Photospheric Polarity Inversion Lines Research Articles

Prominence fine structures in weakly twisted and highly twisted magnetic flux ropes

Context. Many prominences are supported by magnetic flux ropes. One important question is how we can determine whether the flux rope is weakly twisted or highly twisted. Aims. In this paper, we attempt to decipher whether prominences supported by weakly twisted and highly twisted flux ropes can manifest different features so that we might distinguish the two types of magnetic structures based on their appearance. Methods. We performed pseudo three-dimensional simulations of two magnetic flux ropes with different twists. Results. We find that the resulting two prominences differ in many aspects. The prominence supported by a weakly twisted flux rope is composed mainly of transient threads (∼82.8%), forming high-speed flows inside the prominence, and its horns are evident. Conversely, the prominence supported by a highly twisted flux rope consists mainly of stable quasi-stationary threads (∼60.6%), including longer independently trapped threads and shorter magnetically connected threads. Our simulations also reveal that the prominence spine deviates from the flux rope axis in the vertical direction and from the photospheric polarity inversion line projected on the solar surface, especially for the weakly twisted magnetic flux rope. Conclusions. The two types of prominences differ significantly in appearance. Our results also suggest that a piling-up of short threads in highly twisted flux ropes might account for the vertical-like threads in some prominences.

The Mechanism for the Energy Buildup Driving Solar Eruptive Events

Abstract The underlying origin of solar eruptive events (SEEs), ranging from giant coronal mass ejections to small coronal-hole jets, is that the lowest-lying magnetic flux in the Sun’s corona undergoes continual buildup of stress and free energy. This magnetic stress has long been observed as the phenomenon of “filament channels:” strongly sheared magnetic field localized around photospheric polarity inversion lines. However, the mechanism for the stress buildup—the formation of filament channels—is still debated. We present magnetohydrodynamic simulations of a coronal volume that is driven by transient, cellular boundary flows designed to model the processes by which the photosphere drives the corona. The key feature of our simulations is that they accurately preserve magnetic helicity, the topological quantity that is conserved even in the presence of ubiquitous magnetic reconnection. Although small-scale random stress is injected everywhere at the photosphere, driving stochastic reconnection throughout the corona, the net result of the magnetic evolution is a coherent shearing of the lowest-lying field lines. This highly counterintuitive result—magnetic stress builds up locally rather than spreading out to attain a minimum energy state—explains the formation of filament channels and is the fundamental mechanism underlying SEEs. Furthermore, this process is likely to be relevant to other astrophysical and laboratory plasmas.

THE ORIGIN OF NET ELECTRIC CURRENTS IN SOLAR ACTIVE REGIONS

There is a recurring question in solar physics about whether or not electric currents are neutralized in active regions (ARs). This question was recently revisited using three-dimensional (3D) magnetohydrodynamic (MHD) numerical simulations of magnetic flux emergence into the solar atmosphere. Such simulations showed that flux emergence can generate a substantial net current in ARs. Another source of AR currents are photospheric horizontal flows. Our aim is to determine the conditions for the occurrence of net vs. neutralized currents with this second mechanism. Using 3D MHD simulations, we systematically impose line-tied, quasi-static, photospheric twisting and shearing motions to a bipolar potential magnetic field. We find that such flows: (1) produce both {\it direct} and {\it return} currents, (2) induce very weak compression currents - not observed in 2.5D - in the ambient field present in the close vicinity of the current-carrying field, and (3) can generate force-free magnetic fields with a net current. We demonstrate that neutralized currents are in general produced only in the absence of magnetic shear at the photospheric polarity inversion line - a special condition rarely observed. We conclude that, as magnetic flux emergence, photospheric flows can build up net currents in the solar atmosphere, in agreement with recent observations. These results thus provide support for eruption models based on pre-eruption magnetic fields possessing a net coronal current.

FILAMENT CHANNEL FORMATION VIA MAGNETIC HELICITY CONDENSATION

A major unexplained feature of the solar atmosphere is the accumulation of magnetic shear, in the form of filament channels, at photospheric polarity inversion lines (PILs). In addition to free energy, this shear also represents magnetic helicity, which is conserved under reconnection. In this paper, we address the problem of filament channel formation and show how they acquire their shear and magnetic helicity. The results of 3D simulations using the Adaptively Refined Magnetohydrodynamics Solver (ARMS) are presented that support the model of filament channel formation by magnetic helicity condensation developed by \citet{Antiochos13}. We consider the supergranular twisting of a quasi-potential flux system that is bounded by a PIL and contains a coronal hole (CH). The magnetic helicity injected by the small-scale photospheric motions is shown to inverse-cascade up to the largest allowable scales that define the closed flux system: the PIL and the CH. This process produces field lines that are both sheared and smooth, and are sheared in opposite senses at the PIL and the CH. The accumulated helicity and shear flux are shown to be in excellent quantitative agreement with the helicity-condensation model. We present a detailed analysis of the simulations, including comparisons of our analytical and numerical results, and discuss their implications for observations.

HELICITY CONDENSATION AS THE ORIGIN OF CORONAL AND SOLAR WIND STRUCTURE

Three of the most important and most puzzling features of the Sun's atmosphere are the smoothness of the closed field corona, the accumulation of magnetic shear at photospheric polarity inversion lines (PIL), and the complexity of the slow wind. We propose that a single process, helicity condensation, is the physical mechanism giving rise to all three features. A simplified model is presented for how helicity is injected and transported in the closed corona by magnetic reconnection. With this model we demonstrate that helicity must condense onto PILs and coronal hole boundaries, and estimate the rate of helicity accumulation at PILs and the loss to the wind. Our results can account for many of the observed properties of the closed corona and wind.

Multiwavelength Observations of a Failed Flux Rope in the Eruption and Associated M-Class Flare from NOAA AR 11045

We present the multi-wavelength observations of a flux rope that was trying to erupt from NOAA AR 11045 and the associated M-class solar flare on 12 February 2010 using space and ground based observations from TRACE, STEREO, SOHO/MDI, Hinode/XRT and BBSO. While the flux rope was rising from the active region, an M1.1/2F class flare was triggered nearby one of its footpoints. We suggest that the flare triggering was due to the reconnection of a rising flux rope with the surrounding low-lying magnetic loops. The flux rope reached a projected height of ~0.15 Rs with a speed of ~90 km/s while the soft X-ray flux enhanced gradually during its rise. The flux rope was suppressed by an overlying field and the filled plasma moved towards the negative polarity field to the west of its activation site. We find the first observational evidence of the initial suppression of a flux rope due to a remnant filament visible both at chromospheric and coronal temperatures that evolved couple of days before at the same location in the active region. SOHO/MDI magnetograms show the emergence of a bipole ~12 h prior to the flare initiation. The emerged negative polarity moved towards the flux rope activation site, and flare triggering near the photospheric polarity inversion line (PIL) took place. The motion of the negative polarity region towards PIL helped in the build-up of magnetic energy at the flare and flux rope activation site. This study provides a unique observational evidence of a rising flux rope that failed to erupt due to a remnant filament and overlying magnetic field, as well as associated triggering of an M-class flare.

Some properties of the joining of solar filaments

Some possibilities for the reconnection of magnetic-field lines of solar filaments that approach when the photospheric polarity inversion lines change their positions, are discussed. The interaction between filaments depends on their internal properties, which are determined by the filament chirality, or the sign of the helicity of the filament magnetic field. In quadrupolar magnetic configurations, filaments with the same chirality can exchange their halves. Filaments with opposite chirality rupture after the reconnection of the polarity inversion lines, since the two fragments of the different filaments cannot be connected continuously. The morphology and connectivity of the filaments are analyzed using daily Hα filtergrams obtained over the period of maximum activity of the 23rd solar cycle. Examples of alterations of the filament connectivity occuring during the evolution of photospheric fields are presented.

Solar Cycle Variation of Magnetic Flux Ropes in a Quasi-Static Coronal Evolution Model

The structure of electric current and magnetic helicity in the solar corona is closely linked to solar activity over the 11-year cycle, yet is poorly understood. As an alternative to traditional current-free “potential-field” extrapolations, we investigate a model for the global coronal magnetic field which is non-potential and time-dependent, following the build-up and transport of magnetic helicity due to flux emergence and large-scale photospheric motions. This helicity concentrates into twisted magnetic flux ropes, which may lose equilibrium and be ejected. Here, we consider how the magnetic structure predicted by this model – in particular the flux ropes – varies over the solar activity cycle, based on photospheric input data from six periods of cycle 23. The number of flux ropes doubles from minimum to maximum, following the total length of photospheric polarity inversion lines. However, the number of flux rope ejections increases by a factor of eight, following the emergence rate of active regions. This is broadly consistent with the observed cycle modulation of coronal mass ejections, although the actual rate of ejections in the simulation is about a fifth of the rate of observed events. The model predicts that, even at minimum, differential rotation will produce sheared, non-potential, magnetic structure at all latitudes.

Topologically driven coronal dynamics – a mechanism for coronal hole jets

Abstract. Bald patches are magnetic topologies in which the magnetic field is concave up over part of a photospheric polarity inversion line. A bald patch topology is believed to be the essential ingredient for filament channels and is often found in extrapolations of the observed photospheric field. Using an analytic source-surface model to calculate the magnetic topology of a small bipolar region embedded in a global magnetic dipole field, we demonstrate that although common in closed-field regions close to the solar equator, bald patches are unlikely to occur in the open-field topology of a coronal hole. Our results give rise to the following question: What happens to a bald patch topology when the surrounding field lines open up? This would be the case when a bald patch moves into a coronal hole, or when a coronal hole forms in an area that encompasses a bald patch. Our magnetostatic models show that, in this case, the bald patch topology almost invariably transforms into a null point topology with a spine and a fan. We argue that the time-dependent evolution of this scenario will be very dynamic since the change from a bald patch to null point topology cannot occur via a simple ideal evolution in the corona. We discuss the implications of these findings for recent Hinode XRT observations of coronal hole jets and give an outline of planned time-dependent 3-D MHD simulations to fully assess this scenario.

Prominence height shows the proximity of an ejection

The onset of a coronal mass ejection (CME) is not preceded by any specific form of activity that could be recognized several days before the event. The most probable initial magnetic configuration of a CME is a flux rope consisting of twisted field lines which fills the whole volume of the dark cavity stretched in the corona along the photospheric polarity inversion line. Cold dense prominence matter accumulates in the lower parts of helical flux tubes, which serve as magnetic traps in the gravitation field. So, prominences and filaments are the best tracers of the flux ropes in the corona long before the beginning of eruption. A twisted flux rope is held by the tension of field lines of photospheric sources until parameters of the system reach critical values and catastrophe happens. The flux rope height above the photosphere is one of these parameters and it is revealed by the height of the filament. We had analyzed some 80 of filaments and found that eruptive prominences were near the limit of stability a few days before eruptions. We believe that a comparison of the real heights of prominences with the calculated critical heights could be helpful for revealing the filaments that are prone to erupt.

CMEs Associated with Eruptive Prominences: How to Predict?

Solar prominences can be viewed as pre-eruptive states of coronal mass ejections (CMEs). Eruptive prominences are the phenomena most related to CMEs observed in the lower layers of the solar atmosphere. The most probable initial magnetic configuration of a CME is a flux rope consisting of twisted field lines which fills the whole volume of the dark cavity stretched in the corona along the photospheric polarity inversion line. Cold dense prominence matter accumulates in the lower parts of helical flux tubes, which serve as magnetic traps in the gravitation field. Coronal cavity is rather inconvenient feature for observation owing to reduced emission, so prominences and filaments are the best tracers of the flux ropes in the corona long before the beginning of eruption. Thus, the problem of the CME prediction can be reduced to the analysis of the filament equilibrium and estimation of the stability store. The height of a prominence (or a filament when observed against the disk) increases with its age and the death of a filament is usually an eruption which is followed by a CME. The filament height, then, can be a measure of its age and its readiness for eruption. In inverse-polarity models the equilibrium height of a filament is related to the value of the filament electric current. The stronger the electric current, the greater the height of the filament. However, the equilibrium and stability of a filament depend not only on its current but also on the characteristics of the external magnetic field. In order to estimate the probability of eruption, we should therefore compare the observed prominence height with a value characterizing the photospheric magnetic field. This value is the critical height, which can be found in the distribution of the magnetic field vertical gradient above the polarity inversion line. We had analyzed three dozens of filaments and found that eruptive prominences were near the limit of stability a few days before eruptions. We believe that the comparison of the real heights of prominences with the calculated critical heights could be a basis for predicting filament eruptions and following CMEs.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Dynamical Formation and Stability of Helical Prominence Magnetic Fields

We numerically simulated an initially bipolar magnetic field subjected to shear motions concentrated near and parallel to the photospheric polarity inversion line. The simulations yield three principal results: (1) For footpoint displacements comparable to the bipole's depth, the sheared core field acquires a dipped geometry that can support cool prominence material against gravity. This confirms previous force-free equilibrium models for forming dipped prominence fields by differential shear and extends them to much larger applied shears and time-dependent dynamics with dissipation. (2) At larger shears, we discover a new mechanism for forming the helical magnetic fields of prominences. It entails a two-step process of magnetic reconnection in the corona. First, flux in the sheared core reconnects with flux in the unsheared, restraining arcade, producing new pairs of interlinked field lines. Second, as these interlinked fields continue to be sheared, they are brought together and reconnect again, producing helical field threading and enveloping the body of the prominence. This mechanism can account for the twist that is often observed in both quiescent and erupting prominences. (3) Even for very large shears, the dipped, helical structure settles into an apparently stable equilibrium, despite the substantial amount of reconnection and twist in the magnetic field. We conclude that neither a kink instability of the helical core field, nor a tether-cutting instability of the restraining arcade, is operating in our low-lying model prominence. This concurs with both observations and a theoretical model for prominence stability.

The Filament Instability in a Sheared Field

Filaments and prominences are classical examples of condensations of nongravitational origin, their formation being due to the thermal instability. The physics of this instability in a uniform plasma at coronal temperatures, which exhibits increasing radiative output as it cools, is well known (Field, 1965). Filaments, however, form in regions of sheared magnetic fields, as evidenced by their location above photospheric polarity-inversion lines and their occurrence after a period of increasing fibril inclination (Tandberg-Hanssen, 1974). Physically, the importance of the field structure is readily understood if one recalls the capability of magnetic field of strongly collimating the local heat conduction. Thus, the preferred locations for the development of the thermal instability will be those where the field configuration inhibits the stabilizing effects of thermal conduction on the growing temperature perturbation.

The dynamics of filament formation - The thermal instability in a sheared magnetic field

There exists no good model which explains the initial temporal and spatial development of filaments and prominences. It is believed that they form in conditions of sheared coronal magnetic fields, as evidenced by their location above photospheric polarity-inversion lines and their occurrence after a period of increasing fibrille inclination. The general mechanism giving rise to thermal instability in a uniform medium such as the solar corona, which exhibits increasing radiative output as it cools, is well known. In this brief Letter we describe a self-consistent calculation of this instability in a nonuniform magnetic field, which shows the dynamic response of the pressure, density, and temperature to the competing effects of optically thin radiation and field-collimated thermal conduction. The condensation of characteristic ''knife-blade'' filaments is then demonstrated and their observable temporal and spatial scales detailed.