DECREASING SUNSPOT MAGNETIC FIELDS EXPLAIN UNIQUE 10.7 cm RADIO FLUX

Livingston, W.; Penn, Matthew; Svalgaard, Leif

doi:10.1088/2041-8205/757/1/l8

Cited by 104 publications

(130 citation statements)

References 19 publications

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“…Watson et al (2011) Regardless of whether 2012 or 2013 is adopted as the maximum of cycle 24, we find a positive temporal gradient of the magnetic field strength with time in the rising phase of this cycle that is three times larger than the one-σ error margin. This is inconsistent with Livingston et al (2012), who claimed a monotonic decrease of the sunspot field strength through the whole cycle.…”

Section: Temporal Variation Of the Umbral Field Strengthcontrasting

confidence: 73%

“…We used a Kolmogorov-Smirnov test (Press et al 1992) to measure the distance of the sample of the magnetic field strengths of sunspots against a given distribution to determine the best distribution for each cycle. The distribution of the umbral field strength is well represented by a Gaussian (same as, e.g., Livingston et al 2012). The widths of our distributions (Gaussian width =375 and 364 ± 20 G for cycles 23 and 24) are close to the 320 G reported by Livingston et al (2012).…”

Section: Long-term Variation Of the Umbral Field Strength And Intensitysupporting

confidence: 73%

“…The onset of cycle 24 reverses the linear trend of the umbral intensity, as seen in the right panel of Fig. 3, in contrast to the finding of Livingston et al (2012). This figure shows that the umbral intensity has a cyclic variation: it starts to increase again from 2012 onward, and at the same time, the field strength decreases.…”

Section: Temporal Evolutionmentioning

confidence: 58%

“…Rezaei et al (2012), Pevtsov et al (2014), and Schad (2014) reported a cyclic variation of the field strength, but found no significant trace of a longterm variation in their data. Livingston et al (2012) predicted a monotonic decrease of sunspot field strength that should lead to the disappearance of sunspots in cycle 25. Rezaei et al (2012, hereafter Paper I) studied 185 sunspots observed by the Tenerife Infrared Polarimeter (TIP-I and TIP-II, respectively; Martínez Pillet et al 1999;Collados et al 2007) between 1999 and 2011.…”

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: Temporal Variation Of the Umbral Field Strengthcontrasting

confidence: 73%

Section: Long-term Variation Of the Umbral Field Strength And Intensitysupporting

confidence: 73%

Section: Temporal Evolutionmentioning

confidence: 58%

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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“…It is traced with several quantities, including the number and area of sunspots (Hoyt & Schatten 1996;Hathaway et al 2002;Usoskin 2008;Hathaway 2010). The cyclic variation of umbral properties such as intensity and magnetic field strength was subject of several investigations (Livingston 2002;Penn & Livingston 2006;Penn & MacDonald 2007;Penn & Livingston 2011;Livingston et al 2012;Pevtsov et al 2013). Among others, Albregtsen & Maltby (1978) and Albregtsen et al (1984) found that the umbral intensity fluctuates in phase with the solar cycle such that the umbrae are brighter at the end of a cycle.…”

Section: Introductionmentioning

confidence: 99%

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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DECREASING SUNSPOT MAGNETIC FIELDS EXPLAIN UNIQUE 10.7 cm RADIO FLUX

Cited by 104 publications

References 19 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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