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1.
We present a steady-state model for reconnecting current sheets, which relates the central values of temperature, density and pressure within the sheet to the prescribed external values of these parameters as well as the magnetic field strength and inflow velocity (or reconnection rate). The simplifying feature of our model is the assumption of quasi-one-dimensionality so that only variations across the sheet at the centre of symmetry are considered in detail. The dimensions of the sheet, the spatial profiles and their variation with the prescribed dimensionless parameters are obtained from the model. We also obtain the conditions on the dimensionless parameters for the existence of a steady state. A beta-limitation is discovered, such that steady reconnection is impossible when the plasma beta is too small. Also, the sheet dimensions may be an order of magnitude larger than previously thought. Finally, these general results are applied to the emerging flux model for solar flares. A state of thermal nonequilibrium ensues when the current sheet between the emerging and ambient flux reaches a critical height. The effect of the beta-limitation is to make this critical height decrease with increasing magnetic field strength.Now at A.W.R.E., Aldermaston, Berks., England. 相似文献
2.
Yohkoh observations strongly suggest that electron acceleration in solar flares occurs in magnetic reconnection regions in the corona above the soft X-ray flare loops. Unfortunately, models for particle acceleration in reconnecting current sheets predict electron energy gains in terms of the reconnection electric field and the thickness of the sheet, both of which are extremely difficult to measure. It can be shown, however, that application of Ohm's law in a turbulent current sheet, combined with energy and Maxwell's equations, leads to a formula for the electron energy gain in terms of the flare power output, the magnetic field strength, the plasma density and temperature in the sheet, and its area. Typical flare parameters correspond to electron energies between a few tens of keV and a few MeV. The calculation supports the viewpoint that electrons that generate the continuum gamma-ray and hard X-ray emissions in impulsive solar flares are accelerated in a large-scale turbulent current sheet above the soft X-ray flare loops. 相似文献
3.
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
4.
Ü.D. Göker 《New Astronomy》2012,17(2):130-136
A Lagrangian Remap (LareXd) Code is employed to investigate the shock wave formation in the current sheet of a solar coronal magnetic loop and its effect on the magnetic reconnection. We constructed the slow shock structure in the presence of viscosity and heat conduction parallel and perpendicular to the magnetic field and pairs of slow shocks propagate away from the central current sheet, the so-called diffusion region. Significant jumps in plasma density, pressure, velocity and magnetic field occur across the main shock while the temperature appears in the foreshock. In the presence of dissipative effects, the distinct jumps disappear and the shock profiles show smooth transition between the downstream and the upstream regions while the plasma density and the pressure show very narrow and a sharp decrease with time. These results can be applied to the heating of the solar corona, the structure of the magnetic reconnection and the solar wind. 相似文献
5.
Numerical simulations of the helical (m=1) kink instability of an arched, line-tied flux rope demonstrate that the helical deformation enforces reconnection between
the legs of the rope if modes with two helical turns are dominant as a result of high initial twist in the range Φ≳6π. Such a reconnection is complex, involving also the ambient field. In addition to breaking up the original rope, it can form
a new, low-lying, less twisted flux rope. The new flux rope is pushed downward by the reconnection outflow, which typically
forces it to break as well by reconnecting with the ambient field. The top part of the original rope, largely rooted in the
sources of the ambient flux after the break-up, can fully erupt or be halted at low heights, producing a “failed eruption.”
The helical current sheet associated with the instability is squeezed between the approaching legs, temporarily forming a
double current sheet. The leg – leg reconnection proceeds at a high rate, producing sufficiently strong electric fields that
it would be able to accelerate particles. It may also form plasmoids, or plasmoid-like structures, which trap energetic particles
and propagate out of the reconnection region up to the top of the erupting flux rope along the helical current sheet. The
kinking of a highly twisted flux rope involving leg – leg reconnection can explain key features of an eruptive but partially
occulted solar flare on 18 April 2001, which ejected a relatively compact hard X-ray and microwave source and was associated
with a fast coronal mass ejection. 相似文献
6.
We employ a 2 1/2-dimensional reconnection model to analyse different aspects of the energy release in two-ribbon flares. In particular, we investigate in which way the systematic change of inflow region variables, associated with the vertical elongation of current sheet, affects the flare evolution. It is assumed that as the transversal magnetic field decreases, the ambient plasma-to-magnetic pressure ratio increases, and the reconnection rate diminishes. As the transversal field decreases due to the arcade stretching, the energy release enhances and the temperature rises. Furthermore, the magnetosonic Mach number of the reconnection outflow increases, providing the formation of fast mode standing shocks above the flare loops and below the erupting flux rope. Eventually, in the limit of a very small transversal field the reconnection becomes turbulent due to a highly non-linear response of the system to small fluctuations of the transversal field. The turbulence results in the energy release fragmentation which increases the release efficiency, and is likely to be responsible for the impulsive phase of the flare. On the other hand, as the current sheet stretches to larger heights, the ambient plasma-to-magnetic pressure ratio increases which causes a gradual decrease of the reconnection rate, energy release rate, and temperature in the late phase of flare. The described magnetohydrodynamical changes affect also the electron distribution function in space and time. At large reconnection rates (impulsive phase of the flare) the ratio of the inflow-to-outflow magnetic field strength is much smaller than at lower reconnection rates (late phase of the flare), i.e., the corresponding loss-cone angle becomes narrower. Consequently, in the impulsive phase a larger fraction of energized electrons can escape from the current sheet downwards to the chromosphere and upwards into the corona – the dominant flare features are the foot-point hard X-ray sources and type III radio bursts. On the other hand, at low reconnection rates, more particles stay trapped in the outflow region, and the thermal conduction flux becomes strongly reduced. As a result, a superhot loop-top, and above-the-loop plasma appears, as sometimes observed, to be a dominant feature of the gradual phase. 相似文献
7.
Magnetic reconnection at the photospheric boundary is an essential part of some theories for prominence formation. We consider a simple model for reconnection in this region. Parameters of the reconnecting current sheet are expressed in terms of the concentration and temperature of the outside dense and cold plasma, magnetic field intensity, and velocity of convective flows at the photosphere. The reconnection process is shown to be most efficient in a layer several hundred kilometers thick coinciding with the temperature minimum region of the solar atmosphere. The calculated upward flux of matter through the current sheet ( 1011–1012 g s–1) is amply sufficient for prominence formation in the upper chromosphere or lower corona. 相似文献
8.
A model for solar quiescent prominences nested in a Figure 8 magnetic field topology is developed. This topology is argued to be the natural consequence of the distention of bipolar regions upward into the corona. If this distention is slow enough so that hydrostatic equilibrium holds approximately along the field lines, the transverse gas pressure forces fall exponentially with height whereas the inward Lorentz forces fall as a power law. At a low height in the corona, the pressure forces cannot balance the Lorentz forces provided the field lines remain tied to the photosphere and an inward collapse with subsequent reconnection at the point of closest approach should occur. Because of initial shear in the magnetic field, the reconnection would produce isolated helices above the point of reconnection since field lines would not interact with themselves but with their neighbors. This resulting topology produces a field above the elevated neutral line which is opposite in polarity to that of the photospheric field as in the current sheet models of Kuperus and Tandberg-Hanssen (1967). Raadu and Kuperus (1973), Kuperus and Raadu (1974), and Raadu (1979) and in agreement with recent observations of Leroy (1982), and Leroy et al. (1983).Assuming the isolated helices formed by reconnection are insulated from coronal thermal conduction and heating, the radiative cooling process and condensation is considered for the temperature range of 104-6000 K. This condensation results in a steady downflow to the bottom of the helices as the temperature scale-height falls, thus forming a dense, cool, prominence at the bottom of the helical configuration resting on the elevated neutral line with the remainder of the helix being essentially evacuated of material. We identify this neutral line at the bottom of the prominence with the sharp lower edge often seen when viewing quiescent prominences side-on and the evacuated helix with the coronal cavity observed around prominences when seen during total eclipses.Downflow speeds associated with the condensation process are calculated for prominence temperatures and yield velocities in the range of the observed downflows of about 1 km s–1.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
9.
E. R. Priest 《Solar physics》1976,47(1):41-75
Current sheets have been suggested as the site for flare energy release because they can convert magnetic energy very rapidly into both heat and directed plasma energy. Also they contain electric fields with the potential of accelerating particles to high energies.The basic properties of current sheets are first reviewed. For instance, magnetic flux may be carried into a current sheet and annihilated. An exact solution for such a process in an infinitely long sheet has been found; it describes the annihilation of fields which are inclined at any angle, not just 180°. Moreover, field lines which are expelled from the ends of a current sheet can be described as having been reconnected. The only workable model for fast reconnection in the solar atmosphere, namely Petschek's mechanism, has recently been put on a firm foundation; it gives a reconnection rate which depends on the electrical conductivity but is typically a tenth or a hundredth of the Alfvén speed. A current sheet may be formed when the sources of an initially potential field start to move; a simple analytic technique for finding the position and shape of such a sheet in two dimensions now exists. Finally, a sheet with no transverse magnetic field component is subject to the tearing-mode instability, which rapidly produces a series of loops in the field.The main ways in which current sheets have been used for solar flare models is described. Syrovatskii's mechanism relies on the increase of the electric current density during the formation of a sheet, to a value in excess of the critical value j
* for the onset of microinstabilities. But Anzer has recently demonstrated that the critical value is most unlikely to be reached during the initial formation process. Sturrock, on the other hand, has advocated the occurrence of the tearing-mode instability in an open streamer-like configuration (which may result from the eruption of a force-free field). But recent observations do not point to that as the relevant configuration. Rather, they suggest that flares are triggered by the emergence of new magnetic flux from below the solar photosphere. This has led Heyvaerts, Priest, and Rust (1976) to propose a new emerging flux model, according to which, as more and more flux emerges, so reconnection occurs, producing some preflare heating. When the current sheet reaches such a height (around the transition region) that its current density exceeds j
*, then the impulsive phase of the flare is triggered. The main phase is caused by an enhanced level of magnetic energy conversion in a turbulent current sheet. The type of flare depends on the magnetic environment in which the emerging flux finds itself. A surge flare results if the flux appears near a strong unipolar region such as a simple sunspot, whereas a two ribbon flare may be produced by flux emergence near an active region filament, in which case the main phase energy is released from the field that surrounds the filament. 相似文献
10.
Numerical simulation of magnetic field reconnection at IMF sector boundaries shows that the reconnection line may be carried by the solar wind out of the region of the anomalous resistivity. This makes it possible to observe magnetic loops at the Earth's orbit open to the Sun as well as from it. Besides, it is shown that the current sheet in the vicinity of the reconnection line has to split into two currents.Experimental data on the structure of the sector boundaries are analyzed, and it is shown that the currents at sector boundaries are indeed often splitted.The thickness of the splitted boundaries may amount to 18×106 km; taking into account this value, the heliocentric distance of the region of anomalous resistivity in the interplanetary current sheet is estimated as 0.4–0.5 AU.The probability of observing magnetic loops open towards the Sun seems to be greater than that of loops open from the Sun, which suggests an essential asymmetry of the field reversal regions. 相似文献
11.
Simple models for the MHD eruption of a solar prominence are presented, in which the prominence is treated as a twisted magnetic flux tube that is being repelled from the solar surface by magnetic pressure forces. The effects of different physical assumptions to deal with this magneto-hydrodynamically complex phenomenon are evaluated, such as holding constant the prominence current, radius, flux or twist or modelling the prominence as a current sheet. Including a background magnetic field allows the prominence to be in equilibrium initially with an Inverse Polarity and then to erupt due to magnetic non-equilibrium when the background magnetic field is too small or the prominence twist is too great. The electric field at the neutral point below the prominence rapidly increases to a maximum value and then declines. Including the effect of gravity also allows an equilibrium with Normal Polarity to exist. Finally, an ideal MHD solution is found which incorporates self-consistently a current sheet below the prominence and which implies that a prominence will still erupt and form a current sheet even if no reconnection occurs. When reconnection is allowed it is, therefore, driven by the eruption. 相似文献
12.
From observations of two-ribbon solar flares, we present a new line of evidence that magnetic reconnection is of key importance in magnetospheric substorms. We infer that in substorms reconnection of closed field lines in the near-Earth thinned plasma sheet both initiates and is driven by the overall MHD instability that drives the tailward expulsion of the reconnected closed field (0 loops). The general basis for this inference is the longstanding notion that two-ribbon flares and substorms are essentially similar phenomena, driven by similar processes. We give an array of observed similarities that substantiate this view. More specifically, our inference for substorms is drawn from observations of filament eruptions in two-ribbon flares, from which we conclude that the heart of the overall instability consists of reconnection and eruption of the closed magnetic field in and around the filament. We propose that essentially the same overall instability operates in substorms. Our point is not that the magnetic field configuration or the microphysics in substorms is identical to that in two-ribbon flares, but that the overall instability results from essentially the same combination of reconnection and eruption of closed magnetic field. 相似文献
13.
Interaction of weak shock waves with a current sheet is investigated by a two-dimensional numerical magnetohydrodynamic model. In accordance with solar coronal conditions, a ratio of thermal to magnetic pressures of 0.1 and a shock Alfvén Mach number slightly above 1 are considered. It is found that even weak shock waves trigger magnetic field reconnection in current sheets. Based on this result, it is suggested that drifting chains of type I radio bursts are radio manifestations of the interactions of weakly super-Alfvénic shock waves with pre-existing current sheets distributed in an active region. This model of type I noise storms is then discussed in connection with the concept of nanoflares (localized reconnections) and the heating of the solar corona. 相似文献
14.
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
15.
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
16.
We outline a method to determine the direction of solar open flux transport that results from the opening of magnetic clouds
(MCs) by interchange reconnection at the Sun based solely on in-situ observations. This method uses established findings about i) the locations and magnetic polarities of emerging MC footpoints, ii) the hemispheric dependence of the helicity of MCs, and iii) the occurrence of interchange reconnection at the Sun being signaled by uni-directional suprathermal electrons inside MCs.
Combining those observational facts in a statistical analysis of MCs during solar cycle 23 (period 1995 – 2007), we show that
the time of disappearance of the northern polar coronal hole (1998 – 1999), permeated by an outward-pointing magnetic field,
is associated with a peak in the number of MCs originating from the northern hemisphere and connected to the Sun by outward-pointing
magnetic field lines. A similar peak is observed in the number of MCs originating from the southern hemisphere and connected
to the Sun by inward-pointing magnetic field lines. This pattern is interpreted as the result of interchange reconnection
occurring between MCs and the open field lines of nearby polar coronal holes. This reconnection process closes down polar
coronal hole open field lines and transports these open field lines equatorward, thus contributing to the global coronal magnetic
field reversal process. These results will be further constrainable with the rising phase of solar cycle 24. 相似文献
17.
We present a theory of filament eruption before the impulsive phase of solar flares. We show that the upward motion of the magnetic X-point tracing the filament eruption begins several minutes before the impulsive phase of the flare, where the explosive magnetic reconnection starts at the X-point magnetic field configuration located under the filament. No change occurs in the character of the motion of the X-point during the onset of the explosive magnetic reconnection. The upward speed of the X-point is about 110 km s-1 at the onset of the impulsive phase. We give an important condition leading to filament eruptions, which relate to the state of the current sheet under the filament, where the magnetic energy can be released. 相似文献
18.
Two kinematic models of line-tied reconnection are considered which describe the motion of a magnetic neutral line (NL) during the main phase of a two-ribbon solar flare and during the recovery phase of a magnetospheric substorm in the geomagnetic tail. The models are kinematic in that they use only the magnetic induction equation, which suffices to determine the position and velocity of the NL as functions of time if the rate of reconnection is prescribed. The solar flare model shows that the observed large decrease in the rate at which “post”-flare loops rise upward from the photosphere during the main phase does not require a corresponding decrease in the rate of reconnection. Instead it is found that a constant rate of reconnection can account for the motion of the loops for almost the entire period during which they are observed. By contrast, application of the same procedures to the recovery phase of the magnetospheric substorm in the tail predicts a slightly increasing speed of NL motion if the rate of reconnection is constant. Furthermore, it is found that the motion of the NL relative to the ambient medium may account for much of the observed asymmetry in the magnetic field in the plasma sheet during recovery. Due to this motion, the plasma sheet thickness may be up to 4 times smaller and the normal magnetic field component up to 2 times weaker in the region tailward of the NL than in the corresponding region earthward of the NL. 相似文献
19.
Traditional models for particle acceleration by magnetic reconnection in solar flares assumed a constant electric field in
a steady reconnecting magnetic field. Although this assumption may be justified during the gradual phase of flares, the situation
is different during the impulsive phase. Observed rapid variations in flare emissions imply that reconnection is non-steady
and a time-varying electric field is present in a reconnecting current sheet. This paper describes exploratory calculations
of charged particle orbits in an oscillating electric field present either at a neutral plane or a neutral line of two-dimensional
magnetic field. A simple analytical model makes it possible to explain the effects of particle trapping and resonant acceleration
previously noted by Petkaki and MacKinnon in a numerical simulation. As an application, electron acceleration to X-ray generating
energies in impulsive solar flares is discussed within the context of the model. 相似文献
20.
Characteristic times for heating and cooling of the thermal X-ray plasma in solar flares are estimated from the time profile of the thermal X-ray burst and from the temperature, emission measure and over-all length scale of the flare-heated plasma at thermal X-ray maximum. The heating is assumed to be due to magnetic field reconnection, and the cooling is assumed to be due to heat conduction and radiation. Temperatures and emission measures derived from UCSD OSO-7 X-ray flare observations are used, and length scales are obtained from Big Bear large-scale Hα filtergrams for 17 small (subflare to Class 1) flares. The empirical values obtained for the characteristic times imply (1) that flares are produced by magnetic field reconnection, (2) that conduction cooling of the thermal X-ray plasma dominates radiative cooling and (3) that reconnection heating and conduction cooling of the thermal X-ray plasma are approximately in balance at thermal X-ray maximum. This model in combination with the data gives estimates for the electron number density (1010–1011 cm?3) and the magnetic field strength (10–100 G) in the thermal X-ray plasma and for the total thermal energy generated in a subflare (≈ 1030 erg for an Hα area ≈ 1 square degree) which agree with previous observational and theoretical estimates obtained by others. 相似文献