Non-potential Magnetic Field Research Articles

Can the Parker Solar Probe Detect a CME-flare Current Sheet?

A current sheet (CS) is the central structure in the disrupting magnetic configuration during solar eruptions. More than 90% of the free magnetic energy (the difference between the energy in the nonpotential magnetic field and that in the potential one) stored in the coronal magnetic field beforehand is converted into the heating and kinetic energy of the plasma, as well as accelerating charged particles, by magnetic reconnection occurring in the CS. However, the detailed physical properties and fine structures of the CS are still unknown, since there is no relevant information obtained via in situ detections. The Parker Solar Probe (PSP) may provide us with such information should it traverse a CS in an eruption. The perihelion of PSP’s final orbit is located at about 10 solar radii from the center of the Sun, so it can observe the CS at a very close distance, or even traverse the CS, which would provide us with a unique opportunity to look into the fine properties and structures of the CS, helping to reveal the detailed physics of large-scale reconnection that would have been impossible before. We evaluate the probability that PSP can traverse a CS, and examine the orbit of a PSP-like spacecraft that has the highest probability to traverse a CS.

Combining magneto-hydrostatic constraints with Stokes profiles inversions

Context. Electric currents play an important role in the energy balance of the plasma in the solar atmosphere. They are also indicative of non-potential magnetic fields and magnetic reconnection. Unfortunately, the direct measuring of electric currents has traditionally been riddled with inaccuracies. Aims. We study how accurately we can infer electric currents under different scenarios. Methods. We carry out increasingly complex inversions of the radiative transfer equation for polarized light applied to Stokes profiles synthesized from radiative three-dimensional magnetohydrodynamic (MHD) simulations. The inversion yields the magnetic field vector, B, from which the electric current density, j, is derived by applying Ampere’s law. Results. We find that the retrieval of the electric current density is only slightly affected by photon noise or spectral resolution. However, the retrieval steadily improves as the Stokes inversion becomes increasingly elaborated. In the least complex case (a Milne-Eddington-like inversion applied to a single spectral region), it is possible to determine the individual components of the electric current density (jx, jy, jz) with an accuracy of σ = 0.90 − 1.00 dex, whereas the modulus (∥j∥) can only be determined with σ = 0.75 dex. In the most complicated case (with multiple spectral regions, a large number of nodes, Tikhonov vertical regularization, and magnetohydrostatic equilibrium), these numbers improve to σ = 0.70 − 0.75 dex for the individual components and σ = 0.5 dex for the modulus. Moreover, in regions where the magnetic field is above 300 gauss, ∥j∥ can be inferred with an accuracy of σ = 0.3 dex. In general, the x and y components of the electric current density are retrieved slightly better than the z component. In addition, the modulus of the electric current density is the best retrieved parameter of all, and thus it can potentially be used to detect regions of enhanced Joule heating. Conclusions. The fact that the accuracy does not worsen with decreasing spectral resolution or increasing photon noise, and instead increases as the Stokes inversion complexity grows, suggests that the main source of errors in the determination of electric currents is the lack of realism in the inversion model employed to determine variations in the magnetic field along the line of sight at scales smaller than the photon mean-free path, along with the intrinsic limitations of the model due to radiative transfer effects.

Impact of subsurface convective flows on the formation of sunspot magnetic field and energy build-up

ABSTRACT Strong solar flares occur in δ-spots characterized by the opposite-polarity magnetic fluxes in a single penumbra. Sunspot formation via flux emergence from the convection zone to the photosphere can be strongly affected by convective turbulent flows. It has not yet been shown how crucial convective flows are for the formation of δ-spots. The aim of this study is to reveal the impact of convective flows in the convection zone on the formation and evolution of sunspot magnetic fields. We simulated the emergence and transport of magnetic flux tubes in the convection zone using radiative magnetohydrodynamics code r2d2. We carried out 93 simulations by allocating the twisted flux tubes to different positions in the convection zone. As a result, both δ-type and β-type magnetic distributions were reproduced only by the differences in the convective flows surrounding the flux tubes. The δ-spots were formed by the collision of positive and negative magnetic fluxes on the photosphere. The unipolar and bipolar rotations of the δ-spots were driven by magnetic twist and writhe, transporting magnetic helicity from the convection zone to the corona. We detected a strong correlation between the distribution of the non-potential magnetic field in the photosphere and the position of the downflow plume in the convection zone. The correlation could be detected 20–30 h before the flux emergence. The results suggest that high free energy regions in the photosphere can be predicted even before the magnetic flux appears in the photosphere by detecting the downflow profile in the convection zone.

Evolution of the Nonpotential Magnetic Field in the Solar Active Region 12673 Based on a Nonlinear Force-free Modeling

Abstract Active region (AR) 12673 has produced many M-class and several X-class flares, one of which being an X9.3 flare, which is recorded as the largest solar flare in solar cycle 24. We studied the evolution of the three-dimensional flare-productive magnetic field within AR 12673, using a time series of nonlinear force-free field extrapolations of every 12 hr from 2017 September 4 00:00 UT to 6 00:00 UT. Our analysis found that three magnetic flux ropes (MFRs) formed by September 4, one of which produced the X9.3 flare on September 6. One MFR has positive magnetic twist, which is a different sign from the other two MFRs. Since the temporal evolution of the magnetic flux of the MFR accumulating the positive twist is consistent with the profile of the GOES X-ray flux well observed from September 4 to 6, we suggest that the formation of the MFR having the positive twist is closely related to the occurrence of the M-class flares, including an M5.5 flare. We further found a magnetic null in the magnetic field surrounding the MFRs, in particular above the MFR having positive twist before the M5.5 flare, which is the largest M-flare observed during this period. By comparing with Atmospheric Imaging Assembly 1600 Å images, we found that the footpoints of the overlying field lines are anchored to the area where the brightening was initially observed. Therefore, we suggest that reconnection induced by the torus instability of the positively twisted MFR at the null possibly drove the M5.5 flare.

Designing a New Coronal Magnetic Field Energy Diagnostic

Abstract In the solar corona, the free energy, i.e., the excess in magnetic energy over a ground-state potential field, forms the reservoir of energy that can be released during solar flares and coronal mass ejections. Such free energy provides a measure of the magnetic field nonpotentiality. Recent theoretical and observational studies indicate that the presence of nonpotential magnetic fields is imprinted into the structures of infrared, off-limb, coronal polarization. In this paper, we investigate the possibility of exploiting such observations for mapping and studying the accumulation and release of coronal free magnetic energy, with the goal of developing a new tool for identifying “hot spots” of coronal free energy such as those associated with twisted and/or sheared coronal magnetic fields. We applied forward modeling of infrared coronal polarimetry to three-dimensional models of nonpotential and potential magnetic fields. From these we defined a quantitative diagnostic of nonpotentiality that in the future could be calculated from a comparison of infrared, off-limb, coronal polarization observations from, e.g., the Coronal Multi-channel Polarimeter or the Daniel K. Inouye Solar Telescope, and the corresponding polarization signal forward-modeled from a potential field extrapolated from photospheric magnetograms. We considered the relative diagnostic potential of linear and circular polarization, and the sensitivities of these diagnostics to coronal density distributions and assumed boundary conditions of the potential field. Our work confirms the capacity of polarization measurements for diagnosing nonpotentiality and free energy in the solar corona.

Eruptions from coronal hole bright points: Observations and non-potential modelling

Context. We report on the third part of a series of studies on eruptions associated with small-scale loop complexes named coronal bright points (CBPs). Aims. A single case study of a CBP in an equatorial coronal hole with an exceptionally large size is investigated to expand on our understanding of the formation of mini-filaments, their destabilisation, and the origin of the eruption triggering the formation of jet-like features recorded in extreme ultraviolet (EUV) and X-ray emission. We aim to explore the nature of the so-called micro-flares in CBPs associated with jets in coronal holes and mini coronal mass ejections in the quiet Sun. Methods. Co-observations from the Atmospheric Imaging Assembly (AIA) and Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory as well as GONG Hα images are used together with a non-linear force free field (NLFFF) relaxation approach, where the latter is based on a time series of HMI line-of-sight magnetograms. Results. A mini-filament (MF) that formed beneath the CBP arcade about 3−4 h before the eruption is seen in the Hα and EUV AIA images to lift up and erupt triggering the formation of an X-ray jet. No significant photospheric magnetic flux concentration displacement (convergence) is observed and neither is magnetic flux cancellation between the two main magnetic polarities forming the CBP in the time period leading to MF lift-off. The CBP micro-flare is associated with three flare kernels that formed shortly after the MF lift-off. No observational signature is found for magnetic reconnection beneath the erupting MF. The applied NLFFF modelling successfully reproduces both the CBP loop complex as well as the magnetic flux rope that hosts the MF during the build-up to the eruption. Conclusions. The applied NLFFF modelling is able to clearly show that an initial potential field can be evolved into a non-potential magnetic field configuration that contains free magnetic energy in the region that observationally hosts the eruption. The comparison of the magnetic field structure shows that the magnetic NLFFF model contains many of the features that can explain the different observational signatures found in the evolution and eruption of the CBP. In the future, it may eventually indicate the location of destabilisation that results in the eruptions of flux ropes.

Investigation of the Middle Corona with SWAP and a Data-Driven Non-Potential Coronal Magnetic Field Model

The large field-of-view of the Sun Watcher using Active Pixel System detector and Image Processing (SWAP) instrument onboard the PRoject for Onboard Autonomy 2 (PROBA2) spacecraft provides a unique opportunity to study extended coronal structures observed in the EUV in conjunction with global coronal magnetic field simulations. A global non-potential magnetic field model is used to simulate the evolution of the global corona from 1 September 2014 to 31 March 2015, driven by newly emerging bipolar active regions determined from Helioseismic and Magnetic Imager (HMI) magnetograms. We compare the large-scale structure of the simulated magnetic field with structures seen off-limb in SWAP EUV observations. In particular, we investigate how successful the model is in reproducing regions of closed and open structures, the scale of structures, and compare the evolution of a coronal fan observed over several rotations. The model is found to accurately reproduce observed large-scale, off-limb structures. When discrepancies do arise they mainly occur off the east solar limb due to active regions emerging on the far side of the Sun, which cannot be incorporated into the model until they are observed on the Earth-facing side. When such “late” active region emergences are incorporated into the model, we find that the simulated corona self-corrects within a few days, so that simulated structures off the west limb more closely match what is observed. Where the model is less successful, we consider how this may be addressed, through model developments or additional observational products.

“False” Sigmoids in the Solar Corona

As a rule, sigmoidal structures in the colar corona, i.e., structures having the appearance of an S or reversed S, are taken to be evidence for twisting of the magnetic-field lines and the presence of electric currents in the corona. However, these formations can also exist in a potential field. The S-like shapes of solar filaments, for example, are determined by the distribution of photospheric magnetic fields, which do not depend on coronal currents. Sigmoidal field lines can appear in a fairly complex potential field in the vicinities of null singular points. Such field lines can display shear, which is also often taken as manifestation of a non-potential field. Thus, one should treat some evidence for non-potential magnetic fields in the corona cautiously, and rule out other origins for the appearance of “suspicious” structures before drawing conclusions about possible non-potentiality of the fields.

Helical Twisting Number and Braiding Linkage Number of Solar Coronal Loops

Abstract Coronal loops in active regions are often characterized by quasi-circular and helically twisted (sigmoidal) geometries, which are consistent with dipolar potential field (PF) models in the former case, and with nonlinear force-free field models with vertical currents in the latter case. Alternatively, Parker-type nanoflare models of the solar corona hypothesize that a braiding mechanism operates between unresolved loop strands, which is a more complex topological model. In this study we use the vertical-current approximation of a nonpotential magnetic field solution (that fulfils the divergence-free and force-free conditions) to characterize the number of helical turns N twist in twisted coronal loops. We measure the helical twist in 15 active regions observed with Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager/SDO (Solar Dynamic Observatory) and find a mean nonpotentiality angle (between the potential and nonpotential field directions) of μ NP = 15° ± 3°. The resulting mean rotational twist angle is φ = 49° ± 11°, which corresponds to N twist = φ/360° = 0.14 ± 0.03 turns with respect to the untwisted PF, with an absolute upper limit of N twist ≲ 0.5, which is far below the kink instability limit of . The number of twist turns N twist corresponds to the Gauss linkage number N link in braiding topologies. We conclude that any braided topology (with ) cannot explain the observed stability of loops in a force-free corona, nor the observed low twist number. Parker-type nanoflaring can thus occur in non-force-free environments only, such as in the chromosphere and transition region.

Observing the simulations: applying ZDI to 3D non-potential magnetic field simulations

The large-scale magnetic fields of stars can be obtained with the Zeeman-Doppler-Imaging (ZDI) technique, but their interpretation is still challenging as the contribution of the small-scale field or the reliability of the reconstructed field properties is still not fully understood. To quantify this, we use 3D non-potential magnetic field simulations for slowly rotating solar-like stars as inputs to test the capabilities of ZDI. These simulations are based on a flux transport model connected to a non-potential coronal evolution model using the observed solar flux emergence pattern. We first compare four field prescriptions regarding their reconstruction capabilities and investigate the influence of the spatial resolution of the input maps on the corresponding circularly polarised profiles. We then generate circularly polarised spectra based on our high resolution simulations of three stellar models with different activity levels, and reconstruct their large-scale magnetic fields using a non-potential ZDI code assuming two different stellar inclination angles. Our results show that the ZDI technique reconstructs the main features of slowly rotating solar-like stars but with $\sim\,$one order of magnitude less magnetic energy. The large-scale field morphologies are recovered up to harmonic modes $\ell \sim 5$, especially after averaging over several maps for each stellar model. While ZDI is not able to reproduce the input magnetic energy distributions across individual harmonic modes, the fractional energies across the modes are generally within $20\,\%$ agreement. The fraction of axisymmetric and toroidal field tends to be overestimated for stars with solar flux emergence patterns for more pole-on inclination angles.

A new technique for observationally derived boundary conditions for space weather

Context. In recent years, space weather research has focused on developing modelling techniques to predict the arrival time and properties of coronal mass ejections (CMEs) at the Earth. The aim of this paper is to propose a new modelling technique suitable for the next generation of Space Weather predictive tools that is both efficient and accurate. The aim of the new approach is to provide interplanetary space weather forecasting models with accurate time dependent boundary conditions of erupting magnetic flux ropes in the upper solar corona. Methods. To produce boundary conditions, we couple two different modelling techniques, MHD simulations and a quasi-static non-potential evolution model. Both are applied on a spatial domain that covers the entire solar surface, although they extend over a different radial distance. The non-potential model uses a time series of observed synoptic magnetograms to drive the non-potential quasi-static evolution of the coronal magnetic field. This allows us to follow the formation and loss of equilibrium of magnetic flux ropes. Following this a MHD simulation captures the dynamic evolution of the erupting flux rope, when it is ejected into interplanetary space. Results.The present paper focuses on the MHD simulations that follow the ejection of magnetic flux ropes to 4 R⊙. We first propose a technique for specifying the pre-eruptive plasma properties in the corona. Next, time dependent MHD simulations describe the ejection of two magnetic flux ropes, that produce time dependent boundary conditions for the magnetic field and plasma at 4 R⊙ that in future may be applied to interplanetary space weather prediction models. Conclusions. In the present paper, we show that the dual use of quasi-static non-potential magnetic field simulations and full time dependent MHD simulations can produce realistic inhomogeneous boundary conditions for space weather forecasting tools. Before a fully operational model can be produced there are a number of technical and scientific challenges that still need to be addressed. Nevertheless, we illustrate that coupling quasi-static and MHD simulations in this way can significantly reduce the computational time required to produce realistic space weather boundary conditions.

Onset mechanism of solar eruptions

Solar eruptions are the most energetic phenomena observed in the solar system observed as flares, coronal mass ejections (CMEs) and filament/prominence eruption. The helically twisted flux tube is widely thought to be the source and driver of solar eruptions and to carry the plasma into the interplanetary space. Those may eventually reach the magnetosphere and cause strong disturbances of the geomagnetic field. Therefore, the understanding of the onset of solar eruptions is important not only in the framework of solar physics but also for the space weather forecast. In this paper, we report on new insight into the onset mechanism of solar eruptions recently obtained from our new studies. We perform the studies in terms of the observational approach with state-of-the-art solar physics satellites and the numerical one with the latest super computer system. We specified two types of small magnetic perturbations of the photospheric magnetic field. These can enhance the magnetic reconnection in the pre-existing non-potential magnetic field, which produces a large flux tube and then drives the eruption. We further confirmed that this reconnection is a key process for the eruption in our latest data-constrained simulation. We report our latest results and our interpretation of the onset mechanism of solar eruptions.

Observations and Modelling of the Pre-flare Period of the 29 March 2014 X1 Flare

On 29 March 2014, NOAA Active Region (AR) 12017 produced an X1 flare that was simultaneously observed by an unprecedented number of observatories. We have investigated the pre-flare period of this flare from 14:00 UT until 19:00 UT using joint observations made by the Interface Region Imaging Spectrometer (IRIS) and the Hinode Extreme Ultraviolet Imaging Spectrometer (EIS). Spectral lines providing coverage of the solar atmosphere from the chromosphere to the corona were analysed to investigate pre-flare activity within the AR. The results of the investigation have revealed evidence of strongly blue-shifted plasma flows, with velocities up to 200~mbox{km},mbox{s}^{-1}, being observed 40 minutes prior to flaring. These flows are located along the filament present in the active region and are both spatially discrete and transient. In order to constrain the possible explanations for this activity, we undertake non-potential magnetic field modelling of the active region. This modelling indicates the existence of a weakly twisted flux rope along the polarity inversion line in the region where a filament and the strong pre-flare flows are observed. We then discuss how these observations relate to the current models of flare triggering. We conclude that the most likely drivers of the observed activity are internal reconnection in the flux rope, early onset of the flare reconnection, or tether-cutting reconnection along the filament.

TRACING THE CHROMOSPHERIC AND CORONAL MAGNETIC FIELD WITH AIA, IRIS, IBIS, AND ROSA DATA

ABSTRACT The aim of this study is to explore the suitability of chromospheric images for magnetic modeling of active regions. We use high-resolution images ( ), from the Interferometric Bidimensional Spectrometer in the Ca ii 8542 Å line, the Rapid Oscillations in the Solar Atmosphere instrument in the Hα 6563 Å line, the Interface Region Imaging Spectrograph in the 2796 Å line, and compare non-potential magnetic field models obtained from those chromospheric images with those obtained from images of the Atmospheric Imaging Assembly in coronal (171 Å, etc.) and in chromospheric (304 Å) wavelengths. Curvi-linear structures are automatically traced in those images with the OCCULT-2 code, to which we forward-fitted magnetic field lines computed with the Vertical-current Approximation Nonlinear Force Free Field code. We find that the chromospheric images: (1) reveal crisp curvi-linear structures (fibrils, loop segments, spicules) that are extremely well-suited for constraining magnetic modeling; (2) that these curvi-linear structures are field-aligned with the best-fit solution by a median misalignment angle of –7°; (3) the free energy computed from coronal data may underestimate that obtained from cromospheric data by a factor of –4, (4) the height range of chromospheric features is confined to km, while coronal features are detected up to h = 35,000 km; and (5) the plasma-β parameter is for all traced features. We conclude that chromospheric images reveal important magnetic structures that are complementary to coronal images and need to be included in comprehensive magnetic field models, something that is currently not accomodated in standard NLFFF codes.

DYNAMIC FLARING NON-POTENTIAL FIELDS ON QUIET SUN NETWORK SCALES

ABSTRACT We report on the identification of dynamic flaring non-potential structures on quiet Sun (QS) supergranular network scales. Data from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory allow for the high spatial and temporal resolution of this diverse class of compact structures. The rapidly evolving non-potential events presented here, with lifetimes <10 minutes, are on the order of 10″ in length. Thus, they contrast significantly with well-known active region (AR) non-potential structures such as high-temperature X-ray and EUV sigmoids (>100″) and micro-sigmoids (>10″) with lifetimes on the order of hours to days. The photospheric magnetic field environment derived from the Helioseismic and Magnetic Imager shows a lack of evidence for these flaring non-potential fields being associated with significant concentrations of bipolar magnetic elements. Of much interest to our events is the possibility of establishing them as precursor signatures of eruptive dynamics, similar to notions for AR sigmoids and micro-sigmoids, but associated with uneventful magnetic network regions. We suggest that the mixed network flux of QS-like magnetic environments, though unresolved, can provide sufficient free magnetic energy for flaring non-potential plasma structuring. The appearance of non-potential magnetic fields could be a fundamental process leading to self-organized criticality in the QS-like supergranular network and contribute to coronal heating, as these events undergo rapid helicial and vortical relaxations.

Blind Stereoscopy of the Coronal Magnetic Field

We test the feasibility of 3D coronal-loop tracing in stereoscopic EUV image pairs, with the ultimate goal of enabling efficient 3D reconstruction of the coronal magnetic field that drives flares and coronal mass ejections (CMEs). We developed an automated code designed to perform triangulation of coronal loops in pairs (or triplets) of EUV images recorded from different perspectives. The automated (or blind) stereoscopy code includes three major tasks: (i) automated pattern recognition of coronal loops in EUV images, (ii) automated pairing of corresponding loop patterns from two different aspect angles, and (iii) stereoscopic triangulation of 3D loop coordinates. We perform tests with simulated stereoscopic EUV images and quantify the accuracy of all three procedures. In addition we test the performance of the blind stereoscopy code as a function of the spacecraft-separation angle and as a function of the spatial resolution. We also test the sensitivity to magnetic non-potentiality. The automated code developed here can be used for analysis of existing {\sl Solar TErrestrial RElationship Observatory (STEREO)} data, but primarily serves for a design study of a future mission with dedicated diagnostics of non-potential magnetic fields. For a pixel size of 0.6\arcsec (corresponding to the {\sl Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA)} spatial resolution of 1.4\arcsec), we find an optimum spacecraft-separation angle of $\alpha_s \approx 5^\circ$.

Hard X-ray emitting energetic electrons and photospheric electric currents

The energy released during solar flares is believed to be stored in non-potential magnetic fields associated with electric currents flowing in the corona. While no measurements of coronal electric currents are presently available, maps of photospheric electric currents can now be derived from SDO/HMI observations. Photospheric electric currents have been shown to be the tracers of the coronal electric currents. Particle acceleration can result from electric fields associated with coronal electric currents. We revisit here some aspects of the relationship between particle acceleration in solar flares and electric currents in the active region. We study the relation between the energetic electron interaction sites in the solar atmosphere, and the magnitudes and changes of vertical electric current densities measured at the photospheric level, during the X2.2 flare on February 15 2011 in AR NOAA 11158. X-ray images from RHESSI are overlaid on magnetic field and electric current density maps calculated from the spectropolarimetric measurements of SDO/HMI using the UNNOFIT inversion and Metcalf disambiguation codes. X-ray images are also compared with EUV images from SDO/AIA to complement the flare analysis. Part of the elongated X-ray emissions from both thermal and non-thermal electrons overlay the elongated narrow current ribbons observed at the photospheric level. A new X-ray source at 50-100 keV (produced by non-thermal electrons) is observed in the course of the flare and is cospatial with a region in which new vertical photospheric currents appeared during the same period (increase of 15%). These observational results are discussed in the context of the scenarios in which magnetic reconnection (and subsequent plasma heating and particle acceleration) occurs at current-carrying layers in the corona.

High-speed photospheric material flow observed at the polarity inversion line of a δ-type sunspot producing an X5.4 flare on 2012 March 7

Abstract Solar flares abruptly release the free energy stored as a non-potential magnetic field in the corona and may be accompanied by eruptions of the coronal plasma. Formation of a non-potential magnetic field and the mechanisms for triggering the onset of flares are still poorly understood. In particular, photospheric dynamics observed near those polarity inversion lines that are sites of major flare production have not been well observed with high spatial resolution spectro-polarimetry. This paper reports on a remarkable high-speed material flow observed along the polarity inversion line located between flare ribbons at the main energy release side of an X5.4 flare on 2012 March 7. Observations were carried out by the spectro-polarimeter of the Solar Optical Telescope on board Hinode. The high-speed material flow was observed in the horizontally oriented magnetic field formed nearly parallel to the polarity inversion line. This flow persisted from at least six hours before the onset of the flare, and continued for at least several hours after the onset of the flare. Observations suggest that the observed material flow represents neither the emergence nor convergence of the magnetic flux. Rather, it may be considered to be material flow working both to increase the magnetic shear along the polarity inversion line and to develop magnetic structures favorable for the onset of the eruptive flare.

The Evolution of the Net Twist Current and the Net Shear Current in Active Region NOAA 10930

The electric current exists because of the non-potential magnetic field in solar active regions. We present the evolution of net current in the solar active region NOAA 10930 as the sum of shear current and twist current by using 27 high-resolution vector magnetograms obtained with Hinode/SOT-SP during 9 – 15 December 2006. This active region was highly eruptive and produced a large number of flares ranging from B to X class. We derived local distribution of shear and twist current densities in this active region and studied the evolution of net shear current (NSC) and net twist current (NTC) in the N-polarity and S-polarity regions separately. We found the following: i) The twist current density was dominant in the umbrae. ii) The footpoint of the emerging flux rope showed a dominant twist current. iii) The shear current density and twist current density appeared in alternate bands around the umbrae. iv) On the scale of the active region, NTC was always larger than NSC. v) Both NTC and NSC decreased after the onset of an X3.4 class flare that occurred on 13 December 2006.

NON-POTENTIAL FIELDS IN THE QUIET SUN NETWORK: EXTREME-ULTRAVIOLET AND MAGNETIC FOOTPOINT OBSERVATIONS

The quiet Sun (QS) magnetic network is known to contain dynamics which are indicative of non-potential fields. Non-potential magnetic fields forming S-shaped loop arcades can lead to the breakdown of static activity and have only been observed in high temperature X-ray coronal structures—some of which show eruptive behavior. Thus, analysis of this type of atmospheric structuring has been restricted to large-scale coronal fields. Here we provide the first identification of non-potential loop arcades exclusive to the QS supergranulation network. High-resolution Atmospheric Imaging Assembly data from the Solar Dynamics Observatory have allowed for the first observations of fine-scale S-shaped loop arcades spanning the network. We have investigated the magnetic footpoint flux evolution of these arcades from Heliospheric and Magnetic Imager data and find evidence of evolving footpoint flux imbalances accompanying the formation of these non-potential fields. The existence of such non-potentiality confirms that magnetic field dynamics leading to the build up of helicity exist at small scales. QS non-potentiality also suggests a self-similar formation process between the QS network and high temperature corona and the existence of self-organized criticality (SOC) in the form of loop-pair reconnection and helicity dissipation. We argue that this type of behavior could lead to eruptive forms of SOC as seen in active region (AR) and X-ray sigmoids if sufficient free magnetic energy is available. QS magnetic network dynamics may be considered as a coronal proxy at supergranular scales, and events confined to the network can even mimic those in coronal ARs.