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Pevtsov A.A.,U.S. National Solar Observatory | Nagovitsyn Y.A.,Russian Academy of Sciences | Tlatov A.G.,Kislovodsk Solar Station of Pulkovo Observatory | Rybak A.L.,Russian Academy of Sciences
Astrophysical Journal Letters | Year: 2011

Recent studies indicate that a maximum field strength in sunspots shows a gradual decrease over the last several years. By extrapolating this trend, Penn & Livingston proposed that sunspots may completely disappear in the not-so-distant future. To verify these recent findings, we employ historic synoptic data sets from seven observatories in the former USSR covering the period from 1957 to 2011 (from 1998 to 2011, observations were taken at only one observatory). Our results indicate that while sunspot field strengths rise and wane with solar cycle, there is not a long-term trend that would suggest a gradual decrease in sunspot magnetic fields over the four and a half solar cycles covered by these observations. © 2011. The American Astronomical Society. All rights reserved..

Tlatov A.G.,Kislovodsk Solar Station of Pulkovo Observatory | Vasil'Eva V.V.,Kislovodsk Solar Station of Pulkovo Observatory | Pevtsov A.A.,U.S. National Solar Observatory
Astrophysical Journal | Year: 2010

We employ synoptic full disk longitudinal magnetograms to study latitudinal distribution and orientation (tilt) of magnetic bipoles in the course of sunspot activity during cycles 21, 22, and 23. The data set includes daily observations from the National Solar Observatory at Kitt Peak (1975-2002) and Michelson Doppler Imager on board the Solar and Heliospheric Observatory (MDI/SOHO, 1996-2009). Bipole pairs were selected on the basis of proximity and flux balance of two neighboring flux elements of opposite polarity. Using the area of the bipoles, we have separated them into small quiet-Sun bipoles (QSBs), ephemeral regions (ERs), and active regions (ARs). We find that in their orientation, ERs and ARs follow Hale-Nicholson polarity rule. As expected, AR tilts follow Joy's law. ERs, however, show significantly larger tilts of opposite sign for a given hemisphere. QSBs are randomly oriented. Unlike ARs, ERs also show a preference in their orientation depending on the polarity of the large-scale magnetic field. These orientation properties may indicate that some ERs may form at or near the photosphere via the random encounter of opposite polarity elements, while others may originate in the convection zone at about the same location as ARs. The combined latitudinal distribution of ERs and ARs exhibits a clear presence of Spörer's butterfly diagram (equatorward drift in the course of a solar cycle). ERs extend the ARs' "wing" of the butterfly diagram to higher latitudes. This high latitude extension of ERs suggests an extended solar cycle with the first magnetic elements of the next cycle developing shortly after the maximum of the previous cycle. The polarity orientation and tilt of ERs may suggest the presence of poloidal fields of two configurations (new cycle and old cycle) in the convection zone at the declining phase of the sunspot cycle. © 2010. The American Astronomical Society.

Tlatov A.G.,Kislovodsk Solar Station of Pulkovo Observatory | Pevtsov A.A.,U.S. National Solar Observatory
Solar Physics | Year: 2014

We applied automatic identification of sunspot umbrae and penumbrae to daily observations from the Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) to study their magnetic flux density (B) and area (A). The results confirm an already known logarithmic relationship between the area of sunspots and their maximum flux density. In addition, we find that the relation between average magnetic flux density (Bavg) and sunspot area shows a bimodal distribution: for small sunspots and pores (A≤20 millionth of solar hemisphere, MSH), Bavg ≈ 800 G(gauss), and for large sunspots (A≥100 MSH), Bavg is about 600 G. For intermediate sunspots, average flux density linearly decreases from about 800 G to 600 G. A similar bimodal distribution was found in several other integral parameters of sunspots. We show that this bimodality can be related to different stages of sunspot penumbra formation and can be explained by the difference in average inclination of magnetic fields at the periphery of small and large sunspots. © 2013 Springer Science+Business Media Dordrecht.

Pevtsov A.A.,U.S. National Solar Observatory | Bertello L.,U.S. National Solar Observatory | Tlatov A.G.,Kislovodsk Solar Station of Pulkovo Observatory | Kilcik A.,Big Bear Solar Observatory | And 2 more authors.
Solar Physics | Year: 2014

Measurements from the Mount Wilson Observatory (MWO) were used to study the long-term variations of sunspot field strengths from 1920 to 1958. Following a modified approach similar to that presented in Pevtsov et al. (Astrophys. J. Lett. 742, L36, 2011), we selected the sunspot with the strongest measured field strength for each observing week and computed monthly averages of these weekly maximum field strengths. The data show the solar cycle variation of the peak field strengths with an amplitude of about 500 - 700 gauss (G), but no statistically significant long-term trends. Next, we used the sunspot observations from the Royal Greenwich Observatory (RGO) to establish a relationship between the sunspot areas and the sunspot field strengths for cycles 15 - 19. This relationship was used to create a proxy of the peak magnetic field strength based on sunspot areas from the RGO and the USAF/NOAA network for the period from 1874 to early 2012. Over this interval, the magnetic field proxy shows a clear solar cycle variation with an amplitude of 500 - 700 G and a weaker long-term trend. From 1874 to around 1920, the mean value of magnetic field proxy increases by about 300 - 350 G, and, following a broad maximum in 1920 - 1960, it decreases by about 300 G. Using the proxy for the magnetic field strength as the reference, we scaled the MWO field measurements to the measurements of the magnetic fields in Pevtsov et al. (2011) to construct a combined data set of maximum sunspot field strengths extending from 1920 to early 2012. This combined data set shows strong solar cycle variations and no significant long-term trend (the linear fit to the data yields a slope of - 0.2±0.8 G year-1). On the other hand, the peak sunspot field strengths observed at the minimum of the solar cycle show a gradual decline over the last three minima (corresponding to cycles 21 - 23) with a mean downward trend of ≈ 15 G year-1. © 2013 Springer Science+Business Media Dordrecht.

Bertello L.,U.S. National Solar Observatory | Pevtsov A.,U.S. National Solar Observatory | Tlatov A.,Kislovodsk Solar Station of Pulkovo Observatory | Singh J.,Indian Institute of Astrophysics
Solar Physics | Year: 2016

Long-term synoptic observations in the resonance line of Ca ii K constitute a fundamental database for a variety of retrospective analyses of the state of the solar magnetism. Synoptic Ca ii K observations began in late 1904 at the Kodaikanal Observatory in India. In the early 1970s, the National Solar Observatory (NSO) at Sacramento Peak (USA) started a new program of daily Sun-as-a-star observations in the Ca ii K line. Today the NSO is continuing these observations through its Synoptic Optical Long-term Investigations of the Sun (SOLIS) facility. These different data sets can be combined into a single disk-integrated Ca ii K index time series that describes the average properties of the chromospheric emission over several solar cycles. We present such a Ca ii K composite and discuss its correlation with the new entirely revised sunspot number data series. For this preliminary investigation, the scaling factor between pairs of time series was determined assuming a simple linear model for the relationship between the monthly mean values during the duration of overlapping observations. © 2016 Springer Science+Business Media Dordrecht

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