Coupling of small-and large-scale filament eruptions

We present observations of the eruption of a large-scale quiescent filament(LF) that is associated with the formation and eruption of a miniature filament(MF). As a result of convergence and subsequent cancelation of opposite-polarity magnetic flux, MF was formed just below the spine of the LF's right segment. Probably triggered by a nearby newly emerging flux, MF underwent a failed eruption immediately after its full development, which first ejected away from the spine of LF and then drained back to the Sun.This eruption no sooner started than the overlying LF's right segment began to rise slowly and the LF's other parts were also disturbed, and eventually the whole LF erupted bodily and quickly. These observations suggest that the MF can serve as an intermediary that links the photospheric small-scale magnetic-field activities to the eruption of the overlying large filament. It appears that, rather than directly interacting with the supporting magnetic field of LF, small-scale flux cancelation and emergence in the LF's channel can manifest themselves as the formation and eruption of MF and so indirectly affect the stability of LF.     

The two types of flare-associated filament eruptions

Using 15 years of high-resolution solar film obtained at Big Bear Solar Observatory we studied flare-associated filament eruptions. In addition to the classical type eruption consisting of expansion and breakup, we find evidence of another type where a layer is shed from the filament and erupts while the inversion line filament below (or, what is left of it) remains in place. Both types of eruptions are presented in the paper. It is hoped that the new evidence will shed new light on the understanding of the role of filaments in flares.     

A circuit model for filament eruptions and two-ribbon flares

We derive a circuit model for solar filament eruptions and two-ribbon flares which reproduces the slow energy build up and eruption of the filament, and the energy dissipation in a current sheet at the top of post-flare loops during the two-ribbon flare. In our model the free magnetic energy is concentrated in a current through the filament, another current through an underlying current sheet, and surface return currents. The magnetic field configuration, generated by these currents and a general photospheric background field, has a topology similar to the field topology derived from observations.We consider two circuits, that of the filament and its return current, and that of the current sheet and its return current. These circuits are inductively coupled and free energy stored in the filament in the pre-flare phase is found to be transferred to the sheet during the impulsive phase, and rapidly dissipated there. A comparable amount of magnetic energy is converted into kinetic energy of the ejected filament. The basic equations of the model are the momentum equations for the filament and the current sheet, and the induction equations for the filament and sheet circuits. The derivation of the equations is an extension of previous models by Kuperus and Raadu, Van Tend and Kuperus, Syrovatskii, and Kaastra. The set of equations is closed in the sense that only the initial conditions and a number of parameters, all related to pre-flare observables, are needed to calculate the evolution of the system. The pre-flare observations we need to determine these parameters, are: (1) a magnetogram, (2) an picture, (3) a measurement of the coronal density in the region, and (4) estimates of the photospheric velocity fields in the region.In the solutions for the evolution of the filament current sheet system we distinghuish 4 phases: (1) a slow energy build up, lasting for almost two days, during which the filament evolves quasi-statically, (2) a metastable state, lasting for about three hours, during which the filament is susceptible to flare triggers, and during which a current sheet emerges, (3) the eruptive phase, with strong acceleration of the filament, during which a large current is induced and dissipated in the current sheet, and energy is injected in the post-flare loops, and finally (4) a post-flare phase, in which the filament acceleration declines and the current sheet vanishes.From further numerical work we derive the following conclusions: (1) The magnetic flux input into the filament circuit has to surpass a certain threshold for an eruption to occur. Below that threshold we find solutions representing quiescent filaments. (2)Flare triggers are neither necessary nor sufficient for an eruption, but may set off the eruption during the metastable state. (3) The model reproduces the increase in shear in the filament prior to the eruption, through adecline of the filament current, in contrast to most models for filament eruptions. (4) The ratio of energy lost as kinetic energy of ejecta to the energy radiated away in the post-flare loops is sensitively dependent on the resistance of the current sheet. (5) Flare prediction is possible with this model, but the potential for triggering during the metastable state complicates the prediction of the exact moment of eruption.Former NAS/NRC Resident Research Associate.ST Systems Corporation.     

Magnetic interactions during sympathetic solar eruptions

We present the first evidence for occurrences of magnetic interactions between a jet, a filament and coronal loops during a complex event, in which two flares sequen-tially occurred at different positions of the same active region and were closely associated with two successive coronal mass ejections (CMEs), respectively. The coronal loops were located outside but nearby the filament channel before the flares. The jet, originating from the first flare during its rise phase, not only hit the filament body but also met one of the ends of the loops. The filament then underwent an inclined eruption followed by the second flare and met the same loop end once more. Both the jet and the filament erup- tion were accompanied by the development of loop disturbances and the appearances of brightenings around the meeting site. In particular, the erupting filament showed clear manifestations of interactions with the loops. After a short holdup, only its portion passed through this site, while the other portion remained at the same place. Following the fila-ment eruption and the loop disappearance, four dimmings were formed and located near their four ends. This is a situation that we define as "quadrupolar dimmings." It appears that the two flares consisted of a sympathetic pair physically linked by the interaction between the jet and the filament, and their sympathy indicated that of the two CMEs.Moreover, it is very likely that the two sympathetic CMEs were simultaneously associ-ated with the disappearing loops and the quadrupole dimmings.     

Magnetic polarimetric refraction in the solar corona

We present expressions which describe the angular displacement of radio sources due to refraction in a magnetized plasma. The main objective of the present paper is to take into account the combined effect of gradients of the electron density and the magnetic field. We use the geometrical optics approximation for the determination of the angular broadening of the radiation. The expressions obtained are applied to the case of the solar corona.     

Magnetic helicity in stellar dynamos: new numerical experiments

The theory of large scale dynamos is reviewed with particular emphasis on the magnetic helicity constraint in the presence of closed and open boundaries. In the presence of closed or periodic boundaries, helical dynamos respond to the helicity constraint by developing small scale separation in the kinematic regime, and by showing long time scales in the nonlinear regime where the scale separation has grown to the maximum possible value. A resistively limited evolution towards saturation is also found at intermediate scales before the largest scale of the system is reached. Larger aspect ratios can give rise to different structures of the mean field which are obtained at early times, but the final saturation field strength is still decreasing with decreasing resistivity. In the presence of shear, cyclic magnetic fields are found whose period is increasing with decreasing resistivity, but the saturation energy of the mean field is in strong super‐equipartition with the turbulent energy. It is shown that artificially induced losses of small scale field of opposite sign of magnetic helicity as the large scale field can, at least in principle, accelerate the production of large scale (poloidal) field. Based on mean field models with an outer potential field boundary condition in spherical geometry, we verify that the sign of the magnetic helicity flux from the large scale field agrees with the sign of α. For solar parameters, typical magnetic helicity fluxes lie around 10⁴⁷ Mx² per cycle.     

Martens-Kuin models of normal and inverse polarity filament eruptions and coronal mass ejections

An analysis is made of the Martens-Kuin filament eruption model in relation to observations of coronal mass ejections (CMEs). The field lines of this model are plotted in the vacuum or infinite resistivity approximation with two background fields. The first is the dipole background field of the model and the second is the potential streamer model of Low. The assumption is made that magnetic field evolution dominates compression or other effects which is appropriate for a low- coronal plasma. The Martens-Kuin model predicts that, as the filament erupts, the overlying coronal magnetic field lines rise in a manner inconsistent with observations of CMEs associated with eruptive filaments. Initially, the bright arc of a CME broadens in time much more slowly than the dark cavity between it and the filament, whereas in the model they broaden at the same rate or the bright arc broadens more rapidly than the dark cavity, depending on the background field. Thus, this model and, by generalization the whole class of so-called Kuperus-Raadu configurations in which a neutral point occurs below the filament, are of questionable utility for CME modeling. An alternate case is considered in which the directions of currents in the Martens-Kuin model are reversed resulting in a so-called normal polarity configuration of the filament magnetic field. In this case, a neutral line occurs above the current-carrying filament. The background field lines now distort to support the filament and help eject it. While the vacuum field results make this configuration appear very promising, a full two- or more-dimensional MHD simulation is required to properly analyze the dynamics resulting from this configuration.Presently NRC Senior Research Associate at NOAA, Space Environment Laboratory, Boulder, Colorado, U.S.A.At the NASA National Space Data Center.     

CME's,filament eruptions,and sheared low-lying magnetic loops in multipolar magnetic configuration

The finite element method was used to solve a partial differential equation (magnetostatic equation) for multipolar magnetic regions. It is found that the height of magnetic field lines above the magnetic neutral line of a central strong bipolar magnetic field decreases as the field lines' footpoints approach the neutral line and also with increased magnetic shear. Both the electric current density and plasma pressure in the sheared low-lying loops are high. We suggest that the sheared low-lying loops may store the energies of large coronal mass ejections (CMEs) and filament eruptions. In addition, it is found that a lower pressure area exists above the low-lying loops and that it is similar in morphology to a coronal cavity. Above the lower pressure area there is a higher pressure area, which may be the source of CMEs. In this area magnetic shear leads to magnetic reconnection, which may be the cause of high coronal temperature.     

A time series of filament eruptions observed by three eyes from space: from failed to successful eruptions

We present stereoscopic observations of six sequential eruptions of a filament in the active region NOAA 11045 on 2010 Feb 8,with the advantage of the STEREO twin viewpoints in combination with Earth's viewpoint from SOHO instruments and ground-based telescopes.The last one of the six eruptions is a coronal mass ejection,but the others are not.The flare in this successful one is more intense than in the others.Moreover,the velocity of filament material in the successful one is also the largest among them.In...     

Magnetic field evolution during prominence eruptions and two-ribbon flares

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.     

Magnetic reconnection in the corona and the loop prominence phenomenon

Many classes of transient solar phenomena, such as flares, flare sprays, and eruptive prominences, cause major disruptions in the magnetic geometry of the overlying corona. Typically, the results from Skylab indicate that pre-existing closed magnetic loops in the corona are torn open by the force of the disruption. We examine here some of the theoretical consequences to be expected during the extended relaxation phase which must follow such events. This phase is characterized by a gradual reconnection of the outward-distended field lines. In particular, the enhanced coronal expansion which occurs on open field lines just before they reconnect appears adequate to supply the large downward mass fluxes observed in Ha loop prominence systems that form during the post-transient relaxation. In addition, this enhanced flow may produce nonrecurrent high speed streams in the solar wind after such events. Calculations of the relaxation phase for representative field geometries and the resulting flow configurations are described.New address: Los Alamos Scientific Laboratory, Los Alamos, N.M. 87545, U.S.A.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A theory of filament eruptions before the impulsive phase of solar flares

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.     

Magnetic fields and the structure of the solar corona

Several different mathematical methods are described which use the observed line-of-sight component of the photospheric magnetic field to determine the magnetic field of the solar corona in the current-free (or potential-field) approximation. Discussed are (1) a monopole method, (2) a Legendre polynomial expansion assuming knowledge of the radial photospheric magnetic field, (3) a Legendre polynomial expansion obtained from the line-of-sight photospheric field by a least-meansquare technique, (4) solar wind simulation by zero-potential surfaces in the corona, (5) corrections for the missing flux due to magnetograph saturation. We conclude (1) that the field obtained from the monopole method is not consistent with the given magnetic data because of non-local effects produced by monopoles on a curved surface, (2) that the field given by a Legendre polynomial (which is fitted to the measured line-of-sight magnetic field) is a rigorous and self-consistent solution with respect to the available data, (3) that it is necessary to correct for the saturation of the magnetograph (at about 80 G) because fields exceeding 80 G provide significant flux to the coronal field, and (4) that a zero-potential surface at 2.5 solar radii can simulate the effect of the solar wind on the coronal magnetic field.     

Magnetic fields and the structure of the solar corona

During the eclipse of 12 November 1966, the solar corona was photographed at an effective wavelength of 6500 Å with an f/16, 11.1 cm aperture camera. Reduction of the observations yields coronal radiances and polarizations from 1.4 to 3.5 solar radii. Standard techniques are used for the separation of F and K-coronas, determination of coronal electron densities and temperatures, and estimation of the orientation of the major streamers in space.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

An investigation of the large-scale magnetic field variations in the corona prior to and after CME eruptions

The magnetic field changes in the corona at the site of coronal mass ejections (CMEs) have been investigated using the potential field-source surface model. It is shown that a CME is accompanied by the opening of closed field lines that formed the streamer's helmet base prior to the onset of a coronal disturbance. Two to three days after the appearance of the CME, the field configuration at the location of the coronal ejection reverts approximately to the state pre-existing before the generation of the CME. The appearance of small transient open magnetic tubes has been found after eruption of the coronal mass. These magnetic tubes seem to be the analogs for transient coronal holes.Taking into account the results of calculations of the field changes in the neighbourhood of the CME occurrence site, we have suggested a possible mechanism governing the spatio-temporal correlation between some flares and CMEs. Also, a possible mechanism has been proposed for field reconfiguration in the corona, leading to loss of the equilibrium of the magnetic configuration and to the subsequent generation of a CME in the region of coronal streamer chains separating coronal holes with same-polarity magnetic field.     

Magnetic field and current sheets in the corona above active regions

A new method for the calculation of coronal magnetic field is proposed and it is shown to reproduce the EUV features in the corona as observed by Skylab experiments satisfactorily well. One of the remarkable points is that it reproduces the loopy threads in the active region corona and also the large scale field lines connecting active regions. The existence of coronal current is expected wherever the present coronal-current-free model fails to represent the feature. A method of calculating the coronal sheet-current is also developed with the purpose of knowing the shape of the current sheet and the amount of magnetic stress energy stored due the the presence of it by comparing the calculated field configuration with the observed local distortion of the EUV threads. This may be used in pinning down the possible site of the flare and in discussing the flare occurrence in terms of the energy stored there.During the preparation of this work, Poletto et al. (1975) calculated the magnetic field shape in Schmidt's method to compare with the soft X-ray feature obtained by Skylab.     

Magnetic gradient:a natural driver of solar eruptions

It is well-known that a gradient will inevitably drive a flow.For example,a density-gradient may drive a diffusion flow,an electrical potential-gradient may drive an electric current in plasmas,and so on.What flow will be driven when a magnetic-gradient occurs in solar atmospheric plasmas?Considering the ubiquitous distribution of magnetic-gradient in solar plasma loops,this work demonstrates that the magnetic-gradient pumping(MGP) mechanism is valid,even in the partial ionized solar photosphere and chromosphere as well as in the corona.The magnetic gradient drives energetic particle upflows that carry and convey kinetic energy from the underlying atmosphere to move upwards,accumulate around the looptop and increase the temperature and pressure,produce ballooning instabilities,and finally it leads to magnetic reconnections and eruptions around the looptop.This mechanism may explain the formation of the hot cusp-structures that can be observed above flaring loops in most preflare phases;therefore,the magneticgradient should be a natural driver of solar eruptions.Furthermore,we may also apply this mechanism to explain many other astrophysical phenomena,such as the cold of sunspot and the hot above it,the formation of solar plasma jets,type-II spicule,and fast solar wind above coronal holes,and also the fast plasma jets related to white dwarfs,neutron stars and black holes.     

Magnetic reconnection and energy release in the solar corona by Taylor relaxation

The heating of the solar corona by resistive turbulence of coronal magnetic fields is considered. The theory of this process, based on the Taylor-Heyvaerts-Priest hypothesis and a magnetic relaxation equation, is developed. Such an approach allows one to obtain the successive magnetic reconnection configurations and energy balance of the coronal magnetic field in response to prescribed motions of the photospheric footpoints. Two specific models of the coronal magnetic configuration are investigated, namely an array of closely packed flux tubes and a two-dimensional magnetic arcade.