On a Possible Explanation of the Long-Term Decrease in Sunspot Field Strength

Nagovitsyn, Yu. A.; Pevtsov, Alexei A.; Livingston, W.

doi:10.1088/2041-8205/758/1/l20

Cited by 75 publications

(52 citation statements)

References 8 publications

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“…We claim this because the determined PDF of umbral areas is consistent with previous studies, both for the whole cycle and for different portions of the solar cycle. Although Nagovitsyn, Pevtsov, and Livingston (2012) suggest that the distribution of the total sunspot areas (including penumbrae) varies with time, we find no support for changes in the umbral area PDF with time. The classification and identification of penumbra in a synoptic data set is quite complicated, as penumbrae associated with different umbrae are often merged and/or too close to each other.…”

Section: Discussioncontrasting

confidence: 96%

On the Collective Magnetic Field Strength and Vector Structure of Dark Umbral Cores Measured by the Hinode Spectropolarimeter

Schad

2013

Sol Phys

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We study 7530 sunspot umbrae and pores measured by the Hinode Spectropolarimeter (SP) between November 2006 and November 2012. We primarily seek confirmation of the long term secular decrease in the mean magnetic field strength of sunspot umbrae found by Penn and Livingston (2011, IAU Symp. 273,126) between 1998 and 2011. The excellent SP photometric properties and full vector magnetic field determinations from full-Stokes Milne-Eddington inversions are used to address the interrelated properties of the magnetic field strength and brightness temperature for all umbral cores. We find non-linear relationships between magnetic field strength and umbral temperature (and continuum contrast), as well as between umbral radius and magnetic field strength. Using disambiguated vector data, we find that the azimuths measured in the umbral cores reflect an organization weakly influenced by Joy's law. The large selection of umbrae displays a log-normal size spectrum similar to earlier solar cycles. Influenced by the amplitude of the solar cycle and the nonlinear relationship between umbral size and core magnetic field strength, the distribution of core magnetic field strengths, fit most effectively with a skew-normal distribution, shows a weak solar cycle dependence. Yet, the mean magnetic field strength does not show a significant long term trend.

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Section: Discussioncontrasting

confidence: 96%

On the Collective Magnetic Field Strength and Vector Structure of Dark Umbral Cores Measured by the Hinode Spectropolarimeter

Schad

2013

Sol Phys

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“…A large daily variation is seen throughout the solar cycle with a tendency toward larger intensities during time of low activity, when large spots are rare or absent. The lower envelope of the data points (corresponding to the largest spot present on the Sun at any given time) shows a solar cycle shape in agreement with the results of Nagovitsyn et al (2012). A first-and second-order fit to the data is overplotted.…”

Section: Sunspot Propertiessupporting

confidence: 81%

Temporal Stability of Sunspot Umbral Intensities: 1986-2012

Toma

Chapman

Cookson

et al. 2013

ApJ

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We examine the relative intensity of sunspot umbrae during the period from 1986 to 2012 using photometric images from the San Fernando Observatory. We confirm the presence of a relationship between the mean umbral core intensity and the mean sunspot area, as found in previous studies, and do not find a notable change in this relationship between cycles 22 and 23. We looked for a possible time variation in the sunspot umbral contrast during the 27 yr covering cycles 22, 23, and the rise of cycle 24, and we did not find a significant change. These findings do not indicate that sunspots have become less dark during cycles 23 and 24.

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“…BOG noted that the distribution function does not change from one cycle to the next. This is in contrast to other umbral properties such as umbral brightness (Albregtsen & Maltby 1978;Albregtsen et al 1984) or magnetic field strength Rezaei et al 2012a), which vary within the solar cycle or show a longterm trend (Nagovitsyn et al 2012). …”

Section: Temporal Invariance Of the Size Distributionmentioning

confidence: 85%

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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On a Possible Explanation of the Long-Term Decrease in Sunspot Field Strength

Cited by 75 publications

References 8 publications

On the Collective Magnetic Field Strength and Vector Structure of Dark Umbral Cores Measured by the Hinode Spectropolarimeter

On the Collective Magnetic Field Strength and Vector Structure of Dark Umbral Cores Measured by the Hinode Spectropolarimeter

Temporal Stability of Sunspot Umbral Intensities: 1986-2012

Properties of sunspot umbrae observed in cycle 24

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