Formation Of Filament Channels Research Articles

Comprehensive Analysis of a Filament-embedding Solar Active Region at Different Stages of Evolution

Active regions are the brightest structures seen in the solar corona, so their physical properties hold important clues to the physical mechanisms underlying coronal heating. In this work, we present a comprehensive study for a filament-embedding active region as determined from observations from multiple facilities including the Chinese Hα Solar Explorer. We find three types of dynamic features that correspond to different thermal and magnetic properties, i.e., the overlying loops—1 MK cool loops, the moss region—2–3 MK hot loops’ footprints, and the sigmoidal filament. The overlying cool loops, which have a potential field, always show Doppler blueshifts at the east footprint and Doppler redshifts at the west, indicating a pattern of “siphon flow.” The moss-brightening regions, which sustain the hot loops that have a moderate sheared field, always show downward Doppler redshifts at the chromosphere, which could be a signature of plasma condensing into the inner region adjacent to the filament. The sigmoidal filament, which has strongly sheared field lines along the polarity inversion line, however, shows a different Doppler velocity pattern in its middle part, i.e., an upward Doppler blueshift at the double-J-shaped stage indicating tether-cutting reconnection during the filament channel formation and then a downward redshift showing the plasma condensation for the sigmoidal filament formation. The present work shows overall properties of the filament-embedding active region, constraining the heating mechanisms of different parts of the active region and providing hints regarding the mass loading of the embedded filament.

What Determines Active Region Coronal Plasma Composition?

The chemical composition of the solar corona is different from that of the solar photosphere, with the strongest variation being observed in active regions (ARs). Using data from the Extreme Ultraviolet (EUV) Imaging Spectrometer (EIS) on Hinode, we present a survey of coronal elemental composition as expressed in the first ionization potential (FIP) bias in 28 ARs of different ages and magnetic flux content, which are at different stages in their evolution. We find no correlation between the FIP bias of an AR and its total unsigned magnetic flux or age. However, there is a weak dependence of FIP bias on the evolutionary stage, decreasing from 1.9 to 2.2 in ARs with spots to 1.5–1.6 in ARs that are at more advanced stages of the decay phase. FIP bias shows an increasing trend with average magnetic flux density up to 200 G, but this trend does not continue at higher values. The FIP bias distribution within ARs has a spread between 0.4 and 1. The largest spread is observed in very dispersed ARs. We attribute this to a range of physical processes taking place in these ARs, including processes associated with filament channel formation. These findings indicate that, while some general trends can be observed, the processes influencing the composition of an AR are complex and specific to its evolution, magnetic configuration, or environment. The spread of FIP bias values in ARs shows a broad match with that previously observed in situ in the slow solar wind.

The Role of Magnetic Helicity in Coronal Heating

Abstract One of the greatest challenges in solar physics is understanding the heating of the Sun’s corona. Most theories for coronal heating postulate that free energy in the form of magnetic twist/stress is injected by the photosphere into the corona where the free energy is converted into heat either through reconnection or wave dissipation. The magnetic helicity associated with the twist/stress, however, is expected to be conserved and appear in the corona. In previous works, we showed that the helicity associated with the small-scale twists undergoes an inverse cascade via stochastic reconnection in the corona and ends up as the observed large-scale shear of filament channels. Our “helicity condensation” model accounts for both the formation of filament channels and the observed smooth, laminar structure of coronal loops. In this paper, we demonstrate, using helicity- and energy-conserving numerical simulations of a coronal system driven by photospheric motions, that the model also provides a natural mechanism for heating the corona. We show that the heat generated by the reconnection responsible for the helicity condensation process is sufficient to account for the observed coronal heating. We study the role that helicity injection plays in determining coronal heating and find that, crucially, the heating rate is only weakly dependent on the net helicity preference of the photospheric driving. Our calculations demonstrate that motions with 100% helicity preference are least efficient at heating the corona; those with 0% preference are most efficient. We discuss the physical origins of this result and its implications for the observed corona.

A Model for Energy Buildup and Eruption Onset in Coronal Mass Ejections

Abstract Coronal mass ejections (CMEs) and eruptive flares (EFs) are the most energetic explosions in the solar system. Their underlying origin is the free energy that builds up slowly in the sheared magnetic field of a filament channel. We report the first end-to-end numerical simulation of a CME/EF, from zero-free-energy initial state through filament channel formation to violent eruption, driven solely by the magnetic-helicity condensation process. Helicity is the topological measure of linkages between magnetic flux systems, and is conserved in the corona, building up inexorably until it is ejected into interplanetary space. Numerous investigations have demonstrated that helicity injected by small-scale vortical motions, such as those observed in the photosphere, undergoes an inverse cascade from small scales to large, “condensing” at magnetic-polarity boundaries. Our new results verify that this process forms a filament channel within a compact bipolar region embedded in a background dipole field, and show for the first time that a fast CME eventually occurs via the magnetic-breakout mechanism. We further show that the trigger for explosive eruption is reconnection onset in the flare current sheet that develops above the polarity inversion line: this reconnection forms flare loops below the sheet and a CME flux rope above, and initiates high-speed outward flow of the CME. Our findings have important implications for magnetic self-organization and explosive behavior in solar and other astrophysical plasmas, as well as for understanding and predicting explosive solar activity.

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 ROLE OF MAGNETIC HELICITY IN STRUCTURING THE SOLAR CORONA

ABSTRACT Two of the most widely observed and striking features of the Sun's magnetic field are coronal loops, which are smooth and laminar, and prominences or filaments, which are strongly sheared. Loops are puzzling because they show little evidence of tangling or braiding, at least on the quiet Sun, despite the chaotic nature of the solar surface convection. Prominences are mysterious because the origin of their underlying magnetic structure—filament channels—is poorly understood at best. These two types of features would seem to be quite unrelated and wholly distinct. We argue that, on the contrary, they are inextricably linked and result from a single process: the injection of magnetic helicity into the corona by photospheric motions and the subsequent evolution of this helicity by coronal reconnection. In this paper, we present numerical simulations of the response of a Parker (1972) corona to photospheric driving motions that have varying degrees of helicity preference. We obtain four main conclusions: (1) in agreement with the helicity condensation model of Antiochos (2013), the inverse cascade of helicity by magnetic reconnection in the corona results in the formation of filament channels localized about polarity inversion lines; (2) this same process removes most complex fine structure from the rest of the corona, resulting in smooth and laminar coronal loops; (3) the amount of remnant tangling in coronal loops is inversely dependent on the net helicity injected by the driving motions; and (4) the structure of the solar corona depends only on the helicity preference of the driving motions and not on their detailed time dependence. We discuss the implications of our results for high-resolution observations of the corona.

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.

The Interplay between Electronic and Ionic Transport in the Resistive Switching Process of Random Access Memory Devices

In resistive random access memory devices (ReRAM), in order to achieve high endurance and good retention characteristics repetitive control over the resistance change between low resistance (LRS) and high resistance (HRS) states is desired. The binary transition metal oxides TiOx, NiOx, HfOx, AlOx, TaOx have been specifically promising to achieve this goal and are the preferential candidates for the next generation of resistance change based devices. To address the critical issues of ReRAM operation, however, atomistic modeling approaches based on quantum mechanical principles are needed to achieve the optimization of material properties and structures of ReRAM cells, as well as of selection devices and memory array configurations.To explain the process of resistance change observed experimentally, various switching mechanisms were proposed recently and discussed in the technical literature. Generally, in transition metal oxide system such as TiO2, HfO2, NiO, Al2O3, Ta2O5, the diffusion of oxygen vacancies to cluster and create filaments [1-8] and the diffusion of oxygen atoms away from the oxide region to form a thin interfacial reduced oxide, are considered the key ionic processes taking place during forming and switching. Filamentary models for transition metal oxides had been proposed theoretically and the formation energy implications of a conductive filament channel formation corresponding to the “ON” state or LRS [4-9] was investigated for TiO2, HfO2 and Al2O3 (Fig. 1). It was found that during the clustering process of oxygen vacancies, conductive channels are formed by electron delocalization trends induced by the vacancies. The accompanying electronic effects of charge trapping under applied electrical field during the switching process were also recently addressed [7-9]. Hole injection into a reduced transition metal oxide containing a formed filament were found to favor the dissolution, since isolated vacancies prefer the 2+ charge state configuration. On the other hand electron injection induces filament formation and a transition of vacancy charge state to 1+or neutral configuration. The energy barriers of oxygen vacancy out-diffusion from a filament in various charge states are shown in Fig. 2 as a function of chemical potential and Fermi level control. In addition, preferential impurity doping [10] in these types of systems can favorably affect the forming of the conductive filaments and the transition process between the “ON” and “OFF” states (Fig. 3). In conclusion, several competing processes may undergo simultaneously at the microscopic level during the switching of ReRAM devices. Atomistic simulations based on density functional theory can provide an accurate description of the role of each component and describe the mechanism of ReRAM switching.[1] B. Magyari-Köpe, S.G. Park, H.-D. Lee and Y. Nishi, J. Mater. Sci. 47, pp 7498, 2012. [2] S.G. Park, B. Magyari-Köpe, and Y. Nishi, Phys. Rev. B, 82, 115109, 2010. [3] H.-D. Lee, B. Magyari-Köpe, and Y. Nishi, Phys. Rev. B 81, 193202, 2010. [4] S.G. Park, B. Magyari-Köpe, and Y. Nishi, Tech. Digest VLSI Tech. Symp., 2011. [5] B. Magyari-Köpe, M. Tendulkar, S.G. Park, H.-D. Lee, and Y. Nishi, Nanotechnology 22, 254029, 2011. [6] S.G. Park, B. Magyari-Köpe, and Y. Nishi, IEEE Electron Device Letters, 32, 197, 2011. [7] K. Kamiya, M.Y. Yang, S.G. Park, B. Magyari-Köpe, Y. Nishi, M. Niwa, and K. Shiraishi, Appl. Phys. Lett.,100, 073502, 2012. [8] K. Kamiya, M.Y. Yang, B. Magyari-Köpe, M. Niwa, Y. Nishi, and K. Shiraishi, IEEE Trans. Electron Devices, vol. 60, pp. 3400, 2013. [9] K. Kamiya, M.Y. Yang, B. Magyari-Köpe, M. Niwa, Y. Nishi, and K. Shiraishi, Proc. IEEE IEDM, pp. 478, 2012. [10] L. Zhao, S.G. Park, B. Magyari-Köpe and Y. Nishi, Tech. Digest VLSI Tech. Symp. , 2013.

Fe XII STALKS AND THE ORIGIN OF THE AXIAL FIELD IN FILAMENT CHANNELS

Employing Fe XII images and line-of-sight magnetograms, we deduce the direction of the axial field in high-latitude filament channels from the orientation of the adjacent stalklike structures. Throughout the rising phase of the current solar cycle 24, filament channels poleward of latitude 30° overwhelmingly obeyed the hemispheric chirality rule, being dextral (sinistral) in the northern (southern) hemisphere, corresponding to negative (positive) helicity. During the deep minimum of 2007-2009, the orientation of the Fe XII stalks was often difficult to determine, but no obvious violations of the rule were found. Although the hemispheric trend was still present during the maximum and early declining phase of cycle 23 (2000-2003), several high-latitude exceptions were identified at that time. From the observation that dextral (sinistral) filament channels form through the decay of active regions whose Fe XII features show a counterclockwise (clockwise) whorl, we conclude that the axial field direction is determined by the intrinsic helicity of the active regions. In contrast, generation of the axial field component by the photospheric differential rotation is difficult to reconcile with the observed chirality of polar crown and circular filament channels, and with the presence of filament channels along the equator. The main role of differential rotation in filament channel formation is to expedite the cancellation of flux and thus the removal of the transverse field component. We propose further that, rather than being ejected into the heliosphere, the axial field is eventually resubmerged by flux cancellation as the adjacent unipolar regions become increasingly mixed.

On the Formation of Filament Channels

From the Hα archive of the Big Bear Solar Observatory (BBSO) we have selected three examples showing fibril structures that change their orientation, over 1 or 2 days, from nearly perpendicular to nearly parallel to the polarity inversion line (PIL). In one case, the filament channel forms within a single decaying bipole; in the other two cases, it forms along the boundary between an active region and its surroundings. Comparing the Hα filtergrams with magnetograms from the Michelson Doppler Imager (MDI), we find that the fibrils become aligned with the PIL as supergranular convection brings opposite-polarity magnetic flux together; shearing motions along the PIL, when present, act mainly to accelerate the rate of diffusive annihilation. We conclude that the reorientation of the fibrils is due to the cancellation and submergence of the transverse field component (B⊥), leaving behind the preexisting axial field component (B∥). The latter may have been generated by photospheric differential rotation over longer timescales, or else was already present when the flux emerged. The filament channel forms slowly if B∥/B⊥ is initially small, as along the internal neutral line of a newly emerged bipole, but may appear within hours if this ratio is initially substantial, as where the dipole-like loops of an active region curve around its periphery. In all of our examples, filaments form within a day or so after the fibrils become aligned with the PIL, while barbs appear at a later stage, as flux elements continue to diffuse across the PIL and cancel with the majority-polarity flux on the other side.

Mean Field Model for the Formation of Filament Channels on the Sun

The coronal magnetic field is subject to random footpoint motions that cause small-scale twisting and braiding of field lines. We present a mean field theory describing the effects of such small-scale twists on the large-scale coronal field. This theory assumes that the coronal field is force free, with electric currents flowing parallel or antiparallel to magnetic field lines. Random footpoint motions are described in terms of diffusion of the mean magnetic field at the photosphere. The appropriate mean field equations are derived, and a numerical method for solving these equations in three dimensions is presented. Preliminary results obtained with this method are also presented. In particular the formation of filament channels is studied. Filament channels are regions where the coronal magnetic field is strongly aligned with the underlying polarity inversion line in the photosphere. It is found that magnetic flux cancellation plays an important role in the formation of such channels. Various models of the coronal field are presented, including some in which the axial field is assumed to originate from below the photosphere. The models reproduce many of the observed features of filament channels, but the observed hemisphere pattern of dextral and sinistral channels remains a mystery.

Magnetic Flux Transport and the Formation of Filament Channels on the Sun

Observations of filaments and filament channels on the Sun indicate that the magnetic fields in these structures exhibit a large-scale organization: filament channels in the northern hemisphere predominantly have axial fields directed to the right when viewed from the positive polarity side of the channel (dextral orientation), while those in the south have axial fields directed to the left (sinistral orientation). In this paper we attempt to explain this pattern in terms of the most natural mechanism, namely, solar differential rotation acting on already emerged magnetic fields. We develop a model of global magnetic flux transport that includes the effects of differential rotation, meridional flow, and magnetic diffusion on photospheric and coronal fields. The model is applied to National Solar Observatory/Kitt Peak data on the photospheric magnetic flux distribution. We also present results from a simulation of solar activity over a period of two solar cycles, which gives a buildup of flux at the poles of a magnitude, in agreement with observations. We find that differential rotation acting on initially north-south oriented polarity inversion lines (PILs) does produce axial fields consistent with the observed hemispheric pattern. The fields associated with switchbacks in the PILs are predicted to have a definite orientation: the high-latitude lead arms of the switchbacks are preferentially sinistral (dextral) in the north (south), while the lower latitude return arms are, in agreement with observations, preferentially dextral (sinistral). The predicted orientation of fields at the polar crown, however, appear to be in conflict with observations. Further observational studies are needed to determine whether the model can explain the observed hemispheric pattern.

Magnetic Flux Transport and Formation of Filament Channels

AbstractWe present a model of the transport of photospheric and coronal magnetic fields with the aim of explaining the observed global pattern of dextral and sinistral filaments on the quiet sun. The model is based on the assumption that the axial field in a filament channel originates in the surrounding coronal arcade. The model predicts that switchbacks in the polarity inversion line have opposite chirality on the two arms of the switchback: the higher latitude “lead” arm has sinistral (dextral) orientation in the North (South), while the lower latitude “return” arm has dextral (sinistral) orientation. This is in conflict with prominence observations which show that the chirality is the same on the two arms of a switchback. The model predicts the wrong sign of the axial field in polar crown filaments. This suggests that axial field in quiescent filaments are not produced by differential rotation acting on the surrounding coronal arcades.

Force-Free Models of a Filament Channel in which a Filament Forms

AbstractOver the years few examples of the formation of filament channels or filaments have been observed. One such example was recently seen by Gaizauskas et al. (1997). This paper constructs force-free models of the main stages of formation.