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Acoustic Power Absorption and its Relation with Vector Magnetic Field of a Sunspot  

Sanjay Gosain   Submitted: 2010-08-11 21:05

The distribution of acoustic power over sunspots shows an enhanced absorption near the umbra-penumbra boundary. Earlier studies revealed that the region of enhanced absorption coincides with the region of strongest transverse potential field. The aim of this paper is to (i) utilize the high-resolution vector magnetograms derived using Hinode SOT/SP observations and study the relationship between the vector magnetic field and power absorption and (ii) study the variation of power absorption in sunspot penumbrae due to the presence of spine-like radial structures. It is found that (i) both potential and observed transverse fields peak at a similar radial distance from the center of the sunspot, and (ii) the magnitude of the transverse field, derived from Hinode observations, is much larger than the potential transverse field derived from SOHO/MDI longitudinal field observations. In the penumbra, the radial structures called spines (intra-spines) have stronger (weaker) field strength and are more vertical (horizontal). The absorption of acoustic power in the spine and intra-spine shows different behaviour with the absorption being larger in the spine as compared to the intra-spine.

Authors: Sanjay Gosain, S. K. Mathew and P. Venkatakrishnan
Projects: Hinode/SOT

Publication Status: In Press Solar Physics, Topical Issue on Helio-and-Astroseismology
Last Modified: 2010-08-12 09:47
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Evolution of twist-shear and dip-shear during X-class flare of 13 December 2006: Hinode observations  

Sanjay Gosain   Submitted: 2010-07-15 04:34

The non-potentiality (NP) of the solar magnetic fields is measured tradi- tionally in terms of magnetic shear angle i.e., the angle between observed and potential field azimuth. Here, we introduce another measure of shear that has not been studied earlier in solar active regions, i.e. the one that is associated with the inclination angle of the magnetic field. This form of shear, which we call as the ?dip-shear?, can be calculated by taking the difference between the observed and potential field inclination. In this Letter, we study the evolution of dip-shear as well as the conventional twist-shear in a -sunspot using high- resolution vector magnetograms from Hinode space mission. We monitor these shears in a penumbral region located close to flare site during 12 and 13 December 2006. It is found that: (i) the penumbral area close to the flaring site shows high value of twist-shear and dip-shear as compared to other parts of penumbra, (ii) after the flare the value of dip-shear drops in this region while the twist-shear in this region tends to increase after the flare, (iii) the dip-shear and twist-shear are correlated such that pixels with large twist-shear also tend to exhibit large dip- shear, and (iv) the correlation between the twist-shear and dip-shear is tighter after the flare. The present study suggests that monitoring twist-shear during the flare alone is not sufficient but we need to monitor it together with dip-shear.

Authors: Sanjay Gosain and P. Venkatakrishnan
Projects: Hinode/SOT

Publication Status: Accepted for publication in ApJ Letters
Last Modified: 2010-07-15 07:47
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On the estimate of magnetic non-potentiality of sunspots derived using SOT/SP observations: Effect of polarimetric noise  

Sanjay Gosain   Submitted: 2010-07-14 13:25

The accuracy of Milne-Eddington (ME) inversions, used to retrieve the magnetic field vector, depends upon the signal-to-noise ratio (SNR) of the spectropolarimetric observations. The SNR in real observations varies from pixel to pixel, therefore the accuracy of the field vector also varies over the map. The aim of this work is to study the effect of polarimetric noise on the inference of magnetic field vector and the magnetic non-potentiality of a real sunspot. To this end, we use Hinode SOT/SP vector magnetogram of a real sunspot NOAA 10933 as an input to generate synthetic Stokes profiles under ME model assumptions. We then add normally-distributed polarimetric noise of the level 0.5% of continuum intensity to these synthetic profiles and invert them again using ME code. This process is repeated 100 times with different realizations of noise. It is found that within most of the sunspot area (> 90% area) the spread in the (i) field strength is less than 8 Gauss, (ii) field inclination is less than 1 degree, and (iii) field azimuth is less than 5 degrees. Further, we determine the uncertainty in the magnetic non-potentiality of a sunspot as determined by the force-free parameter α _g and Spatially Averaged Signed Shear Angle (SASSA). It is found that for the sunspot studied here these parameters are α _ g = -3.5?0.37 (?10-9 m^-1) and SASSA =-1.68?0.014 degrees. This suggests that the SASSA is a less dispersion non-potentiality parameter as compared to α _g. Further, we examine the effect of increasing noise levels viz. 0.01, 0.1, 0.5 and 1% of continuum intensity and find that SASSA is less vulnerable to noise as compared to α _ g parameter.

Authors: Sanjay Gosain, Sanjiv Kumar Tiwari and P. Venkatakrishnan
Projects: Hinode/SOT

Publication Status: Accepted for publication in ApJ
Last Modified: 2010-07-15 07:48
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A Technique for Removing Background Features in SECCHI-EUVI He II 304 Å Filtergrams: Application to the Filament Eruption of 22 May 2008  

Sanjay Gosain   Submitted: 2010-01-27 02:58

The STEREO mission has been providing stereoscopic view of filament eruptions in EUV. The clearest view during a filament eruption is seen in He II 304 Å observations. One of the main problems visualizing filament dynamics in He II 304 Å is the strong background contrast due to surface features. We present a technique that removes background features and leaves behind only the filamentary structure, as seen by STEREO-A and B. The technique uses a pair of STEREO He II 304 Å images observed simultaneously. The STEREO-B image is geometrically transformed to STEREO-A view so that the background images appear similar. Filaments being elevated structures, i.e., not lying on the same spherical surface as background features, do not appear similar in the transformed view. Thus, subtracting the two images cancels the background but leaves behind the filament structure. We apply this technique to study the dynamics of the filament eruption event of 22 May 2008, which was observed by STEREO and followed by several ground-based observatories participating in the Joint Observing Programme (JOP 178).

Authors: Guy Artzner, Sanjay Gosain, Brigitte Schmieder
Projects: STEREO

Publication Status: To appear in Solar Physics (Topical Issue on Solar Image Processing Techniques)
Last Modified: 2010-01-27 14:03
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Estimation of width and inclination of a filament sheet using He II 304 Å observations by STEREO/EUVI  

Sanjay Gosain   Submitted: 2010-01-18 03:21

The STEREO mission has been providing stereoscopic view of the filament eruptions in EUV wavelengths. The most extended view during filament eruptions is seen in He II 304 Å observations, as the filament spine appears darker and sharper. The projected filament width appears differently when viewed from different angles by STEREO satellites. Here, we present a method for estimating the width and inclination of the filament sheet using He II 304 Å observations by STEREO-A and B satellites from the two viewpoints. The width of the filament sheet, when measured from its feet to its apex, gives estimate of filament height above the chromosphere.

Authors: Sanjay Gosain and Brigitte Schmieder
Projects: STEREO

Publication Status: To appear in Annales Geophysicae
Last Modified: 2010-01-18 13:06
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HINODE OBSERVATIONS OF COHERENT LATERAL MOTION OF PENUMBRAL FILAMENTS DURING A X-CLASS FLARE  

Sanjay Gosain   Submitted: 2009-11-04 03:08

The X-3.4 class flare of 13 December 2006 was observed with a high cadence of 2 minutes at 0.2 arc-sec resolution by HINODE/SOT FG instrument. The flare ribbons could be seen in G-band images also. A careful analysis of these observations after proper registration of images show flare related changes in penumbral filaments of the associated sunspot, for the first time. The observations of sunspot deformation, decay of penumbral area and changes in magnetic flux during large flares have been reported earlier in the literature. In this Letter, we report lateral motion of the penumbral filaments in a sheared region of the -sunspot during the X-class flare. Such shifts have not been seen earlier. The lateral motion occurs in two phases, (i) motion before the flare ribbons move across the penumbral filaments and (ii) motion afterwards. The former motion is directed away from expanding flare ribbons and lasts for about four minutes. The latter motion is directed in the opposite direction and lasts for more than forty minutes. Further, we locate a patch in adjacent opposite polarity spot moving in opposite direction to the penumbral filaments. Together these patches represent conjugate foot-points on either side of the polarity inversion line (PIL), moving towards each other. This converging motion could be interpreted as shrinkage of field lines.

Authors: S. Gosain, P. Venkatakrishnan and Sanjiv Kumar Tiwari
Projects: Hinode/SOT

Publication Status: Accepted in ApJ Letters
Last Modified: 2009-11-04 08:42
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3D evolution of a filament disappearance event observed by STEREO  

Sanjay Gosain   Submitted: 2009-10-06 03:04

A filament disappearance event was observed on 22 May 2008 during our recent campaign JOP 178. The filament, situated in the southern hemisphere, showed sinistral chirality consistent with the hemispheric rule. The event was well observed by several observatories in particular by THEMIS. One day before the disappearance, Hα observations showed up and down flows in adjacent locations along the filament, which suggest plasma motions along twisted flux rope. THEMIS and GONG observations show shearing photospheric motions leading to magnetic flux canceling around barbs. STEREO A, B spacecraft with separation angle 52.4 degrees, showed quite different views of this untwisting Flux rope in He II 304 Å images. Here, we reconstruct the 3D geometry of the Filament during its eruption phase using STEREO EUV He II 304 Å images and find that the filament was highly inclined to the solar normal. The He II 304 Å movies show individual threads, which oscillate and rise to an altitude of about 120 Mm with apparent velocities of about 100 km s-1, during the rapid evolution phase. Finally, as the flux rope expands into the corona, the Filament disappears by becoming optically thin to undetectable levels. No CME was detected by STEREO, only a faint CME was recorded by LASCO at the beginning of the disappearance phase at 02:00 UT, which could be due to partial filament eruption. Further, STEREO Fe XII 195 A images showed bright loops beneath the filament prior to the disappearance phase, suggesting magnetic reconnection below the flux rope.

Authors: S. Gosain, B. Schmieder, P. Venkatakrishnan, R. Chandra, G. Artzner
Projects: GONG,SoHO-MDI,STEREO,THEMIS/MTR

Publication Status: Solar Physics (In Press)
Last Modified: 2009-10-06 05:10
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Abstracts by Author
Acoustic Power Absorption and its Relation with Vector Magnetic Field of a Sunspot
Evolution of twist-shear and dip-shear during X-class flare of 13 December 2006: Hinode observations
On the estimate of magnetic non-potentiality of sunspots derived using SOT/SP observations: Effect of polarimetric noise
A Technique for Removing Background Features in SECCHI-EUVI He II 304 A Filtergrams: Application to the Filament Eruption of 22 May 2008
Estimation of width and inclination of a filament sheet using He II 304 AA observations by STEREO/EUVI
HINODE OBSERVATIONS OF COHERENT LATERAL MOTION OF PENUMBRAL FILAMENTS DURING A X-CLASS FLARE
3D evolution of a filament disappearance event observed by STEREO

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