Variation in sunspot properties between 1999 and 2011 as observed with the Tenerife Infrared Polarimeter

Rezaei, Reza; Beck, C.; Schmidt, W.

doi:10.1051/0004-6361/201118635

Cited by 46 publications

(62 citation statements)

References 63 publications

(82 reference statements)

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“…Since the mainly vertical field lines in umbra partially quench the convection, several authors searched for a relation between umbral intensity and umbral field strength. Kopp & Rabin (1992), Penn et al (2002), Schad & Penn (2010), and Rezaei et al (2012) presented this relation for different samples of sunspots in the near-infrared wavelength range, while among others, Norton & Gilman (2004), Leonard & Choudhary (2008), Pevtsov et al (2011), Watson et al (2014), Kiess et al (2014), and Schad (2014) made similar analyses in the visible wavelength range.…”

Section: Introductionmentioning

confidence: 66%

“…These authors reported a nonlinear decrease of the Article published by EDP Sciences A43, page 1 of 9 intensity with umbral radius. Schad & Penn (2010) and Rezaei et al (2012) used 0.87 and 1.56 μm continuum intensities and reported nonlinear and linear relations between minimum intensity and umbral size, respectively. There are contradictory reports about temporal variation of the umbral intensity.…”

Section: Introductionmentioning

confidence: 99%

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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: Introductionmentioning

confidence: 66%

Section: Introductionmentioning

confidence: 99%

Variation in sunspot properties between 1999 and 2014

Rezaei

Beck

Lagg

et al. 2015

A&A

Self Cite

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“…Livingston (2002, their Fig. 2) and Rezaei et al (2012a, their Fig. 3) also did not find field strengths higher than about 4 kG in cycles 22 and 23, respectively.…”

Section: Maximum Field Strength and Strength Of A Cyclementioning

confidence: 81%

“…6). SP fit a power law function to the relation of field strength to intensity, while Rezaei et al (2012a) used a linear relation alone. The power law exponent in our fit (-0.26) is lower than the one presented by SP because of the weaker umbral contrast in the near infrared.…”

Section: Relation Of Field Strength To Intensitymentioning

confidence: 99%

“…The coalescence of the emerged flux elements in the simulations agrees with observations of Bernasconi et al (2002) and Schlichenmaier et al (2010a), for example. The tetheredballoon model of Spruit (1981) predicts that the coalescence of small flux patches (Pariat et al 2004;Rezaei et al 2012a) is a surface phenomenon since these flux elements are deeply anchored. In other words, the emergence of large flux tubes does not fundamentally affect their integrity.…”

Section: Introductionmentioning

confidence: 99%

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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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A new solar signal: Average maximum sunspot magnetic fields independent of activity cycle

Livingston

Watson

2015

Geophysical Research Letters

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Over the past 5 years, 2010 to April 2015, we observed 4176 sunspot umbrae in the infrared (IR) to measure maximum magnetic field strengths from the Zeeman splitting of Fe 15,648.5 Å. Herein we distinguish “field strengths” from “field flux.” Field strengths range from 1500 G in pores to 3500+ in large spots. We made one observation per spot per observing day, ignoring spot size. We show that in the IR no activity cycle dependence on average maximum field strength (2070 ± 20 G) has been found. A similar analysis of 17,450 spots observed in space by the Helioseismic and Magnetic Imager reveals a similar cycle independence (2050 ± 0.18 G). We conclude that the average maximum umbral fields are constant with time and independent of the activity cycle within our time coverage.

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Variation in sunspot properties between 1999 and 2011 as observed with the Tenerife Infrared Polarimeter

Cited by 46 publications

References 63 publications

Variation in sunspot properties between 1999 and 2014

Variation in sunspot properties between 1999 and 2014

Properties of sunspot umbrae observed in cycle 24

A new solar signal: Average maximum sunspot magnetic fields independent of activity cycle

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