Global Twist of Sunspot Magnetic Fields Obtained From High-Resolution Vector Magnetograms

Tiwari, Sanjiv K.; Venkatakrishnan, P.; Sankarasubramanian, K.

doi:10.1088/0004-637x/702/2/l133

Cited by 58 publications

(80 citation statements)

References 40 publications

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“…The current density is of mixed sign (salt and pepper) in the umbra of S-polarity sunspot while it is in the form of fibrils in the penumbra, alternatively changing their sign azimuthally as has been reported by Su, Sakurai & Suematsu (2009) and Tiwari, Venkatakrishnan, & Sankarasubramanian (2009). Apart from these there is a region of dominant sign of current density in the S and N-polarity regions.…”

Section: Spatial Map Of Vertical Current Densitymentioning

confidence: 63%

Evolution of Currents of Opposite Signs in the Flare-Productive Solar Active Region Noaa 10930

Ravindra

Venkatakrishnan

Tiwari

et al. 2011

ApJ

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Analysis of a time series of high spatial resolution vector magnetograms of the active region NOAA 10930 available from SOT/SP on-board Hinode revealed that there is a mixture of upward and downward currents in the two foot-points of an emerging flux-rope. The flux emergence rate is almost the same in both the polarities. We observe that along with an increase in magnetic flux, the net current in each polarity increases initially for about three days after which it decreases. This net current is characterized by having exactly opposite signs in each polarities while its magnitude remains almost the same most of the time.The decrease of net current in both the polarities is due to the increase of current having a sign opposite to that of the net current. The dominant current, with same sign as the net current, is seen to increase first and then decreases during the major X-class flares. Evolution of non-dominant current appears to be a necessary condition for a flare initiation. The above observations can have a plausible explanation in terms of the superposition of two different force-free states resulting in non-zero Lorentz force in the corona. This Lorentz force then push the coronal plasma and might facilitate the magnetic reconnection required for flares. Also, the evolution of the net current is found to follow the evolution of magnetic shear at the polarity inversion line.

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Section: Spatial Map Of Vertical Current Densitymentioning

confidence: 63%

Evolution of Currents of Opposite Signs in the Flare-Productive Solar Active Region Noaa 10930

Ravindra

Venkatakrishnan

Tiwari

et al. 2011

ApJ

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“…In addition, the azimuth value on the central axis of the filament was taken as an offset and removed, so that the value of the azimuth along the major axis is always zero. This correction was necessary to remove the background caused by a global twist in the sunspot's magnetic field (which is <5 • ; see, e.g., Tiwari et al 2009) and any remaining calibration errors in the data.…”

Section: Filament Selection De-stretching and Length-normalizationmentioning

confidence: 99%

Structure of sunspot penumbral filaments: a remarkable uniformity of properties

Tiwari

Noort

Lagg

et al. 2013

A&A

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205

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Context. The sunspot penumbra comprises numerous thin, radially elongated filaments that are central for heat transport within the penumbra, but whose structure is still not clear. Aims. We aim to investigate the fine-scale structure of these penumbral filaments. Methods. We perform a depth-dependent inversion of spectropolarimetric data of a sunspot very close to solar disk center obtained by Solar Optical Telescope/Spectropolarimeter onboard the Hinode spacecraft. We have used a recently developed, spatially coupled 2D inversion scheme, which allows us to analyze the fine structure of individual penumbral filaments up to the diffraction limit of the telescope.Results. Filaments of different sizes in all parts of the penumbra display very similar magnetic field strengths, inclinations, and velocity patterns. The temperature structure is also similar, although the filaments in the inner penumbra have cooler tails than those in the outer penumbra. The similarities allowed us to average all these filaments and to subsequently extract the physical properties common to all of them. This average filament shows upflows associated with an upward-pointing field at its inner, umbral end (head) and along its axis, as well as downflows along the lateral edge and strong downflows in the outer end (tail) associated with a nearly vertical, strong, and downward-pointing field. The upflowing plasma is significantly, i.e., up to 800 K, hotter than the downflowing plasma. The hot, tear-shaped head of the averaged filament can be associated with a penumbral grain. The central part of the filament shows nearly horizontal fields with strengths in the range of 1 kG. The field above the filament converges, whereas a diverging trend is seen in the deepest layers near the head of the filament. The fluctuations in the physical parameters along and across the filament increase rapidly with depth. Conclusions. We put forward a unified observational picture of a sunspot penumbral filament. It is consistent with such a filament being a magneto-convective cell, in line with recent magnetohydrodynamic simulations. The uniformity of its properties over the penumbra sets constraints on penumbral models and simulations. The complex and inhomogeneous structure of the filament provides a natural explanation for a number of long-running controversies in the literature.

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“…Photospheric vector magnetic fields of ARs reveal that on an average solar ARs have a small but statistically significant mean twist in magnetic helicity [2,11] that is left-handed (right-handed) in the northern (southern) hemisphere. However, some theoretical [12] and observational [13] analyses show opposite hemispheric trends during the beginning of a cycle.…”

Section: Introductionmentioning

confidence: 99%

Kinetic and magnetic helicities in solar active regions

Maurya

Ambastha

2011

J. Phys.: Conf. Ser.

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Abstract. We have studied the kinetic and magnetic helicities in sub-photospheric flows and photospheric magnetic fields, respectively, of a sample of 91 ARs of solar cycle 23. Hemispheric trend is investigated in the kinetic helicity of sub-photospheric flows averaged in the depth range of 2.5-12 Mms. Magnetic helicity parameters for the ARs are derived using photospheric vector magnetograms to examine their correlation with the corresponding kinetic helicities. We found no significant association between the two helicity parameters.

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Global Twist of Sunspot Magnetic Fields Obtained From High-Resolution Vector Magnetograms

Cited by 58 publications

References 40 publications

Evolution of Currents of Opposite Signs in the Flare-Productive Solar Active Region Noaa 10930

Evolution of Currents of Opposite Signs in the Flare-Productive Solar Active Region Noaa 10930

Structure of sunspot penumbral filaments: a remarkable uniformity of properties

Kinetic and magnetic helicities in solar active regions

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