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1.
This paper is the second in a series of studies working towards constructing a realistic, evolving, non-potential coronal
model for the solar magnetic carpet. In the present study, the interaction of two magnetic elements is considered. Our objectives
are to study magnetic energy build-up, storage and dissipation as a result of emergence, cancellation, and flyby of these
magnetic elements. In the future these interactions will be the basic building blocks of more complicated simulations involving
hundreds of elements. Each interaction is simulated in the presence of an overlying uniform magnetic field, which lies at
various orientations with respect to the evolving magnetic elements. For these three small-scale interactions, the free energy
stored in the field at the end of the simulation ranges from 0.2 – 2.1×1026 ergs, whilst the total energy dissipated ranges from 1.3 – 6.3×1026 ergs. For all cases, a stronger overlying field results in higher energy storage and dissipation. For the cancellation and
emergence simulations, motion perpendicular to the overlying field results in the highest values. For the flyby simulations,
motion parallel to the overlying field gives the highest values. In all cases, the free energy built up is sufficient to explain
small-scale phenomena such as X-ray bright points or nanoflares. In addition, if scaled for the correct number of magnetic
elements for the volume considered, the energy continually dissipated provides a significant fraction of the quiet Sun coronal
heating budget. 相似文献
2.
There are four key processes that dictate the behaviorof the magnetic flux concentrations that form the so-called `magnetic carpet' of the quiet photosphere. These processes are emergence, cancellation, coalescence, and fragmentation. Rates of emergence have been estimated from observations, but the rates of cancellation, coalescence, and fragmentation are much more difficult to determine observationally. A model is set up to simulate an area of magnetic carpet in the quiet Sun. In the model there are three imposed parameters: the rate of emergence of new flux, the distribution of emerged flux and the rate of fragmentation of flux concentrations. The rate of cancellation and the rate of coalescence are deduced from the model. From the simulations it is estimated that the average emergence rate of new flux in the quiet Sun must be between 6×10–6 and 10– 5 Mx cm–2 s–1 to maintain an absolute flux density of between 2.5 and 3 G. For this rate of emergence a fragmentation rate of more than 12×10–5 s–1 is required to produce the observed exponential index for the number density of flux concentrations. This is equivalent to each fragment canceling more than once every 200 minutes. The rate of cancellation is calculated from the model and is found naturally to be equivalent to the rate of emergence. However, it is found that the frequency of cancellation is much greater than the frequency of emergence. In fact, it is likely that there are several orders of magnitude more cancellation events than emergence events. This implies that flux is injected in relatively large concentrations whereas cancellation occurs though the disappearance of many small concentrations. 相似文献
3.
This study aims to quantify characteristic features of the bipolar flux appearance of solar intranetwork (IN) magnetic elements.
To attack this problem, we use the Narrowband Filter Imager (NFI) magnetograms from the Solar Optical Telescope (SOT) on board Hinode; these data are from quiet and enhanced network areas. Cluster emergence of mixed polarities and IN ephemeral regions (ERs)
are the most conspicuous forms of bipolar flux appearance within the network. Each of the clusters is characterized by a few
well-developed ERs that are partially or fully coaligned in magnetic axis orientation. On average, the sampled IN ERs have
a total maximum unsigned flux of several 1017 Mx, a separation of 3 – 4 arcsec, and a lifetime of 10 – 15 minutes. The smallest IN ERs have a maximum unsigned flux of
several 1016 Mx, separations of less than 1 arcsec, and lifetimes as short as 5 minutes. Most IN ERs exhibit a rotation of their magnetic
axis of more than 10 degrees during flux emergence. Peculiar flux appearance, e.g., bipole shrinkage followed by growth or the reverse, is not unusual. A few examples show repeated shrinkage–growth or growth–shrinkage,
like magnetic floats in the dynamic photosphere. The observed bipolar behavior seems to carry rich information on magnetoconvection
in the subphotospheric layer. 相似文献
4.
We analyse data from Hinode spacecraft taken over two 54-minute periods during the emergence of AR 11024. We focus on small-scale portions within the
observed solar active region and discover the appearance of very distinctive small-scale and short-lived dark features in
Ca ii H chromospheric filtergrams and Stokes I images. The features appear in regions with close-to-zero longitudinal magnetic field, and are observed to increase in length
before they eventually disappear. Energy release in the low chromospheric line is detected while the dark features are fading.
Three complete series of these events are detected with remarkably similar properties, i.e. lifetime of ≈ 12 min, maximum length and area of 2 – 4 Mm and 1.6 – 4 Mm2, respectively, and all with associated brightenings. In time series of magnetograms a diverging bipolar configuration is
observed accompanying the appearance of the dark features and the brightenings. The observed phenomena are explained as evidencing
elementary flux emergence in the solar atmosphere, i.e. small-scale arch filament systems rising up from the photosphere to the lower chromosphere with a length scale of a few solar
granules. Brightenings are explained as being the signatures of chromospheric heating triggered by reconnection of the rising
loops (once they have reached chromospheric heights) with pre-existing magnetic fields, as well as being due to reconnection/cancellation
events in U-loop segments of emerging serpentine fields. The characteristic length scale, area and lifetime of these elementary
flux emergence events agree well with those of the serpentine field observed in emerging active regions. We study the temporal
evolution and dynamics of the events and compare them with the emergence of magnetic loops detected in quiet Sun regions and
serpentine flux emergence signatures in active regions. The physical processes of the emergence of granular-scale magnetic
loops seem to be the same in the quiet Sun and active regions. The difference is the reduced chromospheric emission in the
quiet Sun attributed to the fact that loops are emerging in a region of lower ambient magnetic field density, making interactions
and reconnection less likely to occur. Incorporating the novel features of granular-scale flux emergence presented in this
study, we advance the scenario for serpentine flux emergence. 相似文献
5.
In this paper we study the evolution of magnetic fields of a 1F/2.4C solar flare and following magnetic flux cancellation. The data are Big Bear Solar Observatory and SOHO/MDI observations of active region NOAA 8375. The active region produced a multitude of subflares, many of them being clustered along the moat boundary in the area with mixed polarity magnetic fields. The study indicates a possible connection between the flare and the flux cancellation. The cancellation rate, defined from the data, was found to be 3×1019 Mx h–1. We observed strong upward directed plasma flows at the cancellation site. Suggesting that the cancellation is a result of reconnection process, we also found a reconnection rate of 0.5 km s–1, which is a significant fraction of Alfvén speed. The reconnection rate indicates a regime of fast photospheric reconnection happening during the cancellation. 相似文献
6.
Magnetic reconnection in the temperature minimum region of the solar photosphere can account for the canceling magnetic features
on the Sun. Litvinenko (1999a) showed that a reconnection model explains the quiet-Sun features with the magnetic flux cancelation
rate of order 1017 Mx hr−1. In this paper the model is applied to cancelation in solar active regions, which is characterized by a much larger rate
of cancelation ∖ ge1019 Mx hr−1. In particular, the evolution of a photospheric canceling feature observed in an active region on July 2, 1994 is studied.
The theoretical predictions are demonstrated to be in reasonable agreement with the measured speed of approaching magnetic
fragments, the magnetic field in the fragments, and the flux cancelation rate, deduced from the combined Big Bear Hα time-lapse
images and videomagnetograms calibrated against the daily NSO/Kitt Peak magnetogram. Of particular interest is the prediction
that photospheric reconnection should lead to a significant upward mass flux and the formation of a solar filament. Hα observations
indeed showed a filament that had one of its ends spatially superposed with the canceling feature.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005284116353 相似文献
7.
The structure of electric current and magnetic helicity in the solar corona is closely linked to solar activity over the 11-year cycle, yet is poorly understood. As an alternative to traditional current-free “potential-field” extrapolations, we investigate a model for the global coronal magnetic field which is non-potential and time-dependent, following the build-up and transport of magnetic helicity due to flux emergence and large-scale photospheric motions. This helicity concentrates into twisted magnetic flux ropes, which may lose equilibrium and be ejected. Here, we consider how the magnetic structure predicted by this model – in particular the flux ropes – varies over the solar activity cycle, based on photospheric input data from six periods of cycle 23. The number of flux ropes doubles from minimum to maximum, following the total length of photospheric polarity inversion lines. However, the number of flux rope ejections increases by a factor of eight, following the emergence rate of active regions. This is broadly consistent with the observed cycle modulation of coronal mass ejections, although the actual rate of ejections in the simulation is about a fifth of the rate of observed events. The model predicts that, even at minimum, differential rotation will produce sheared, non-potential, magnetic structure at all latitudes. 相似文献
8.
S. Vargas Domínguez D. MacTaggart L. Green L. van Driel-Gesztelyi A. W. Hood 《Solar physics》2012,278(1):33-45
Recent studies of NOAA active region 10953, by Okamoto et al. (Astrophys. J. Lett.
673, 215, 2008; Astrophys. J.
697, 913, 2009), have interpreted photospheric observations of changing widths of the polarities and reversal of the horizontal magnetic
field component as signatures of the emergence of a twisted flux tube within the active region and along its internal polarity
inversion line (PIL). A filament is observed along the PIL and the active region is assumed to have an arcade structure. To
investigate this scenario, MacTaggart and Hood (Astrophys. J. Lett.
716, 219, 2010) constructed a dynamic flux emergence model of a twisted cylinder emerging into an overlying arcade. The photospheric signatures
observed by Okamoto et al. (2008, 2009) are present in the model although their underlying physical mechanisms differ. The model also produces two additional signatures
that can be verified by the observations. The first is an increase in the unsigned magnetic flux in the photosphere at either
side of the PIL. The second is the behaviour of characteristic photospheric flow profiles associated with twisted flux tube
emergence. We look for these two signatures in AR 10953 and find negative results for the emergence of a twisted flux tube
along the PIL. Instead, we interpret the photospheric behaviour along the PIL to be indicative of photospheric magnetic cancellation
driven by flows from the dominant sunspot. Although we argue against flux emergence within this particular region, the work
demonstrates the important relationship between theory and observations for the successful discovery and interpretation of
signatures of flux emergence. 相似文献
9.
This paper considers the hemispheric pattern of solar filaments using newly developed simulations of the real photospheric
and 3D coronal magnetic fields over a six-month period, on a global scale. The magnetic field direction in the simulation
is compared directly with the chirality of observed filaments, at their observed locations. In our model the coronal field
evolves through a continuous sequence of nonlinear force-free equilibria, in response to the changing photospheric boundary
conditions and the emergence of new magnetic flux. In total 119 magnetic bipoles with properties matching observed active
regions are inserted. These bipoles emerge twisted and inject magnetic helicity into the solar atmosphere. When we choose
the sign of this active-region helicity to match that observed in each hemisphere, the model produces the correct chirality
for up to 96% of filaments, including exceptions to the hemispheric pattern. If the emerging bipoles have zero helicity, or
helicity of the opposite sign, then this percentage is much reduced. In addition, the simulation produces a higher proportion
of filaments with the correct chirality after longer times. This indicates that a key element in the evolution of the coronal
field is its long-term memory, and the build-up and transport of helicity from low to high latitudes over many months. It
highlights the importance of continuous evolution of the coronal field, rather than independent extrapolations at different
times. This has significant consequences for future modelling such as that related to the origin and development of coronal
mass ejections. 相似文献
10.
The associations of flares to flux emergence and cancellation have been further examined and clarified with the aid of complete time sequences of vector magnetograms of an active region for a 4-day period around the central meridian passage.It is found that the emergence of new flux and its driven flux cancellation with existing flux is a wholly inseparable, elementary process in the active region, favorable for flare occurence. The early discovery ofstructures magnetique evolutive (Martreset al., 1968) is confirmed and identified to be the net result of this process.All events of flux cancellation appear in the interface of two topologically separated magnetic loops. Direct indications of magnetic reconnection between two cancelling components in the photospheric layer are identified. The cancellation is most likely a slow reconnection in the lower atmosphere of the Sun. The quite popular view of interpreting flux cancellation as a pure flux submergence could not fit the magnetic topology learned from alignments of the transverse magnetic field. In this sense, the association of flares to flux cancellation seems to represent a coupling of the slow reconnection in the lower atmosphere to the fast reconnection higher in the corona.This slow reconnection can even take place below the photosphere. In one case, an inferred sub-photospheric reconnection eventually prevents one pole of an emerging flux region with the polarity opposite to the background from showing up at the photospheric level.Six of all eight flares which appeared in this period are spatially and temporally associated with the emergence of new flux and its driven cancellation. They might be divided into two groups. The first group of flares appears at the early phase of flux emergence and in close proximity to the cancelling site between new and old flux; the second ones appear after several hours of flux cancellation, centering around the cancelling site. 相似文献
11.
High-cadence, high-resolution magnetograms have shown that the quiet-Sun photosphere is very dynamic in nature. It is comprised
of discrete magnetic fragments which are characterized by four key processes – emergence, coalescence, fragmentation and cancellation.
All of this will have consequences for the magnetic field in the corona above.
The aim of this study is to gauge the effect of the behavior of the photospheric flux fragments on the quiet-Sun corona. By
considering a sequence of observed magnetograms, photospheric flux fragments are represented by a series of point sources
and the resulting potential field arising from them is examined. It is found that the quiet-Sun coronal flux is generally
recycled on time scales considerably shorter than the corresponding time scales for the recycling of photospheric flux. From
the motions of photospheric fragments alone, a recycling time of coronal flux of around 3 h is found. However, it is found
that the amount of reconnection driven by the motions of fragments is comparable to the amount driven by emergence and cancellation
of flux, resulting in a net flux replacement time for the corona of only 1.4 h.
The technique used in this study was briefly presented in a short research letter (R. M. Close et al., Astrophys. J., 612, L81, 2004); here the technique is discussed in far greater depth. Furthermore, an estimate is made of the currents required
to flow along separator field lines in order to sustain the observed heating rates (assuming separator reconnection is the
key mechanism by which the solar corona is heated). 相似文献
12.
The solar wind conditions at one astronomical unit (AU) can be strongly disturbed by interplanetary coronal mass ejections
(ICMEs). A subset, called magnetic clouds (MCs), is formed by twisted flux ropes that transport an important amount of magnetic
flux and helicity, which is released in CMEs. At 1 AU from the Sun, the magnetic structure of MCs is generally modeled by
neglecting their expansion during the spacecraft crossing. However, in some cases, MCs present a significant expansion. We
present here an analysis of the huge and significantly expanding MC observed by the Wind spacecraft during 9 – 10 November 2004. This MC was embedded in an ICME. After determining an approximate orientation for
the flux rope using the minimum variance method, we obtain a precise orientation of the cloud axis by relating its front and
rear magnetic discontinuities using a direct method. This method takes into account the conservation of the azimuthal magnetic
flux between the inbound and outbound branches and is valid for a finite impact parameter (i.e., not necessarily a small distance between the spacecraft trajectory and the cloud axis). The MC is also studied using dynamic
models with isotropic expansion. We have found (6.2±1.5)×1020 Mx for the axial flux and (78±18)×1020 Mx for the azimuthal flux. Moreover, using the direct method, we find that the ICME is formed by a flux rope (MC) followed
by an extended coherent magnetic region. These observations are interpreted by considering the existence of a previously larger
flux rope, which partially reconnected with its environment in the front. We estimate that the reconnection process started
close to the Sun. These findings imply that the ejected flux rope is progressively peeled by reconnection and transformed
to the observed ICME (with a remnant flux rope in the front part). 相似文献
13.
Using high resolution KPNO magnetograms and sequences of simultaneous S-054 soft X-ray solar images we have compared the properties of X-ray bright points (XBP) and ephemeral active regions (ER). All XBP appear on the magnetograms as bipolar features, except for very newly emerged or old and decayed XBP. We find that the separation of the magnetic bipoles increases with the age of the XBP, with an average emergence growth rate of 2.2 ± 0.4 km s–1. The total magnetic flux in a typical XBP living about 8 hr is found to be 2 x 1019 Mx. A proportionality is found between XBP lifetime and total magnetic flux, equivalent to 1020 Mx per day of lifetime.Operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation. 相似文献
14.
The aim of this paper is to determine the flux emergence rate due to small-scale magnetic features in the quiet Sun using
high-resolution Hinode SOT NFI data. Small-scale magnetic features are identified in the data using two different feature identification methods
(clumping and downhill); then three methods are applied to detect flux emergence events. The distribution of the intranetwork
peak emerged fluxes is determined. When combined with previous emergence results, from ephemeral regions to sunspots, the
distribution of all fluxes are found to follow a power-law distribution which spans nearly seven orders of magnitude in flux
(1016 – 1023 Mx) and 18 orders of magnitude in frequency. The power-law fit to all these data is of the form
|
\fracdNdY = \fracn0Y0\fracYY0-2.7,\frac{\mathrm{d}N}{\mathrm{d}\Psi} = \frac{n_0}{\Psi_0}\frac{\Psi}{\Psi _0}^{-2.7}, 相似文献
15.
Active region NOAA 8038 was observed from 10 to 13 May, 1997 using the USO solar video magnetograph. During this period, the active region was mostly inactive, and gave rise to only a single notable flare of 1N/C1.3 class on May 12, 1997/04:45 UT. The flare occurred in a weak field location, but new emerging fluxes were observed prior to the flare onset. Horizontal motions of the network photospheric magnetic fluxes were inferred using USO and SOHO magnetograms, and velocities in the range 300–800 m s–1 were estimated. The initial flare brightening was observed at the flux cancellation site where magnetic field gradients were found to increase. Detailed analyses of flux motions, cancellation and their relation with the flare are presented. 相似文献
16.
Three-dimensional, quasi-static evolutions of coronal magnetic fields driven by photospheric flux emergence are modeled by a class of analytic force-free magnetic fields. Our models relate commonly observed photospheric magnetic phenomena, such as the formation and growth of sunspots, the emergence of an X-type separator, and the collision and merging of sunspots, to the three-dimensional magnetic fields in the corona above. By tracking the evolution in terms of a continuous sequence of force-free states, we show that flux emergence and submergence along magnetic neutral lines in the photosphere are essential processes in all these photospheric phenomena. The analytic solutions we present have a parametric regime within which the magnetic energy attained by an evolving force-free field may be of the order of 1030 ergs to several 1031 ergs, depending on the magnetic environment into which an emerging flux intrudes. The commonly used indicators of magnetic shear in magnetogram interpretation are discussed in terms of field connectivity in our models. It is demonstrated that the crossing angle of the photospheric transverse magnetic field with the neutral line may not be a reliable indicator of the magnetic shear in the coronal field above, due to the complexity of three-dimensionality. The poorly understood constraint of magnetic-helicity conservation on the availability of magnetic free energy for a flare is briefly discussed.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
17.
G. Valori L. M. Green P. Démoulin S. Vargas Domínguez L. van Driel-Gesztelyi A. Wallace D. Baker M. Fuhrmann 《Solar physics》2012,278(1):73-97
We study the flux emergence process in NOAA active region 11024, between 29 June and 7 July 2009, by means of multi-wavelength
observations and nonlinear force-free extrapolation. The main aim is to extend previous investigations by combining, as much
as possible, high spatial resolution observations to test our present understanding of small-scale (undulatory) flux emergence,
whilst putting these small-scale events in the context of the global evolution of the active region. The combination of these
techniques allows us to follow the whole process, from the first appearance of the bipolar axial field on the east limb, until
the buoyancy instability could set in and raise the main body of the twisted flux tube through the photosphere, forming magnetic
tongues and signatures of serpentine field, until the simplification of the magnetic structure into a main bipole by the time
the active region reaches the west limb. At the crucial time of the main emergence phase high spatial resolution spectropolarimetric
measurements of the photospheric field are employed to reconstruct the three-dimensional structure of the nonlinear force-free
coronal field, which is then used to test the current understanding of flux emergence processes. In particular, knowledge
of the coronal connectivity confirms the identity of the magnetic tongues as seen in their photospheric signatures, and it
exemplifies how the twisted flux, which is emerging on small scales in the form of a sea-serpent, is subsequently rearranged
by reconnection into the large-scale field of the active region. In this way, the multi-wavelength observations combined with
a nonlinear force-free extrapolation provide a coherent picture of the emergence process of small-scale magnetic bipoles,
which subsequently reconnect to form a large-scale structure in the corona. 相似文献
18.
Canceling magnetic features are commonly believed to result from magnetic reconnection in the low atmosphere. According to the Sweet–Parker type reconnection model, the rate of flux cancellation in a canceling magnetic feature is related to the converging speed of each pole. To test this prediction observationally, we have analyzed the time variation of two canceling magnetic features in detail using the high-resolution magnetograms taken by the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO). As a result, we have obtained the rate and converging speed of flux cancellation in each feature: 1.3×1018 Mx hr–1 (or 1.1×106 G cm s–1 per unit contact length) and 0.35 km s–1 in the smaller one, and 3.5×1018 Mx hr–1 (1.2×106 G cm s–1) and 0.27 km s–1 in the bigger one. The observed speeds are found to be significantly bigger than the theoretically expected ones, but this discrepancy can be resolved if uncertainty factors such as low area filling factor of magnetic flux and low electric conductivity are taken into account. 相似文献
19.
The north – south asymmetries (NSA) of three solar activity indices are derived and mutually compared over a period of more
than five solar cycles (1945 – 2001). A catalogue of the hemispheric sunspot numbers, the data set of the coronal green line
brightness developed by us, and the magnetic flux derived from the NSO/KP data (1975 – 2001) are treated separately within
the discrete low- and mid-latitude zones (5° – 30°, 35° – 60°). The calculated autocorrelations, cross-correlations, and regressions
between the long-term NSA data sets reveal regularities in the solar activity phenomenon. Namely, the appearance of a distinct
quasi-biennial oscillation (QBO) is evident in all selected activity indices. Nevertheless, a smooth behavior of QBO is derived
only when sufficient temporal averaging is performed over solar cycles. The variation in the significance and periodicity
of QBO allows us to conclude that the QBO is not persistent over the whole solar cycle. A similarity in the photospheric and
coronal manifestations of the NSA implies that their mutual relation will also show the QBO. A roughly two-year periodicity
is actually obtained, but again only after significant averaging over solar cycles. The derived cross-correlations are in
fact variable in degree of correlation as well as in changing periodicity. A clear and significant temporal shift of 1 – 2
months in the coronal manifestation of the magnetic flux asymmetry relative to the photospheric manifestation is revealed
as a main property of their mutual correlation. This shift can be explained by the delayed large-scale coronal manifestation
in responding to the emergence of the magnetic flux in the photosphere. The reliability of the derived results was confirmed
by numerical tests performed by selecting different numerical values of the used parameters. 相似文献
20.
In this paper the origin and evolution of the Sun's open magnetic flux are considered for single magnetic bipoles as they are transported across the Sun. The effects of magnetic flux transport on the radial field at the surface of the Sun are modeled numerically by developing earlier work by Wang, Sheeley, and Lean (2000). The paper considers how the initial tilt of the bipole axis () and its latitude of emergence affect the variation and magnitude of the surface and open magnetic flux. The amount of open magnetic flux is estimated by constructing potential coronal fields. It is found that the open flux may evolve independently from the surface field for certain ranges of the tilt angle. For a given tilt angle, the lower the latitude of emergence, the higher the magnitude of the surface and open flux at the end of the simulation. In addition, three types of behavior are found for the open flux depending on the initial tilt angle of the bipole axis. When the tilt is such that ge2° the open flux is independent of the surface flux and initially increases before decaying away. In contrast, for tilt angles in the range –16°<<2° the open flux follows the surface flux and continually decays. Finally, for le–16° the open flux first decays and then increases in magnitude towards a second maximum before decaying away. This behavior of the open flux can be explained in terms of two competing effects produced by differential rotation. Firstly, differential rotation may increase or decrease the open flux by rotating the centers of each polarity of the bipole at different rates when the axis has tilt. Secondly, it decreases the open flux by increasing the length of the polarity inversion line where flux cancellation occurs. The results suggest that, in order to reproduce a realistic model of the Sun's open magnetic flux over a solar cycle, it is important to have accurate input data on the latitude of emergence of bipoles along with the variation of their tilt angles as the cycle progresses. 相似文献
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