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Kim S.,Nobeyama Solar Radio Observatory NAOJ | Nakariakov V.M.,University of Warwick | Nakariakov V.M.,Russian Academy of Sciences | Shibasaki K.,Nobeyama Solar Radio Observatory NAOJ
Astrophysical Journal Letters | Year: 2012

Analysis of the microwave data, obtained in the 17GHz channel of the Nobeyama Radioheliograph during the M1.6 flare on 2010 November 4, revealed the presence of 12.6 minute oscillations of the emitting plasma density. The oscillations decayed with the characteristic time of about 15 minutes. Similar oscillations with the period of about 13.8 minutes and the decay time of 25 minutes are also detected in the variation of EUV emission intensity measured in the 335 Å channel of the Solar Dynamics Observatory/Atmospheric Imaging Assembly. The observed properties of the oscillations are consistent with the oscillations of hot loops observed by the Solar and Heliospheric Observatory/Solar Ultraviolet Measurement of Emitted Radiation (SUMER) in the EUV spectra in the form of periodic Doppler shift. Our analysis presents the first direct observations of the slow magnetoacoustic oscillations in the microwave emission of a solar flare, complementing accepted interpretations of SUMER hot loop oscillations as standing slow magnetoacoustic waves. © 2012 The American Astronomical Society. All rights reserved.

Reznikova V.E.,Nobeyama Solar Radio Observatory NAOJ | Shibasaki K.,Nobeyama Solar Radio Observatory NAOJ | Sych R.A.,Key Laboratory of Solar Activity | Sych R.A.,Institute of Solar Terrestrial Physics | And 2 more authors.
Astrophysical Journal | Year: 2012

Three-minute oscillations over a sunspot's umbra in AR 11131 were observed simultaneously in UV/EUV emission by the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) and in radio emission by the Nobeyama Radioheliograph (NoRH). We use 24hr series of SDO and 8hr series of NoRH observations to study spectral, spatial, and temporal variations of pulsations in the 5-9mHz frequency range at different layers of the solar atmosphere. High spatial and temporal resolution of SDO/AIA in combination with long-duration observations allowed us to trace the variations of the cutoff frequency and spectrum of oscillations across the umbra. We found that higher frequency oscillations are more pronounced closer to the umbra's center, while the lower frequencies concentrate on the peripheral parts. We interpreted this discovery as a manifestation of variation of the magnetic field inclination across the umbra at the level of temperature minimum. Possible implications of this interpretation for the diagnostics of sunspot atmospheres are discussed. © 2012. The American Astronomical Society. All rights reserved.

Chorley N.,University of Warwick | Foullon C.,University of Warwick | Hnat B.,University of Warwick | Nakariakov V.M.,University of Warwick | And 2 more authors.
Astronomy and Astrophysics | Year: 2011

Long period oscillations in the microwave radiation intensity generated over the sunspot of NOAA AR 10330 are studied with the Nobeyama Radioheliograph as the sunspot passes over the solar disk, over the course of 9 days (06-15 April 2003). Periodogram, Fourier and global wavelet analyses reveal the presence of a significant oscillatory component in the range P ≃ 50-120 min over the course of the observations. The spectral amplitudes of five significant Fourier components in the range P = 50-150 min are also seen to be stable over the observations, when the data are not affected by changes in magnetic configuration in the region. The ground-based nature of the instrument naturally introduces long data gaps in such long duration observations and the presence of the gaps does not allow any conclusion as to the stability of the phases of the oscillations. As a model to explain the persistence of the dominant long periods, a simple oscillator with a nonlinear driving term is proposed. The spectral difference between distinct peaks within, e.g. the 3 min spectral band, is expected to be able to resonate with the long period one hour oscillations. © 2011 ESO.

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.

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.

Kim S.,Nobeyama Solar Radio Observatory NAOJ | Masuda S.,Nagoya University | Shibasaki K.,Nobeyama Solar Radio Observatory NAOJ | Bong S.-C.,Korea Astronomy and Space Science Institute
Publications of the Astronomical Society of Japan | Year: 2013

We found systematic microwave source motions along a flare-arcade using Nobeyama Radioheliograph (NoRH) 17 GHz images. The motions were associated with an X-class disk flare that occurred on 2011 February 15. For this study, we also used EUV images from Atmospheric Imaging Assembly (AIA) and magnetograms from Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory, and multi-channel microwave data from Nobeyama Radio Polarimeters (NoRP) and Korean Solar Radio Burst Locator (KSRBL). We traced centroids of the microwave source observed by NoRH 17 GHz during the flare, and found two episodes of the motion based on several facts: (1) The microwave source moved systematically along the flare-arcade, which was observed by the AIA 94 ?A channel, in a direction parallel to the neutral line. (2) The period of each episode was 5 min and 14 min, respectively. (3) Estimated parallel speed was 34 kms-1 for the first episode and 22 km s-1 for the second episode. The spectral slope of the microwave flux above 10 GHz obtained by NoRP and KSRBL was negative for both episodes, and for the last phase of the second episodes it was flat with a flux of 150 sfu. The negative spectrum and the flat with high flux indicate that the gyrosynchrotron emission from accelerated electrons was dominant during the source motions. The sequential images from the AIA 304 ?A and 94 ?A channels revealed that there were successive plasma eruptions, and each eruption was initiated just before the start time of the microwave sources motion. Based on the results, we suggest that the microwave source motion manifests the displacement of the particle acceleration site caused by plasma eruptions © 2013. Astronomical Society of Japan.

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.

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.

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.

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