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1.
Magnetic Energy of Force-Free Fields with Detached Field Lines 总被引：2，自引：0，他引：2
Guo-Qiang Li You-Qiu HuSchool of Earth Space Sciences University of Science Technology of China Hefei 《中国天文和天体物理学报》2003,3(6):555-562
Using an axisymmetrical ideal MHD model in spherical coordinates, we present a numerical study of magnetic configurations characterized by a levitating flux rope embedded in a bipolar background field whose normal field at the solar surface is the same or very close to that of a central dipole. The characteristic plasma β (the ratio between gas pressure and magnetic pressure) is taken to be sosmall (β= 10^-4) that the magnetic field is close to being force-free. The system as a whole is then let evolve quasi-statically with a slow increase of either the annular magnetic flux or the axial magnetic flux of the rope, and the total magneticenergy of the system grows accordingly. It is found that there exists an energy threshold: the flux rope sticks to the solar surface in equilibrium if the magneticenergy of the system is below the threshold, whereas it loses equilibrium if the threshold is exceeded. The energy threshold is found to be larger than that of thecorresponding fully-open magnetic field by a factor of nearly 1.08 irrespective as towhether the background field is completely closed or partly open, or whether the magnetic energy is enhanced by an increase of annular or axial flux of the rope.This gives an example showing that a force-free magnetic field may have an energy larger than the corresponding open field energy if part of the field lines is allowed tobe detached from the solar surface. The implication of such a conclusion in coronal mass ejections is briefly discussed and some comments are made on the maximum energy of force-free magnetic fields. 相似文献
2.
Using a 2.5-D, time-dependent ideal MHD model in Cartesian coordinates, we carried out numerical simulations to investigate
the equilibrium and evolution properties of a magnetic configuration that consists of a coronal magnetic flux rope and a partly
open photospheric background field, which is equivalent to that produced by a two-patch magnetic source on the photospheric
surface. The axial and annular magnetic fluxes of the flux rope are given and fixed. The global magnetic configuration evolves
in response to three types of changes of the background field: decreasing of the distance between the two sources, shrinking
of the size of each source, and increasing of the shear in the closed component of the background field. As a result, the
geometrical parameters of the flux rope, i.e. the height of the rope axis, the half-width of the rope and the length of the
vertical current sheet below the rope, change due to the variation of the background field. It is shown that for a given coronal
magnetic flux rope in a partly open background field, the variation of the geometrical parameters of the flux rope displays
a catastrophic behavior, namely, there exists a critical point for each case, at which an infinitesimal change of the background
field leads to a loss of equilibrium, and thus a jump of the flux rope. The implication of such a catastrophe in solar active
phenomena is briefly discussed. 相似文献
3.
Using a 2.5-D, time-dependent ideal MHD model in Cartesian coordinates, a numerical study is carried out to find equilibrium solutions associated with a magnetic flux rope in the corona. The ambient magnetic field is partially open, consisting of a closed arcade in the center and an open field at the flank. The coronal magnetic flux rope is characterized by its magnetic properties, including the axial and annular magnetic fluxes and the magnetic helicity, and its geometrical features, including the height of the rope axis, the halfwidth of the rope and the length of the vertical current sheet below the rope. It is shown that for a given partially open ambient magnetic field, the dependence of the geometrical features on the magnetic properties displays a catastrophic behavior, namely, there exists a certain critical point, across which an infinitesimal enhancement of the magnetic parameters causes a finite jump of the geometrical parameters for the rope. The amplitude of the jump depends on the extent to which the ambient magnetic field in open, and approaches to zero when the ambient magnetic field becomes completely closed. The implication of such a catastrophe in solar active phenomena is briefly discussed. 相似文献
4.
As demonstrated by many previous studies, a system consisting of an isolated coronal flux rope and a surrounding background
magnetic field exhibits a catastrophic behavior. In particular, if the magnetic field of the system is force-free and axisymmetric
in spherical geometry, the magnetic energy at the catastrophic point, referred to as the catastrophic energy threshold, is
found to be larger than the corresponding partly or fully open field energy. This paper takes an octapole field as the background
and introduces a flux rope within the central arcade of the octapole field. A relaxation method based on time-dependent ideal
magnetohydrodynamic (MHD) simulations is used to find axisymmetric force-free field solutions in spherical geometry associated
with the flux rope system. With respect to an increase of either the annular flux Φp or the axial flux Φϕ of the rope, the system exhibits a catastrophic behavior as expected, and the catastrophic energy threshold is larger than
that of the corresponding partly open field, in which the central arcade is opened up, but the remainder remains closed. For
a given octapole field, the energy threshold depends on either Φp or Φϕ at the catastrophic point, and it increases with increasing Φp or decreasing Φϕ. On the other hand, the extent to which the central bipolar component of the octapole field is open also affects the energy
threshold. These results differ from those for the bipolar background field case, in which the catastrophic energy threshold
is almost independent of the magnetic properties of the flux rope at the catastrophic points and the extent to which the background
field is open. The reason for such a difference is briefly discussed. 相似文献
5.
Govind Dubey Bart van der Holst Stefaan Poedts 《Journal of Astrophysics and Astronomy》2006,27(2-3):159-166
The initiation of solar Coronal Mass Ejections (CMEs) is studied in the framework of numerical magnetohydrodynamics (MHD).
The initial CME model includes a magnetic flux rope in spherical, axi-symmetric geometry. The initial configuration consists
of a magnetic flux rope embedded in a gravitationally stratified solar atmosphere with a background dipole magnetic field.
The flux rope is in equilibrium due to an image current below the photosphere. An emerging flux triggering mechanism is used
to make this equilibrium system unstable. When the magnetic flux emerges within the filament below the flux rope, this results
in a catastrophic behavior similar to previous models. As a result, the flux rope rises and a current sheet forms below it.
It is shown that the magnetic reconnection in the current sheet below the flux rope in combination with the outward curvature
forces results in a fast ejection of the flux rope as observed for solar CMEs. We have done a parametric study of the emerging
flux rate. 相似文献
6.
Coronal Flux Rope Equilibria in Closed Magnetic Fields 总被引：1，自引：0，他引：1
Zhen Wang You-Qiu HuSchool of Earth Space Sciences University of Science Technology of China 《中国天文和天体物理学报》2003,3(3):241-246
Using a 2.5-dimensional ideal MHD model in Cartesian coordinates,we investigate the equilibrium properties of coronal magnetic flux ropes in background magnetic fields that are completely closed.The background fields are produced by a dipole,a quadrupole,and an octapole,respectively,located below the photosphere at the same depth.A magnetic flux rope is then launched from below the photo-sphere,and its magnetic properties,i.e,the annular magnetic fluxφp and the axial magnetic fluxφz,are controlled by a single emergence parameter.The whole sys-tem eventually evolves into equilibrium,and the resultant flux rope is characterized by three geometrical parameters:the height of the rope axis,the half-width of the rope,and the length of the vertical current sheet below the rope.It is found that the geometrical parameters increase monotonically and continuously with increasing φp and φz:no catastrophe occurs.Moreover,there exists a steep segment in the profiles of the geometrical parameters versus either φp or φz,and the faster the background field decays with height,the larger both the gradient and the growth amplitude within the steep segment will be. 相似文献
7.
Coronal Magnetic Flux Rope Equilibria and Magnetic Helicity 总被引：1，自引：0，他引：1
1 INTRODUCTIONObservations show that the magnetic helicity of solar magnetic structures has a predominantsign in each hemisphere of the Sun, positive in the southern hemisphere and negative in thenorthern, regardless of the solar cycle (Rust, 1994). The magnetic helicity is strictly conservedin the frame of ideal MHD (WOltjer, 1958), and approximately conserved in the presence ofresistive dissipation and magnetic reconnection in a highly conductive plajsma (Taylor, 1974;Berger, 1984; H… 相似文献
8.
A major solar active event called Bastille Day Event occurred in AR 9077 on July 14, 2000. Simultaneous occurrence of a filament
eruption, a flare and a coronal mass ejection was observed in this event. Previous analyses of this event show that before
the event, there existed an activation and eruption of a huge trans-equatorial filament, which might play a crucial role in
triggering the Bastille Day event. This implies that independent flux systems are closely related to and affect each other,
which has encouraged us to investigate the catastrophic behavior of a multiple coronal flux rope system with the use of a
2.5-D time-dependent MHD model. A force-free field that contains three separate coronal flux ropes is taken to be the initial
state. Starting from this state, we increase either the annular or the axial flux of a certain flux rope to examine the catastrophic
behavior of the system in two regimes, the ideal MHD regime and the resistive MHD regime. It is found that a catastrophe occurs
if the flux exceeds a certain critical value, or the magnetic energy of the system exceeds a certain threshold: the rope of
interest breaks away from the base and escapes to infinity, leaving a current sheet below. Moreover, the destiny of the remainder
flux ropes relies on whether reconnection takes place across the current sheet. In the ideal MHD regime, i.e., in the absence of reconnection, these ropes remain to be attached to the base in equilibrium, whereas in the resistive MHD
regime they abruptly erupt upward during reconnection and escape to infinity. Reconnection causes the field lines to close
back to the base and thus changes the background field outside the attached flux ropes in such a way that the constraint on
these ropes is substantially relaxed and the corresponding catastrophic energy threshold is reduced accordingly, leading to
a catastrophic eruption of these ropes. Since magnetic reconnection is generally inevitable when a current sheet forms and
develops through an eruption of one flux rope, the eruption of this flux rope must lead to an eruption of the others. This
provides an example to demonstrate the interaction between several independent magnetic flux systems in different regions,
as implied by the Bastille Day event, and may serve as a possible mechanism for sympathetic events occurring on the Sun. 相似文献
9.
A numerical study of the pre-ejection, magnetically-sheared corona as a free boundary problem 总被引：1，自引：0，他引：1
A class of magnetostatic equilibria with axial symmetry outside a unit sphere in the presence of plasma pressure and an r
–2 gravitational field is constructed. The structure contains a localized current-carrying region confined by a background bipolar potential field, and the shape of the region changes subject to the variation of the electric current. The continuity requirement for the magnetic field and plasma pressures at the outer boundary of the cavity defines a free boundary problem, which is solved numerically using a spectral boundary scheme. The model is then used to study the expansion of the current-carrying region, caused by the buildup of magnetic shear, against the background confining field. The magnetic shear in our model is induced by the loading of an azimuthal field, accompanied by a depletion of plasma density.We show that due to the additional effect of confinement by the dense surrounding plasma, the energy of the magnetic field can exceed the energy of its associated open field, presumably a necessary condition for the onset of coronal mass ejections. (However, the plasma beta of the confining fluid is higher than that in the outer boundary of a realistic helmet-streamer structure.) Furthermore, under the assumption that coronal mass ejections are driven by magnetic buoyancy, the result from our model study lends further support to the notion of a suspended magnetic flux rope in the low-density cavity of a helmet-streamer as a promising pre-ejection configuration.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
10.
Large-scale solar eruptions, known as coronal mass ejections (CMEs), are regarded as the main drivers of space weather. The exact trigger mechanism of these violent events is still not completely clear; however, the solar magnetic field indisputably plays a crucial role in the onset of CMEs. The strength and morphology of the solar magnetic field are expected to have a decisive effect on CME properties, such as size and speed. This study aims to investigate the evolution of a magnetic configuration when driven by the emergence of new magnetic flux in order to get a better insight into the onset of CMEs and their magnetic structure. The three-dimensional, time-dependent equations for ideal magnetohydrodynamics are numerically solved on a spherical mesh. New flux emergence in a bipolar active region causes destabilisation of the initial stationary structure, finally resulting in an eruption. The initial magnetic topology is suitable for the ??breakout?? CME scenario to work. Although no magnetic flux rope structure is present in the initial condition, highly twisted magnetic field lines are formed during the evolution of the system as a result of internal reconnection due to the interaction of the active region magnetic field with the ambient field. The magnetic energy built up in the system and the final speed of the CME depend on the strength of the overlying magnetic field, the flux emergence rate, and the total amount of emerged flux. The interaction with the global coronal field makes the eruption a large-scale event, involving distant parts of the solar surface. 相似文献
11.
E. Palmerio E. K. J. Kilpua A. W. James L. M. Green J. Pomoell A. Isavnin G. Valori 《Solar physics》2017,292(2):39
A key aim in space weather research is to be able to use remote-sensing observations of the solar atmosphere to extend the lead time of predicting the geoeffectiveness of a coronal mass ejection (CME). In order to achieve this, the magnetic structure of the CME as it leaves the Sun must be known. In this article we address this issue by developing a method to determine the intrinsic flux rope type of a CME solely from solar disk observations. We use several well-known proxies for the magnetic helicity sign, the axis orientation, and the axial magnetic field direction to predict the magnetic structure of the interplanetary flux rope. We present two case studies: the 2 June 2011 and the 14 June 2012 CMEs. Both of these events erupted from an active region, and despite having clear in situ counterparts, their eruption characteristics were relatively complex. The first event was associated with an active region filament that erupted in two stages, while for the other event the eruption originated from a relatively high coronal altitude and the source region did not feature a filament. Our magnetic helicity sign proxies include the analysis of magnetic tongues, soft X-ray and/or extreme-ultraviolet sigmoids, coronal arcade skew, filament emission and absorption threads, and filament rotation. Since the inclination of the post-eruption arcades was not clear, we use the tilt of the polarity inversion line to determine the flux rope axis orientation and coronal dimmings to determine the flux rope footpoints, and therefore, the direction of the axial magnetic field. The comparison of the estimated intrinsic flux rope structure to in situ observations at the Lagrangian point L1 indicated a good agreement with the predictions. Our results highlight the flux rope type determination techniques that are particularly useful for active region eruptions, where most geoeffective CMEs originate. 相似文献
12.
Model calculations are presented for the rising motion of the top section of a prominence loop, which is represented by a straight flux rope immersed in a coronal medium permeated with a bipolar magnetic field. Initially the prominence is at rest, in equilibrium with the surrounding coronal medium. When the magnetic monopoles that account for the source current for the bipolar field strengthen, the upward hydromagnetic buoyancy force overcomes the downward gravitational force so that the prominence is initiated into rising motion. The illustrative examples show that prominences can move away from the solar surface by the action of the hydromagnetic buoyancy force, which is preponderant with the diamagnetic force due to the current carried by the prominence interacting with the coronal magnetic field produced by the photospheric currents, if the changes in the photospheric magnetic field are sufficiently large. 相似文献
13.
Xie Xiao-yan Ziegler Udo Mei Zhi-xing Wu Ning Lin Jun 《Chinese Astronomy and Astrophysics》2018,42(4):550-574
On the basis of the catastrophe model developed by Isenberg et al., we have used the NIRVANA code to perform the magnetohydrodynamics (MHD) numerical experiments to look into the various behaviors of the coronal magnetic configuration that includes a current-carrying flux rope for modelling the prominence levitation in the corona. These behaviors include the evolution of the equilibrium height of magnetic flux rope with the background magnetic field, the corresponding interior equilibrium of magnetic flux rope, the dynamic properties of magnetic flux rope after the system loses equilibrium, as well as the impact of the reference radius on the equilibrium height of magnetic flux rope. In our calculations, an empirical model of the coronal density distribution given by Sittler & Guhathakurta is used, and the physical dissipation is included. Our experiments show that a deviation between the simulated equilibrium height of magnetic flux rope and the theoretical result of Isenberg et al. exists, but it is not apparent, and the evolutionary features of the two results are similar. If the magnetic flux rope is initially located at the stable branch of the theoretical equilibrium curve, the magnetic flux rope will quickly reach the equilibrium position after several rounds of oscillations as a result of the self-adjustment of the system; when the system is located at the critical point it will quickly lose equilibrium and evolve to the eruptive state; the impact of the variation of reference radius on the equilibrium height of magnetic flux rope is consistent with the prediction of the theory; in the eruptive state, the kinetic properties of magnetic flux rope are consistent with the results given by the Lin-Forbes model and observation, and the fast-mode shock in front of the magnetic flux rope is observed in our experiments; furthermore, because that the dissipation is included in our numerical experiments, the energy conversion from the magnetic energy to other forms of energy is very apparent in the eruptive process. 相似文献
14.
The well-known Chandrasekhar-Prendergastmagnetostatic solution for a sphericalmagnetic vortex with axial symmetry squeezed
externally by a potential magnetic field is of considerable interest both for describing the magnetic field of a star as a
whole and for modeling solar active phenomena (flares, coronal spiders, etc.). This solution is generalized to the case of
a uniform gravity field. In contrast to the Chandrasekhar-Prendergast model, the dependence of the plasma density in the spherical
vortex on magnetic flux appears in the new solution. This expands considerably the class of magnetoplasma equilibria being
analyzed and allows new scenarios for energy release in solar flares and coronal spiders to be proposed. 相似文献
15.
The rate of two-dimensional flux pile-up magnetic reconnection is known to be severely limited by gas pressure in a low-beta plasma of the solar corona. As earlier perturbational calculations indicated, however, the pressure limitation should be less restrictive for three-dimensional flux pile-up. In this paper the maximum rate of reconnection is calculated in the approximation of reduced magnetohydrodynamics (RMHD), which is valid in the solar coronal loops. The rate is calculated for finite-magnitude reconnecting fields in the case of a strong axial field in the loop. Gas pressure effects are ignored in RMHD but a similar limitation on the rate of magnetic merging exists. Nevertheless, the magnetic energy dissipation rate and the reconnection electric field can increase by several orders of magnitude as compared with strictly two-dimensional pile-up. Though this is still not enough to explain the most powerful solar flares, slow coronal transients with energy release rates of order 1025– 1026 erg s–1and heating of quiet coronal loops are within the compass of the model. 相似文献
16.
We present a new approach to the theory of large-scale solar eruptive phenomena such as coronal mass ejections and two-ribbon flares, in which twisted flux tubes play a crucial role. We show that it is possible to create a highly nonlinear three-dimensional force-free configuration consisting of a twisted magnetic flux rope representing the magnetic structure of a prominence (surrounded by an overlaying, almost potential, arcade) and exhibiting an S-shaped structure, as observed in soft X-ray sigmoid structures. We also show that this magnetic configuration cannot stay in equilibrium and that a considerable amount of magnetic energy is released during its disruption. Unlike most previous models, the amount of magnetic energy stored in the configuration prior to its disruption is so large that it may become comparable to the energy of the open field. 相似文献
17.
The onset stage of coronal mass ejections (CMEs) is difficult to observe and is poorly studied. In spite of their practical importance, methods for CME predictions with sufficient lead times are only in the nascent stages of development. The most probable CME mechanism is a catastrophic loss of equilibrium of a large-scale current system in the corona (a flux rope). A twisted magnetic rope is maintained by the tension of field lines of photospheric sources until parameters of the system reach critical values and the equilibrium is lost. Unfortunately, there is low-density plasma (coronal cavity) in most of the rope volume; thus, it is difficult to observe a rope. However, the lower parts of the helical field lines of a rope are fine traps for the dense cold plasma of prominences. Thus, prominences are the best tracers of flux ropes in the corona. The maximal height up to which the rope is in stable equilibrium can be found by analyzing the distribution of the magnetic field generated by photospheric sources in the corona. Comparing this critical height with the actually observed prominence height, one can estimate the probability of the loss of equilibrium by a magnetic rope with a following eruption of prominences and coronal mass ejections. 相似文献
18.
《天文和天体物理学研究(英文版)》2020,(10)
Magnetic flux ropes are characterized by coherently twisted magnetic field lines, which are ubiquitous in magnetized plasmas. As the core structure of various eruptive phenomena in the solar atmosphere, flux ropes hold the key to understanding the physical mechanisms of solar eruptions,which impact the heliosphere and planetary atmospheres. The strongest disturbances in the Earth's space environments are often associated with large-scale flux ropes from the Sun colliding with the Earth's magnetosphere, leading to adverse, sometimes catastrophic, space-weather effects. However, it remains elusive as to how a flux rope forms and evolves toward eruption, and how it is structured and embedded in the ambient field. The present paper addresses these important questions by reviewing current understandings of coronal flux ropes from an observer's perspective, with an emphasis on their structures and nascent evolution toward solar eruptions, as achieved by combining observations of both remote sensing and in-situ detection with modeling and simulation. This paper highlights an initiation mechanism for coronal mass ejections(CMEs) in which plasmoids in current sheets coalesce into a 'seed' flux rope whose subsequent evolution into a CME is consistent with the standard model, thereby bridging the gap between microscale and macroscale dynamics. 相似文献
19.
We study the interaction between an erupting solar filament and a nearby coronal hole, based on multi-viewpoint observations from the Solar Dynamics Observatory and STEREO. During the early evolution of the filament eruption, it exhibits a clockwise rotation that brings its easternmost leg in contact with the oppositely aligned field at the coronal hole boundary. The interaction between the two magnetic-field systems is manifested as the development of a narrow contact layer in which we see enhanced EUV brightening and bi-directional flows, suggesting that the contact layer is a region of strong and ongoing magnetic reconnection. The coronal mass ejection (CME) resulting from this eruption is highly asymmetric, with its southern portion opening up to the upper corona, while the northern portion remains closed and connected to the Sun. We suggest that the erupting flux rope that made up the filament reconnected with both the open and closed fields at the coronal hole boundary via interchange reconnection and closed-field disconnection, respectively, which led to the observed CME configuration. 相似文献
20.
A. W. James L. M. Green E. Palmerio G. Valori H. A. S. Reid D. Baker D. H. Brooks L. van Driel-Gesztelyi E. K. J. Kilpua 《Solar physics》2017,292(5):71
Coronal mass ejections (CMEs) are one of the primary manifestations of solar activity and can drive severe space weather effects. Therefore, it is vital to work towards being able to predict their occurrence. However, many aspects of CME formation and eruption remain unclear, including whether magnetic flux ropes are present before the onset of eruption and the key mechanisms that cause CMEs to occur. In this work, the pre-eruptive coronal configuration of an active region that produced an interplanetary CME with a clear magnetic flux rope structure at 1 AU is studied. A forward-S sigmoid appears in extreme-ultraviolet (EUV) data two hours before the onset of the eruption (SOL2012-06-14), which is interpreted as a signature of a right-handed flux rope that formed prior to the eruption. Flare ribbons and EUV dimmings are used to infer the locations of the flux rope footpoints. These locations, together with observations of the global magnetic flux distribution, indicate that an interaction between newly emerged magnetic flux and pre-existing sunspot field in the days prior to the eruption may have enabled the coronal flux rope to form via tether-cutting-like reconnection. Composition analysis suggests that the flux rope had a coronal plasma composition, supporting our interpretation that the flux rope formed via magnetic reconnection in the corona. Once formed, the flux rope remained stable for two hours before erupting as a CME. 相似文献