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Ermolli I.,National institute for astrophysics | Shibasaki K.,Nobeyama Solar Radio Observatory NAOJ | Tlatov A.,Kislovodsk Mountain Astronomical Station of the Pulkovo Observatory | van Driel-Gesztelyi L.,University College London | And 2 more authors.
Space Science Reviews | Year: 2014

A variety of indices have been proposed in order to represent the many different observables modulated by the solar cycle. Most of these indices are highly correlated with each other owing to their intrinsic link with the solar magnetism and the dominant eleven year cycle, but their variations may differ in fine details, as well as on short- and long-term trends. In this paper we present an overview of the indices that are often employed to describe the many features of the solar cycle, moving from the ones referring to direct observations of the inner solar atmosphere, the photosphere and chromosphere, to those deriving from measurements of the transition region and solar corona. For each index, we summarize existing measurements and typical use, and for those that quantify physical observables, we describe the underlying physics. © 2014, Springer Science+Business Media Dordrecht. Source


Shibasaki K.,Nobeyama Solar Radio Observatory NAOJ | Alissandrakis C.E.,University of Ioannina | Pohjolainen S.,University of Turku
Solar Physics | Year: 2011

Solar radio emission provides valuable information on the structure and dynamics of the solar atmosphere above the temperature minimum. We review the background and most recent observational and theoretical results on the quiet Sun and active region studies, covering the entire radio range from millimeter to decameter wavelengths. We examine small- and large-scale structures, at short and long time scales, as well as synoptic aspects. Open questions and challenges for the future are also identified. © 2011 Springer Science+Business Media B.V. Source


Bakunina I.A.,National Research University Higher School of Economics | Abramov-Maximov V.E.,Russian Academy of Sciences | Nakariakov V.M.,Russian Academy of Sciences | Nakariakov V.M.,Kyung Hee University | And 7 more authors.
Publications of the Astronomical Society of Japan | Year: 2013

Long-term oscillations of microwave emission generated in sunspot magnetospheres are detected with the Nobeyama Radioheliograph (NoRH) at a frequency of 17 GHz, and the Siberian Solar Radio Telescope (SSRT) at 5.7 GHz. Significant periodicities in the range of 22-170 min are found in the variation of the emission intensity, polarisation and the degree of circular polarisation. Periods of the oscillations are not stable: they are different in different sunspots and in the same sunspot on different days. A cross-correlation analysis shows the presence of common significant periods in both NoRH and SSRT data. The cross-correlation coefficients are typically lower than 0.5, which can be attributed to the different heights of the emission formation, and different mechanisms for the emission generation (gyroresonance and thermal bremstrahlung at 17 GHz, and pure gyroresonance at 5.7 GHz). The observational results are consistent with the global sunspot oscillation model © 2013. Astronomical Society of Japan. Source


Shibasaki K.,Nobeyama Solar Radio Observatory NAOJ
Publications of the Astronomical Society of Japan | Year: 2013

The Nobeyama Radioheliograph has been observing the Sun at a frequency of 17 GHz regularly since 1992, providing synthesized full-disk images. This long period of continuous and consistent operation, providing wellcalibrated data of a uniform standard, makes possible long-term studies of solar activity, from full-disk down to the angular resolution of the instrument. By using about 7200 daily, full-disk images, it has been possible to generate a radio version of the butterfly diagram, which differs significantly from the sunspot butterfly diagram. The polar regions are bright at 17 GHz, with their brightness well-correlated with the polar magnetic field strengths. Both are anti-correlated with activity at low latitudes, such as active regions and solar flares. The 17 GHz butterfly diagram shows both high and low-latitude activity. The brightness of both these facets of solar activity shows a significant decline over the 20+ years observations that have been made. In the northern hemisphere, the radio brightnesses at low and high latitudes are strongly anti-correlated. However, this anti-correlation is weak in the southern hemisphere. We find a weakening of the synchronization of activity between the northern and southern hemispheres, and also between high and low latitude activity in the southern hemisphere. Possible causes of polar brightening and the meaning with respect to the general scenario of solar activity are discussed. © 2013. Astronomical Society of Japan. Source


Abramov-Maximov V.E.,Russian Academy of Sciences | Efremov V.I.,Russian Academy of Sciences | Parfinenko L.D.,Russian Academy of Sciences | Solov'ev A.A.,Russian Academy of Sciences | Shibasaki K.,Nobeyama Solar Radio Observatory NAOJ
Publications of the Astronomical Society of Japan | Year: 2013

We present an investigation of oscillatory processes with periods in the range of several tens of minutes for some single sunspots of a new solar cycle, observed in 2010-2011 at the same time intervals in the optical and radio ranges. We used magnetograms from SDO/HMI with a cadence of 45 s, and radio images at a frequency of 17 GHz obtained with the Nobeyama Radioheliograph (NoRH). Radio images in intensity (Stokes parameter I ) and circular polarization (Stokes parameter V ) were synthesized with a cadence of ten seconds and ten-second averaging. Time profiles obtained with NoRH and SDO/HMI show a correlation between the radio emission of sunspots and a magnetic field. Wavelet spectra and cross-wavelet transforms give similar oscillation periods: 30-40 min, 60-70 min, 100-110 min, and 150-200 min. The same periods found by fundamentally different methods from ground-based and space observations confirm the solar nature of these oscillations. One of the possible interpretations of our results is that detected oscillations reflect eigen oscillations of a sunspot as a whole predicted by the shallow sunspot model © 2013. Astronomical Society of Japan. Source

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