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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

OBSERVATIONS OF MULTIPLE SURGES ASSOCIATED WITH MAGNETIC ACTIVITIES IN AR 10484 ON 2003 OCTOBER 25

We present a multi-wavelength study of recurrent surges observed in H{alpha}, UV (Solar and Heliospheric Observatory (SOHO)/EIT), and Radio (Learmonth, Australia) from the super-active region NOAA 10484 on 2003 October 25. Several bright structures visible in H{alpha} and UV corresponding to subflares are also observed at the base of each surge. Type III bursts are triggered and RHESSI X-ray sources are evident with surge activity. The major surge consists of bunches of ejective paths forming a fan-shaped region with an angular size of ( Almost-Equal-To 65 Degree-Sign ) during its maximum phase. The ejection speed reaches up to {approx}200 km s{sup -1}. The SOHO/Michelson Doppler Imager magnetograms reveal that a large dipole emerges from the east side of the active region on 2003 October 18-20, a few days before the surges. On 2003 October 25, the major sunspots were surrounded by 'moat regions' with moving magnetic features (MMFs). Parasitic fragmented positive polarities were pushed by the ambient dispersion motion of the MMFs and annihilated with negative polarities at the borders of the moat region of the following spot to produce flares and surges. A topology analysis of the global Sun using Potential Field Source Surface shows that the fan structures visible in the EIT 171 A images follow magnetic field lines connecting the present active region to a preceding active region in the southeast. Radio observations of Type III bursts indicate that they are coincident with the surges, suggesting that magnetic reconnection is the driver mechanism. The magnetic energy released by the reconnection is transformed into plasma heating and provides the kinetic energy for the ejections. A lack of a radio signature in the high corona suggests that the surges are confined to follow the closed field lines in the fans. We conclude that these cool surges may have some local heating effects in the closed loops, but probably play a minor role in global coronal heating and the surge material does not escape to the solar wind.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

DETECTION OF FLUX EMERGENCE, SPLITTING, MERGING, AND CANCELLATION OF NETWORK FIELD. I. SPLITTING AND MERGING

Frequencies of magnetic patch processes on the supergranule boundary, namely, flux emergence, splitting, merging, and cancellation, are investigated through automatic detection. We use a set of line-of-sight magnetograms taken by the Solar Optical Telescope (SOT) on board the Hinode satellite. We found 1636 positive patches and 1637 negative patches in the data set, whose time duration is 3.5 hr and field of view is 112'' Multiplication-Sign 112''. The total numbers of magnetic processes are as follows: 493 positive and 482 negative splittings, 536 positive and 535 negative mergings, 86 cancellations, and 3 emergences. The total numbers of emergence and cancellation are significantly smaller than those of splitting and merging. Further, the frequency dependence of the merging and splitting processes on the flux content are investigated. Merging has a weak dependence on the flux content with a power-law index of only 0.28. The timescale for splitting is found to be independent of the parent flux content before splitting, which corresponds to {approx}33 minutes. It is also found that patches split into any flux contents with the same probability. This splitting has a power-law distribution of the flux content with an index of -2 as a time-independent solution. These results support that the frequency distribution of the flux content in the analyzed flux range is rapidly maintained by merging and splitting, namely, surface processes. We suggest a model for frequency distributions of cancellation and emergence based on this idea.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

CALIBRATING 100 YEARS OF POLAR FACULAE MEASUREMENTS: IMPLICATIONS FOR THE EVOLUTION OF THE HELIOSPHERIC MAGNETIC FIELD

Although the Sun's polar magnetic fields are thought to provide important clues for understanding the 11 year sunspot cycle, including the observed variations of its amplitude and period, the current database of high-quality polar field measurements spans relatively few sunspot cycles. In this paper, we address this deficiency by consolidating Mount Wilson Observatory polar faculae data from four data reduction campaigns, validating it through a comparison with facular data counted automatically from Michelson Doppler Imager (MDI) intensitygrams, and calibrating it against polar field measurements taken by the Wilcox Solar Observatory and average polar field and total polar flux calculated using MDI line-of-sight magnetograms. Our results show that the consolidated polar facular measurements are in excellent agreement with both polar field and polar flux estimates, making them an ideal proxy to study the evolution of the polar magnetic field. Additionally, we combine this database with sunspot area measurements to study the role of the polar magnetic flux in the evolution of the heliospheric magnetic field (HMF). We find that there is a strong correlation between HMF and polar flux at solar minimum and that, taken together, polar flux and sunspot area are better at explaining the evolution of the HMF during the last century than sunspot area alone.

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DOE OSTI2012

Spatial structure of sunspot oscillations observed with SDO/AIA

The spatial structure of sunspot oscillations and its variation with frequency and height have been studied using data from SDO/AIA for two well-developed sunspots observed in 2010. Computation of potential magnetic fields together with line-of-sight and vector magnetograms from SDO/HMI allowed us to interpret discovered features of spatial structure. Namely, we have found that (1) expansion of the magnetic field lines above the sunspot causes a gradual broadening of the area occupied by the 3 minute oscillations with height, and (2) variation of magnetic field inclination across the sunspot causes a decrease in the pulsation frequency with distance from the center of the umbra. We have shown that the transformation from 3 minute umbral oscillations to 5 minute penumbral waves can be explained by variation of the acoustic cutoff frequency across the sunspot.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE COUPLED EVOLUTION OF ELECTRONS AND IONS IN CORONAL MASS EJECTION-DRIVEN SHOCKS

We present simulations of coronal mass ejections (CMEs) performed with a new two-temperature coronal model developed at the University of Michigan, which is able to address the coupled thermodynamics of the electron and proton populations in the context of a single fluid. This model employs heat conduction for electrons, constant adiabatic index ({gamma} = 5/3), and includes Alfven wave pressure to accelerate the solar wind. The Wang-Sheeley-Arge empirical model is used to determine the Alfven wave pressure necessary to produce the observed bimodal solar wind speed. The Alfven waves are dissipated as they propagate from the Sun and heat protons on open magnetic field lines to temperatures above 2 MK. The model is driven by empirical boundary conditions that includes GONG magnetogram data to calculate the coronal field, and STEREO/EUVI observations to specify the density and temperature at the coronal boundary by the Differential Emission Measure Tomography method. With this model, we simulate the propagation of fast CMEs and study the thermodynamics of CME-driven shocks. Since the thermal speed of the electrons greatly exceeds the speed of the CME, only protons are directly heated by the shock. Coulomb collisions low in the corona couple the protons and electrons allowing heat exchange between the two species. However, the coupling is so brief that the electrons never achieve more than 10% of the maximum temperature of the protons. We find that heat is able to conduct on open magnetic field lines and rapidly propagates ahead of the CME to form a shock precursor of hot electrons.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A GPU-COMPUTING APPROACH TO SOLAR STOKES PROFILE INVERSION

We present a new computational approach to the inversion of solar photospheric Stokes polarization profiles, under the Milne-Eddington model, for vector magnetography. Our code, named GENESIS, employs multi-threaded parallel-processing techniques to harness the computing power of graphics processing units (GPUs), along with algorithms designed to exploit the inherent parallelism of the Stokes inversion problem. Using a genetic algorithm (GA) engineered specifically for use with a GPU, we produce full-disk maps of the photospheric vector magnetic field from polarized spectral line observations recorded by the Synoptic Optical Long-term Investigations of the Sun (SOLIS) Vector Spectromagnetograph (VSM) instrument. We show the advantages of pairing a population-parallel GA with data-parallel GPU-computing techniques, and present an overview of the Stokes inversion problem, including a description of our adaptation to the GPU-computing paradigm. Full-disk vector magnetograms derived by this method are shown using SOLIS/VSM data observed on 2008 March 28 at 15:45 UT.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A METHOD FOR DATA-DRIVEN SIMULATIONS OF EVOLVING SOLAR ACTIVE REGIONS

We present a method for performing data-driven simulations of solar active region formation and evolution. The approach is based on magnetofriction, which evolves the induction equation assuming that the plasma velocity is proportional to the Lorentz force. The simulations of active region (AR) coronal field are driven by temporal sequences of photospheric magnetograms from the Helioseismic Magnetic Imager instrument on board the Solar Dynamics Observatory (SDO). Under certain conditions, the data-driven simulations produce flux ropes that are ejected from the modeled AR due to loss of equilibrium. Following the ejection of flux ropes, we find an enhancement of the photospheric horizontal field near the polarity inversion line. We also present a method for the synthesis of mock coronal images based on a proxy emissivity calculated from the current density distribution in the model. This method yields mock coronal images that are somewhat reminiscent of images of ARs taken by instruments such as SDO's Atmospheric Imaging Assembly at extreme ultraviolet wavelengths.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A NON-RADIAL ERUPTION IN A QUADRUPOLAR MAGNETIC CONFIGURATION WITH A CORONAL NULL

We report one of the several homologous non-radial eruptions from NOAA active region (AR) 11158 that are strongly modulated by the local magnetic field as observed with the Solar Dynamic Observatory. A small bipole emerged in the sunspot complex and subsequently created a quadrupolar flux system. Nonlinear force-free field extrapolation from vector magnetograms reveals its energetic nature: the fast-shearing bipole accumulated {approx}2 Multiplication-Sign 10{sup 31} erg free energy (10% of AR total) over just one day despite its relatively small magnetic flux (5% of AR total). During the eruption, the ejected plasma followed a highly inclined trajectory, over 60 Degree-Sign with respect to the radial direction, forming a jet-like, inverted-Y-shaped structure in its wake. Field extrapolation suggests complicated magnetic connectivity with a coronal null point, which is favorable of reconnection between different flux components in the quadrupolar system. Indeed, multiple pairs of flare ribbons brightened simultaneously, and coronal reconnection signatures appeared near the inferred null. Part of the magnetic setting resembles that of a blowout-type jet; the observed inverted-Y structure likely outlines the open field lines along the separatrix surface. Owing to the asymmetrical photospheric flux distribution, the confining magnetic pressure decreases much faster horizontally than upward. This special field geometry likely guided the non-radial eruption during its initial stage.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

OBSERVATIONS AND MAGNETIC FIELD MODELING OF A SOLAR POLAR CROWN PROMINENCE

We present observations and magnetic field modeling of the large polar crown prominence that erupted on 2010 December 6. Combination of Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) and STEREO{sub B}ehind/EUVI allows us to see the fine structures of this prominence both at the limb and on the disk. We focus on the structures and dynamics of this prominence before the eruption. This prominence contains two parts: an active region part containing mainly horizontal threads and a quiet-Sun part containing mainly vertical threads. On the northern side of the prominence channel, both AIA and EUVI observe bright features which appear to be the lower legs of loops that go above then join in the filament. Filament materials are observed to frequently eject horizontally from the active region part to the quiet-Sun part. This ejection results in the formation of a dense-column structure (concentration of dark vertical threads) near the border between the active region and the quiet Sun. Using the flux rope insertion method, we create nonlinear force-free field models based on SDO/Helioseismic and Magnetic Imager line-of-sight magnetograms. A key feature of these models is that the flux rope has connections with the surroundings photosphere, so its axial flux varies along the filament path. The height and location of the dips of field lines in our models roughly replicate those of the observed prominence. Comparison between model and observations suggests that the bright features on the northern side of the channel are the lower legs of the field lines that turn into the flux rope. We suggest that plasma may be injected into the prominence along these field lines. Although the models fit the observations quiet well, there are also some interesting differences. For example, the models do not reproduce the observed vertical threads and cannot explain the formation of the dense-column structure.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE BUILDUP OF A SCALE-FREE PHOTOSPHERIC MAGNETIC NETWORK

We use a global Monte Carlo simulation of the formation of the solar photospheric magnetic network to investigate the origin of the scale invariance characterizing magnetic flux concentrations visible on high-resolution magnetograms. The simulations include spatially and temporally homogeneous injection of small-scale magnetic elements over the whole photosphere, as well as localized episodic injection associated with the emergence and decay of active regions. Network elements form in response to cumulative pairwise aggregation or cancellation of magnetic elements, undergoing a random walk on the sphere and advected on large spatial scales by differential rotation and a poleward meridional flow. The resulting size distribution of simulated network elements is in very good agreement with observational inferences. We find that the fractal index and size distribution of network elements are determined primarily by these post-emergence surface mechanisms, and carry little or no memory of the scales at which magnetic flux is injected in the simulation. Implications for models of dynamo action in the Sun are briefly discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE ABRUPT CHANGES IN THE PHOTOSPHERIC MAGNETIC AND LORENTZ FORCE VECTORS DURING SIX MAJOR NEUTRAL-LINE FLARES

We analyze the spatial and temporal variations of the abrupt photospheric magnetic changes associated with six major flares using 12 minute, 0.''5 pixel{sup -1} vector magnetograms from NASA's Helioseismic and Magnetic Imager instrument on the Solar Dynamics Observatory satellite. The six major flares occurred near the main magnetic neutral lines of four active regions, NOAA 11158, 11166, 11283, and 11429. During all six flares the neutral-line field vectors became stronger and more horizontal, in each case almost entirely due to strengthening of the horizontal field components parallel to the neutral line. In all six cases the neutral-line pre-flare fields were more vertical than the reference potential fields, and collapsed abruptly and permanently closer to potential-field tilt angles during every flare, implying that the relaxation of magnetic stress associated with non-potential tilt angles plays a major role during major flares. The shear angle with respect to the reference potential field did not show such a pattern, demonstrating that flare processes do not generally relieve magnetic stresses associated with photospheric magnetic shear. The horizontal fields became significantly and permanently more aligned with the neutral line during the four largest flares, suggesting that the collapsing field is on average more aligned with the neutral line than the pre-flare neutral-line field. The vertical Lorentz force had a large, abrupt, permanent downward change during each of the flares, consistent with loop collapse. The horizontal Lorentz force changes acted mostly parallel to the neutral line in opposite directions on each side, a signature of the fields contracting during the flare, pulling the two sides of the neutral line toward each other. The greater effect of the flares on field tilt than on shear may be explained by photospheric line-tying.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE MORPHOLOGIC PROPERTIES OF MAGNETIC NETWORKS OVER THE SOLAR CYCLE 23

The morphologic properties of the magnetic networks during Carrington Rotations (CRs) 1955-2091 (from 1999 to 2010) have been analyzed by applying the watershed algorithm to magnetograms observed by the Michelson Doppler Interferometer on board the Solar and Heliospheric Observatory spacecraft. We find that the average area of magnetic cells on the solar surface at lower latitudes (within {+-}50 Degree-Sign ) is smaller than that at higher latitudes (beyond {+-}50 Degree-Sign ). Statistical analysis of these data indicates that the magnetic networks are fractal in nature and the average fractal dimension is D{sub f} = 1.253 {+-} 0.011. We also find that both the fractal dimension and the size of the magnetic networks are anti-correlated with the sunspot area. This is perhaps because a strong magnetic field can suppress spatially modulated oscillation and compress the boundaries of network cells, leading to smoother cell boundaries. The fractal dimension of the cell deviates from that predicted from an isobar of Kolmogorov k {sup -5/3} homogeneous turbulence.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

STUDY OF THE THREE-DIMENSIONAL CORONAL MAGNETIC FIELD OF ACTIVE REGION 11117 AROUND THE TIME OF A CONFINED FLARE USING A DATA-DRIVEN CESE-MHD MODEL

We apply a data-driven magnetohydrodynamics (MHD) model to investigate the three-dimensional (3D) magnetic field of NOAA active region (AR) 11117 around the time of a C-class confined flare that occurred on 2010 October 25. The MHD model, based on the spacetime conservation-element and solution-element scheme, is designed to focus on the magnetic field evolution and to consider a simplified solar atomsphere with finite plasma {beta}. Magnetic vector-field data derived from the observations at the photosphere is inputted directly to constrain the model. Assuming that the dynamic evolution of the coronal magnetic field can be approximated by successive equilibria, we solve a time sequence of MHD equilibria based on a set of vector magnetograms for AR 11117 taken by the Helioseismic and Magnetic Imager on board the Solar Dynamic Observatory around the time of the flare. The model qualitatively reproduces the basic structures of the 3D magnetic field, as supported by the visual similarity between the field lines and the coronal loops observed by the Atmospheric Imaging Assembly, which shows that the coronal field can indeed be well characterized by the MHD equilibrium in most cases. The magnetic configuration changes very little during the studied time interval of 2 hr. A topological analysis reveals that the small flare is correlated with a bald patch (BP, where the magnetic field is tangent to the photosphere), suggesting that the energy release of the flare can be understood by magnetic reconnection associated with the BP separatrices. The total magnetic flux and energy keep increasing slightly in spite of the flare, while the computed magnetic free energy drops during the flare by {approx}10{sup 30} erg, which seems to be adequate in providing the energy budget of a minor C-class confined flare.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

OBSERVATION OF HIGH-SPEED OUTFLOWS IN CORONAL LOOPS ASSOCIATED WITH PHOTOSPHERIC MAGNETIC FIELD EVOLUTION

Using SDO/AIA instruments, we provide an EUV observation of two adjacent loop strands (Loops 1 and 2) with one side of their footpoints rooted in the boundaries of active region (AR) NOAA 11158 and the other side in the quiet-Sun regions. The AR footpoints of Loop 1 were located in monopolar magnetic areas and those of Loop 2 in mixed polar areas (SDO/HMI magnetograms). There were no apparent outflows found in Loop 1 in 10 hr of observations, whereas in Loop 2, the outflows were detected throughout the whole observation with an average speed of 120-150 km s{sup -1}. We find clear evidence of magnetic reconnections occurring in the AR footpoints of Loop 2 (the opposite magnetic polarities came close and then a part of them disappeared) and magnetic flux dispersal in the quiet-Sun footpoints (a patch of positive polarities decayed with time). Furthermore, with Hinode/SOT observations, there were no significant Ca II H brightenings detected at the loop footpoints of Loop 2 at the chromospheric heights in response to those of the AIA 171 A and 304 A channels when four strong outflow events took place in the loops, which seem to differ from the conclusions of previous studies. In other studies, the rapid coronal outflows along the coronal loops were found to originate from the chromosphere through transient events (e.g., type II spicules).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ON THE RELATIONSHIP BETWEEN THE CORONAL MAGNETIC DECAY INDEX AND CORONAL MASS EJECTION SPEED

Numerical simulations suggest that kink and torus instabilities are two potential contributors to the initiation and prorogation of eruptive events. A magnetic parameter called the decay index (i.e., the coronal magnetic gradient of the overlying fields above the eruptive flux ropes) could play an important role in controlling the kinematics of eruptions. Previous studies have identified a threshold range of the decay index that distinguishes between eruptive and confined configurations. Here we advance the study by investigating if there is a clear correlation between the decay index and coronal mass ejection (CME) speed. Thirty-eight CMEs associated with filament eruptions and/or two-ribbon flares are selected using the H{alpha} data from the Global H{alpha} Network. The filaments and flare ribbons observed in H{alpha} associated with the CMEs help to locate the magnetic polarity inversion line, along which the decay index is calculated based on the potential field extrapolation using Michelson Doppler Imager magnetograms as boundary conditions. The speeds of CMEs are obtained from the LASCO C2 CME catalog available online. We find that the mean decay index increases with CME speed for those CMEs with a speed below 1000 km s{sup -1} and stays flat around 2.2 for the CMEs with higher speeds. In addition, we present a case study of a partial filament eruption, in which the decay indices show different values above the erupted/non-erupted part.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

NON-NEUTRALIZED ELECTRIC CURRENT PATTERNS IN SOLAR ACTIVE REGIONS: ORIGIN OF THE SHEAR-GENERATING LORENTZ FORCE

Using solar vector magnetograms of the highest available spatial resolution and signal-to-noise ratio, we perform a detailed study of electric current patterns in two solar active regions (ARs): a flaring/eruptive and a flare-quiet one. We aim to determine whether ARs inject non-neutralized (net) electric currents in the solar atmosphere, responding to a debate initiated nearly two decades ago that remains inconclusive. We find that well-formed, intense magnetic polarity inversion lines (PILs) within ARs are the only photospheric magnetic structures that support significant net current. More intense PILs seem to imply stronger non-neutralized current patterns per polarity. This finding revises previous works that claim frequent injections of intense non-neutralized currents by most ARs appearing in the solar disk but also works that altogether rule out injection of non-neutralized currents. In agreement with previous studies, we also find that magnetically isolated ARs remain globally current-balanced. In addition, we confirm and quantify the preference of a given magnetic polarity to follow a given sense of electric currents, indicating a dominant sense of twist in ARs. This coherence effect is more pronounced in more compact ARs with stronger PILs and must be of sub-photospheric origin. Our results yield a natural explanation of the Lorentz force, invariably generating velocity and magnetic shear along strong PILs, thus setting a physical context for the observed pre-eruption evolution in solar ARs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

OBSERVATIONS FROM SDO, HINODE, AND STEREO OF A TWISTING AND WRITHING START TO A SOLAR-FILAMENT-ERUPTION CASCADE

We analyze data from SDO (AIA, HMI), Hinode (SOT, XRT, EIS), and STEREO (EUVI) of a solar eruption sequence of 2011 June 1 near 16:00 UT, with an emphasis on the early evolution toward eruption. Ultimately, the sequence consisted of three emission bursts and two filament ejections. SDO/AIA 304 A images show absorbing-material strands initially in close proximity which over {approx}20 minutes form a twisted structure, presumably a flux rope with {approx}10{sup 29} erg of free energy that triggers the resulting evolution. A jump in the filament/flux rope's displacement (average velocity {approx}20 km s{sup -1}) and the first burst of emission accompanies the flux-rope formation. After {approx}20 more minutes, the flux rope/filament kinks and writhes, followed by a semi-steady state where the flux rope/filament rises at ({approx}5 km s{sup -1}) for {approx}10 minutes. Then the writhed flux rope/filament again becomes MHD unstable and violently erupts, along with rapid (50 km s{sup -1}) ejection of the filament and the second burst of emission. That ejection removed a field that had been restraining a second filament, which subsequently erupts as the second filament ejection accompanied by the third (final) burst of emission. Magnetograms from SDO/HMI and Hinode/SOT, and other data, reveal several possible causes for initiating the flux-rope-building reconnection, but we are not able to say which is dominant. Our observations are consistent with magnetic reconnection initiating the first burst and the flux-rope formation, with MHD processes initiating the further dynamics. Both filament ejections are consistent with the standard model for solar eruptions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ON THE INJECTION OF HELICITY BY THE SHEARING MOTION OF FLUXES IN RELATION TO FLARES AND CORONAL MASS EJECTIONS

An investigation of helicity injection by photospheric shear motions is carried out for two active regions (ARs), NOAA 11158 and 11166, using line-of-sight magnetic field observations obtained from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We derived the horizontal velocities in the ARs from the differential affine velocity estimator (DAVE) technique. Persistent strong shear motions at maximum velocities in the range of 0.6-0.9 km s{sup -1} along the magnetic polarity inversion line and outward flows from the peripheral regions of the sunspots were observed in the two ARs. The helicities injected in NOAA 11158 and 11166 during their six-day evolution period were estimated as 14.16 Multiplication-Sign 10{sup 42} Mx{sup 2} and 9.5 Multiplication-Sign 10{sup 42} Mx{sup 2}, respectively. The estimated injection rates decreased up to 13% by increasing the time interval between the magnetograms from 12 minutes to 36 minutes, and increased up to 9% by decreasing the DAVE window size from 21 Multiplication-Sign 18 to 9 Multiplication-Sign 6 pixel{sup 2}, resulting in 10% variation in the accumulated helicity. In both ARs, the flare-prone regions (R2) had inhomogeneous helicity flux distribution with mixed helicities of both signs and coronal mass ejection (CME) prone regions had almost homogeneous distribution of helicity flux dominated by a single sign. The temporal profiles of helicity injection showed impulsive variations during some flares/CMEs due to negative helicity injection into the dominant region of positive helicity flux. A quantitative analysis reveals a marginally significant association of helicity flux with CMEs but not flares in AR 11158, while for the AR 11166, we find a marginally significant association of helicity flux with flares but not CMEs, providing evidence of the role of helicity injection at localized sites of the events. These short-term variations of helicity flux are further discussed in view of possible flare-related effects. This study suggests that flux motions and spatial distribution of helicity injection are important to understanding the complex nature of the magnetic flux system of the AR, and how it can lead to conditions favorable for eruptive events.

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