Multiwavelength Study of a Solar Eruption from AR NOAA 11112 I. Flux Emergence, Sunspot Rotation and Triggering of a Solar Flare

We analyze the multiwavelength observations of an M2.9/1N flare that occurred in the active region (AR) NOAA 11112 in the vicinity of a huge filament system on 16 October 2010. SDO/HMI magnetograms reveal the emergence of a bipole (within the existing AR) 50 hours prior to the flare event. During the emergence, both the positive and negative sunspots in the bipole show translational as well as rotational motion. The positive-polarity sunspot shows significant motion/rotation in the south-westward/clockwise direction, and we see continuously pushing/sliding of the surrounding opposite-polarity field region. On the other hand, the negative-polarity sunspot moves/rotates in the westward/anticlockwise direction. The positive-polarity sunspot rotates ≈?70^° within 30 hours, whereas the one with negative polarity rotates ≈?20^° within 10 hours. SDO/AIA 94 Å EUV images show the emergence of a flux tube in the corona, consistent with the emergence of the bipole in HMI. The footpoints of the flux tube were anchored in the emerging bipole. The initial brightening starts at one of the footpoints (western) of the emerging loop system, where the positive-polarity sunspot pushes/slides towards a nearby negative-polarity field region. A high speed plasmoid ejection (speed ≈?1197 km?s^?1) was observed during the impulsive phase of the flare, which suggests magnetic reconnection of the emerging positive-polarity sunspot with the surrounding opposite-polarity field region. The entire AR shows positive-helicity injection before the flare event. Moreover, the newly emerging bipole reveals the signature of a negative (left-handed) helicity. These observations provide unique evidence of the emergence of twisted flux tubes from below the photosphere to coronal heights, triggering a flare mainly due to the interaction between the emerging positive-polarity sunspot and a nearby negative-polarity sunspot by the shearing motion of the emerging positive sunspot towards the negative one. Our observations also strongly support the idea that the rotation can most likely be attributed to the emergence of twisted magnetic fields, as proposed by recent models.     

The Magnetic Helicity Budget of a cme-Prolific Active Region

Coronal mass ejections (CMEs) are thought to be the way by which the solar corona expels accumulated magnetic helicity which is injected into the corona via several methods. DeVore (2000) suggests that a significant quantity is injected by the action of differential rotation, however Démoulin et al. (2002b), based on the study of a simple bipolar active region, show that this may not be the case. This paper studies the magnetic helicity evolution in an active region (NOAA 8100) in which the main photospheric polarities rotate around each other during five Carrington rotations. As a result of this changing orientation of the bipole, the helicity injection by differential rotation is not a monotonic function of time. Instead, it experiences a maximum and even a change of sign. In this particular active region, both differential rotation and localized shearing motions are actually depleting the coronal helicity instead of building it. During this period of five solar rotations, a high number of CMEs (35 observed, 65 estimated) erupted from the active region and the helicity carried away has been calculated, assuming that each can be modeled by a twisted flux rope. It is found that the helicity injected by differential rotation (–7×10⁴² Mx²) into the active region cannot provide the amount of helicity ejected via CMEs, which is a factor 5 to 46 larger and of the opposite sign. Instead, it is proposed that the ejected helicity is provided by the twist in the sub-photospheric part of the magnetic flux tube forming the active region.     

Twisted Flux Tube Emergence Evidenced in Longitudinal Magnetograms: Magnetic Tongues

Bipolar active regions (ARs) are thought to be formed by twisted flux tubes, as the presence of such twist is theoretically required for a cohesive rise through the whole convective zone. We use longitudinal magnetograms to demonstrate that a clear signature of a global magnetic twist is present, particularly, during the emergence phase when the AR is forming in a much weaker pre-existing magnetic field environment. The twist is characterised by the presence of elongated polarities, called “magnetic tongues”, which originate from the azimuthal magnetic field component. The tongues first extend in size before retracting when the maximum magnetic flux is reached. This implies an apparent rotation of the magnetic bipole. Using a simple half-torus model of an emerging twisted flux tube having a uniform twist profile, we derive how the direction of the polarity inversion line and the elongation of the tongues depend on the global twist in the flux rope. Using a sample of 40 ARs, we verify that the helicity sign, determined from the magnetic polarity distribution pattern, is consistent with the sign derived from the photospheric helicity flux computed from magnetogram time series, as well as from other proxies such as sheared coronal loops, sigmoids, flare ribbons and/or the associated magnetic cloud observed in situ at 1 AU. The evolution of the tongues observed in emerging ARs is also closely similar to the evolution found in recent MHD numerical simulations. We also found that the elongation of the tongue formed by the leading magnetic polarity is significantly larger than that of the following polarity. This newly discovered asymmetry is consistent with an asymmetric Ω-loop emergence, trailing the solar rotation, which was proposed earlier to explain other asymmetries in bipolar ARs.     

Evaluation of a Selected Case of the Minimum Dissipative Rate Method for Non-Force-Free Solar Magnetic Field Extrapolations

The minimum dissipative rate (MDR) method for deriving a coronal non-force-free magnetic field solution is partially evaluated. These magnetic field solutions employ a combination of three linear (constant-α) force-free-field solutions with one being a potential field (i.e., α=0). The particular case of the solutions where the other two α’s are of equal magnitude but of opposite sign is examined. This is motivated by studying the SOLIS (Synoptic Optical Long-term Investigation of the Sun (SOLIS), a National Solar Observatory facility) vector magnetograms of AR 10987, which show a global α value consistent with an α=0 value as evaluated by (∇×B) _z /B _z over the region. Typical of the current state of the observing technology, there is no definitive twist for input into the general MDR method. This suggests that the special α case, of two α’s with equal magnitudes and opposite signs, is appropriate given the data. Only for an extensively twisted active region does a dominant, nonzero α normally emerge from a distribution of local values. For a special set of conditions, is it found that (i) the resulting magnetic field is a vertically inflated magnetic field resulting from the electric currents being parallel to the photosphere, similar to the results of Gary and Alexander (Solar Phys. 186:123, 1999), and (ii) for α≈(α _max/2), the Lorentz force per unit volume normalized by the square of the magnetic field is on the order of 1.4×10⁻¹⁰ cm⁻¹. The Lorentz force (F _L) is a factor of ten higher than that of the magnetic force d(B ²/8π)/dz, a component of F _L. The calculated photospheric electric current densities are an order of magnitude smaller than the maximum observed in all active regions. Hence both the Lorentz force density and the generated electric current density seem to be physically consistent with possible solar dynamics. The results imply that the field could be inflated with an overpressure along the neutral line. However, the implementation of this or any other extrapolation method using the electric current density as a lower boundary condition must be done cautiously, with the current magnetography.     

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

We present the multiwavelength 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-based 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 near 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 ≈0.15R _⊙ with a speed of ≈90 km s⁻¹ 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 found 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 a couple of days earlier 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 the PIL helped in the build-up of magnetic energy at the flare and flux rope activation site. This study provides 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.     

Relationship between Rotating Sunspots and Flares

Active Region (AR) NOAA 10486 was a super AR in the declining phase of solar cycle 23. Dominated by the rapidly rotating positive polarity of an extensive δ sunspot, it produced several powerful flare-CMEs. We study the evolution and properties of the rotational motion of the major poles of positive polarities and estimate the accumulated helicity injected by them. We also present two homologous flares that occurred in the immediate periphery of the rotating sunspots. The main results are as follows: i) anticlockwise rotational motions are identified in the main poles of positive polarities in the AR; the fastest of them is about 220° for six days. ii) The helicity injection inferred from such rotational motion during the interval from October 25 to 30 is about − 3.0×10⁴³ Mx², which is comparable that calculated by the local correlation tracking (LCT) method (− 5.2×10⁴³ Mx²) in the whole AR. It is suggested that both methods reveal the essential topological properties of the AR, even if the former includes only the major poles and the fine features of the magnetic field are neglected. iii) It is found that there is a good spatial and temporal correspondence between the onset of two homologous CME-associated flares and the rotational motion of sunspots. This suggests that the rotational motions of sunspots not only relate to the transport of magnetic energy and complexity from the low atmosphere to the corona but may also play a key role in the onset of the homologous flares. Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users.     

Conservation of both current and helicity in a quadrupolar model for solar flares

A model for a solar flare, involving magnetic reconnection transferring flux and current between current-carrying magnetic loops connecting two pairs of footpoints, is generalized to include conservation of magnetic helicity during reconnection, as well as conservation of current at all four footpoints. For a set of force-free loops, with the ith loop having flux F _i and current I _i, the self and mutual helicities are proportional to the self and mutual inductances with the constant of proportionality determined by α_i=F _i/μ₀ I _i. In a constant-α model, the change in magnetic energy is proportional to the change in helicity, and conservation of helicity implies conservation of magnetic energy, so that a flare cannot occur. In a quadrupolar model, with α₁>α₂ initially, α₁ increases and α₂ decreases when flux and current are transferred from loops 1 and 2 to loops 3 and 4. A model that conserves both current and helicity is constructed; it depends on the initial αs, and otherwise is somewhat simpler than when helicity is neglected.     

<Emphasis Type="Italic">In Situ</Emphasis> Signatures of Interchange Reconnection between Magnetic Clouds and Open Magnetic Fields: A Mechanism for the Erosion of Polar Coronal Holes?

We outline a method to determine the direction of solar open flux transport that results from the opening of magnetic clouds (MCs) by interchange reconnection at the Sun based solely on in-situ observations. This method uses established findings about i) the locations and magnetic polarities of emerging MC footpoints, ii) the hemispheric dependence of the helicity of MCs, and iii) the occurrence of interchange reconnection at the Sun being signaled by uni-directional suprathermal electrons inside MCs. Combining those observational facts in a statistical analysis of MCs during solar cycle 23 (period 1995 – 2007), we show that the time of disappearance of the northern polar coronal hole (1998 – 1999), permeated by an outward-pointing magnetic field, is associated with a peak in the number of MCs originating from the northern hemisphere and connected to the Sun by outward-pointing magnetic field lines. A similar peak is observed in the number of MCs originating from the southern hemisphere and connected to the Sun by inward-pointing magnetic field lines. This pattern is interpreted as the result of interchange reconnection occurring between MCs and the open field lines of nearby polar coronal holes. This reconnection process closes down polar coronal hole open field lines and transports these open field lines equatorward, thus contributing to the global coronal magnetic field reversal process. These results will be further constrainable with the rising phase of solar cycle 24.     

Deflection of Coronal Rays by Remote CMEs: Shock Wave or Magnetic Pressure?

We analyze five events of the interaction of coronal mass ejections (CMEs) with the remote coronal rays located up to 90° away from the CME as observed by the SOHO/LASCO C2 coronagraph. Using sequences of SOHO/LASCO C2 images, we estimate the kink propagation in the coronal rays during their interaction with the corresponding CMEs ranging from 180 to 920 km s⁻¹ within the interval of radial distances from 3 R _⊙ to 6 R _⊙. We conclude that all studied events do not correspond to the expected pattern of shock wave propagation in the corona. Coronal ray deflection can be interpreted as the influence of the magnetic field of a moving flux rope within the CME. The motion of a large-scale flux rope away from the Sun creates changes in the structure of surrounding field lines, which are similar to the kink propagation along coronal rays. The retardation of the potential should be taken into account since the flux rope moves at a high speed, comparable with the Alfvén speed.     

Magnetic Helicity Budget of Solar-Active Regions from the Photosphere to Magnetic Clouds

We have analyzed the long-term evolution of two active regions (ARs) from their emergence through their decay using observations from several instruments on board SoHO (MDI, EIT and LASCO) and Yohkoh/SXT. We have computed the evolution of the relative coronal magnetic helicity combining data from MDI and SXT with a linear force-free model of the coronal magnetic field. Next, we have computed the injection of helicity by surface differential rotation using MDI magnetic maps. To estimate the depletion of helicity we have counted all the CMEs of which these ARs have been the source, and we have evaluated their magnetic helicity assuming a one to one correspondence with magnetic clouds with an average helicity contain. When these three values (variation of coronal magnetic helicity, injection by differential rotation and ejection via CMEs) are compared, we find that surface differential rotation is a minor contributor to the helicity budget since CMEs carry away at least 10 times more helicity than the one differential rotation can provide. Therefore, the magnetic helicity flux needed in the global balance should come from localized photospheric motions that, at least partially, reflect the emergence of twisted flux tubes. We estimate that the total helicity carried away in CMEs can be provided by the end-to-end helicity of the flux tubes forming these ARs. Therefore, we conclude that most of the helicity ejected in CMEs is generated below the photosphere and emerges with the magnetic flux.     

Predictions of Energy and Helicity in Four Major Eruptive Solar Flares

In order to better understand the solar genesis of interplanetary magnetic clouds (MCs), we model the magnetic and topological properties of four large eruptive solar flares and relate them to observations. We use the three-dimensional Minimum Current Corona model (Longcope, 1996, Solar Phys. 169, 91) and observations of pre-flare photospheric magnetic field and flare ribbons to derive values of reconnected magnetic flux, flare energy, flux rope helicity, and orientation of the flux-rope poloidal field. We compare model predictions of those quantities to flare and MC observations, and within the estimated uncertainties of the methods used find the following: The predicted model reconnection fluxes are equal to or lower than the reconnection fluxes inferred from the observed ribbon motions. Both observed and model reconnection fluxes match the MC poloidal fluxes. The predicted flux-rope helicities match the MC helicities. The predicted free energies lie between the observed energies and the estimated total flare luminosities. The direction of the leading edge of the MC’s poloidal field is aligned with the poloidal field of the flux rope in the AR rather than the global dipole field. These findings compel us to believe that magnetic clouds associated with these four solar flares are formed by low-corona magnetic reconnection during the eruption, rather than eruption of pre-existing structures in the corona or formation in the upper corona with participation of the global magnetic field. We also note that since all four flares occurred in active regions without significant pre-flare flux emergence and cancelation, the energy and helicity that we find are stored by shearing and rotating motions, which are sufficient to account for the observed radiative flare energy and MC helicity.     

Progressive Transformation of a Flux Rope to an ICME

The solar wind conditions at one astronomical unit (AU) can be strongly disturbed by interplanetary coronal mass ejections (ICMEs). A subset, called magnetic clouds (MCs), is formed by twisted flux ropes that transport an important amount of magnetic flux and helicity, which is released in CMEs. At 1 AU from the Sun, the magnetic structure of MCs is generally modeled by neglecting their expansion during the spacecraft crossing. However, in some cases, MCs present a significant expansion. We present here an analysis of the huge and significantly expanding MC observed by the Wind spacecraft during 9 – 10 November 2004. This MC was embedded in an ICME. After determining an approximate orientation for the flux rope using the minimum variance method, we obtain a precise orientation of the cloud axis by relating its front and rear magnetic discontinuities using a direct method. This method takes into account the conservation of the azimuthal magnetic flux between the inbound and outbound branches and is valid for a finite impact parameter (i.e., not necessarily a small distance between the spacecraft trajectory and the cloud axis). The MC is also studied using dynamic models with isotropic expansion. We have found (6.2±1.5)×10²⁰ Mx for the axial flux and (78±18)×10²⁰ Mx for the azimuthal flux. Moreover, using the direct method, we find that the ICME is formed by a flux rope (MC) followed by an extended coherent magnetic region. These observations are interpreted by considering the existence of a previously larger flux rope, which partially reconnected with its environment in the front. We estimate that the reconnection process started close to the Sun. These findings imply that the ejected flux rope is progressively peeled by reconnection and transformed to the observed ICME (with a remnant flux rope in the front part).     

Comparison of Helicity Signs in Interplanetary CMEs and Their Solar Source Regions

If all coronal mass ejections (CMEs) have flux ropes, then the CMEs should keep their helicity signs from the Sun to the Earth according to the helicity conservation principle. This study presents an attempt to answer the question from the Coordinated Data Analysis Workshop (CDAW), “Do all CMEs have flux ropes?”, by using a qualitative helicity sign comparison between interplanetary CMEs (ICMEs) and their CME source regions. For this, we select 34 CME–ICME pairs whose source active regions (ARs) have continuous SOHO/MDI magnetogram data covering more than 24 hr without data gap during the passage of the ARs near the solar disk center. The helicity signs in the ARs are determined by estimation of cumulative magnetic helicity injected through the photosphere in the entire source ARs. The helicity signs in the ICMEs are estimated by applying the cylinder model developed by Marubashi (Adv. Space. Res., 26, 55, 2000) to 16 second resolution magnetic field data from the MAG instrument onboard the ACE spacecraft. It is found that 30 out of 34 events (88 %) are helicity sign-consistent events, while four events (12 %) are sign-inconsistent. Through a detailed investigation of the source ARs of the four sign-inconsistent events, we find that those events can be explained by the local helicity sign opposite to that of the entire AR helicity (28 July 2000 ICME), incorrectly reported solar source region in the CDAW list (20 May 2005 ICME), or the helicity sign of the pre-existing coronal magnetic field (13 October 2000 and 20 November 2003 ICMEs). We conclude that the helicity signs of the ICMEs are quite consistent with those of the injected helicities in the AR regions from where the CMEs erupted.     

Dirac neutrino magnetic moment and possible time evolution of the neutrino signal from a supernova

We analyze the influence of neutrino helicity conversion, ν _L → ν _R, on the neutrino flux from a supernova attributable to the interaction of the Dirac neutrino magnetic moment with a magnetic field.We show that if the neutrino has a magnetic moment in the interval 10⁻¹³μ_B < μ_ν < 10⁻¹²μ_B and provided that a magnetic field of ∼10¹³–10¹⁴ G exists in the supernova envelope, a peculiar kind of time evolution of the neutrino signal from the supernova attributable to the resonance transition ν _L → ν _R in the magnetic field of the envelope can appear.     

Scaling laws in decaying helical hydromagnetic turbulence

We study the evolution of growth and decay laws for the magnetic field coherence length ξ, energy E_M and magnetic helicity H in freely decaying 3D MHD turbulence. We show that with certain assumptions, self‐similarity of the magnetic power spectrum alone implies that ξ ∼ t^1/2. This in turn implies that magnetic helicity decays as H ∼ t^–2s, where s = (ξ_diff/ξ_H)², in terms of ξ_diff, the diffusion length scale, and ξ_H, a length scale defined from the helicity power spectrum. The relative magnetic helicity remains constant, implying that the magnetic energy decays as E_M ∼ t^–1/2–2s. The parameter s is inversely proportional to the magnetic Reynolds number Re_M, which is constant in the self‐similar regime. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nonhelical mean‐field dynamos in a sheared turbulence

Mechanisms of nonhelical large‐scale dynamos (shear‐current dynamo and effect of homogeneous kinetic helicity fluctuations with zero mean) in a homogeneous turbulence with large‐scale shear are discussed. We have found that the shearcurrent dynamo can act even in random flows with small Reynolds numbers. However, in this case mean‐field dynamo requires small magnetic Prandtl numbers (i.e., when Pm < Pm^cr < 1). The threshold in the magnetic Prandtl number, Pm^cr = 0.24, is determined using second order correlation approximation (or first‐order smoothing approximation) for a background random flow with a scale‐dependent viscous correlation time τ_c = (νk 2)^–1 (where ν is the kinematic viscosity of the fluid and k is the wave number). For turbulent flows with large Reynolds numbers shear‐current dynamo occurs for arbitrary magnetic Prandtl numbers. This dynamo effect represents a very generic mechanism for generating large‐scale magnetic fields in a broad class of astrophysical turbulent systems with large‐scale shear. On the other hand, mean‐field dynamo due to homogeneous kinetic helicity fluctuations alone in a sheared turbulence is not realistic for a broad class of astrophysical systems because it requires a very specific random forcing of kinetic helicity fluctuations that contains, e.g., low‐frequency oscillations. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

How Can a Negative Magnetic Helicity Active Region Generate a Positive Helicity Magnetic Cloud?

The geoeffective magnetic cloud (MC) of 20 November 2003 was associated with the 18 November 2003 solar active events in previous studies. In some of these, it was estimated that the magnetic helicity carried by the MC had a positive sign, as did its solar source, active region (AR) NOAA 10501. In this article we show that the large-scale magnetic field of AR 10501 has a negative helicity sign. Since coronal mass ejections (CMEs) are one of the means by which the Sun ejects magnetic helicity excess into interplanetary space, the signs of magnetic helicity in the AR and MC must agree. Therefore, this finding contradicts what is expected from magnetic helicity conservation. However, using, for the first time, correct helicity density maps to determine the spatial distribution of magnetic helicity injections, we show the existence of a localized flux of positive helicity in the southern part of AR 10501. We conclude that positive helicity was ejected from this portion of the AR leading to the observed positive helicity MC.     

Study of Ribbon Separation and Magnetic Reconnection Rates in a Two-Ribbon Flare

In this paper, we present a study of the correlation between the speed of flare ribbon separation and the magnetic flux density during the 10 April 2001 solar flare. The study includes the section of the neutral line containing the flare core and its peripheral area. This event shows clear two-ribbon structure and inhomogeneous magnetic fields along the ribbons, so the spatial correlation and distribution of the flare and magnetic parameters can be studied. A weak negative correlation is found between the ribbon separation speed (V _r) and the longitudinal magnetic flux density (B _z ). This correlation is the weakest around the peak of the flare. Spatially, the correlation is also weakest at the positions of the hard X-ray (HXR) sources. In addition, we estimate the magnetic reconnection rate (electric field strength in the reconnection region E _rec) by combining the speed of flare ribbons and the longitudinal magnetic flux density. During flare evolution, the time profiles of the magnetic reconnection rate are similar to that of the ribbon separation speed, and the speeds of ribbon separation are relatively slow in the strong magnetic fields (i.e., V _r is negatively correlated with B _z ). However, along the flare ribbons, E _rec fluctuates in a small range except near the HXR source. A localized enhancement of the reconnection rate corresponds to the position of the HXR source.     

Modeling and Measuring the Flux Reconnected and Ejected by the Two-Ribbon Flare/CME Event on 7 November 2004

Observations of the large two-ribbon flare on 7 November 2004 made using SOHO and TRACE data are interpreted in terms of a three-dimensional magnetic field model. Photospheric flux evolution indicates that ?1.4×10⁴³ Mx² of magnetic helicity was injected into the active region during the 40-hour buildup prior to the flare. The magnetic model places a lower bound of 8×10³¹ ergs on the energy stored by this motion. It predicts that 5×10²¹ Mx of flux would need to be reconnected during the flare to release the stored energy. This total reconnection compares favorably with the flux swept up by the flare ribbons, which we measure using high-time-cadence TRACE images in 1?600 Å. Reconnection in the model must occur in a specific sequence that would produce a twisted flux rope containing significantly less flux and helicity (10²¹ Mx and ?3×10⁴² Mx², respectively) than the active region as a whole. The predicted flux compares favorably with values inferred from the magnetic cloud observed by Wind. This combined analysis yields the first quantitative picture of the flux processed through a two-ribbon flare and coronal mass ejection.