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Stellmacher, G.; Wiehr, E.

doi:10.1023/a:1012268112384

Cited by 25 publications

(45 citation statements)

References 7 publications

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“…This was first attributed to stray light by Zwaan (1965), but later studies found a real relation between the size and intensity in sunspot umbrae (McIntosh 1981;Stellmacher & Wiehr 1988;Martinez Pillet & Vazquez 1993). Mathew et al (2007), Wesolowski et al (2008), Leonard & Choudhary (2008), de Toma et al (2013, Schad (2014), and Kiess et al (2014) studied the variation of the umbral intensity in the visible wavelength range.…”

Section: Continuum Intensity As a Function Of Umbral Sizementioning

confidence: 99%

Variation in sunspot properties between 1999 and 2014

Rezaei

Beck

Lagg

et al. 2015

A&A

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Aims. We study the variation in the magnetic field strength, area, and continuum intensity of umbrae in solar cycles 23 and 24. Methods. We analyzed a sample of 374 sunspots observed from 1999 until 2014 with the Tenerife Infrared Polarimeter at the German Vacuum Tower Telescope and the Facility InfRared Spectropolarimeter at the Dunn Solar Telescope. The sample of field strength, area, and intensities was used to trace any long-term or cyclic trend of umbral properties in the last 15 years. Results. Sunspots are systematically weaker, that is, have a weaker field strength and stronger continuum intensity, toward the end of cycle 23 than they had at the maximum of cycle 23. The linear trend reverses with the onset of cycle 24. We find that the field strength decreases in the declining phase of cycle 23 by about 112 (±16) G yr −1 , while it increases in the rising phase of cycle 24 by about 138 (±72) G yr −1 . The umbral intensity shows the opposite trend: the intensity increases with a rate of 0.7 (±0.3)% of I c yr −1 toward the end of cycle 23 and decreases with a rate of 3.8 (±1.5)% of I c yr −1 toward the maximum of cycle 24. The distribution of the umbral maximum field strength in cycle 24 is similar to that of cycle 23, but is slightly shifted toward lower values by about 80 G, corresponding to a possible long-term gradient in umbral field strength of about 7 ± 4 G yr −1 . If instead of the maximum umbral field we consider the average value over the entire umbra, the distribution shifts by about 44 Gauss. Conclusions. The umbral brightness decreases in the rising stage of a solar cycle, but increases from maximum toward the end of the cycle. Our results do not indicate a drastic change of the solar cycle toward a grand minimum in the near future.

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Section: Continuum Intensity As a Function Of Umbral Sizementioning

confidence: 99%

Variation in sunspot properties between 1999 and 2014

Rezaei

Beck

Lagg

et al. 2015

A&A

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“…Line core intensity contrast decreases towards the limb as the contribution to the spectral line from magnetic elements diminishes from absorption in the non-magnetic part of the atmosphere transversed by the rays . Another probable cause of the centre-to-limb decline is the spatial displacement of the line core with respect to the corresponding continuum towards the limb caused by the difference in formation height and oblique viewing geometry (Stellmacher & Wiehr 1991). The line core intensity enhancement arising from temperature excess in the middle photosphere and Zeeman splitting discussed here is not to be confused with that from the centre-to-limb broadening of the Fe I 6173 Å line mentioned in Sect.…”

Section: Line Core Intensity Contrastmentioning

confidence: 99%

Intensity contrast of solar network and faculae

Yeo

Solanki

Krivova

2013

A&A

105

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Aims. This study aims at setting observational constraints on the continuum and line core intensity contrast of network and faculae, specifically, their relationship with magnetic field and disc position. Methods. Full-disc magnetograms and intensity images by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) were employed. Bright magnetic features, representing network and faculae, were identified and the relationship between their intensity contrast at continuum and line core with magnetogram signal and heliocentric angle examined. Care was taken to minimize the inclusion of the magnetic canopy and straylight from sunspots and pores as network and faculae.Results. In line with earlier studies, network features, on a per unit magnetic flux basis, appeared brighter than facular features. Intensity contrasts in the continuum and line core differ considerably, most notably, they exhibit opposite centre-to-limb variations. We found this difference in behaviour to likely be due to the different mechanisms of the formation of the two spectral components. From a simple model based on bivariate polynomial fits to the measured contrasts we confirmed spectral line changes to be a significant driver of facular contribution to variation in solar irradiance. The discrepancy between the continuum contrast reported here and in the literature was shown to arise mainly from differences in spatial resolution and treatment of magnetic signals adjacent to sunspots and pores. Conclusions. HMI is a source of accurate contrasts and low-noise magnetograms covering the full solar disc. For irradiance studies it is important to consider not just the contribution from the continuum but also from the spectral lines. In order not to underestimate long-term variations in solar irradiance, irradiance models should take the greater contrast per unit magnetic flux associated with magnetic features with low magnetic flux into account.

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“…The underestimation of stray light in earlier works was noticed by Zwaan (1965), A&A 565, A52 (2014) who argued that much of the dependency of the continuum intensity on the sunspot size can be explained by stray light (see also Rossbach & Schröter 1970). More recent investigations challenged the idea that the continuum-area dependency is an artefact of stray light (McIntosh 1981;Stellmacher & Wiehr 1988;Martinez Pillet & Vazquez 1993). Most recent observations indicate that the continuum-area dependency is real and exists even after removal of stray light (Mathew et al 2007;Wesolowski et al 2008;Schad & Penn 2010;Rezaei et al 2012a).…”

Section: Introductionmentioning

confidence: 96%

Properties of sunspot umbrae observed in cycle 24

Kiess

Rezaei

Schmidt

2014

A&A

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Aims. There is an ongoing debate whether the solar activity cycle is overlaid with a long-term decline that may lead to another grand minimum in the near future. We used the size, intensity, and magnetic field strength of sunspot umbrae to compare the present cycle 24 with the previous one. Methods. We used data of the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory and selected all sunspots between May 2010 and October 2012, using one image per day. We created two subsets of this dataset with a manual tracking algorithm, both without duplication. One contains each sunspot (910 umbrae within 488 spots) and was used to analyze the distribution of umbral areas, selected with an automated thresholding method. The other subset contains 205 fully evolved sunspots. We estimated their magnetic field and the total magnetic flux and discuss the relations between umbral size, minimum continuum intensity, maximum field strength, and total magnetic flux. Results. We find non-linear relations between umbral minimum intensity and size and between maximum magnetic field strength and size. The field strength scales linearly with the intensity and the umbral size scales roughly linearly with the total magnetic flux, while the size and field strength level off with stronger flux. When separated into hemispheres and averaged temporally, the southern umbrae show a temporal increase in size and the northern umbrae remain constant. We detected no temporal variation in the umbral mean intensity. The probability density function of the umbral area in the ascending phase of the current solar cycle is similar to that of the last solar cycle. Conclusions. From our investigation of umbral area, magnetic field, magnetic flux, and umbral intensity of the sunspots of the rising phase of cycle 24, we do not find a significant difference to the previous cycle, and hence no indication for a long-term decline of solar activity.

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Cited by 25 publications

References 7 publications

Variation in sunspot properties between 1999 and 2014

Variation in sunspot properties between 1999 and 2014

Intensity contrast of solar network and faculae

Properties of sunspot umbrae observed in cycle 24

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