been found. The relations
between the parameters of a two-dimensional discontinuous flow have the simplest form in a frame of reference where the magnetic
field lines (B) are parallel to the matter velocity (u)—the deHoffmann-Teller frame. The question about the transformation of the magnetic field configuration when passing to a
“laboratory” frame of reference where (v · B) ≠ 0, i.e., an electric field is present, is considered in this connection. The result is applied to the analytical solution
of the problem on the magnetic field structure in the vicinity of a reconnecting current sheet obtained previously by Bezrodnykh
et al. The regions of nonevolutionary shocks are shown to appear near the endpoints of a current sheet with reverse currents. 相似文献