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1.
The Sun’s magnetic field is the primary factor determining the structure and evolution of the solar corona. Here, magnetic
topology is used in combination with a Green’s function method to model the global coronal magnetic field with a spherical
photosphere. We focus on the case of three negative flux sources and one positive source, completing our previous categorisation
of the topological states and bifurcations that are present in quadrupolar configurations in a spherical geometry. Three fundamental
varieties of topological state are found, with three types of bifurcation taking one to the other. A comparison to the equivalent
results for a planar photosphere is then carried out, and the differences between the two cases are explained. 相似文献
2.
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
3.
Previous studies of the source regions of solar wind sampled by ACE and Ulysses showed that some solar wind originates from open magnetic flux rooted in active regions. These solar wind sources were labeled active-region sources when the open flux was from a strong field region with no corresponding coronal hole in the NSO He 10830 Å synoptic coronal-hole maps. Here, we present a detailed investigation of several of these active-region sources using ACE and Ulysses solar wind data, potential field models of the corona, and solar imaging data. We find that the solar wind from these active-region sources has distinct signatures, e.g., it generally has a higher oxygen charge state than wind associated with helium-10830 Å coronal-hole sources, indicating a hotter source region, consistent with the active region source interpretation. We compare the magnetic topology of the open field lines of these active-region sources with images of the hot corona to search for corresponding features in EUV and soft X-ray images. In most, but not all, cases, a dark area is seen in the EUV and soft X-ray image as for familiar coronal-hole sources. However, in one case no dark area was evident in the soft X-ray images: the magnetic model showed a double dipole coronal structure consistent with the images, both indicating that the footpoints of the open field lines, rooted deep within the active region, lay near the separatrix between loops connecting to two different opposite polarity regions. 相似文献
4.
Previous studies of the source regions of solar wind sampled by ACE and Ulysses showed that some solar wind originates from open magnetic flux rooted in active regions. These solar wind sources were labeled active-region sources when the open flux was from a strong field region with no corresponding coronal hole in the NSO He 10830 Å synoptic coronal-hole maps. Here, we present a detailed investigation of several of these active-region sources using ACE and Ulysses solar wind data, potential field models of the corona, and solar imaging data. We find that the solar wind from these active-region sources has distinct signatures, e.g., it generally has a higher oxygen charge state than wind associated with helium-10830 Å coronal-hole sources, indicating a hotter source region, consistent with the active region source interpretation. We compare the magnetic topology of the open field lines of these active-region sources with images of the hot corona to search for corresponding features in EUV and soft X-ray images. In most, but not all, cases, a dark area is seen in the EUV and soft X-ray image as for familiar coronal-hole sources. However, in one case no dark area was evident in the soft X-ray images: the magnetic model showed a double dipole coronal structure consistent with the images, both indicating that the footpoints of the open field lines, rooted deep within the active region, lay near the separatrix between loops connecting to two different opposite polarity regions. 相似文献
5.
Solar flares occur due to the sudden release of energy stored in active-region magnetic fields. To date, the precursors to
flaring are still not fully understood, although there is evidence that flaring is related to changes in the topology or complexity
of an active-region’s magnetic field. Here, the evolution of the magnetic field in active region NOAA 10953 was examined using
Hinode/SOT-SP data over a period of 12 hours leading up to and after a GOES B1.0 flare. A number of magnetic-field properties and
low-order aspects of magnetic-field topology were extracted from two flux regions that exhibited increased Ca ii H emission during the flare. Pre-flare increases in vertical field strength, vertical current density, and inclination angle
of ≈ 8° toward the vertical were observed in flux elements surrounding the primary sunspot. The vertical field strength and
current density subsequently decreased in the post-flare state, with the inclination becoming more horizontal by ≈ 7°. This
behavior of the field vector may provide a physical basis for future flare-forecasting efforts. 相似文献
6.
7.
It is important to understand the complex topology of the magnetic field in the solar corona in order to be able to comprehend
the mechanisms which give rise to phenomena such as coronal loop structures and x-ray bright points. A key feature of the
magnetic topology is a separator. A magnetic separator is a field line which connects two magnetic null points, places where
the magnetic field becomes zero. A stable magnetic separator is important as it is the intersection of two separatrix surfaces.
These surfaces divide the magnetic field lines into regions of different connectivity, so a separator usually borders four
regions of field-line connectivity. This work examines the topological behaviour of separators that appear in a magnetic field
produced by a system of magnetic sources lying in a plane (the photosphere). The questions of how separators arise and are
destroyed, the topological conditions for which they exist, how they interact and their relevance to the coronal magnetic
field are addressed. 相似文献
8.
Helioseismic techniques such as ring-diagram analysis have often been used to determine the subsurface structural differences
between solar active and quiet regions. Results obtained by inverting the frequency differences between the regions are usually
interpreted as the sound-speed differences between them. These in turn are used as a measure of temperature and magnetic-field
strength differences between the two regions. In this paper we first show that the “sound-speed” difference obtained from
inversions is actually a combination of sound-speed difference and a magnetic component. Hence, the inversion result is not
directly related to the thermal structure. Next, using solar models that include magnetic fields, we develop a formulation
to use the inversion results to infer the differences in the magnetic and thermal structures between active and quiet regions.
We then apply our technique to existing structure inversion results for different pairs of active and quiet regions. We find
that the effect of magnetic fields is strongest in a shallow region above 0.985R
⊙ and that the strengths of magnetic-field effects at the surface and in the deeper (r<0.98R
⊙) layers are inversely related (i.e., the stronger the surface magnetic field the smaller the magnetic effects in the deeper layers, and vice versa). We also find that the magnetic effects in the deeper layers are the strongest in the quiet regions, consistent with the
fact that these are basically regions with weakest magnetic fields at the surface. Because the quiet regions were selected
to precede or follow their companion active regions, the results could have implications about the evolution of magnetic fields
under active regions. 相似文献
9.
Yu. V. Glagolevskij 《Astrophysics》2010,53(4):536-543
Models of the magnetic field of the He-w star HD142301 are constructed. Observational data are well described by the model
of a dipole shifted by 0.6 stellar radii transverse to the axis. The phase dependence of the HeI λ4026? line, however, corresponds
better to a model assuming the presence of four monopoles (or two dipoles) shifted by 0.4 stellar radii from the center. The
distance between the monopoles in both models is comparable to the star’s radius, which indicates that the source of the magnetic
field is “long” dipoles, rather than “point” dipoles. 相似文献
10.
Kenneth H. Schatten 《Solar physics》2009,255(1):3-38
Photospheric ephemeral regions (EPRs) cover the Sun like a magnetic carpet. From this, we update the Babcock – Leighton solar
dynamo. Rather than sunspot fields appearing in the photosphere de novo from eruptions originating in the deep interior, we consider that sunspots form directly in the photosphere by a rapid accumulation
of like-sign field from EPRs. This would only occur during special circumstances: locations and times when the temperature
structure is highly superadiabatic and contains a large subsurface horizontal magnetic field (only present in the Sun’s lower
latitudes). When these conditions are met, superadiabatic percolation occurs, wherein an inflow and downflow of gas scours
the surface of EPRs to form active regions. When these conditions are not met, magnetic elements undergo normal percolation,
wherein magnetic elements move about the photosphere in Brownian-type motions. Cellular automata (CA) models are developed
that allow these processes to be calculated and thereby both small-scale and large-scale models of magnetic motions can be
obtained. The small-scale model is compared with active region development and Hinode observations. The large-scale CA model offers a solar dynamo, which suggests that fields from decaying bipolar magnetic regions
(BMRs) drift on the photosphere driven by subsurface magnetic forces. These models are related to observations and are shown
to support Waldmeier’s findings of an inverse relationship between solar cycle length and cycle size. Evidence for significant
amounts of deep magnetic activity could disprove the model presented here, but recent helioseismic observations of “butterfly
patterns” at depth are likely just a reflection of surface activity. Their existence seems to support the contention made
here that the field and flow separate, allowing cool, relatively field-free downdrafts to descend with little field into the
nether worlds of the solar interior. There they heat by compression to form a hot solar-type Santa Ana wind deep below active
regions. 相似文献
11.
This paper reviews the contrasting properties of radio and EUV/X-ray observations for the study of the solar atmosphere. The
emphasis is placed on explaining the nature of radio observations to an EUV/X-ray audience. Radio emission is produced by
mechanisms which are well-understood within classical physics. Bremsstrahlung tends to be dominant at low frequencies, while
gyro-resonance emission from strong magnetic fields produces bright sources at higher frequencies. At most radio frequencies
the images of the Sun are dominated almost everywhere by bremsstrahlung opacity, which may be optically thick or thin depending
on circumstances. Where gyro-resonance sources are present they may be used as sensitive probes of the regions above active
regions where magnetic field strengths exceed several hundred gauss, and this unique capability is one of the strengths of
radio observations. Typically a gyro-resonance radio source shows the temperature on an optically thick surface of constant
magnetic field within the corona. Since each radio frequency corresponds to a different magnetic field strength, the coronal
structure can be `peeled away' by using different frequencies. The peculiarities of radio observing techniques are discussed
and contrasted with EUV/X-ray techniques. Radio observations are strong at determining temperatures and coronal magnetic field
strengths while EUV/X-ray observations better sense densities and reveal coronal magnetic field lines: in this way the two
wavelength domains are nicely complementary. 相似文献
12.
A magnetic field model is constructed for the extremely slow rotator γEqu based on measurements of its magnetic field over
many years and using the “magnetic charge” method. An analysis of γEqu and of all the data accumulated up to the present on
the magnetic field parameters of chemically peculiar stars leads to some interesting conclusions, of which the main ones are:
the fact that the axis of rotation and the dipole axis are not parallel in γEqu and the other slowly rotating magnetic stars
which we have studied previously is one of the signs that the braking of CP stars does not involve the participation of the
magnetic field as they evolve “to the main sequence.” The axes of the magnetic field dipole in slow rotators are oriented
arbitrarily with respect to their axes of rotation. The substantial photometric activity of these CP stars also argues against
these axes being close. The well-known absence of sufficiently strong magnetic fields in the Ae/Be Herbig stars also presents
difficulties for the hypothesis of “magnetic braking” in the “pre-main sequence” stages of evolution. The inverse relation
between the average surface magnetic field Bs and the rotation period P is yet another fact in conflict with the idea that
the magnetic field is involved in the braking of CP stars. We believe that angular momentum loss involving the magnetic field
can hardly have taken place during evolution immediately prior “to the main sequence,” rather the slow rotation of CP stars
most likely originates from protostellar clouds with low angular momentum. Some of the slowly rotating stars have a central
dipole magnetic field configuration, while others have a displaced dipole configuration, where the displacement can be toward
the positive or the negative magnetic pole.
__________
Translated from Astrofizika, Vol. 49, No. 2, pp. 251–262 (May 2006). 相似文献
13.
C. Beveridge 《Solar physics》2006,236(1):41-57
Magnetic topology is a powerful tool for constraining certain physical properties of a given magnetic configuration, including
the strengths and locations of current sheets, relative helicity and the magnetic free energy available for reconnection.
A critical feature of magnetic topology is the separator, a field line bordering several different regions of connectivity.
With existing methods, these field lines are at best computationally expensive and at worst impossible to find. A new method
is presented for finding the Minimal Separator Set, all of the separators that necessarily exist in a configuration, and to
use this information in combination with the optical analogy and a simulated annealing method to ‘cool’ an initial guess for
each separator into a good approximation. 相似文献
14.
In this paper we present a further study of the Ml class flare observed on October 22, 2002. We focus on the SOHO Coronal
Diagnostic Spectrometer (CDS) spectral observations performed during a multi-wavelength campaign with TRACE and ground-based
instruments (VTT, THEMIS). Strong blue-shifts are observed in the CDS coronal lines in flare kernels during the impulsive
phase of this flare. From a careful wavelength calibration we deduce upflows of 140 km/s for the Fe XIX flare emission, with
a pattern of progressively smaller flows at lower temperatures. Large line-widths were observed, especially for the Fe XIX
line, which indicate the existence of turbulent velocities. The strong upflows correspond to full shifts of the line profiles.
These flows are observed at the initial phase of the flare, and correspond to the “explosive evaporation”. The regions of
the blueshifted kernels, a few arc seconds away from the flare onset location, could be explained by the chain reaction of
successive magnetic reconnections of growing emerging field line with higher and higher overlying field. This interpretation
is evidenced by the analysis of the magnetic topology of the active region using a linear force-free-field extrapolation of
THEMIS magnetograms. 相似文献
15.
V. A. Perebeinos N. N. Stepanian V. G. Fainshtein G. V. Rudenko 《Bulletin of the Crimean Astrophysical Observatory》2011,107(1):43-50
Height variation of the magnetic field structure over groups of sunspots for heights ranging from the photosphere to the source
surface (R = 2.5 Ro, where Ro is the radius of the Sun) is examined. For all heights, starting from the photospheric level, groups of
sunspot are shown as being independent of long-lived boundaries of large-scale structures rotating with a period shorter than
the Carrington period. At heights of 1–1.5 Ro, there is a clear relation between sunspot groups and boundaries separating
the head and tail sunspots in the groups (the Hale boundaries). The rotation periods of these structures are close to the
Carrington period, their lifespan being less than three to five rotations. The maximal intensity of the solar magnetic field
drops by two orders when height increases from H = 1 to H = 1.1 Ro. Further decrease in intensity proceeds gradually (dropping by one order from H = 1.1 to 2.5 Ro). The results obtained can be considered as evidence that large-scale magnetic field structures and long-lived
boundries between them (the lines dividing polarities of the magnetic field or zero lines) all exist irrespective of sunspot
fields being generated by other sources than sunspots. At the photospheric level, active regions fields are superimposed on
these structures. 相似文献
16.
C. E. Parnell 《Solar physics》2007,242(1-2):21-41
The magnetic fields within the solar atmosphere have a complex topology owing to the fragmentary nature with which they thread the solar surface. The topologies of the potential magnetic fields containing only a few (up to four) point photospheric sources have been classified. For small numbers of sources determining the connectivity of source pairs is equivalent to counting the number of flux domains. As the numbers of sources increase this, however, is no longer the case. Instead, a pair of connected sources can have more than one distinct flux domain linking them. We call these multiply connected source pairs. Pairs of nulls connected by more than one separator are called multiply connected null pairs. Multiply connected source and null pairs go hand-in-hand such that two separators connecting the same pair of nulls immediately implies multiple flux domains linking the same source pair and vice versa. It is found that multiply connected source pairs are common not only in fairly complex potential magnetic fields but more interestingly in the resistive-MHD evolution of both simple and complex magnetic fields. Magnetic energy release is often significant around separators. Thus fields with multiply connected source pairs, which naturally have more separators, (i) have more sites for intense energy release and (ii) are likely to release energy more quickly than other magnetic fields. Moreover, the combination of multiply connected source and null pairs can give rise to a situation where flux is reconnected repeatedly between two flux domains. 相似文献
17.
D. D. Ryutov J. O. Kane A. Mizuta M. W. Pound B. A. Remington 《Astrophysics and Space Science》2005,298(1-2):183-190
The thermal pressure inside molecular clouds is insufficient for maintaining the pressure balance at an ablation front at
the cloud surface illuminated by nearby UV stars. Most probably, the required stiffness is provided by the magnetic pressure.
After surveying existing models of this type, we concentrate on two of them: the model of a quasi-homogeneous magnetic field
and the recently proposed model of a “magnetostatic turbulence”. We discuss observational consequences of the two models,
in particular, the structure and the strength of the magnetic field inside the cloud and in the ionized outflow. We comment
on the possible role of reconnection events and their observational signatures. We mention laboratory experiments where the
most significant features of the models can be tested. 相似文献
18.
The Sun's coronal magnetic field is highly complex and provides the driving force for many dynamical processes. The topology of this complex field is made up mainly of discrete topological building blocks produced by small numbers of magnetic fragments. In this work we develop a method for predicting the possible topologies due to a potential field produced by three photospheric sources, and describe how this model accurately predicts the results of Brown and Priest (1999). We then sketch how this idea may be extended to more general non-symmetric configurations. It is found that, for the case of positive total flux, a local separator bifurcation may take place with three positive sources or with one positive and two negative sources, but not for two positive sources and one negative. 相似文献
19.
“EIT waves” are a wavelike phenomenon propagating in the corona, which was initially observed in the extreme ultraviolet (EUV)
wavelength by the EUV Imaging Telescope (EIT). Their nature is still elusive, with the debate on-going between fast-mode wave
model and non-wave model. In order to distinguish between these models, we investigate the relation between the EIT wave velocity
and the local magnetic field in the corona. It is found that the two parameters show significant negative correlation in most
of the EIT wave fronts, i.e., the EIT wave propagates more slowly in the regions of stronger magnetic field. Such a result poses a big challenge to the
fast-mode wave model, which would predict a strong positive correlation between the two parameters. However, it is demonstrated
that such a result can be explained by the fieldline stretching model, i.e., that “EIT waves” are the propagation of apparent brightenings, which are generated by successive stretching of closed magnetic
field lines pushed by the erupting flux rope during coronal mass ejections (CMEs). 相似文献
20.
Pre-main sequence stars are bright UV (UV) sources compared with their main sequence analogues. The source of this excess
is the high energy processes associated with the physics of accretion/outflow during early stellar evolution. In this review,
the main sources of UV excess are described as well as the most significant “unknowns” in the field. Special emphasis is made
on the results from the last observations carried out with the Hubble Space Telescope and on the relevance of future dedicated
monitoring programs with the World Space Observatory-UV. 相似文献