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1.
The system of transfer equations of the four Stokes parameters I, Q, U, V under the action of the magneto-optical effect (i.e. the Unno-Beckers equations) are numerically solved in this paper for the magneto-sensitive lines FeI λλ 6302.499 and 5324.191 using an appropriate sunspot model. The errors in the expressions for the coefficients r and W in Beckers' paper [2] have been corrected for. From the results of calculations, features of the profiles of the Stokes parameters dependent on the magnetic vector have been isolated. Our computations also show that the magneto-optical effect should be taken into consideration in the measurement of the vector magnetic fields.
In the fourth section of this paper we have established a simple and convenient method for obtaining-information on the magnetic vector (including the field strength B, its inclination to the line of sight γ and its azimuth χ) from the profiles of the Stokes parameters. It consists of three steps: (1) The value of B is determined from the distance of the highest point in the V-profile from the central line. (2) γ is then found from Vmax, i.e maximum value of V. (3) Lastly, the angle χ is found from Q0, i.e. the value of Q at line centre. 相似文献
2.
Fletcher Lyndsay López Fuentes Marcelo C. Mandrini Cristina H. Schmieder Brigitte Démoulin Pascal Mason Helen E. Young Peter R. Nitta Nariaki 《Solar physics》2001,203(2):255-287
We present multi-instrument observations of active region (AR) 8048, made between 3 June and 5 June 1997, as part of the SOHO
Joint Observing Program 33. This AR has a sigmoid-like global shape and undergoes transient brightenings in both soft X-rays
and transition region (TR) lines. We compute a magneto-hydrostatic model of the AR magnetic field, using as boundary condition
the photospheric observations of SOHO/MDI. The computed large-scale magnetic field lines show that the large-scale sigmoid
is formed by two sets of coronal loops. Shorter loops, associated with the core of the SXT emission, coincide with the loops
observed in the hotter CDS lines. These loops reveal a gradient of temperature, from 2 MK at the top to 1 MK at the ends.
The field lines most closely matching these hot loops extend along the quasi-separatrix layers (QSLs) of the computed coronal
field. The TR brightenings observed with SOHO/CDS can also be associated with the magnetic field topology, both QSL intersections
with the photosphere, and places where separatrices issuing from bald patches (sites where field lines coming from the corona
are tangent to the photosphere) intersect the photosphere. There are, furthermore, suggestions that the element abundances
measured in the TR may depend on the type of topological structure present. Typically, the TR brightenings associated with
QSLs have coronal abundances, while those associated with BP separatrices have abundances closer to photospheric values. We
suggest that this difference is due to the location and manner in which magnetic reconnection occurs in two different topological
structures.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1013302317042 相似文献
3.
The main theoretical studies of the process involved in solar flares have been made in the two-dimensional approximation. However, the preliminary studies made with three field components suggest that reconnection could take place in the separatrices, the separator (intersection of separatrices) being a privileged location for this process. As a consequence the sites of flare kernels must be located on the intersections of the separatrices with the photosphere. Therefore, in order to understand the role of interacting large-scale structures in solar flares, we have analysed the topology of three-dimensional potential and linear force-free fields. The magnetic field has been modelled by a distribution of charges or dipoles located below the photosphere. This modelling permits us to define the field connectivity by the charges or the dipoles at both ends of every field line.We found that the appearance of a separator above the photosphere is more likely when a parasitic bipole emerges outside the axis that joins the main polarities and when the field lines are characteristic of a field created by dipoles. The separatrices derived in the potential and force-free hypothesis have different shapes. However, in the strong field regions where flares usually occur, the separatrices of the potential and force-free field models become closer. This property makes possible the use of the potential field, as a first estimate, for computing the location in the photosphere of the separatrices and for comparing this location with the position of observed H kernels. Displacements of the separatrices of a force-free field result from modifications of the free energy of the field. Then force-free fields have the further capability of predicting the kernel displacement. In all cases a configuration suitable for prominence support is found above the separator. 相似文献
4.
We study transverse loop oscillations triggered by 17flares and filament destabilizations; only 2 such cases have been reported
in the literature until now. Oscillation periods are estimated to range over a factor of ∼15, with most values between 2 and
7 min. The oscillations are excited by filament destabilizations or flares (in 6% of the 255 flares inspected, ranging from
about C3 to X2). There is no clear dependence of oscillation amplitude on flare magnitude. Oscillations occur in loops that
close within an active region, or in loops that connect an active region to a neighboring region or to a patch of strong flux
in the quiet Sun. Some magnetic configurations are particularly prone to exhibit oscillations: two active regions showed two,
and one region even three, distinct intervals with loop oscillations. The loop oscillations are not a resonance that builds
up: oscillations in loops that are excited along their entire length are likely to be near the fundamental resonance mode
because of that excitation profile, but asymmetrically excited oscillations clearly show propagating waves that are damped
too quickly to build up a resonance, and some cases show multiple frequencies. We discuss evidence that all oscillating loops
lie near magnetic separatrices that outline the large-scale topology of the field. All magnetic configurations are more complicated
than a simple bipolar region, involving mixed-polarities in the interior or vicinity of the region; this may reflect that
the exciting eruptions occur only in such environments, but this polarity mixing likely also introduces the large-scale separatrices
that are involved. Often the oscillations occur in conjunction with gradual adjustments in loop positions in response to the
triggering event. We discuss the observations in the context of two models: (a) transverse waves in coronal loops that act
as wave guides and (b) strong sensitivity to changes in the field sources for field lines near separatrices. Properties that
favor model b are (1) the involvement of loops at or near separatrices that outline the large-scale topology of the field,
(2) the combined occurrence of oscillations and loop translations, (3) the small period spread and similar decay time scale
in a set of oscillating loops in one well-observed event, and (4) the existence of loops oscillating in antiphase with footpoints
close together in two cases. All other properties are compatible with either model, except the fact that almost all of the
oscillations start away from the triggering event, suggestive of an outward-pushing exciting wave more in line with model
a. The spread in periods from event to event suggests that the oscillations may reflect the properties of some driver mechanism
that is related to the flare or mass ejection.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014957715396 相似文献
5.
P. Démoulin C. H. Mandrini M. G. Rovira J. C. Hénoux M. E. Machado 《Solar physics》1994,150(1-2):221-243
We present a detailed analysis of the magnetic topology of AR 2776 together with Hα UV, X-rays, and radio observations of the November 5, 1980 flares in order to understand the role of the active region large-scale
topology on the flare process. As at present the coronal magnetic field is modeled by an ensemble of sub-photospheric sources
whose positions and intensities are deduced from a least-square fit between the computed and observed longitudinal magnetic
fields. Charges and dipole representations are shown to lead to similar modeling of the magnetic topology provided that the
number of sources is great enough. However, for AR 2776, departure from a potential field has to be taken into account, therefore
a linear force-free field extrapolation is used.
The locations of the four bright off-band Hα kernels in quadrupolar active regions have been studied previously. In this new study the active region is bipolar and shows
a two-ribbon structure. We show that these two ribbons are a consequence of the bipolar photospheric field (the four kernels
of quadrupolar regions merge into two bipolar regions). The two ribbons are found to be located at the intersection of the
separatrices with the chromosphere when the shear, deduced from the fibril direction, is taken into account.
This study supports the hypothesis that magnetic energy is stored in field-aligned currents and released by magnetic reconnection
at the location of the separator, before being transported along field lines to the chromospheric level. It is also possible
that part of the magnetic energy could be stored and released on the separatrices. Our study shows that meeting just one of
two conditions- the presence of intense coronal currents or of a separator in a magnetic field configuration - is not sufficient
for flaring. In order to release the stored energy, the coronal currents need either to be formed along the separatrices or
to be transported towards the separator or separatrices. The location of the observed photospheric current concentrations
on the computed separatrices supports this view.
Member of the Carrera del Investigador Científico, CONICET. 相似文献
6.
Mandrini C. H. DÉmoulin P. BagalÁ L. G. Van Driel-Gesztelyi L. HÉnoux J.C. Schmieder B. Rovira M. G. 《Solar physics》1997,174(1-2):229-240
A conventional view of magnetic reconnection is mainly based on the 2-D picture of an X-type neutral point, or on the extension of it to 3-D, and it is thought to be accompanied by flux transport across separatrices (places where the field-line mapping is discontinuous). This view is too restrictive when we realize the variety of configurations that are seen flaring. We designed an algorithm, called Source Method (SM), to determine the magnetic topology of active regions (ARs). The observed photospheric field was extrapolated to the corona using subphotospheric sources and the topology was defined by the link between these sources. H flare brightenings were found to be located at the intersection with the chromosphere of the separatrices so defined. These results and the knowledge we gained on the properties of magnetic field-line linkage, led us to generalize the concept of separatrices to quasi-separatrix layers (QSLs) and to design a new method (quasi-separatrix layers method, QSLM) to determine the magnetic topology of ARs. QSLs are regions where the magnetic field-line linkage changes drastically (discontinuously when they behave like separatrices) and the QSLM can be applied to ARs where the photospheric field has been extrapolated using any kind of technique. In this paper we apply the QSLM to observed flaring regions presenting very different configurations and also to a decaying AR where a minor phenomenon, like an X-ray bright point (XBP), is observed. We find that the locations of flare and XBP brightenings are related to the properties of the field-line linkage of the underlying magnetic region, as expected from recent developments of 3-D magnetic reconnection. The extrapolated coronal field lines representing the structures involved in the analyzed events have their photospheric footpoints located at both sides of QSLs. Our results strongly support the hypothesis that magnetic reconnection is at work in various coronal phenomena, ranging from the less energetic ones to large-scale eruptions. 相似文献
7.
L. van Driel-Gesztelyi A. Hofmann P. Démoulin B. Schmieder G. Csepura 《Solar physics》1994,149(2):309-330
Through coordinated observations made during the Max'91 campaign in June 1989 in Potsdam (magnetograms), Debrecen (white light and H), and Meudon (MSDP), we follow the evolution of the sunspot group in active region NOAA 5555 for 6 days. The topology of the coronal magnetic field is investigated by using a method based on the concept of separatrices - applied previously (Mandriniet al., 1991) to a magnetic region slightly distorted by field-aligned currents. The present active region differs by having significant magnetic shear. We find that the H flare kernels and the main photospheric electric current cells are located close to the intersection of the separatrices with the chromosphere, in a linear force-free field configuration adapted to the observed shear. Sunspot motions, strong currents, isolated polarities, or intersecting separatrices are not in themselves sufficient to produce a flare. A combination of them all is required. This supports the idea that flares are due to magnetic reconnection, when flux tubes with field-aligned currents move towards the separatrix locations. 相似文献
8.
9.
L. G. Bagalá C. H. Mandrini M. G. Rovira P. Démoulin J. C. Hénoux 《Solar physics》1995,161(1):103-121
We analyze the UV and X-ray data obtained by the SMM satellite for the flare starting at 02:36 UT on November 12, 1980 in AR 2779. From a detailed revision of the Ov emission, we find that the observations are compatible with energy being released in a zone above the magnetic inversion line of the AR intermediate bipole. This energy is then transported mainly by conduction towards the two distant kernels located in the AR main bipole. One of these kernels is first identified in this paper. Accelerated particles contribute to the energy transport only during the impulsive phase.We model the observed longitudinal magnetic field by means of a discrete number of subphotospheric magnetic poles, and derive the magnetic field overall topology. As in previous studies of chromospheric flares, the Ov kernels are located along the intersection of the computed separatrices with the photosphere. Especially where the field-line linkage changes discontinuously, these kernels can be linked in pairs by lines that extend along separatrices. Our results agree with the hypothesis of magnetic energy released by magnetic reconnection occurring on separatrices. 相似文献
10.
The dynamics of interacting coronal loops and arcades have recently been highlighted by observations from theYohkoh satellite and may represent a viable mechanism for heating the solar corona. Here such an interaction is studied using two-dimensional
resistive magnetohydrodynamic (MHD) simulations. Initial potential field structures evolve in response to imposed photospheric
flows. In addition to the anticipated current sheet about theX-point separating the colliding flux systems, significant current layers are found to lie all the way along the separatrices
that intersect at theX-point and divide the coronal magnetic field into topologically distinct regions. Shear flows across the separatrices are
also observed. Both of these features are shown to be compatible with recent analytical studies of two-dimensional linear
steady-state magnetic reconnection, even though the driven system that has been simulated is not strictly ‘open’ in the sense
implied by steady-state calculations. The implications for future steady-state models are also discussed. The presence of
the neutral point also brings into question any constant-density approximations that have previously been used for such quasi-steady
coronal evolution models. This results from the intimate coupling between the neutral point and its separatrices communicated
via the gas pressure.
In terms of the detailed energetics during the arcade evolution, preliminary results reveal that on the order of 3% of the
energy injected by the footpoint motions is lost purely through ohmic dissipation. We would therefore anticipate a local hot
spot between the interacting flux systems, and a brightening distributed along the length of any separatrix field lines. Furthermore,
as the resistivityη is reduced, the flux annihilation rate and the ohmic dissipation rate are found to scale independently ofη. 相似文献
11.
Tyan Yeh 《Solar physics》1978,56(2):439-447
The topological structure of the coronal-interplanetary magnetic field is determined by the arrangement of the neutral lines at the coronal base. It is characterized by nested separatrices, which are interfaces between closed and open field lines or between oppositely directed open field lines, in the coronal-interplanetary space. In the neighborhoods of these separatrices there are important electric currents. These currents form the basis for the sheet-current model.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
12.
Wynn C. G. Ho † David L. Kaplan ‡ Philip Chang Matthew van Adelsberg Alexander Y. Potekhin 《Monthly notices of the Royal Astronomical Society》2007,375(3):821-830
RX J1856.5−3754 is one of the brightest nearby isolated neutron stars (INSs), and considerable observational resources have been devoted to it. However, current models are unable to satisfactorily explain the data. We show that our latest models of a thin, magnetic, partially ionized hydrogen atmosphere on top of a condensed surface can fit the entire spectrum, from X-rays to optical, of RX J1856.5−3754, within the uncertainties. In our simplest model, the best-fitting parameters are an interstellar column density N H ≈ 1 × 1020 cm−2 and an emitting area with R ∞ ≈ 17 km (assuming a distance to RX J1856.5−3754 of 140 pc), temperature T ∞ ≈ 4.3 × 105 K , gravitational redshift z g ∼ 0.22 , atmospheric hydrogen column y H ≈ 1 g cm−2 , and magnetic field B ≈ (3–4) × 1012 G ; the values for the temperature and magnetic field indicate an effective average over the surface. We also calculate a more realistic model, which accounts for magnetic field and temperature variations over the NS surface as well as general relativistic effects, to determine pulsations; we find that there exist viewing geometries that produce pulsations near the currently observed limits. The origin of the thin atmospheres required to fit the data is an important question, and we briefly discuss mechanisms for producing these atmospheres. Our model thus represents the most self-consistent picture to date for explaining all the observations of RX J1856.5−3754. 相似文献
13.
Haimin Wang 《Solar physics》1992,140(1):85-98
This paper studies the evolution of vector magnetic fields in the active region Boulder No. 6233 during an 11-hour observing period and its relationship to an X-3 flare on August 27, 1990.We observed the evolution of magnetic fields, which includes magnetic shear build-up, directly in high-resolution vector magnetograph movies. The magnetic shear is observed to be built up in two ways: (1) shear motion between two poles of opposite magnetic polarities and (2) direct collision of two poles of opposite polarities. When two magnetic elements of opposite polarities are canceling, the field lines are observed to turn from direct connection (potential) to a sheared configuration during the process.An X-3 flare occurred at 2100 UT. The vector magnetic structure showed an unexpected pattern of changes during and after the flare. The shear (defined as the angle between the measured transverse field and the calculated potential field) in the area covering two major footpoints increased rapidly coinciding with the burst of GOES X-ray flux. While the flare faded away in about one hour, the high shear status dropped slowly for the remainder of the observing period. Immediately after the flare, new flux emerged more rapidly and the flow speed of several magnetic elements increased near the flare footpoints.In this active region and a few other flare-productive regions we have studied recently, we always find rapid and complicated flow motions near the sites where flares occur. Photospheric flows appear to be another important factor for the production of flares. 相似文献
14.
The analyses have been made with the emphasis on four existing criteria for calibrating the azimuth of the photospheric transverse magnetic field measured with a heliomagnetograph. The results indicate that the potential criterion, Krall's criterion, i.e. Bt · Bz < 0 and Wu-Ai's criterion, i.e.
, are applicable to different cases, and the criterion based on the assumption about azimuth continuity of the transverse field is unreliable for the magnetograms with discrete data. On the basis of these analyses, a synthesized method for determining the azimuth of the transverse magnetic field on solar photosphere has been suggested in this paper. The main points of the method are as follows:
Full-size image (1K)
The transverse magnetic field observed with a heliomagnetograph could be calibrated, respectively, by means of potential criterion and Krall's criterion, and two different results could be obtained. By comparing the both results with one another, we can find that the azimuth distributions of transverse field are the same in some areas, which are named as areas with certain transverse field (CA), and different in other areas, which are named as areas with uncertain transverse field(UA). In order to determine the transverse field in UA, we introduce an assumption that the values of (the factor of force-free magnetic field) at neighbouring points are close. According to this assumption, the distribution of in UA could be determined through extrapolating from the CA, and hence the azimuth distribution of the transverse field in UA could be determined as well.
An observational example shown in this paper preliminarily demonstrates the availability of the synthesized method. 相似文献
15.
Shiv K. Sethi 《Monthly notices of the Royal Astronomical Society》2003,342(3):962-970
We study the effect of large-scale tangled magnetic fields on the galaxy two-point correlation function in the redshift space. We show that (i) the magnetic field effects can be comparable to the gravity-induced clustering for present magnetic field strength B 0 ≃ 5 × 10−8 G , (ii) the absence of this signal from the present data gives an upper bound B 0 ≲ 3 × 10−8 G and (iii) the future data can probe the magnetic fields of ≃10−8 G . A comparison with other constraints on the present magnetic field shows that they are marginally compatible. However, if the magnetic fields corresponding to B 0 ≃ 10−8 G existed at the last scattering surface, they will cause unacceptably large cosmic microwave background radiation anisotropies. 相似文献
16.
Based on photospheric vector magnetograms obtained at Huairou Solar Observing Station, we have studied the evolution of magnetic
nonpotentiality in NOAA AR 9077 from 11 to 15 July 2000. We focus our analysis on the daily change of nonpotential characteristics
in the photospheric magnetic field preceding the `Bastille Day' flare. We have identified the following evolving patterns:
(1) The shear-angle distribution underwent dramatic change in the filament channel. At a key site of the filament environment,
the magnetic shear changed sign from positive to negative. (2) The old current systems disrupted, and new but weaker systems
formed before this major event. Similar changes are identified for the longitudinal current helicity. (3) The source field
weakened before the flare, and the density of free magnetic energy decreased at the photospheric level. These obvious changes
lasted at least nine hours before the `Bastille Day' flare, and they took place where a large amount of magnetic flux disappeared
in magnetic flux cancellation. The site of dramatic changes is also found to coincide with the base of helical magnetic ropes
which were seen in a 3-D force-free reconstruction. We suggest that the observed evolution of the magnetic nonpotentiality
represents a continuous transportation of magnetic energy and complexity from the lower atmosphere to the corona. This transportation
seems to be responsible for the energy build-up for the major flare. Moreover, the slow magnetic reconnection in the lower
atmosphere, observed as magnetic flux cancellation, appears to play a key role in this energy build-up process.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014258426134 相似文献
17.
Kohji Tomisaka 《Monthly notices of the Royal Astronomical Society》1998,298(3):797-810
The importance of the interstellar magnetic field is studied in relation to the evolution of superbubbles with a three-dimensional (3D) numerical magnetohydrodynamical (MHD) simulation. A superbubble is a large supernova remnant driven by sequential supernova explosions in an OB association. Its evolution is affected by the density stratification in the galactic disc. After the superbubble size reaches 2–3 times the density scaleheight, it expands preferentially in the z -direction, until finally it can punch out a hole in the gas disc (blowout). On the other hand, the magnetic field running parallel to the galactic disc has the effect of preventing it from expanding in the direction perpendicular to the field. The density stratification and the magnetic fields have completely opposite effects on the evolution of the superbubble. We present results of a 3D MHD simulation in which both effects are included. As a result, it is concluded that when the magnetic field has a much larger scaleheight than the density, even for a model in which the bubble would blow out from the disc if the magnetic field were absent, a magnetic field with a strength of 5 μG can confine the bubble in | z |≲300 pc for ≃ 20 Myr (confinement). In a model in which the field strength decreases in the halo as B ∝ ρ1/2 , the superbubble eventually blows out like a model with B = 0 even if the magnetic field in the mid-plane is as strong as B = 5 μG. 相似文献
18.
The quiet-Sun magnetic field emerges through the solar photosphere in a multitude of mixed-polarity magnetic concentrations and is subsequently tangled up into intricate regions of interconnecting flux. Moreover, since these discrete concentrations are likely to be extremely small in size, with fluxes of around only 1017 Mx, the number of such flux sources in, say, a supergranule, will be extremely large. The flux-tube tectonics model of Priest, Heyvaerts, and Title (2002) demonstrated how the formation and dissipation of current sheets along the separatrices that separate the regions of different connectivity are likely to make an important contribution to coronal heating. Since the full complexity of the magnetic field is below present observable scales, this study examines the effect of having the magnetic flux emerge through configurations structured on smaller and smaller scales. It is found that, by fixing the amount of flux emerging into a given 2D region, the main factors influencing the current build-up along the separatrices are the number of sources through which the flux emerges and the spatial distribution of the sources on the photosphere. The free energy (i.e., that above potential) is stored lower and lower in the atmosphere as the complexity of the system increases. A simple comparison is then made between coronal heating by separator currents and by separatrix currents. It is found that both result in comparable amounts of energy release, with separatrix heating being the more dominant. 相似文献
19.
On 18 May, 1994, a subflare was observed in AR 7722 in X-rays by Yohkoh/SXT and in H at National Astronomical Observatory of Japan. The associated brightenings are due to small-scale magnetic energy release, triggered by parasitic fluxes emerging and moving at the edge of leading sunspots. Using the magnetohydrostatic equations derived by Low (1992), we model the magnetic field configuration by extrapolation of the Kitt Peak photospheric field, taking into account the effects of pressure and gravity. H flare kernels are shown to be located at computed separatrices associated with field lines which are tangent to the photosphere, namely 'bald patches (BPs). This is evidence that BPs can be involved in flares, and that current sheets can be dissipated in low levels of the solar atmosphere. The presence of dense plasma which is supported against gravity in the magnetic dips above BPs is correlated to dark elongated features observed in H. Mass flows in these flat fibrils are discussed in the context of energy release in the BP separatrices. The effect of the plasma on the computed magnetic configuration is shown to be of secondary importance with respect to the topology of the field. 相似文献
20.
V. S. Beskin I. V. Kuznetsova & R. R. Rafikov 《Monthly notices of the Royal Astronomical Society》1998,299(2):341-348
The stationary axisymmetric outflow from a rotating sphere with a (split) monopole magnetic field is considered. The stream equation describing the outflow is linearized in terms of the Michel magnetization parameter σ−1 ≪ 1, which allows a self-consistent analysis of the direct problem. It is shown that for a finite σ the fast magnetosonic surface is located at a finite distance ∼ σ1/3 R L ( R L = c /ΩF is the light cylinder). We have also found that the particle energy at the fast surface is just equal to the Michel value γ ∼ 1/3σ. The particle acceleration and magnetic field collimation are shown to become ineffective outside the fast magnetosonic surface. 相似文献