Properties of sunspot umbrae observed in cycle 24

Kiess, C.; Rezaei, Reza; Schmidt, Wolfgang

doi:10.1051/0004-6361/201321119

Cited by 44 publications

(47 citation statements)

References 127 publications

(176 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: 60%

“…In addition, the umbral and penumbral area of each spot was also measured in the full-disk images of MDI (till end of 2009) and HMI (from 2010 onward). The full-disk images of continuum intensity were corrected for center-to-limb effects using the same method as in Kiess et al (2014). A fixed threshold of 0.6 and 0.9 I c was used to select the umbra and penumbra, respectively.…”

Section: Observationsmentioning

confidence: 99%

“…Norton et al (2013) compared the relation of the intensity to the field strength in individual spots and in a small sample of sunspots and found a strong correlation between intensity and field strength in both cases. Norton & Gilman (2004) found that the umbral intensity is a good tracer of the magnetic field strength because the umbral contrast in the visible is higher than in the infrared (see also Kiess et al 2014). Determining the relation of the intensity to the field strength based solely on MDI or HMI data is difficult, however, because for high field strengths, the spectral line splits beyond the spectral range sampled by the five or six sparsely spaced filter measurements.…”

Section: Introductionmentioning

confidence: 99%

“…The high umbral contrast in the visible wavelengths allowed Mathew et al (2007), Schad (2014), and Kiess et al (2014), among others, to study the variation of umbral continuum intensity as a function of size. These authors reported a nonlinear decrease of the Article published by EDP Sciences A43, page 1 of 9 intensity with umbral radius.…”

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: 60%

Section: Observationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Variation in sunspot properties between 1999 and 2014

Rezaei

Beck

Lagg

et al. 2015

A&A

View full text Add to dashboard Cite

show abstract

“…We inverted the Stokes profiles from HMI using the Very Fast Inversion of the Stokes Vector (VFISV, Borrero et al 2007Borrero et al , 2011 code to retrieve the magnetic field vector in the photosphere (see Kiess et al 2014 for a detailed explanation of the inversion setup). The retrieved magnetic field vector was transformed into the local reference frame.…”

Section: Inversion Of Spectropolarimetric Datamentioning

confidence: 99%

Multiwavelength spectropolarimetric observations of an Ellerman bomb

Rezaei

Beck

2015

A&A

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Context. Ellerman bombs (EBs) are enhanced emission in the wings of the Hα line in the solar spectrum. Aims. We study the structure of an EB in the photosphere and chromosphere.Methods. We analyze simultaneous observations of four chromospheric lines (Hα, Ca ii H, Ca ii IR 854 nm, and He i 1083 nm) as well as two photospheric lines (Fe i 630 and Si i 1082.7 nm) along with high-cadence 160 and 170 nm ultraviolet (UV) continuum filtergrams. Full Stokes data from the Helioseismic and Magnetic Imager (HMI) are used to trace the temporal evolution of the magnetic structure.Results. We identify the EB by excess emission in the wings of the Hα line, a brightening in the UV continuum, and large emission peaks in the core of the two Ca ii lines. The EB shows a blueshift in all chromospheric lines, while no shifts are observed in the photospheric lines. The blueshift in the chromospheric layer causes very asymmetric emission peaks in the Ca ii H line. The photospheric Si i spectral line shows a shallower line depth at the location of the EB. The UV continuum maps show that the EB was substantially brighter than its surroundings for about 30 min. The continuum contrast of the EB from 170 nm to 1080 nm shows a power-law dependency on the wavelength. The temperature enhancement amounts to 130 K in the low photosphere and 400 K at the temperature minimum level. This temperature excess is also seen in an LTE inversion of the Ca ii spectra. The total thermal and radiative energy content of the EB is about 10 20 J and 10 18 J in the photosphere and chromosphere, respectively. The HMI data hints at a photospheric magnetic flux cancellation as the driver of the EB. Conclusions. Ellerman bombs release the energy in a height range of several pressure scale heights around the temperature minimum such that they affect both the photosphere and the lower chromosphere.

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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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Properties of sunspot umbrae observed in cycle 24

Cited by 44 publications

References 127 publications

Variation in sunspot properties between 1999 and 2014

Variation in sunspot properties between 1999 and 2014

Multiwavelength spectropolarimetric observations of an Ellerman bomb

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

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