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1.
A. G. Hearn 《Solar physics》1977,51(1):159-168
The main differences between a coronal hole and quiet coronal regions are explained by a reduction of the thermal conduction coefficient by transverse components of the magnetic field in the transition region of quiet coronal regions.Calculations of minimum flux coronae show that if the flux of energy heating the corona is maintained constant while the thermal conductivity in the transition region is reduced, the coronal temperature, the pressure in the transition region and the corona, and the temperature gradient in the transition region all increase. At the same time the intensities of lines emitted from the transition region are almost unchanged. Thus all the main spectroscopically observed differences between coronal holes and quiet coronal regions are explained.The flux of energy heating the corona in both coronal holes and quiet coronal regions is 3.0 × 105 erg cm-2 s-1.The energy lost from coronal holes by the high speed streams in the solar wind is not sufficient to explain the difference in the coronal temperature in coronal holes and quiet coronal regions. The most likely explanation of the high velocity streams in the solar wind associated with coronal holes is that of Durney and Hundhausen. 相似文献
2.
T. Wiegelmann S. K. Solanki J. M. Borrero H. Peter P. Barthol A. Gandorfer V. Martínez Pillet W. Schmidt M. Knölker 《Solar physics》2013,283(2):253-272
Observations with the balloon-borne Sunrise/Imaging Magnetograph eXperiment (IMaX) provide high spatial resolution (roughly 100 km at disk center) measurements of the magnetic field in the photosphere of the quiet Sun. To investigate the magnetic structure of the chromosphere and corona, we extrapolate these photospheric measurements into the upper solar atmosphere and analyze a 22-minute long time series with a cadence of 33 seconds. Using the extrapolated magnetic-field lines as tracer, we investigate temporal evolution of the magnetic connectivity in the quiet Sun’s atmosphere. The majority of magnetic loops are asymmetric in the sense that the photospheric field strength at the loop foot points is very different. We find that the magnetic connectivity of the loops changes rapidly with a typical connection recycling time of about 3±1 minutes in the upper solar atmosphere and 12±4 minutes in the photosphere. This is considerably shorter than previously found. Nonetheless, our estimate of the energy released by the associated magnetic-reconnection processes is not likely to be the sole source for heating the chromosphere and corona in the quiet Sun. 相似文献
3.
Coronal holes (CH) emit significantly less at coronal temperatures than quiet-Sun regions (QS), but can hardly be distinguished in most chromospheric and lower transition region lines. A key quantity for the understanding of this phenomenon is the magnetic field. We use data from SOHO/MDI to reconstruct the magnetic field in coronal holes and the quiet Sun with the help of a potential magnetic model. Starting from a regular grid on the solar surface we then trace field lines, which provide the overall geometry of the 3D magnetic field structure. We distinguish between open and closed field lines, with the closed field lines being assumed to represent magnetic loops. We then try to compute some properties of coronal loops. The loops in the coronal holes (CH) are found to be on average flatter than in the QS. High and long closed loops are extremely rare, whereas short and low-lying loops are almost as abundant in coronal holes as in the quiet Sun. When interpreted in the light of loop scaling laws this result suggests an explanation for the relatively strong chromospheric and transition region emission (many low-lying, short loops), but the weak coronal emission (few high and long loops) in coronal holes. In spite of this contrast our calculations also suggest that a significant fraction of the cool emission in CHs comes from the open flux regions. Despite these insights provided by the magnetic field line statistics further work is needed to obtain a definite answer to the question if loop statistics explain the differences between coronal holes and the quiet Sun. 相似文献
4.
In this review paper we discuss several aspects of magnetic reconnection theory, focusing on the field-line motions that are
associated with reconnection. A new exact solution of the nonlinear MHD equations for reconnective annihilation is presented which represents a two-fold generalization of the previous solutions. Magnetic reconnection at null points by
several mechanisms is summarized, including spine reconnection, fan reconnection and separator reconnection, where it is pointed out that two common features of separator reconnection are the rapid flipping of magnetic field lines
and the collapse of the separator to a current sheet. In addition, a formula for the rate of reconnection between two flux
tubes is derived. The magnetic field of the corona is highly complex, since the magnetic carpet consists of a multitude of
sources in the photosphere. Progress in understanding this complexity may, however, be made by constructing the skeleton of the field and developing a theory for the local and global bifurcations between the different topologies. The eruption
of flux from the Sun may even sometimes be due to a change of topology caused by emerging flux break-out. A CD-ROM attached to this paper presents the results of a toy model of vacuum reconnection, which suggests that rapid flipping
of field lines in fan and separator reconnection is an essential ingredient also in real non-vacuum conditions. In addition,
it gives an example of binary reconnection between a pair of unbalanced sources as they move around, which may contribute significantly to coronal heating. Finally,
we present examples in TRACE movies of geometrical changes of the coronal magnetic field that are a likely result of large-scale
magnetic reconnection.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005248007615 相似文献
5.
Coronal bright points, first identified as X-ray Bright Points (XBPs), are compact, short-lived and associated with small-scale, opposite polarity magnetic flux features. Previous studies have yielded contradictory results suggesting that XBPs are either primarily a signature of emerging flux in the quiet Sun, or of the disappearance of pre-existing flux. With the goal of improving our understanding of the evolution of the quiet Sun magnetic field, we present results of a study of more recent data on XBPs and small-scale evolving magnetic structures. The coordinated data set consists of X-ray images obtained during rocket flights on 15 August and 11 December, 1987, full-disk magnetograms obtained at the National Solar Observatory - Kitt Peak, and time-lapse magnetograms of multiple fields obtained at Big Bear Solar Observatory. We find that XBPs were more frequently associated with pre-existing magnetic features of opposite polarity which appeared to be cancelling than with emerging or new flux regions. Most young, emerging regions were not associated with XBPs. However, some XBPs were associated with older ephemeral regions, some of which were cancelling with existing network or intranetwork poles. Nearly all of the XBPs corresponded to opposite polarity magnetic features which wereconverging towards each other; some of these had not yet begun cancelling. We suggest that most XBPs form when converging flow brings oppositely directed field lines together, leading to reconnection and heating of the newly-formed loops in the low corona. 相似文献
6.
Skylab observations of the Sun in soft X-rays gave us the first possibility to study the development of a complex of activity in the solar corona during its whole lifetime of seven solar rotations. The basic components of the activity complex were permanently interconnected (including across the equator) through sets of magnetic field lines, which suggests similar connections also below the photosphere. However, the visibility of individual loops in these connections was greatly variable and typically shorter than one day. Each brightening of a coronal loop in X-rays seems to be related to a variation in the photospheric magnetic field near its footpoint. Only loops (rarely visible) connecting active regions with remnants of old fields can be seen in about the same shape for many days. The interconnecting X-ray loops do not connect sunspots.We point out several examples of possible reconnections of magnetic field lines, giving rise to the onset of the visibility or, more likely, to sudden enhancements of the loop emission. In one case a new system of loops brightened in X-rays, while the field lines definitely could not have reconnected. Some striking brightenings show association with flares, but the flare occurrence and the loop brightening seem to be two independent consequences of a common triggering action: emergence of new magnetic flux. In old active regions, growing and/or brightened X-ray loops can be seen quite often without any associated flare; thus, the absence of any flaring in the chromosphere does not necessarily mean that the overlying coronal active region is quiet and inactive.We further discuss the birth of the interconnecting loops, their lifetime, altitude, variability in shape in relation to the photospheric magnetic field, the similarity of interconnecting and internal loops in the late stages of active regions, phases of development of an active region as manifested in the corona, the remarkably linear boundary of the X-ray emission after the major flare of 29 July 1973, and a striking sudden change in the large-scale pattern of unipolar fields to the north of the activity complex.The final decay of the complex of activity was accompanied by the penetration of a coronal hole into the region where the complex existed before. 相似文献
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.
The coronal magnetic field above a particular photospheric region will vanish at a certain number of points, called null points.
These points can be found directly in a potential field extrapolation or their density can be estimated from the Fourier spectrum
of the magnetogram. The spectral estimate, in which the extrapolated field is assumed to be random and homogeneous with Gaussian
statistics, is found here to be relatively accurate for quiet Sun magnetograms from SOHO’s MDI. The majority of null points
occur at low altitudes, and their distribution is dictated by high wavenumbers in the Fourier spectrum. This portion of the
spectrum is affected by Poisson noise, and as many as five-sixths of null points identified from a direct extrapolation can
be attributed to noise. The null distribution above 1500 km is found to depend on wavelengths that are reliably measured by
MDI in either its low-resolution or high-resolution mode. After correcting the spectrum to remove white noise and compensate
for the modulation transfer function we find that a potential field extrapolation contains, on average, one magnetic null
point, with altitude greater than 1.5 Mm, above every 322 Mm2 patch of quiet Sun. Analysis of 562 quiet Sun magnetograms spanning the two latest solar minima shows that the null point
density is relatively constant with roughly 10% day-to-day variation. At heights above 1.5 Mm, the null point density decreases
approximately as the inverse cube of height. The photospheric field in the quiet Sun is well approximated as that from discrete
elements with mean flux 〈|φ|〉=1.0×1019 Mx distributed randomly with density n=0.007 Mm−2. 相似文献
9.
The photosphere possesses many small, intense patches of magnetic flux. Each of these patches (or sources) is connected magnetically through the corona to several sources of opposite polarity. An elemental flux loop consists of all of the flux joining one such source to another. We find that each source is connected to twenty other sources,
on average, and that the typical flux and diameter of elemental loops in the corona are 1016 Mx and 200 km; there are approximately 17 separators for each source. We also model a typical large-scale coronal loop consisting
of many elemental loops and determine its complex internal topology. Each upright null lies at the end of about 22 separatrices,
which tend to be clustered together in trunk-like structures, analogous to river-valleys in a geographical contour map. Prone
nulls correspond to saddle points, while their spines are analogous to watersheds. 相似文献
10.
Sensitive observations of the quiet Sun observed by EIT on the SOHO satellite in high-temperature iron-line emission originating in the corona are presented. The thermal radiation of the quiet corona is found to fluctutate significantly, even on the shortest time scale of 2 min and in the faintest pixels. The power spectrum of the emission measure time variations is approximately a power law with an exponent of 1.79±0.08 for the brightest pixels and 1.69±0.08 for the average and the faintest pixels. The more prominent enhancements are identified with previously reported X-ray network flares (Krucker et al., 1997) above the magnetic network of the quiet chromosphere. In coronal EUV iron lines they are amenable to detailed analysis suggesting that the brightenings are caused by additional plasma injected from below and heated to slightly higher temperature than the preexisting corona. Statistical investigations are consistent with the hypothesis that the weaker emission measure enhancements originate from the same parent population. The power input derived from the impulsive brightenings is linearly proportional to the radiative loss in the observed part of the corona. The absolute amount of impulsive input is model-dependent. It cannot be excluded that it can satisfy the total requirement for heating. These observations give strong evidence that a significant fraction of the heating in quiet coronal regions is impulsive. 相似文献
11.
James A. Klimchuk 《Solar physics》2006,234(1):41-77
The question of what heats the solar corona remains one of the most important problems in astrophysics. Finding a definitive
solution involves a number of challenging steps, beginning with an identification of the energy source and ending with a prediction
of observable quantities that can be compared directly with actual observations. Critical intermediate steps include realistic
modeling of both the energy release process (the conversion of magnetic stress energy or wave energy into heat) and the response
of the plasma to the heating. A variety of difficult issues must be addressed: highly disparate spatial scales, physical connections
between the corona and lower atmosphere, complex microphysics, and variability and dynamics. Nearly all of the coronal heating
mechanisms that have been proposed produce heating that is impulsive from the perspective of elemental magnetic flux strands.
It is this perspective that must be adopted to understand how the plasma responds and radiates. In our opinion, the most promising
explanation offered so far is Parker's idea of nanoflares occurring in magnetic fields that become tangled by turbulent convection.
Exciting new developments include the identification of the “secondary instability” as the likely mechanism of energy release
and the demonstration that impulsive heating in sub-resolution strands can explain certain observed properties of coronal
loops that are otherwise very difficult to understand. Whatever the detailed mechanism of energy release, it is clear that
some form of magnetic reconnection must be occurring at significant altitudes in the corona (above the magnetic carpet), so
that the tangling does not increase indefinitely. This article outlines the key elements of a comprehensive strategy for solving
the coronal heating problem and warns of obstacles that must be overcome along the way. 相似文献
12.
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
13.
Takara Nishikawa 《Solar physics》1986,105(2):339-354
Activity in the chromosphere-corona transition region of the quiet Sun is found both at network boundaries and in cell interiors using a time series of the EUV spectroheliograms obtained with the Harvard experiment on Skylab. We identify time-varying sources by subtracting the minimum count at each pixel in the time series from the counts at any time. Larger flux enhancements in emission lines occur only at the network boundary, though the cell interiors also have variable intensities. Time-varying sources in the cell interior appear often in the shape of streaks which seem to originate from sources at the network boundary, or as expanding network boundary sources. It is likely that the sources in the cell interior come from the transition sheaths of chromospheric inhomogenities. A multi-temperature analysis shows that two types of sources occur in the quiet Sun. One is due to heating of cool chromospheric inhomogenities like dark mottles. Sometimes cool matter is heated to coronal temperatures. The typical mass of the coronal material produced is 1011-1012g. The other type seems to be due to draining of transition region material at the network boundary as the result of thermal instabilities. This quiet Sun activity is compatible with the time-varying sources at 6 cm wavelength. 相似文献
14.
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
15.
Carolus J. Schrijver Alan M. Title Hermance J. Hagenaar Richard A. Shine 《Solar physics》1997,175(2):329-340
Much of the magnetic field in solar and stellar photospheres is arranged into clusters of ‘flux tubes’, i.e., clustered into
compact areas in which the intrinsic field strength is approximately a kilogauss. The flux concentrations are constantly evolving
as they merge with or annihilate against other concentrations, or fragment into smaller concentrations. These processes result
in the formation of concentrations containing widely different fluxes. Schrijver et al. (1997, Paper I) developed a statistical
model for this distribution of fluxes, and tested it on data for the quiet Sun. In this paper we apply that model to a magnetic
plage with an average absolute flux density that is 25 times higher than that of the quiet network studied in Paper I. The
model result matches the observed distribution for the plage region quite accurately. The model parameter that determines
the functional form of the distribution is the ratio of the fragmentation and collision parameters. We conclude that this
ratio is the same in the magnetic plage and in quiet network. We discuss the implications of this for (near-)surface convection,
and the applicability of the model to stars other than the Sun and as input to the study of coronal heating. 相似文献
16.
Some recent observations at Pic-du-Midi (Mulleret al., 1992a) suggest that the photospheric footpoints of coronal magnetic field lines occasionally move rapidly with typical velocities of the order 3 km s–1 for about 3 or 4 min. We argue that such occasional rapid footpoint motions could have a profound impact on the heating of the quiet corona. Qualitative estimates indicate that these occasional rapid motions can account for the entire energy flux needed to heat the quiet corona. We therefore carry out a mathematical analysis to study in detail the response of a vertical thin flux tube to photospheric footpoint motions in terms of a superposition of linear kink modes for an isothermal atmosphere. We find the resulting total energy that is asymptotically injected into an isothermal atmosphere (i.e., an atmosphere without any back reflection). By using typical parameter values for fast and slow footpoint motions, we show that, even if the footpoints spend only 2.5% of the time undergoing rapid motions, still these rapid motions could be more efficient in transporting energy to the corona than the slow motions that take place most of the time. 相似文献
17.
Using the correlation between the radiance or Doppler velocity and the extrapolated magnetic field, we determined the emission heights of a set of solar transition region lines in an equatorial coronal hole and in the surrounding quiet Sun region. We found that for all of the six lower-transition-region lines, the emission height is about 4-5 Mm in the equatorial coronal hole, and around 2 Mm in the quiet Sun region. This result confirms the previous findings that plasma with different temperature can coexist at the same layer of transition region. In the quiet Sun region, the emission height of the upper-transition-region line Ne viii is almost the same that of the lower-transition-region line, but in the coronal hole, it is twice as high. This difference reveals that the outflow of Ne Ⅷ is a signature of solar wind in the coronal hole and is just a mass supply to the large loops in the quiet Sun. 相似文献
18.
The magnetic field in the solar corona plays an important role in coronal heating, flaring activity and many other phenomena studied on the Sun. Magnetic topology is frequently used to understand complicated coronal magnetic fields. By calculating the skeleton of a field, it is possible to build up a sophisticated representation of the key elements of a field’s configuration. This paper determines a simple relation between the numbers of separators (X), coronal null points (Nc), flux domains (D) and flux sources (S) in such a configuration: D=X+S−Nc−1. This equation is used to explain the behaviour of some of the bifurcations found in Magnetic Charge Topology, and to show that a one-to-one relationship exists between the number of circuits in the domain graph and the augmented null graph. Finally, it is shown that in quiet-Sun regions, the number of separators is approximately proportional to the number of flux sources. 相似文献
19.
We analysed multifrequency 2-dimensional maps of the solar corona obtained with the Nançay radioheliograph during two solar rotations in 1986. We discuss the emission of the quiet Sun, coronal holes and local sources and its association with chromospheric and coronal features as well as with large-scale magnetic fields. The brightness temperature of the quiet Sun was 5 to 5.5 × 105 K at 164 MHz and 4.5 to 5 × 105 K at 408 MHz. A coronal hole, also detected in the 10830 Å He i line, had a brightness temperature of 4.5 × 105 at 164 and 2.5 × 105 at 408 MHz. We give statistics of source brightness temperatures (on the average 8% above the background at 164 MHz and 14% at 408 MHz), as well as distributions in longitude and latitude. Although we found no significant center-to-limb effect in the brightness temperature, the sources were not visible far from the central meridian (apparently a refraction effect). The brightest sources at 164 MHz were near, but not directly above active regions and had characteristics of faint type I continua. At 408 MHz some sources were observed directly above active regions and one was unambiguously a type I continuum. The majority of the fainter sources showed no association with chromospheric features seen on H synoptic charts, including filaments. Most of them were detected at one frequency only. Sources identified at three frequencies (164, 327, and 408 MHz) were located in regions of enhanced large-scale magnetic field, some of them at the same location as decayed active regions visible one rotation before on synoptic H charts. Multifrequency sources are associated with maxima of the green line corona. The comparison with K-corona synoptic charts shows a striking association of the radio sources with dense coronal regions, associated with the coronal neutral sheet. Furthermore, we detected an enhanced brightness region which surrounds the local sources and is stable over at least one solar rotation. We call this feature a coronal plateau and we identify it with the radio counterpart of the coronal neutral sheet. 相似文献
20.
We study an active region coronal jet that evolved from southward of a major sunspot of NOAA AR12178 on 04 October 2014. This jet is associated with an onset of the GOES C1.4 flare. We use SDO/AIA, SDO/HMI, GONG \(H\upalpha\) and GOES data for analysing the observed event. We term this jet as a two-stage confined eruption of the plasma. In the first stage, some plasma erupts above the compact flaring region. In the second stage, this eruptive jet plasma and associated magnetic field lines interact with another set of distinct magnetic field lines present in its south-east direction. This creates an X-point region, where the second stage of the jet eruption is deflected above it on a curvilinear path into overlying corona. The lower part of the jet is followed by a cool surge eruption, which is visible only in \(H{\upalpha}\) emissions. The magnetic flux cancellation at the footpoint causes the triggering of C-class flare eruption. This flare energy release further triggers first stage of the coronal jet eruption. The second stage of the jet eruption is a consequence of an interaction of two distinct sets of magnetic field lines in the overlying corona. The first stage of the coronal jet and co-spatial but lagging cool surge may have common origin due to the reconnection generated heating pulses. This complex evolution of the coronal jet involves flare heating induced first stage plasma eruption, guiding of jet’s material above a junction of two distinct sets of field lines in the corona, and intra-relationship with cool surge. In effect, it imposes rigid constraints on the existing jet models. 相似文献