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Yohkoh SXT Full-Resolution Observations of Sigmoids: Structure, Formation, and Eruption  

Richard Canfield   Submitted: 2007-08-01 09:23

We study the structure of 107 bright sigmoids using full-resolution (2 farcs 5 pixels) images from the Yohkoh Soft X-Ray Telescope obtained between December 1991 and December 2001. We find that none of these sigmoids are made of single loops of S or inverse-S shape - all comprise a pattern of multiple loops. We also find that all S-shaped sigmoids are made of right-bearing loops, and all S-1-shaped sigmoids, left-bearing loops, without exception. We co-align the SXT images with Kitt Peak magnetograms to determine the magnetic field directions in each sigmoid. We use a potential-field source surface model to determine the direction of the overlying magnetic field. We find that sigmoids for which the relative orientation of these two fields has a parallel component outnumber anti-parallel ones by more than an order of magnitude. We find that the number of sigmoids per AR region varies with the solar cycle in a manner that is consistent with this finding. Finally, those few sigmoids that are anti-parallel erupt roughly twice as often as those that are parallel. We briefly discuss the implications of these results in terms of formation and eruption mechanisms of flux tubes and sigmoids.

Authors: Richard C. Canfield, Maria D. Kazachenko, Loren W. Acton, D. H. Mackay, Ji Son, Tanya L. Freeman
Projects: None

Publication Status: ApJ Letters, submitted
Last Modified: 2007-08-01 09:24
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Solar Active Region Flux Fragmentation, Subphotospheric Flows, and Flaring  

Richard Canfield   Submitted: 2007-04-24 15:27

We explore the properties of the fragmentation of magnetic flux in solar active regions. We apply gradient-based tessellation to magnetograms of 59 active regions to identify flux fragments. First, we find that the distribution function of flux fragments in these regions is highly consistent with lognormal form, which is the most direct evidence yet obtained that repeated random bifurcation dominates fragmentation and coalescence in all active regions. Second, we apply non-parametric statistical methods to the variance of the lognornal distribution of fragment flux, the flare X-ray energy output of the active regions, and kinetic helicity measurements in the upper convection zone (Komm et al, 2005) to show that there is no significant statistical relationship between the amount of fragmentation of an active region's flux at photospheric levels and the amplitude of either its average kinetic helicity density in the upper convection zone or its X-ray flare energy output.

Authors: Richard C. Canfield and Alexander J. B. Russell
Projects:

Publication Status: ApJ Letters, published
Last Modified: 2007-05-29 15:45
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Spatial Relationship Between Twist in Active Region Magnetic Fields and Solar Flares  

Richard Canfield   Submitted: 2005-01-03 16:36

Twisted magnetic field lines in solar active regions constitute stressed flux systems, the reconnection of which can release the stored (excess) magnetic energy in the form of solar flares. Using co-registered photospheric vector magnetograms and chromospheric Hα images for thirty flares, we explore the relationship between these flares and the magnetic topology of the active regions in which they occur. We find that flares preferentially start in sub-regions that have an high gradient in twist and lie close to chirality inversion lines (which separate regions with twist of opposite handedness). Our results demonstrate that magnetic field topology plays an important role in magnetic reconnection and flares.

Authors: Hahn, M., Gaard, S., Jibben, P., Canfield, R. C., and Nandy, D.
Projects: None

Publication Status: ApJ, submitted.
Last Modified: 2005-01-03 16:36
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On the Tilt and Twist of Solar Active Regions  

Richard Canfield   Submitted: 2004-02-13 11:05

Tilts and twists are two measurable characteristics of solar active regions which can give us information about subsurface physical processes associated with the creation and subsequent evolution of magnetic flux tubes inside the Sun. Using Mees Solar Observatory active-region vector magnetograms and Mt. Wilson Observatory full-disk longitudinal magnetograms we measure both the twist and tilt of the magnetic fields of 368 active regions. In addition to two well-known phenomena, Joy's law and the hemispheric helicity rule, this dataset also shows a lesser-known twist-tilt relationship - which is the focus of this study. We find that those regions that closely follow Joy's law do no show any twist-tilt dependence. The dispersion in tilt angles and the dispersion in twist are also found to be uncorrelated with each other. Both of these results are predicted consequences of convective buffeting of initially untwisted and un-writhed flux tubes through the Sigma-effect. However, we find that regions that strongly depart from Joy's law show significantly larger than average twist and very strong twist-tilt dependence - suggesting that the twist-tilt relationship in these regions is due to the kinking of flux tubes which are initially highly twisted, but not strongly writhed. This implies that some mechanism other than the Sigma-effect (e.g., the solar dynamo or buoyancy instability and flux tube formation) is responsible for imparting the initial twist (at the base of the solar convection zone) for the flux tubes that subsequently become kink-unstable.

Authors: Holder, Z. A., Canfield, R. C., McMullen, R. A., Nandy, D., Howard, R. F., & Pevtsov, A. A
Projects:

Publication Status: ApJ, in press
Last Modified: 2004-04-28 15:58
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Helicity of magnetic clouds and their associated active regions  

Richard Canfield   Submitted: 2003-11-18 11:36

In this work we relate the magnetic and topological parameters of twelve interplanetary magnetic clouds to associated solar active regions. We use a cylindrically symmetric constant- α force-free model to derive field line twist, total current, and total magnetic flux from in situ observations of magnetic clouds. We compare these properties with those of the associated solar active regions, which we infer from solar vector magnetograms. Our comparison of fluxes and currents reveals: (1) the total flux ratios PhiMC / PhiAR tend to be of order unity; (2) the total current ratios IMC / IAR are orders of magnitude smaller; and (3) there is a statistically significant proportionality between them. Our key findings in comparing total twists α L are: (1) the values of α L)MC are typically an order of magnitude greater than those of α L)AR; and (2) there is no statistically significant sign or amplitude relationship between them. These findings compel us to believe that magnetic clouds associated with active region eruptions are formed by magnetic reconnection between these regions and their larger-scale surroundings, rather than simple eruption of pre-existing structures in the corona or chromosphere.

Authors: Leamon, R.J., Canfield, R.C., Jones, S.L., Lambkin, K., Lundberg, B.J., and Pevtsov A.A.
Projects: Soho-EIT,Soho-MDI,Yohkoh-SXT

Publication Status: JGR (submitted)
Last Modified: 2003-11-18 11:39
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Detection of a Taylor-like Plasma Relaxation Process in the Sun  

Richard Canfield   Submitted: 2003-08-26 17:05

The relaxation dynamics of a magnetized plasma system is a subject of fundamental importance in magnetohydrodynamics - with applications ranging from laboratory plasma devices like the toroidal field pinch and spheromaks to astrophysical plasmas, stellar flaring activity and coronal heating. Taylor in 1974 proposed that the magnetic field in a plasma, subject to certain constraints, relaxes to a minimum energy state such that the final magnetic field configuration is a constant α (linear) force-free field - where α is a quantity describing the twist in magnetic field lines. While Taylor's theory was remarkably successful in explaining some intriguing results from laboratory plasma experiments, a clear signature of this mechanism in astrophysical plasmas remained undetected. Here we report observational detection of a relaxation process, similar to what Taylor envisaged, in the magnetic fields of flare-productive solar active regions. The implications of this result for magnetic reconnection and the coronal heating problem are discussed.

Authors: Nandy, D., Hahn, M., Canfield, R.C., and Longcope, D.W.
Projects:

Publication Status: ApJ 597, 73 (2003)
Last Modified: 2003-11-03 10:53
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Preflare Phenomena in Eruptive Flares  

Richard Canfield   Submitted: 2003-08-04 10:23

We report the results of a statistical study of the relationship between eruptive solar flares and an observed Hα preflare phenomenon we call moving blue shift events (MBSEs). The Hα data were gathered using the Mees Solar Observatory CCD imaging spectrograph (MCCD). The 16 events in our dataset were observed by both the MCCD and the Yohkoh Soft X-Ray Telescope (SXT), typically for at least three hours prior to the flare, and in some cases repeatedly for several days prior to the flare. The dataset contains both eruptive and non-eruptive flares, without bias. Focusing on three-hour periods before and after the flares, we found the average rate of MBSEs prior to the flares was ~5 times greater prior to the 11 eruptive flares than prior to the 5 non-eruptive ones. Also, the average rate of MBSEs dropped by a factor of ~6 after the eruptive flares. Earlier studies inferred that MBSEs reflect motions that originate in the readjustment of magnetic fields after magnetic reconnection. From the high correlation between eruptive flares and preflare MBSEs in the several hours prior to such events, we conclude that reconnection in the chromosphere or low corona plays an important role in establishing the conditions that lead to solar flare eruptions.

Authors: Des Jardins, Angela C., Canfield, Richard C.
Projects: Yohkoh-SXT

Publication Status: ApJ, 598, 678, 2003
Last Modified: 2003-11-20 11:23
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Photospheric and Coronal Currents in Solar Active Regions  

Richard Canfield   Submitted: 2002-12-13 11:55

Using photospheric line of sight magnetograms from the National SolarObservatory/Kitt Peak and coronal X-ray images from the Yohkoh Soft X-ray Telescope, we have determined the value of the constant α of the linear force-free-field model that gives the best visual fit to the most distinct coronal X-ray features of 34 active regions. We find that such features in most of these regions are well represented by a linear force-free-field model with a single coronal α value. Only 11% clearly require more than one coronal α value. For 24 well-developed and flare-productive active regions, vector magnetograms are available from the Haleakala Stokes Polarimeter at Mees Solar Observatory. For each of them we determine the single best-fit value of α in the photosphere by three quite different methods, and show that these methods give consistent values. By combining this dataset with that of NSO and SXT, we are able to compare for the first time quantitatively and statistically the observed values of α in the photosphere and corona of these regions. We find that the distribution of photospheric and coronal α values is fully consistent with the hypothesis that the overall twist density of the magnetic fields of flare-productive mature active regions, as measured by the linear force-free field parameter α is the same in the photosphere and the corona. We therefore conclude that the electric currents that create the non-potential structure of such solar coronal active regions are of sub-photospheric origin and pass without significant modification through the photosphere.

Authors: Burnette, A.B., Canfield, R.C, Pevtsov, A.A.
Projects: None

Publication Status: ApJ, published
Last Modified: 2004-04-26 10:35
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On the Origin of Activity in Solar-Type Stars  

Richard Canfield   Submitted: 2002-11-19 21:03

The magnetic flux of solar coronal active regions is thought to originate in strong toroidal magnetic fields generated by a dynamo at the base of the convection zone. Once generated, this magnetic flux rises through the convection zone as discrete buoyant flux tubes, which may be formed into W-shaped loops by their interaction with convective cells and strong downdrafts. The loops are prevented from fragmentation by twist and curvature of their axes, which are writhed by the Coriolis effect and helical convective turbulence. These W-shaped loops emerge through the photosphere to form dipolar sunspot pairs and coronal active regions. These regions’ free energy, relative magnetic helicity, and tendency to flare and erupt reflect the convection zone phenomena that dominate their journey to the surface, in which helical convective turbulence appears to play a primary role. Recent research leads me to suggest a new paradigm for activity in solar-type stars with deep-seated (tachocline) dynamos. In the present paradigm, dynamo models are expected to explain the distribution of activity in the H-R diagram, as reflected in mean chromospheric emission in lower main-sequence stars. In the new paradigm, dynamo action simply generates the flux that is necessary, but not sufficient, for such activity, and the amplitude of activity depends most importantly on the kinetic helicity and turbulence of convection zone flows.

Authors: Richard C. Canfield
Projects:

Publication Status: Advances in Space Research, in press.
Last Modified: 2003-02-06 10:52
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Hemispheric Helicity Trend for Solar Cycle 23  

Richard Canfield   Submitted: 2001-01-05 09:29

Applying the same methods we used in solar cycle 22, we study active-region vector magnetograms, full-disk X-ray images, and full disk line-of-sight magnetograms to derive the helicity of solar magnetic fields in the first four years of solar cycle 23. We find that these three datasets all exhibit the same two key tendencies - significant scatter and weak hemispheric asymmetry - as were observed in solar cycle 22. This supports the interpretation of these tendencies as signatures of the writhing of magnetic flux by turbulence in the convection zone.

Authors: Alexei A. Pevtsov, Richard C. Canfield, Sergei M. Latushko
Projects: None

Publication Status: ApJ 549, L261 (2001)
Last Modified: 2003-11-03 10:58
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Solar Magnetic Fields and Geomagnetic Events  

Richard Canfield   Submitted: 2000-05-25 20:00

Recent interplanetary studies conclude that the large-scale solar dipolar field dominates the solar cycle modulation of the magnetic structure of interplanetary clouds. Other studies lead one to expect that the toroidal fields of active regions, described by the Hale-Nicholson polarity law, play an important role. We have studied the ratio of the geomagnetic A_p index to the sunspot number for solar cycles 17-22. We find no compelling evidence that either the large-scale dipolar field or active regions uniquely modulate this quantity on solar-cycle time scales. In the period 1991 - 1998 the large-scale solar dipolar magnetic field pointed southward. During this period we studied geomagnetic storms temporally associated with the eruption of 18 individual coronal X-ray sigmoids observed with the Yohkoh Soft X-Ray Telescope (SXT). We apply two different models - force-free field (FFF) and coronal flux-rope (CFR) - to infer the magnetic fields in these sigmoids and the geomagnetic consequences of their eruption. We find that if the CFR model is used, eruptions in sigmoids with a southward leading magnetic field component are associated with stronger geomagnetic storms, and northward leading field, weaker storms. The opposite is true if the FFF model is used. From this we infer that the magnetic structure of individual active regions plays a significant role in geomagnetic events, and no simple cycle-dependent generalization is useful in predicting the geomagnetic effects associated with an individual solar eruption.

Authors: Pevtsov, A. A. and Canfield, R. C.
Projects: None

Publication Status: JGR 106, A11, 2591 (2001)
Last Modified: 2003-11-03 11:12
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Vector Magnetic Fields, Sub-surface Stresses, and Evolution of Magnetic Helicity  

Richard Canfield   Submitted: 2000-05-25 20:00

Observations of the strength and spatial distribution of vector magnetic fields in active regions have revealed several fundamental properties of the twist of their magnetic fields. First, the handedness of this twist obeys a hemispheric rule: left-handed in the northern hemisphere, right-handed in the southern. Second, the rule is weak; active regions often disobey it. It is statistically valid only in a large ensemble. Third, the rule itself, and the amplitude of the scatter about the rule, are quantitatively consistent with twisting of fields by turbulence as flux tubes buoy up through the convection zone. Fourth, there is considerable spatial variation of twist within active regions. However, relaxat- relaxation to a linear force-free state, which has been documented amply in laboratory plasmas, is not observed.

Authors: Canfield, R. C. and Pevtsov, A. A.
Projects: None

Publication Status: J. Astrophys. Astron. 21, 213 (2000)
Last Modified: 2003-11-03 11:24
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Coronal Structures as Tracers of Sub-Surface Processes  

Richard Canfield   Submitted: 2000-05-25 19:03

The solar corona - one of the most spectacular celestial shows and yet one of the most challenging puzzles - exhibits a spectrum of structures related to both the quiet Sun and active regions. In spite of dramatic differences in appearance and physical processes, all these structures share a common origin: they all related to the solar magnetic field. The origin of the field is beneath the turbulent convection zone, where the magnetic field is not a tsar but a slave, and one can wonder how much the coronal magnetic field ''remembers'' its dynamo origin. Surprisingly, it does. We will describe several observational phenomena that indicate a close relationship between coronal and sub-photospher- sub-photospheric processes.

Authors: Pevtsov, A. A. and Canfield, R. C
Projects: None

Publication Status: J. Astron. Astrophys, 21, 185 (2000)
Last Modified: 2003-11-03 11:20
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Sigmoids as Precursors of Solar Eruptions  

Richard Canfield   Submitted: 2000-03-22 14:05

Coronal Mass Ejections (CMEs) appear to originate preferentially in regions of the Sun's corona that are sigmoidal, i.e. have sinuous S or reverse-S shapes. Yohkoh solar X-ray images have been studied before and after a modest number of Earth-directed (halo) CMEs. These images tend to show sigmoidal shapes before the eruptions and arcades, cusps, and transient coronal holes after. Using such structures as proxies, it has been shown that there is a relationship between sigmoidal shape and tendency to erupt. Regions in the Sun's corona appear sigmoidal because their magnetic fields are twisted. Some of this twist may originate deep inside the Sun. However, it is significantly modulated by the Coriolis force and turbulent convection as this flux buoys up through the Sun's convection zone. As the result of these phenomena, and perhaps subsequent magnetic reconnection, magnetic flux ropes form. These flux ropes manifest themselves as sigmoids in the corona. Although there are fundamental reasons to expect such flux ropes to be unstable, the physics is not as simple as might first appear, and there exist various explanations for instability. Many gaps need to be filled in before the relationship between sigmoids and CMEs is well enough understood to be a useful predictive tool.

Authors: Canfield, R. C., Hudson, H. S., and Pevtsov, A. A.
Projects: None

Publication Status: IEEE Transactions on Plasma Science, 28, 1786 (2000)
Last Modified: 2003-11-03 10:52
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Abstracts by Author
Yohkoh SXT Full-Resolution Observations of Sigmoids: Structure, Formation, and Eruption
Solar Active Region Flux Fragmentation, Subphotospheric Flows, and Flaring
Spatial Relationship Between Twist in Active Region Magnetic Fields and Solar Flares
On the Tilt and Twist of Solar Active Regions
Helicity of magnetic clouds and their associated active regions
Detection of a Taylor-like Plasma Relaxation Process in the Sun
Preflare Phenomena in Eruptive Flares
Photospheric and Coronal Currents in Solar Active Regions
On the Origin of Activity in Solar-Type Stars
Hemispheric Helicity Trend for Solar Cycle 23
Solar Magnetic Fields and Geomagnetic Events
Vector Magnetic Fields, Sub-surface Stresses, and Evolution of Magnetic Helicity
Coronal Structures as Tracers of Sub-Surface Processes
Sigmoids as Precursors of Solar Eruptions

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Max Millennium Science Mail Archive.
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Max Millennium Flare Catalog

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