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1.
P. Démoulin K.-L. Klein C. P. Goff L. van Driel-Gesztelyi J. L. Culhane C. H. Mandrini S. A. Matthews L. K. Harra 《Solar physics》2007,240(2):301-313
Radio emissions of electron beams in the solar corona and interplanetary space are tracers of the underlying magnetic configuration
and of its evolution. We analyse radio observations from the Culgoora and WIND/WAVES spectrographs, in combination with SOHO/LASCO
and SOHO/MDI data, to understand the origin of a type N burst originating from NOAA AR 10540 on January 20, 2004, and its
relationship with type II and type III emissions. All bursts are related to the flares and the CME analysed in a previous
paper (Goff et al., 2007). A very unusual feature of this event was a decametric type N burst, where a type III-like burst, drifting towards
low frequencies (negative drift), changes drift first to positive, then again to negative. At metre wavelengths, i.e., heliocentric distances ≲1.5R
⊙, these bursts are ascribed to electron beams bouncing in a closed loop. Neither U nor N bursts are expected at decametric
wavelengths because closed quasi-static loops are not thought to extend to distances ≫1.5R
⊙. We take the opportunity of the good multi-instrument coverage of this event to analyse the origin of type N bursts in the
high corona. Reconnection of the expanding ejecta with the magnetic structure of a previous CME, launched about 8 hours earlier,
injects electrons in the same manner as with type III bursts but into open field lines having a local dip and apex. The latter
shape was created by magnetic reconnection between the expanding CME and neighbouring (open) streamer field lines. This particular
flux tube shape in the high corona, between 5R
⊙ and 10R
⊙, explains the observed type N burst. Since the required magnetic configuration is only a transient phenomenon formed by reconnection,
severe timing and topological constraints are present to form the observed decametric N burst. They are therefore expected
to be rare features. 相似文献
2.
Dieter Biskamp 《Astrophysics and Space Science》1996,242(1-2):165-207
A review of the present status of the theory of magnetic reconnection is given. In strongly collisional plasmas reconnection proceeds via resistive current sheets, i.e. quasi-stationary macroscopic Sweet-Parker sheets at intermediate values of the magnetic Reynolds numberR
m
, or mirco-current sheets in MHD turbulence, which develops at highR
m
. In hot, dilute plasmas the reconnection dynamics is dominated by nondissipative effects, mainly the Hall term and electron inertia. Reconnection rates are found to depend only on the ion mass, being independent of the electron inertia and the residual dissipation coefficients. Small-scale whistler turbulence is readily excited giving rise to an anomalous electron viscosity. Hence reconnection may be much more rapid than predicted by conventional resistive theory. 相似文献
3.
In this paper, we present a study of the correlation between the speed of flare ribbon separation and the magnetic flux density
during the 10 April 2001 solar flare. The study includes the section of the neutral line containing the flare core and its
peripheral area. This event shows clear two-ribbon structure and inhomogeneous magnetic fields along the ribbons, so the spatial
correlation and distribution of the flare and magnetic parameters can be studied. A weak negative correlation is found between
the ribbon separation speed (V
r) and the longitudinal magnetic flux density (B
z
). This correlation is the weakest around the peak of the flare. Spatially, the correlation is also weakest at the positions
of the hard X-ray (HXR) sources. In addition, we estimate the magnetic reconnection rate (electric field strength in the reconnection
region E
rec) by combining the speed of flare ribbons and the longitudinal magnetic flux density. During flare evolution, the time profiles
of the magnetic reconnection rate are similar to that of the ribbon separation speed, and the speeds of ribbon separation
are relatively slow in the strong magnetic fields (i.e., V
r is negatively correlated with B
z
). However, along the flare ribbons, E
rec fluctuates in a small range except near the HXR source. A localized enhancement of the reconnection rate corresponds to the
position of the HXR source. 相似文献
4.
Magnetic reconnection can take place between two plasma regions with antiparallel magnetic field components. In a time-dependent reconnection event, the plasma outflow region consists of a leading bulge region and a trailing reconnection layer. Magnetohydrodynamic (MHD) discontinuities, including rotational discontinuities, can be formed in both the bulge region and the trailing layer. In this paper, we suggest that the rotational discontinuities observed in the solar wind may be generated by magnetic reconnection associated with microflares in coronal holes. The structure of the reconnection layer is studied by solving the one-dimensional Riemann problem for the evolution of an initial current sheet after the onset of magnetic reconnection as well as carrying out two-dimensional MHD simulations. As the emerging magnetic flux reconnects with ambient open magnetic fields in the coronal hole, rotational discontinuities are generated in the region with open field lines. It is also found that in the solar corona with a low plasma beta ( 0.01), the magnetic energy is converted through magnetic reconnection mostly into the plasma bulk-flow energy. Since more microflares will generate more rotational discontinuities and also supply more energy to the solar wind, it is expected that the number of rotational discontinuities observed in the solar wind would be an increasing function of solar wind speed. The observation rate of rotational discontinuities generated by microflares is estimated to be dN
RD/dt - f/63 000 s (f > 1) at 1 AU. The present mechanism favors the generation of rotational discontinuities with a large shock normal angle. 相似文献
5.
The concepts of magnetic reconnection that have been developed in two dimensions need to be generalised to three-dimensional configurations. Reconnection may be defined to occur when there is an electric field (E) parallel to field lines (known as potential singular lines) which are potential reconnection locations and near which the field has an X-type topology in a plane normal to that field line. In general there is a continuum of neighbouring potential singular lines, and which one supports reconnection depends on the imposed flow or electric field. For steady reconnection the nearby flow and electric field are severely constrained in the ideal region by the condition that E = 0 there. Potential singular lines may occur in twisted prominence fields or in the complex magnetic configuration above sources of mixed polarity of an active region or a supergranulation cell. When reconnection occurs there is dynamic MHD behaviour with current concentration and strong plasma jetting along the singular line and the singular surfaces which map onto them. 相似文献
6.
A. Hillaris O. Malandraki K.-L. Klein P. Preka-Papadema X. Moussas C. Bouratzis E. Mitsakou P. Tsitsipis A. Kontogeorgos 《Solar physics》2011,273(2):493-509
On 17 January 2005 two fast coronal mass ejections were recorded in close succession during two distinct episodes of a 3B/X3.8
flare. Both were accompanied by metre-to-kilometre type-III groups tracing energetic electrons that escape into the interplanetary
space and by decametre-to-hectometre type-II bursts attributed to CME-driven shock waves. A peculiar type-III burst group
was observed below 600 kHz 1.5 hours after the second type-III group. It occurred without any simultaneous activity at higher
frequencies, around the time when the two CMEs were expected to interact. We associate this emission with the interaction
of the CMEs at heliocentric distances of about 25 R
⊙. Near-relativistic electrons observed by the EPAM experiment onboard ACE near 1 AU revealed successive particle releases
that can be associated with the two flare/CME events and the low-frequency type-III burst at the time of CME interaction.
We compare the pros and cons of shock acceleration and acceleration in the course of magnetic reconnection for the escaping
electron beams revealed by the type-III bursts and for the electrons measured in situ. 相似文献
7.
Lavraud B. Gosling J. T. Rouillard A. P. Fedorov A. Opitz A. Sauvaud J.-A. Foullon C. Dandouras I. Génot V. Jacquey C. Louarn P. Mazelle C. Penou E. Phan T. D. Larson D. E. Luhmann J. G. Schroeder P. Skoug R. M. Steinberg J. T. Russell C. T. 《Solar physics》2009,256(1-2):379-392
We analyze Wind, ACE, and STEREO (ST-A and ST-B) plasma and magnetic field data in the vicinity of the heliospheric current sheet (HCS) crossed by all spacecraft between 22:15 UT on 31 March and 01:25 UT on 1 April 2007 corresponding to its observation at ST-A and ST-B, which were separated by over 1800 R E (or over 1200 R E across the Sun?–?Earth line). Although only Wind and ACE provided good ion flow data in accord with a solar wind magnetic reconnection exhaust at the HCS, the magnetic field bifurcation typical of such exhausts was clearly observed at all spacecraft. They also all observed unambiguous strahl mixing within the exhaust, consistent with the sunward flow deflection observed at Wind and ACE and thus with the formation of closed magnetic field lines within the exhaust with both ends attached to the Sun. The strong dawnward flow deflection in the exhaust is consistent with the exhaust and X-line orientations obtained from minimum variance analysis at each spacecraft so that the X-line is almost along the GSE Z-axis and duskward of all the spacecraft. The observation of strahl mixing in extended and intermittent layers outside the exhaust by ST-A and ST-B is consistent with the formation of electron separatrix layers surrounding the exhaust. This event also provides further evidence that balanced parallel and antiparallel suprathermal electron fluxes are not a necessary condition for identification of closed field lines in the solar wind. In the present case the origin of the imbalance simply is the mixing of strahls of substantially different strengths from a different solar source each side of the HCS. The inferred exhaust orientations and distances of each spacecraft relative to the X-line show that the exhaust was likely nonplanar, following the Parker spiral orientation. Finally, the separatrix layers and exhausts properties at each spacecraft suggest that the magnetic reconnection X-line location and/or reconnection rate were variable in both space and time at such large scales.
相似文献8.
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
9.
The resistive MHD equations are numerically solved in two dimensions for an initial-boundary-value problem which simulates reconnection between an emerging magnetic flux region and an overlying coronal magnetic field. The emerging region is modelled by a cylindrical flux tube with a poloidal magnetic field lying in the same plane as the external, coronal field. The plasma betas of the emerging and coronal regions are 1.0 and 0.1, respectively, and the magnetic Reynolds number for the system is 2 × 103. At the beginning of the simulation the tube starts to emerge through the base of the rectangular computational domain, and, when the tube is halfway into the computational domain, its position is held fixed so that no more flux of plasma enters through the base. Because the time-scale of the emergence is slower than the Alfvén time-scale, but faster than the reconnection time-scale, a region of closed loops forms at the base. These loops are gradually opened and reconnected with the overlying, external magnetic field as time proceeds.The evolution of the plasma can be divided into four phases as follows: First, an initial, quasi-steady phase during which most of the emergence is completed. During this phase, reconnection initially occurs at the slow rate predicted by the Sweet model of diffusive reconnection, but increases steadily until the fast rate predicted by the Petschek model of slow-shock reconnection is approached. Second, an impulsive phase with large-scale, super-magnetosonic flows. This phase appears to be triggered when the internal mechanical equilibrium inside the emerging flux tube is upset by reconnection acting on the outer layers of the flux tube. During the impulsive phase most of the flux tube pinches off from the base to form a cylindrical magnetic island, and temporarily the reconnection rate exceeds the steady-state Petschek rate. (At the time of the peak reconnection rate, the diffusion region at the X-line is not fully resolved, and so this may be a numerical artifact.) Third, a second quasi-steady phase during which the magnetic island created in the impulsive phase is slowly dissipated by continuing, but low-level, reconnection. And fourth, a static, non-evolving phase containing a potential, current-free field and virtually no flow.During the short time in the impulsive phase when the reconnection rate exceeds the steady-state Petschek rate, a pile-up of magnetic flux at the neutral line occurs. At the same time the existing Petschek-slow-mode shocks are shed and replaced by new ones; and, for a while, both new and old sets of slow shocks coexist. 相似文献
10.
We analyze multiple-wavelength observations of a two-ribbon flare exhibiting apparent expansion motion of the flare ribbons
in the lower atmosphere and rising motion of X-ray emission at the top of newly-formed flare loops. We evaluate magnetic reconnection
rate in terms of V
r
B
r by measuring the ribbon-expansion velocity (V
r) and the chromospheric magnetic field (B
r) swept by the ribbons. We also measure the velocity (V
t) of the apparent rising motion of the loop-top X-ray source, and estimate the mean magnetic field (B
t) at the top of newly-formed flare loops using the relation 〈V
t
B
t〉≈〈V
r
B
r〉, namely, conservation of reconnection flux along flare loops. For this flare, B
t is found to be 120 and 60 G, respectively, during two emission peaks five minutes apart in the impulsive phase. An estimate
of the magnetic field in flare loops is also achieved by analyzing the microwave and hard X-ray spectral observations, yielding
B=250 and 120 G at the two emission peaks, respectively. The measured B from the microwave spectrum is an appropriately-weighted value of magnetic field from the loop top to the loop leg. Therefore,
the two methods to evaluate coronal magnetic field in flaring loops produce fully-consistent results in this event. 相似文献
11.
We employ Mariner 10 measurements of the interplanetary magnetic field in the vicinity of Mercury to estimate the rate of magnetic reconnection between the interplanetary magnetic field and the Hermean magnetosphere. We derive a time-series of the open magnetic flux in Mercury's magnetosphere, from which we can deduce the length of the magnetotail. The length of the magnetotail is shown to be highly variable, with open field lines stretching between 15RH and 850RH downstream of the planet (median 150RH). Scaling laws allow the tail length at perihelion to be deduced from the aphelion Mariner 10 observations. 相似文献
12.
A model for a solar flare, involving magnetic reconnection transferring flux and current between current-carrying magnetic
loops connecting two pairs of footpoints, is generalized to include conservation of magnetic helicity during reconnection,
as well as conservation of current at all four footpoints. For a set of force-free loops, with the ith loop having flux F
i and current I
i, the self and mutual helicities are proportional to the self and mutual inductances with the constant of proportionality
determined by αi=F
i/μ0
I
i. In a constant-α model, the change in magnetic energy is proportional to the change in helicity, and conservation of helicity
implies conservation of magnetic energy, so that a flare cannot occur. In a quadrupolar model, with α1>α2 initially, α1 increases and α2 decreases when flux and current are transferred from loops 1 and 2 to loops 3 and 4. A model that conserves both current
and helicity is constructed; it depends on the initial αs, and otherwise is somewhat simpler than when helicity is neglected. 相似文献
13.
Two-dimensional (2D) compressible magnetohydrodynamic simulations are performed to explore the idea that the asymmetric reconnection between newly emerging intranetwork magnetic field flux and pre-existing network flux causes the explosive events in the solar atmosphere. The dependence of the reconnection rate as a function of time on the density and temperature of the emerging flux are investigated. For a Lundquist number of L
u= 5000 we find that the tearing mode instability can lead to the formation and growth of small magnetic islands. Depending on the temperature and density ratio of the emerging plasma, the magnetic island can be lifted upward and convected out of the top boundary, or is suppressed downward and convected out of the top boundary, or is suppressed downward nad submerged below the bottom boundary. The motions of the magnetic islands with different direction are accompanied respectively with upward or downward high velocity flow which might be associated with the red- and blue-shifted components detected in the explosive events. 相似文献
14.
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
15.
Flux pile-up magnetic merging solutions are discussed using the simple robust arguments of traditional steady-state reconnection theory. These arguments determine a unique scaling for the field strength and thickness of the current layer, namely B
s–1/3, l2/3, which are consistent with a variety of plasma inflow conditions. Next we demonstrate that flux pile-up merging can also be understood in terms of exact magnetic annihilation solutions. Although simple annihilation models cannot provide unique reconnection scalings, we show that the previous current sheet scalings derive from an optimized solution in which the peak dynamic and magnetic pressures balance in the reconnection region. The build-up of magnetic field in the current sheet implicit in flux pile-up solutions naturally leads to the idea of saturation. Hydromagnetic pressure effects limit the magnetic field in the sheet, yielding an upper limit on the reconnection rate for such solutions. This rate is still far superior to the Sweet–Parker merging rate, which can be derived by seeking solutions that avoid all forms of saturation. Finally we compare time dependent numerical simulations of the coalescence instability with the optimized flux pile-up models. This comparison suggests that merging driven by the relatively slow approach of large flux systems may be favored in practice. 相似文献
16.
We present results from a global simulation of the interaction of the solar wind with Earth's magnetosphere, ionosphere, and
thermosphere for the Bastille Day geomagnetic storm and compare the results with data. We find that during this event the
magnetosphere becomes extremely compressed and eroded, causing 3 geosynchronous GOES satellites to enter the magnetosheath
for an extended time period. At its extreme, the magnetopause moves at local noon as close as 4.9 R
E to Earth which is interpreted as the consequence of the combined action of enhanced dynamic pressure and strong dayside reconnection
due to the strong southward interplanetary magnetic field component B
z, which at one time reaches a value of −60 nT. The lobes bulge sunward and shield the dayside reconnection region, thereby
limiting the reconnection rate and thus the cross polar cap potential. Modeled ground magnetic perturbations are compared
with data from 37 sub-auroral, auroral, and polar cap magnetometer stations. While the model can not yet predict the perturbations
and fluctuations at individual ground stations, its predictions of the fluctuation spectrum in the 0–3 mHz range for the sub-auroral
and high-latitude regions are remarkably good. However, at auroral latitudes (63° to 70° magnetic latitude) the predicted
fluctuations are slightly too high.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014228230714 相似文献
17.
Transition-region explosive events (TREEs) have long been proposed as a consequence of magnetic reconnection. However, several
critical issues have not been well addressed, such as the location of the reconnection site, their unusually short lifetime
(about one minute), and the recently discovered repetitive behaviour with a period of three to five minutes. In this paper,
we perform MHD numerical simulations of magnetic reconnection, where the effect of five-minute solar p-mode oscillations is examined. UV emission lines are synthesised on the basis of numerical results in order to compare with
observations directly. It is found that several typical and puzzling features of the TREEs with impulsive bursty behaviour
can only be explained if there exist p-mode oscillations and the reconnection site is located in the upper chromosphere at a height range of around 1900 km < h < 2150 km above the solar surface. Furthermore, the lack of proper motions of the high-velocity ejection may be due to a
rapid change of temperature along the reconnection ejecta. 相似文献
18.
In this paper, the Space–Time Conservation Element and Solution Element (CESE) method is applied to 2.5-dimensional resistive
magnetohydrodynamics (MHD) equations in Cartesian coordinates, with the purpose of modeling the magnetic reconnection study.
To show the validity and capacity of its application to MHD reconnection problem, spontaneous fast reconnection and magnetic
reconnection in multiple heliospheric current sheets are studied, which show good consistency with those obtained formerly
by other authors. In order to assess the ∇ ⋅ B = 0 constraint numerically, the contours and evolution of ∇ ⋅ B are analyzed. The numerical results tell us that the CESE numerical scheme not only has good numerical resolution but also
can keep the divergence-free condition for magnetic fields in the reconnection problems during the evolutionary process without
any special treatment. 相似文献
19.
Using a 2-D MHD model, the magnetic field reconnection in the current sheet and corresponding plasma resonance lines (surfaces in 3-D), where the upper-hybrid frequency equals one of harmonics of the electron gyrofrequency, UH=(pe
2+Be
2)1/2=sBe (UH, pe, and Be are the upper hybrid, electron plasma, and cyclotron frequencies, respectively, and s is the integer harmonic number) are computed. Then at selected times and positions in the magnetic reconnection the spatial and time spectra of upper hybrid frequencies along the resonance lines are calculated. These spectra are discussed from the point of view of radio fine structures as narrowband dm-spikes, zebras, and lace bursts. It is shown that not only turbulent plasma outflows, suggested in the paper by Bárta and Karlický (2001), but also perturbed zones near the reconnection slow-mode shocks can be locations of the narrowband dm-spikes (and/or continua). Sources of the lace bursts (i.e. bursts with irregular lines) can be located in the reconnection space, too. On the other hand, the zebras (bursts with regular separations of zebra lines) need to be generated out of strongly perturbed reconnection areas. 相似文献
20.
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