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Sivaraman, K. R.; Gupta, Siddharth; Howard, Robert F.

doi:10.1023/a:1005277515551

Cited by 43 publications

(19 citation statements)

References 15 publications

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“…The flatter profile of the tilt angle law suggested by our simulations is consistent with results from sunspot groups (Howard 1991;Sivaraman et al 1999) as well as with tilt angles from numerical simulations of rising magnetic flux tubes (e.g., Caligari et al 1995). In fact, sunspot groups are very well suited to observationally determine the relevant tilt angles because they represent active regions in their early phases during and after emergence.…”

Section: Discussionsupporting

confidence: 88%

“…Therefore, requiring the consistency of both, B L and B s , with the observational data uniquely fixes the tilt angle law to γ = 0.15λ. This value is consistent with the analysis of sunspot data (Howard 1991;Sivaraman et al 1999) as well as with results from numerical simulations of rising flux tubes (e.g., Caligari et al 1995). Figure 6 gives time-latitude diagrams of the azimuthally averaged radial surface field from the flux-transport model (with f = 0.15, calibrated as described above) and from observational results.…”

Section: Tilt Anglesupporting

confidence: 88%

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Modeling the Sun's open magnetic flux

Schüßler¹,

Baumann

2006

A&A

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Context. The heliospheric magnetic field can be extrapolated from the photospheric field distribution using models based upon various approximations. Such models are required for the reconstruction of the open solar magnetic flux prior to the time of direct measurements on the basis of surface flux transport simulations. Aims. We evaluate the consistency of extrapolation models with direct measurements of the heliospheric magnetic field. Furthermore, we study whether extrapolations on the basis of a surface flux transport model for the photospheric magnetic field reproduce the temporal evolution of the measured near-Earth magnetic field.

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Section: Discussionsupporting

confidence: 88%

Section: Tilt Anglesupporting

confidence: 88%

Modeling the Sun's open magnetic flux

Schüßler¹,

Baumann

2006

A&A

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“…MW data are represented by asterisks connected by the dashed line and KK observations by open circles connected by the solid line. The results for both data sets are in agreement with each other within 1−2σ and are very close to the results obtained by Sivaraman et al (1999), with the mean difference between our points and theirs being |Δ| ∼ 0.1 • and |Δ| ∼ 0.2 • for MW and KK, respectively. These differences are most probably due to different selection criteria applied to the data.…”

Section: Joy's Lawsupporting

confidence: 91%

“…The tilt angle dependence on the latitude was first found by Joy in 1919 (Hale et al 1919) and later confirmed by other authors (Howard 1991;Wang & Sheeley 1991;Sivaraman et al 1999). It provides strong constraints on the magnetic field strength of the flux tubes in the tachocline, which emerge to form the observed active regions (D'Silva & Choudhuri 1993;Schüssler et al 1994).…”

Section: Joy's Lawmentioning

confidence: 52%

Sunspot group tilt angles and the strength of the solar cycle

Dasi-Espuig¹,

Solanki

Krivova³

et al. 2010

A&A

210

231

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Context. It is well known that the tilt angles of active regions increase with their latitude (Joy's law). It has never been checked before, however, whether the average tilt angles change from one cycle to the next. Flux transport models show the importance of tilt angles for the reversal and build up of magnetic flux at the poles, which is in turn correlated to the strength of the next cycle. Aims. Here we analyse time series of tilt angle measurements and look for a possible relationship of the tilt angles with other solar cycle parameters, in order to glean information on the solar dynamo and to estimate their potential for predicting solar activity. Methods. We employed tilt angle data from Mount Wilson and Kodaikanal observatories covering solar cycles 15 to 21. We analyse the latitudinal distribution of the tilt angles (Joy's law), their variation from cycle to cycle, and their relationship to other solar cycle parameters, such as the strength (or total area covered by sunspots in a cycle), amplitude, and length. Results. The two main results of our analysis follow. 1. We find an anti-correlation between the mean normalised tilt angle of a given cycle and the strength (or amplitude) of that cycle, with a correlation coefficient of r c = −0.95 (99.9% confidence level) and r c = −0.93 (99.76% confidence level) for Mount Wilson and Kodaikanal data, respectively. 2. The product of the cycle's averaged tilt angle and the strength of the same cycle displays a significant correlation with the strength of the next cycle (r c = 0.65 at 89% confidence level and r c = 0.70 at 92% confidence level for Mount Wilson and Kodaikanal data, respectively). An even better correlation is obtained between the source term of the poloidal flux in Babcock-Leighton-type dynamos (which contains the tilt angle) and the amplitude of the next cycle. Further we confirm the linear relationship (Joy's law) between the tilt angle and latitude with slopes of 0.26 and 0.28 for Mount Wilson and Kodaikanal data, respectively. In addition, we obtain good positive correlations between the normalised-areaweighted tilt angle and the length of the following cycle, whereas the strength or the amplitude of the next cycle does not appear to be correlated to the tilt angles of the current cycle alone. Conclusions. The results of this study indicate that, in combination with the cycle strength, the active region tilt angles play an important role in building up the polar fields at cycle minimum.

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“…Wilson images and by Sivaraman et al (1999) from Kodaikanal images. From those data, average tilt angles were obtained by Dasi-Espuig et al (2010, 2013 for the solar cycles 15−21.…”

Section: Introductionmentioning

confidence: 99%

Sunspot areas and tilt angles for solar cycles 7–10

Pavai

Arlt

Dasi-Espuig

et al. 2015

A&A

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Aims. Extending the knowledge about the properties of solar cycles into the past is essential for understanding the solar dynamo. This paper aims to estimate areas of sunspots observed by Schwabe in 1825−1867 and to calculate the tilt angles of sunspot groups. Methods. The sunspot sizes in Schwabe's drawings are not to scale and need to be converted into physical sunspot areas. We employed a statistical approach assuming that the area distribution of sunspots was the same in the 19th century as it was in the 20th century. Results. Umbral areas for about 130 000 sunspots observed by Schwabe were obtained, as well as the tilt angles of sunspot groups assuming them to be bipolar. There is, of course, no polarity information in the observations. The annually averaged sunspot areas correlate reasonably with sunspot number. We derived an average tilt angle by attempting to exclude unipolar groups with a minimum separation of the two alleged polarities and an outlier rejection method which follows the evolution of each group and detects the moment it turns unipolar at its decay. As a result, the tilt angles, although displaying considerable scatter, average to 5.• 85 ± 0.• 25, with the leading polarity located closer to the equator, in good agreement with tilt angles obtained from 20th century data sets. Sources of uncertainties in the tilt angle determination are discussed and need to be addressed whenever different data sets are combined. The sunspot area and tilt angle data are provided at the CDS.

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Cited by 43 publications

References 15 publications

Modeling the Sun's open magnetic flux

Modeling the Sun's open magnetic flux

Sunspot group tilt angles and the strength of the solar cycle

Sunspot areas and tilt angles for solar cycles 7–10

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