A Multi-Instrument Analysis of Sunspot Umbrae

Watson, Fraser; Penn, Matthew; Livingston, W.

doi:10.1088/0004-637x/787/1/22

Cited by 22 publications

(17 citation statements)

References 24 publications

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“…Watson et al (2011) studied cycle 23 using SOHO MDI data and reported a long-term trend in the umbral field strength of about −24 G yr −1 . Watson et al (2014) used data of MDI, HMI, and Kitt Peak and found a rate of about −22 G yr −1 between 1996 and 2013, about twice higher than the rate we find.…”

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

confidence: 70%

“…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: 59%

“…Norton & Gilman (2004), Mathew et al (2007), Watson et al (2011Watson et al ( , 2014, and Kiess et al (2014) as well as several other authors used MDI or HMI, that is, space-based two-dimensional spectropolarimeters. These instruments provided and still provide full Stokes measurement of a visible spectral line sampled at five or six filter positions.…”

Section: Diversity Of Measurement Techniques and Resultsmentioning

confidence: 99%

“…The uncertainties in the measured areas are therefore accordingly larger between 1999−2009 than for 2010−2014. Watson et al (2014) also discussed systematic differences between umbral intensities in MDI and HMI data. Our results agree with those of Kiess et al (2014), who reported a linear increase of the umbral area in the rising phase of cycle 24.…”

Section: Temporal Variation Of the Umbral Areamentioning

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: Long-term Variation Of the Umbral Field Strength And Intensitycontrasting

confidence: 70%

Section: Introductionmentioning

confidence: 59%

Section: Diversity Of Measurement Techniques and Resultsmentioning

confidence: 99%

Section: Temporal Variation Of the Umbral Areamentioning

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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“…also, , Watson et al (2009Watson et al ( , 2014, Livingston and Watson (2015), Rezaie et al (2015), there was initial evidence that the average maximum umbral magnetic field strength decreased steadily from 1999-2005. This decrease is now understood to have been biased by a then observational interest in large spots.…”

Section: Umbral Magnetic Fieldsmentioning

confidence: 90%

A Remarkable Recent Transition in the Solar Dynamo

et al. 2016

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We summarize the major aspects of the remarkable, fairly long lasting period (∼ 2005 to ∼ 2010) of low solar activity, that we will call the Transition. It is the transitional stage between the Grand Maximum of the 20th century and a forthcoming (most probably Regular) episode of solar activity. The various kinds of activity in the functioning of the equatorial components of the solar dynamo before and during the Transition are summarized. While the behavior of unipolar magnetic regions and their rest-latitudes already gave very early indications -mid 20th century -of the forthcoming Transition, more such indications became available around 1995 and the main part of it occurred between 2005 and 2010. Some of the inferences are discussed. We submit the hypothesis that the solar tachocline undergoes pulsations and we present some helioseismic evidences. In that scenario we find that its equatorial part has moved downward over a fairly small semi-amplitude (∼ 0.03 solar radii) during the time of the Transition. There are several indications, apart from this 'pulsation', that the tachocline may even be pulsating with still smaller amplitudes in more modes. We speculate about the physical mechanism(s).

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Long‐term variations in sunspot magnetic field–area relation

2017

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Background: Several recent studies indicated possible long-term variations of physical properties of sunspots. Materials and Methods: Using observations of sunspot magnetic field strengths (H) from the Crimean Astrophysical Observatory and area (S) of sunspots from the Kislovodsk Mountain Astronomical Station of Pulkovo Observatory, we investigate the changes in the relation between H and S over the period of about two solar cycles (1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013). The data were fitted by H = A + B log S, where A = (778 ± 46) and B = (778 ± 25). Results: We show that the correlation between H and S varies with the phase of solar cycle, and A coefficient decreases significantly after year 2001, while B coefficient does not change significantly. Furthermore, our data confirm the presence of two distinct populations in the distribution of sunspots (small sunspots with weaker field strength and large sunspots with stronger field). We show that the relative contribution of each component to the distribution of sunspots by their area changes with the phase of solar cycle and on longer-than-cycle periods. Conclusion: We interpret these changes as a signature of a long-term (centennial) variations in the properties of sunspots.

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A Multi-Instrument Analysis of Sunspot Umbrae

Cited by 22 publications

References 24 publications

Variation in sunspot properties between 1999 and 2014

Variation in sunspot properties between 1999 and 2014

A Remarkable Recent Transition in the Solar Dynamo

Long‐term variations in sunspot magnetic field–area relation

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