Magnetic Neutral Line Research Articles

EVOLUTION OF OPTICAL PENUMBRAL AND SHEAR FLOWS ASSOCIATED WITH THE X3.4 FLARE OF 2006 DECEMBER 13

The rapid and irreversible decay of penumbrae related to X-class flares has been found in a number of studies. Since the optical penumbral flows are closely associated with the morphology of sunspot penumbra, we use state-of-the-art Hinode data to track penumbral flows in flaring active regions as well as shear flows close to the flaring neutral line. This paper concentrates on AR 10930 around the time of an X3.4 flare on 2006 December 13. We utilize the seeing-free solar optical telescope G-band data as a tracer to obtain the horizontal component of the penumbral and shear flows by local correlation tracking, and Stokes-V data to register positive and negative magnetic elements along the magnetic neutral line. We find that: (1) an obvious penumbral decay appears in this active region intimately associated with the X3.4 flare; (2) the mean magnitude of the horizontal speeds of the penumbral flows within the penumbral decay areas temporally and spatially varies from 0.6 to 1.1 km s?1; (3) the penumbral flow decreases before the flare eruption in two of the four penumbral decay areas; (4) the mean shear flows along the magnetic neutral line of this ?-sunspot started to decrease before the flare and continue to decrease for another hour after the flare. The magnitude of this flow apparently dropped from 0.6 to 0.3 km s?1. We propose that the decays of the penumbra and the penumbral flow are related to the magnetic rearrangement involved in the coronal mass ejection/flare events.

MAGNETIC CHANGES IN THE COURSE OF THE X7.1 SOLAR FLARE ON 2005 JANUARY 20

Rapid magnetic changes in the course of the X7.1 solar flare on 2005 January 20 at the photosphere in the host active region (AR), NOAA AR 10720, are diagnosed. The database for this study consists of Huairou vector magnetograms, Transition Region and Coronal Explorer (TRACE) white light and UV/EUV images, RHESSI hard X-ray, and Solar and Heliospheric Observatory EUV observations. For such an event that is close to but not on the solar west limb (N12 W58), the projection effects in the observed vector magnetograms are untangled by combining an intuitive geometric analysis and a transformation of the magnetograms into the heliographic coordination system. The magnetic changes in the horizontal magnetic fields are emphasized. We find definitive evidence of weakening in the horizontal magnetic fields in a few isolated patches in the outskirts of this ?-sunspot group and strengthening in the horizontal fields (HFs) in an extended area centralized at the magnetic neutral line between major sunspots of opposite polarities. The rapid magnetic changes take place at the level of 100-300 G, several factors of ten of the noise level. The identified HF changes are consistent with the darkening of inner penumbrae and weakening of outer penumbrae in this ?-sunspot group. The enhanced HFs spatially coincide with the TRACE 1550 ? rope-like structures lying low above the magnetic neutral line. Unexpectedly, during the flare, the lower lying rope-like structures remain in place, though they exhibit episodic disturbance and brightening, while the outer EUV loops are impulsively expanding. The rapid magnetic changes manifest an impulsive input of free magnetic energy in the photosphere, resulting from an impulsive growth of a new emerging flux region (EFR) along the magnetic neutral line. The facts of the increasing core fields in magnetic nonpotentiality, the continued disturbance of the inner rope-like structures, and the breakout of the outer loops during the major flare cannot be interpreted by any single flare model. However, the nature of magnetohydrodynamical catastrophe is clearly implied for the flare triggering.

An interpretation of rapid changes in the magnetic field associated with solar flares

AbstractThe energy source of a flare is the magnetic field in the corona. A topological model of the magnetic field is used here for interpreting the recently discovered drastic changes in magnetic field associated with solar flares. The following observational results are self‐consistently explained: (1) the transverse field strength decreases at outer part of active regions and increases significantly in their centers; (2) the center‐of‐mass positions of opposite magnetic polarities converge towards the magnetic neutral line just after flares onset; (3) the magnetic flux of active regions decreases steadily during the course of flares. For X‐class flares, almost 50% events show such changes. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Lorentz Force: A Possible Driving Force for Sunspot Rotation

Zhao and Kosovichev (Astrophys. J. 591, 446, 2003) found two opposite sub-photospheric vortical flows in the depth range of 0-12 Mm around a fast rotating sunspot. So far there is no theoretical model explaining such flow motions. In this paper, we try to explain this phenomenon from the point of view of magnetic flux tubes interacting with large-scale vortical motions of plasma. In the deeper zone under the photosphere, the magnetic force may be less than the nonmagnetic force of plasma. The vortical flow located there twists the flux tube and magnetic free energy is built up in the tube. In the shallower zone under the photosphere, the magnetic force may be greater than the nonmagnetic force. Thus, part of the stored magnetic free energy is released to drive the plasma to rotate in two opposite directions, e.g., in the depth ranges of 0-3(5) and 9-12 Mm. In addition, we also define a vector of nonpotential magnetic stress tau, which can be related to flare occurrence. It is calculated for the active region NOAA 10930 on 11 December 2006. We find that: i) the integral of its line-of-sight (LOS) stress successively increases around the magnetic neutral line (MNL) prior to and during the flare and decreases to a minimum after the flare; ii) the integral of its transverse stress exceeds the integral of its LOS component by one order of magnitude over the whole field of view; iii) the transverse stress first points toward the MNL, then along it, and finally it points away from it. We need other data to verify whether or not the magnetic energy is transported in the horizontal direction to the neutral line, and then partly changes into the energy in LOS direction before and during the flare.

Cooperative Observation of Ellerman Bombs between the Solar Optical Telescope aboard Hinode and Hida/Domeless Solar Telescope

Abstract High-resolution CaIIH broad-band filter images of NOAA10933 on 2007 January 5 were obtained by the Solar Optical Telescope aboard the Hinode satellite. Many small-scale ($\sim$1") bright points were observed outside the sunspot and inside the emerging flux region. We identified some of these bright points with Ellerman bombs (EBs) by using H$\alpha$ images taken by the Domeless Solar Telescope at Hida observatory. The sub-arcsec structures of two EBs seen in CaIIH were studied in detail. Our observation showed the following two aspects: (1) The CaIIH bright points identified with EBs were associated with the bipolar magnetic field structures, as reported by previous studies. (2) The structure of the CaIIH bright points turned out to consist of the following two parts: a central elongated bright core (0.7" $\times$ 0.5") located along the magnetic neutral line and a diffuse halo (1.2" $\times$ 1.8").

The Origin of Polar Streamers in the Solar Corona

We investigate the large-scale three-dimensional (3D) structure of the solar corona near the maximum of the 23rd solar cycle in an attempt to determine the origin of polar streamers. We use a model that allows us to simulate the quasi-stationary configuration of the large-scale coronal density distribution. The coronal neutral line, as given by the potential field source surface (PFSS) model, serves as a proxy for mid- and low-latitude current sheets. We investigate the contribution of possible polar coronal current sheets associated with large-scale photospheric magnetic neutral lines around the poles of the Sun (polar crown neutral lines). Positions of polar neutral lines are radially extrapolated outward to obtain the configuration of polar current sheets. Coronal plasma sheets are centered around introduced current sheets. Streamer positions during Carrington rotation 1965, near the activity maximum, are calculated. Simulated synoptic maps of the coronal brightness are compared with those obtained from observations by the LASCO C2 coronagraph on board the SOHO spacecraft. We demonstrate that polar streamers are classical streamers situated above low-lying loops (observed by SOHO EIT) connecting the regions of opposite magnetic polarity on two sides of polar crown neutral lines. Polar streamer configurations obtained from our model are close to those observed by LASCO. Our results suggest that the PFSS model cannot adequately describe the configuration of streamers during the epoch of high solar activity. The representation of the streamer belt as a single tilted and warped current sheet becomes questionable. Multiple coronal current sheets may better correspond to the observed streamer configurations.

Moat Flow in the Vicinity of Sunspots for Various Penumbral Configurations

High-resolution time series of sunspots have been obtained with the Swedish 1m Solar Telescope between 2003 and 2006 at different locations on the solar disc. Proper motions in seven different active regions have been studied. The analysis has been done by applying local correlation tracking to every series of sunspots, each of them more than 40 minutes long. The sunspots' shapes include a different variety of penumbral configurations. We report on a systematic behaviour of the large-scale outflows surrounding the sunspots, commonly known as moat flows, that are essentially present only when preceded by a penumbra not tangential but perpendicular to the sunspot border. We present one case for which this rule appears not to be confirmed. We speculate that the magnetic neutral line, which is located in the vicinity of the anomalous region, might be responsible for blocking the outflow. These new results confirm the systematic and strong relation between the moat flows and the existence of penumbrae. A comparative statistical study between moats and standard granulation is also performed.

Addendum to: “Current sheet formation near a hyperbolic magnetic neutral line in a variable-density plasma: An exact solution” [Phys. Lett. A 366 (2007) 97

Addendum to: “Current sheet formation near a hyperbolic magnetic neutral line in a variable-density plasma: An exact solution” [Phys. Lett. A 366 (2007) 97

Electron flat‐top distributions around the magnetic reconnection region

Cluster multisatellite observations provide snapshots of electron distributions around the magnetic neutral line. An isotropic flat‐top‐type electron distribution in phase space is frequently observed around the X line, together with large ion velocities and a Hall quadrupole‐like magnetic field inside the hot and tenuous plasma sheet in the magnetotail. The flat‐top distributions are also associated with a finite magnetic field in the direction normal to the neutral sheet, and the cross‐tail current density is sometimes very small. These results indicate that the flat‐top‐type distribution is mainly located near the outer boundary of the ion diffusion region in the plasma sheet outflow region, before reaching the pileup region with large normal component of the magnetic field. Simultaneously with the flat‐top distributions, strong field‐aligned electron beams mainly toward the X line are occasionally observed. This type of beam is mainly observed in the off‐equatorial plasma sheet and also appears well inside the plasma sheet. Typical energies of the beam are 4–10 keV, which is comparable to the upper edge of flat‐top energy. These highly accelerated electron distributions have a steep decrease in phase space density at the high‐energy end, and it is found that they are not correlated with the increase of the higher‐energy electrons related to suprathermal acceleration (>30 keV). This result indicates that the electron acceleration processes for the flat‐top‐type distributions are different from the suprathermal components, both of which are beyond the conventional MHD outflow acceleration and considered to be associated with some kinetic processes.

The Radio Signature of Magnetic Reconnection for the M-Class Flare of 2004 November 1

In this Letter, we present radio diagnostics for the magnetic field and the density distribution of nonthermal electrons in the microwave source of the M1.1 flare of 2004 November 1. We calculate the nonthermal gyrosynchrotron radiation using observations made at the Nobeyama Radio Observatory to get the longitudinal magnetic field and the magnitude of the transverse magnetic field. The computed longitudinal magnetic field at two hard X-ray footpoints has opposite polarities, which basically agrees with SOHO MDI observations. The main finding is that the magnitude of the transverse magnetic field summed around the magnetic neutral line of the SOHO MDI magnetogram has a short-term impulsive increase during the rising phase of the flare. We propose that the increase of the transverse magnetic field may be considered evidence of the magnetic reconnection process in this flare. The computed mean angle between the magnetic field and line of sight decreases steadily during the flare, clearly indicating the reconfiguration of the magnetic field, presumably the relaxation of the sheared magnetic field. Moreover, the calculated density of nonthermal electrons increases from footpoints to looptop, while the spectral index calculated by the Nobeyama Radio Heliograph at 17 and 34 GHz decreases from footpoints ( soft) to looptop ( hard), which may confirm that the reconnection site is close to the looptop source.

Collisionless magnetic reconnection dynamics with electron inertia and parallel electron compressibility

AbstractCollisionless magnetic reconnection dynamics is considered by including the effects of electron inertia as well as parallel electron compressibility. A fluid treatment is adopted for both electrons and ions. Collisionless plasma dynamics properties near a two-dimensional X-type magnetic neutral line in the steady state are explored. The effects of electron inertia and parallel electron compressibility on the hyperbolicity (or lack thereof) of the magnetic field lines in the neutral layer are discussed. A unified linear tearing-mode formulation incorporating both electron inertia and parallel electron compressibility is given. The parallel-electron-compressibility branch is shown to couple in general to the electron-inertia branch in the presence of resistivity. A sufficient condition for linear stability in the Lyapunov sense for steady states of this collisionless plasma system signifying current confinement is deduced. Bounds on the equilibrium current gradient are shown to constitute sufficient conditions for nonlinear stability in the Lyapunov sense for steady states via nonlinear bounds for a suitable perturbation norm.

Current sheet formation at a magnetic neutral line in Hall magnetohydrodynamics

The dynamics of a plasma in the vicinity of a neutral line of the magnetic field is considered in the framework of incompressible Hall magnetohydrodynamics (MHD). A self-similar solution for the collapse to a current sheet is obtained. Numerical and analytical results are presented. In contrast to the standard incompressible MHD model in two dimensions, the Hall effect leads to the formation of finite-time singularities of the velocity derivatives and the electric current density at the neutral line. Analytical expressions are derived for the collapse time and the form of the solution near the singularity, which agree closely with the numerical results. If the ion skin depth is set to zero, the singularity formation time becomes infinite, corresponding to the standard MHD result. Implications of the results for Hall MHD models of fast magnetic reconnection are discussed.

Two-Dimensional Spectroscopy of Photospheric Shear Flows in a Small δ Spot

In recent high-resolution observations of complex active regions, long-lasting and well-defined regions of strong flows were identified in major flares and associated with bright kernels of visible, near-infrared, and X-ray radiation. These flows, which occurred in the proximity of the magnetic neutral line, significantly contributed to the generation of magnetic shear. Signatures of these shear flows are strongly curved penumbral filaments, which are almost tangential to sunspot umbrae rather than exhibiting the typical radial filamentary structure. Solar active region NOAA 10756 was a moderately complex β δ sunspot group, which provided an opportunity to extend previous studies of such shear flows to quieter settings. We conclude that shear flows are a common phenomenon in complex active regions and δ spots. However, they are not necessarily a prerequisite condition for flaring. Indeed, in the present observations, the photospheric shear flows along the magnetic neutral line are not related to any change of the local magnetic shear. We present high-resolution observations of NOAA 10756 obtained with the 65-cm vacuum reflector at Big Bear Solar Observatory (BBSO). Time series of speckle-reconstructed white-light images and two-dimensional spectroscopic data were combined to study the temporal evolution of the three-dimensional vector flow field in the β δ sunspot group. An hour-long data set of consistent high quality was obtained, which had a cadence of better than 30 seconds and subarcsecond spatial resolution.

A Statistical Study of Rapid Sunspot Structure Change Associated with Flares

We reported recently some rapid changes of sunspot structure in white-light (WL) associated with major flares. We extend the study to smaller events and present here results of a statistical study of this phenomenon. In total, we investigate 403 events from 1998 May 9 to 2004 July 17, including 40 X-class, 174 M-class, and 189 C-class flares. By monitoring the structure of the flaring active regions using the WL observations from the Transition Region and Coronal Explorer (TRACE), we find that segments in the outer sunspot structure decayed rapidly right after many flares; and that, on the other hand, the central part of sunspots near the flare-associated magnetic neutral line became darkened. These rapid and permanent changes are evidenced in the time profiles of WL mean intensity and are not likely resulted from the flare emissions. Our study further shows that the outer sunspot structure decay as well as the central structure darkening are more likely to be detected in larger solar flares. For X-class flares, over 40% events show distinct sunspot structure change. For M- and C-class flares, this percentage drops to 17% and 10%, respectively. The results of this statistical study support our previously proposed reconnection picture, i.e., the flare-related magnetic fields evolve from a highly inclined to a more vertical configuration.

Statistical Correlations between Parameters of Photospheric Magnetic Fields and Coronal Soft X‐Ray Brightness

Using observations of more than 160 active regions, we investigate the relationship between the coronal X-ray brightness, LB, and five parameters derived from the photospheric magnetic fields. The coronal X-ray brightness and the magnetic measures were obtained from co-aligned SFD composite images from the Yohkoh SXT and full-disk magnetograms from the SOHO MDI, respectively. The magnetic parameters are (1) the length of strong-gradient magnetic neutral lines, GNL, (2) the magnetic energy dissipation, , (3) the unsigned line-of-sight magnetic flux, Φ, (4) the horizontal velocities, Vh, of random footpoint motions in the photosphere, and (5) a proxy for the Poynting flux, E = (1/4π) 2, which characterizes the energy flux from the photosphere into the corona due to random footpoint motions. All measures except Vh were analyzed in both the extensive (total) and intensive (average over an area) forms. In addition, we used the area-averaged strong gradient (>50 G) of the magnetic field, ∇Bz, as an intensive form of GNL. We found that the Pearson correlation coefficient between the total X-ray brightness and the total magnetic measures decreases as 0.97, 0.88, 0.86, and 0.47 for Φ, E, , and GNL, respectively. The correlation coefficient between the averaged X-ray brightness and the averaged magnetic measures varied as 0.67, 0.71, 0.57, and 0.49 for , , , and , respectively. We also found that the velocities of the footpoint motions have no dependencies with Φ and LB. We concluded that the observed high correlation between LB and E is mainly due to the magnetic field. The energy of the Poynting flux is in the range 106.7-107.6 ergs cm-2 s-1 for the majority of active regions, which is sufficient to heat the corona due to footpoint random motions of magnetic flux tubes.

Indices for the epochs of the solar activity minima

[1] New solar activity indices for the sunspot cycle minima were obtained from the Hα synoptic charts for 1887–2005, Ca II K observational data of the solar disk at Kodaikanal for 1907–1995. The Apz index determines the area of the polar zone occupied by the magnetic field of one polarity, and A(t) is the sum of squares of dipole and octupole magnetic moments. The L(t) index is the total length of magnetic neutral lines on the Hα synoptic chart, and K(t) characterizes the complexity of the topology pattern of the general magnetic field of the Sun. It is determined by the inverse number of crossings of neutral lines with the meridian net on the Hα chart. The R(t) index characterizes the correlation factor of the magnetic field polarities in the latitude zones ±40◦. The solar rotation index SSPD(t) is determined by the sum of the spectral density of the power of the magnetic field sector structures. The area of the Ca II K bright points at high latitudes is the SK(t) index. It has been shown that the 11-year cycles of Apz(t), A(t), L(t), K(t), R(t), SSPD(t), and SK(t) have maxima at the sunspot activity minimum and that they are ahead of the cycles of Wolf numbers, W (t), by 5–6 years.

Interaction between a Fast Rotating Sunspot and Ephemeral Regions as the Origin of the Major Solar Event on 2006 December 13

The major solar event on 2006 December 13 is characterized by the approximately simultaneous occurrence of a heap of hot ejecta, a great two-ribbon flare, and an extended Earth-directed coronal mass ejection. We examine the magnetic field and sunspot evolution in NOAA AR 10930, the source region of the event, while it transited the solar disk center from December 10 to 13. We find that the obvious changes in the active region associated with the event are the development of magnetic shear, the appearance of ephemeral regions, and fast rotation of a smaller sunspot. Around the area of the magnetic neutral line of the active region, interaction between the fast rotating sunspot and the ephemeral regions triggers continual brightening and finally the major flare. This indicates that only after the sunspot rotates up to 200° does the major event take place. The sunspot rotates at least 240° about its center, the largest sunspot rotation angle that has been reported.

Spatial‐Temporal characteristics of ion beamlets in the plasma sheet boundary layer of magnetotail

The processes of nonadiabatic ion acceleration occurring in the vicinity of magnetic neutral lines produce highly accelerated (up to 2500 km/s) field‐aligned ion beams (beamlets) with transient appearance streaming earthward in the plasma sheet boundary layer (PSBL) of the Earth’s magnetotail. Previous studies of these phenomena based on single spacecraft (s/c) missions supported the view that beamlets are temporal transients, since the typical time of a beamlet observation at a given s/c very rarely exceeds ∼1–2 min. Now multipoint Cluster observations have led to a new understanding of these phenomena with a spatial rather than a temporal structure. On the basis of 3‐year Cluster measurements made in the PSBL, we present statistical evaluation of the beamlet duration (at least 5–15 min) and confirm well‐manifested localization of the beamlet along Z and in some cases along Y directions, i.e., approximately across the lobe magnetic field. Earlier results reporting shorter beamlet observations could be understood by invoking not only PSBL flapping motions but also of an additional effect revealed by Cluster: earthward propagation of kink‐like perturbations along the beamlet filaments. Phase velocity of these perturbations is of the order of the local Alfven velocity (V ∼ 600–1400 km/s) and related fast flappings of localized beamlet structures in the Y‐Z direction significantly decreasing the time of their observation at a given spacecraft. Multipoint observations of beamlets revealed that they represent long‐living (∼5–15 min) plasma filaments elongated along the lobe magnetic field (∼60–100RE) and strongly localized in direction perpendicular to the PSBL‐lobe boundary (∼0.2–0.7RE). In some cases, it was also possible to estimate the width of beamlet in dawn‐dusk direction which was of the order of fractions of RE.

Are Moreton Waves Coronal Phenomena?

We report on permeability characteristics of the upper solar atmosphere due to the progression of a Moreton wave. An exceptional Moreton wave is tracked to cover most of the Sun, following an unusually large solar X-ray flare observed on 2003 October 29. Using Hα intensity and Doppler measurements, the Moreton wave is tracked for as long as 12 minutes. Moving outward, the wave circumnavigates strong-field active regions. The wave sweeps through solar magnetic neutral lines, disrupting material from filament and filament channels, thereby accentuating the visibility of the wave. We establish that the requirement for the visibility of a Moreton wave is the necessary presence of higher density material in the layers of the corona, besides reaffirming that Moreton waves are observed only when the speed of the disturbance exceeds Mach 2. We suggest that the cause can be a removal of significant amount of material from the solar upper atmosphere due to a coronal mass ejection.

Current sheet formation near a hyperbolic magnetic neutral line in a variable-density plasma: An exact solution

Current-sheet formation near a hyperbolic magnetic neutral line in the MHD model is investigated. In this model, the magnetic field lines are swept by the plasma flow toward the X-point. Effects of density-variation of the plasma are included. An exact analytic solution is given. This solution exhibits a finite-time singularity that is peculiar to the variable-density plasma case and this finite-time singularity occurs when there is a plasma density build-up near the magnetic neutral line.