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Küveler, Gerd; Wiehr, E.; Thomas, Dan; Harzer, M.; Bianda, M.; Epple, Alexander; Sütterlin, P.; Weisshaar, E.

doi:10.1023/a:1005077109403

Cited by 17 publications

(15 citation statements)

References 2 publications

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“…Küveler & Wiehr (1985) did not find a sharp change in the Evershed flow at the penumbral boundary. Dialetis et al (1985), Alissandrakis et al (1988), andBörner &Kneer (1992) confirmed this result.…”

Section: Introductionmentioning

confidence: 64%

The flow field in the sunspot canopy

Rezaei

Schlichenmaier

Beck

et al. 2006

A&A

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Aims. We investigate the flow field in the sunspot canopy using simultaneous Stokes vector spectropolarimetry of three sunspots (θ = 27• , 50• , 75• ) and their surroundings in visible (630.15 and 630.25 nm) and near infrared (1564.8 and 1565.2 nm) neutral iron lines. Methods. To calibrate the Doppler shifts, we compare an absolute velocity calibration using the telluric O 2 -line at 630.20 nm and a relative velocity calibration using the Doppler shift of Stokes V profiles in the umbra under the assumption that the umbra is at rest. Both methods yield the same result within the calibration uncertainties (∼150 m s −1 ). We study the radial dependence of Stokes V profiles in the directions of disk center and limb side. Results. Maps of Stokes V profile shifts, polarity, amplitude asymmetry, field strength and magnetic field azimuth provide strong evidence for the presence of a magnetic canopy and for the existence of a radial outflow in the canopy. Conclusions. Our findings indicate that the Evershed flow does not cease abruptly at the white-light spot boundary, but that at least a part of the penumbral Evershed flow continues into the magnetic canopy.Key words. Sun: photosphere -Sun: sunspots -Sun: magnetic fields IntroductionFrom the pioneering work of Evershed (1909), we know that there is a flow field in the sunspot penumbra, which leads to a line shift and line asymmetry. For sunspots outside the disk center, it manifests in blue shifted spectral lines in the center side and red shifted spectral lines in the limb side. The penumbral fine structure has a close relation with the Evershed flow. It is generally accepted that the flow channels are more horizontal than the background field (Solanki 2003). Because spectropolarimetric data has lower spatial resolution than narrow band magnetograms, many questions regarding both the Evershed flow and the fine structure of the penumbra have remained (e.g. Solanki 2003; Bellot Rubio 2004). There are also controversial arguments about the continuation of the Evershed flow outside the sunspot.Brekke & Maltby (1963) studied horizontal variations of the Evershed flow. Investigating 2500 sunspot spectra, they reported that at the outer penumbral boundary "the velocity falls abruptly to zero". Wiehr et al. (1986) observed a sharp decrease of the Evershed effect and magnetic field at the visible boundary of sunspots. Using only Stokes I spectra, they argued that line-core shifts and line asymmetries are "strongly" limited to the continuum boundary of sunspots. Wiehr & Balthasar (1989), Schröter et al. (1989 and Title et al. (1993) confirmed the result of Brekke & Maltby (1963). Wiehr (1996) renewed his argument that the Stokes I profile asymmetries of Ni I 543.6 nm (g = 0.5) and Fe I 543.5 nm (g = 0) "disappear" within less than one arcsec from the penumbral border. Hirzberger & Kneer (2001) observed two sunspots at heliocentric angles of 31• and 20• ; they reported a sharp decrease of the Evershed flow (intensity profile asymmetry) at the penumbral boundary, using Sto...

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Section: Introductionmentioning

confidence: 64%

The flow field in the sunspot canopy

Rezaei

Schlichenmaier

Beck

et al. 2006

A&A

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“…The "beam-exchange technique" requires largely equal location and sharpness of the solar features in the two exposures. The first condition is assured by the high accuracy of the primary image guider (Küveler et al 1998) which compensates drifts in the telescope pointing with high accuracy. Remaining differences in the features' precise locations on the CCD, as well as different image sharpness, were minimized by frame selection among several exposures, yielding a highly similar pair of subimages.…”

Section: Observing Methodsmentioning

confidence: 99%

Solar prominence polarimetry

Wiehr

Bianda

2003

A&A

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Abstract. We measure the resonance polarization in solar prominences in Hα, Hβ and HeD 3 . A two-dimensional set-up with narrow-band filter, polarization analyzer and CCD camera is used to take prominence images in polarized light at high spatial resolution. Placed on a coudé telescope's hour axis, the observations near the equinoxia are free from purely instrumental polarization. Above the 0.1% noise limit, the Balmer lines do not show a polarization in contrast to the HeD 3 line. Here, we determine the complete polarization profile after exchange of filter and CCD with the spectrograph, keeping the polarization analyzer fixed. In most prominences the Stokes-U and -Q profiles are not similar to Stokes-I: occasionally the blue and the red components of the emission are equal or even show a reverse ratio. This fits calculations for magnetic field strengths of the order of 50 Gauß being markedly stronger than commonly assumed.

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“…The first is assured by the primary image guider of the GCT (Küveler et al 1998) which compensates drifts in the telescope pointing. The constant limb sharpness requires highly constant seeing and is assured by the short CCD exposures and by frame selection among several exposures at each limb position.…”

Section: Continuum Polarization Near the Limbmentioning

confidence: 99%

High spatial resolution solar polarimetry with interference filters

Wiehr

Bianda

2003

A&A

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Abstract.A new type of two-dimensional polarimeter is used to measure the center-to-limb variation of the scattering induced polarization in a narrow continuum window up to the extreme limb. The polarimeter is set on the Tenerife Gregory Coudé telescope's hour axis, where the two folding flat mirrors cancel their polarizing effects for zero solar declination at the equinox. The short CCD exposure of only 5 ms allows high spatial resolution images in polarized light. A beam switching technique together with an integration parallel to the solar limb over 20 , yields a high polarimetric accuracy with an rms noise of 2 × 10 −4 . Our results for a continuum window at 4506-4508 Å agree with model calculations down to limb distances of 0. 32 (i.e. cos ϑ < 0.025).

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Cited by 17 publications

References 2 publications

The flow field in the sunspot canopy

The flow field in the sunspot canopy

Solar prominence polarimetry

High spatial resolution solar polarimetry with interference filters

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