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TOPIC: Sunspots
Blobrana


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Posts: 131433
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RE: Sunspots
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Title: Formation of starspots in self-consistent global dynamo models: Polar spots on cool stars
Author: Rakesh K. Yadav, Thomas Gastine, Ulrich R. Christensen, Ansgar Reiners

Observations of cool stars reveal dark spot-like features on their surfaces. Compared to sunspots, starspots can be bigger or cover a larger fraction of the stellar surface. While sunspots appear only at low latitudes, starspots are also found in polar regions, in particular on rapidly rotating stars. Sunspots are believed to result from the eruption of magnetic flux-tubes rising from the deep interior of the Sun. The strong magnetic field locally reduces convective heat transport to the solar surface. Such flux-tube models have also been invoked to explain starspot properties. However, these models use several simplifications and so far the generation of either sunspots or starspots has not been demonstrated in a self-consistent simulation of stellar magnetic convection. Here we show that direct numerical simulations of a distributed dynamo operating in a density-stratified rotating spherical shell can spontaneously generate cool spots. Convection in the interior of the model produces a large scale magnetic field which interacts with near surface granular convection leading to strong concentrations of magnetic flux and formation of starspots. Prerequisites for the formation of sizeable high-latitude spots in the model are sufficiently strong density stratification and rapid rotation. Our model presents an alternate mechanism for starspot formation by distributed dynamo action.

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Blobrana


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Posts: 131433
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Title: Revisiting the Sunspot Number
Author: Frédéric Clette, Leif Svalgaard, José M. Vaquero, Edward W. Cliver

Our knowledge of the long-term evolution of solar activity and of its primary modulation, the 11-year cycle, largely depends on a single direct observational record: the visual sunspot counts that retrace the last 4 centuries, since the invention of the astronomical telescope. Currently, this activity index is available in two main forms: the International Sunspot Number initiated by R. Wolf in 1849 and the Group Number constructed more recently by Hoyt and Schatten (1998a,b). Unfortunately, those two series do not match by various aspects, inducing confusions and contradictions when used in crucial contemporary studies of the solar dynamo or of the solar forcing on the Earth climate. Recently, new efforts have been undertaken to diagnose and correct flaws and biases affecting both sunspot series, in the framework of a series of dedicated Sunspot Number Workshops. Here, we present a global overview of our current understanding of the sunspot number calibration. While the early part of the sunspot record before 1800 is still characterized by large uncertainties due to poorly observed periods, the more recent sunspot numbers are mainly affected by three main inhomogeneities: in 1880-1915 for the Group Number and in 1947 and 1980-2014 for the Sunspot Number. The newly corrected series clearly indicates a progressive decline of solar activity before the onset of the Maunder Minimum, while the slowly rising trend of the activity after the Maunder Minimum is strongly reduced, suggesting that by the mid 18th century, solar activity had already returned to the level of those observed in recent solar cycles in the 20th century. We finally conclude with future prospects opened by this epochal revision of the Sunspot Number, the first one since Wolf himself, and its reconciliation with the Group Number, a long-awaited modernization that will feed solar cycle research into the 21st century.

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Blobrana


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Posts: 131433
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Title: Tracing sunspot groups to determine angular momentum transfer on the Sun
Author: D. Sudar, I. Skokic, D. Rudjak, R. Braja, H. Wöhl

The goal of this paper is to investigate Reynolds stresses and to check if it is plausible that they are responsible for angular momentum transfer toward the solar equator. We also analysed meridional velocity, rotation velocity residuals and correlation between the velocities. We used sunspot groups position measurements from GPR (Greenwich Photographic Result) and SOON/USAF/NOAA (Solar Observing Optical Network/United States Air Force/National Oceanic and Atmospheric Administration) databases covering the period from 1878 until 2011. In order to calculate velocities we used daily motion of sunspot groups. The sample was also limited to ±58\degr in Central Meridian Distance (CMD) in order to avoid solar limb effects. We mainly investigated velocity patterns depending on solar cycle phase and latitude. We found that meridional motion of sunspot groups is toward the centre of activity from all available latitudes and in all phases of the solar cycle. The range of meridional velocities is ±10 m s^-1. Horizontal Reynolds stress is negative at all available latitudes and indicates that there is a minimum value (q= - 3000 m2 s^-2) located at b=±300. In our convention this means that angular momentum is transported toward the solar equator in agreement with the observed rotational profile of the Sun.

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Blobrana


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Posts: 131433
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Title: Forty two years counting spots: solar observations by D. E. Hadden during 1890-1931 revisited
Authors: V. M. S. Carrasco, J. M. Vaquero, M. C. Gallego, R. M. Trigo

We have recovered the sunspot observations made by David E. Hadden during 1890-1931 from Alta, Iowa. We have digitised the available data published by Hadden in different astronomical journals. This data series have been analysed and compared with the standard sunspot number series. Moreover, we provide additional information on two great sunspot groups, previously not described, that originated two important extreme episodes of space weather on February 1892 and September 1898.

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Blobrana


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Posts: 131433
Date:
Sunspot Penumbrae
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Title: A Curious History of Sunspot Penumbrae
Authors: D. H. Hathaway

Daily records of sunspot group areas compiled by the Royal Observatory, Greenwich, from May of 1874 through 1976 indicate a curious history for the penumbral areas of the smaller sunspot groups. On average, the ratio of penumbral area to umbral area in a sunspot group increases from 5 to 6 as the total sunspot group area increases from 100 to 2000 microHem (a microHem is a millionth the area of a solar hemisphere). This relationship does not vary substantially with sunspot group latitude or with the phase of the sunspot cycle. However, for the sunspot groups with total areas <100 microHem, this ratio changes dramatically and systematically through this historical record. The ratio for these smallest sunspots is near 5.5 from 1874 to 1900. After a rapid rise to more than 7 in 1905 it drops smoothly to less than 3 by 1930 and then rises smoothly back to more than 7 in 1961. It then returns to near 5.5 from 1965 to 1976. The smooth variation from 1905 to 1961 shows no indication of any step-like changes that might be attributed to changes in equipment or personnel. The overall level of solar activity was increasing monotonically during this time period when the penumbra-to-umbra area ratio dropped to less than half its peak value and then returned. If this history can be confirmed by other observations (e.g., Mt. Wilson or Kodaikanal) it may impact our understanding of penumbra formation, our dynamo models, and our estimates of historical changes in the solar irradiance.

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Blobrana


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Posts: 131433
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RE: Sunspots
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Title: Direct measurement results of the time lag of LOS-velocity oscillations between two heights in solar faculae and sunspots
Authors: Nikolai Kobanov, Dmitri Kolobov, Arseniy Kustov, Sergey Chupin, Andrey Chelpanov

We present an investigation of line-of-sight (LOS) velocity oscillations in solar faculae and sunspots. To study the phase relations between chromosphere and photosphere oscillations of the LOS velocity, we measured the time lag of the chromospheric signal relative to the photospheric one for several faculae and sunspots in a set of spectral line pairs. The measured time lags are different for different objects. The mean measured delay between the oscillations in the five-minute band in faculae is 50s for the SiI 10827{\AA}-HeI 10830{\AA} pair; for the pair FeI 6569{\AA}-H-alpha 6563{\AA} the mean delay is 20s; for the pair FeI 4551{\AA}-BaII 4554{\AA} the mean delay is 7s; for the pair SiI 8536{\AA}-CaII 8542{\AA} the mean delay is 20s. For the oscillations in the three-minute band in sunspot umbrae the mean delay is 55s for the SiI 10827{\AA}-HeI 10830{\AA} pair; for the Fe I 6569{\AA}-H-alpha 6563{\AA} pair it was not possible to determine the delay; for the FeI 4551{\AA}-BaII 4554{\AA} pair the mean delay is 6s; for the SiI 8536{\AA}-CaII 8542{\AA} pair the mean delay is 21s. Measured delays correspond to the wave propagation speed which significantly exceeds the generally adopted speed of sound in the photosphere. This raises the question of the origin of these oscillations. The possibility that we deal with slow MHD waves is not ruled out.

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Blobrana


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Title: On the surface structure of sunspots
Authors: Morten Franz

A precise knowledge of the surface structure of sunspots is essential to construct adequate input models for helioseismic inversion tools. We summarise our recent findings about the velocity and magnetic field in and around sunspots using HINODE observation. To this end we quantise the horizontal and vertical component of the penumbral velocity field at different levels of precision and study the moat flow around sunspot. Furthermore, we find that a significant amount of the penumbral magnetic fields return below the surface within the penumbra. Finally, we explain why the related opposite polarity signals remain hidden in magnetograms constructed from measurements with limited spectral resolution.

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Blobrana


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Posts: 131433
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Spörer's sunspot drawings
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Title: Digitisation of Spörer's sunspot drawings
Authors: Andrea Diercke, Rainer Arlt, Carsten Denker

Much of our knowledge about the solar dynamo is based on sunspot observations. It is thus desirable to extend the set of positional and morphological data of sunspots into the past. Gustav Sporer observed in Germany from Anklam (1861-1873) and Potsdam (1874-1894). He left detailed prints of sunspot groups, which we digitised and processed to mitigate artefacts left in the print by the passage of time. After careful geometrical correction, the sunspot data are now available as synoptic charts for almost 450 solar rotation periods. Individual sunspot positions can thus be precisely determined and spot areas can be accurately measured using morphological image processing techniques. These methods also allow us to determine tilt angles of active regions (Joy's law) and to assess the complexity of an active region.

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Blobrana


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Posts: 131433
Date:
RE: Sunspots
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Title: The G-O Rule and Waldmeier Effect in the Variations of the Numbers of Large and Small Sunspot Groups
Authors: J. Javaraiah

We have analysed the combined Greenwich and Solar Optical Observing Network (SOON) sunspot group data during the period of 1874-2011 and determined variations in the annual numbers (counts) of the small, large and big sunspot groups (these classifications are made on the basis of the maximum areas of the sunspot groups). We found that the amplitude of an even-numbered cycle of the number of large groups is smaller than that of its immediately following odd-numbered cycle. This is consistent with the well known Gnevyshev and Ohl rule or G-O rule of solar cycles, generally described by using the Zurich sunspot number (Rz). During cycles 12-21 the G-O rule holds good for the variation in the number of small groups also, but it is violated by cycle pair (22, 23) as in the case of Rz. This behaviour of the variations in the small groups is largely responsible for the anomalous behaviour of Rz in cycle pair (22, 23). It is also found that the amplitude of an odd-numbered cycle of the number of small groups is larger than that of its immediately following even-numbered cycle. This can be called as `reverse G-O rule'. In the case of the number of the big groups, both cycle pairs (12, 13) and (22, 23) violated the G-O rule. In many cycles the positions of the peaks of the small, large, and big groups are different and considerably differ with respect to the corresponding positions of the Rz peaks. In the case of cycle 23, the corresponding cycles of the small and large groups are largely symmetric/less asymmetric (Waldmeier effect is weak/absent) with their maxima taking place two years later than that of Rz. The corresponding cycle of the big groups is more asymmetric (strong Waldmeier effect) with its maximum epoch taking place at the same time as that of Rz.

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Blobrana


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Title: Observations of Supersonic Downflows in a Sunspot Light Bridge as Revealed by Hinode
Authors: Rohan E. Louis, Luis R. Bellot Rubio, Shibu K. Mathew, P. Venkatakrishnan

Recent high resolution spectropolarimetric observations from Hinode detected the presence of supersonic downflows in a sunspot light bridge (Louis et al. 2009). These downflows occurred in localised patches, close to regions where the field azimuth changed by a large value. This apparent discontinuity in the field azimuth was seen along a thin ridge running along the western edge of the light bridge. Some, but not all, of these downflowing patches were co-spatial with chromospheric brightness enhancements seen in Ca II H filtergrams. The presence of magnetic inhomogeneities at scales of 0.3 arcsec could facilitate the reconnection of field lines in the lower chromosphere whose signatures might be the supersonic downflows and the brightness enhancements that have been observed.

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