Spatially resolved microwave oscillations above a sunspot

Nindos, A.; Alissandrakis, C. E.; Gelfreikh, G. B.; Bogod, V. M.; Gontikakis, C.

doi:10.1051/0004-6361:20020252

Cited by 45 publications

(34 citation statements)

References 21 publications

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“…It was concluded that the brightness fluctuations of the radio sources could be caused by the density and temperature perturbations in acoustic waves that propagate through the third gyroresonant level (corresponding to the magnetic field of about 2 kG). Nindos et al (2002) used the difference radio maps obtained by VLA at 8.5 and 5 GHz with high spatial resolution. They showed that the amplitude of 3-min oscillations varies irregularly in time, with a rapid rise phase, and a fairly slow decay phase.…”

Section: Introductionmentioning

confidence: 99%

Frequency drifts of 3-min oscillations in microwave and EUV emission above sunspots

Сыч

Zaqarashvili

Nakariakov

et al. 2012

A&A

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Aims. We analysed 3-min oscillations of microwave and extreme ultraviolet (EUV) emission generated at different heights of a sunspot atmosphere, studied the amplitude and frequency modulation of the oscillations, and its relationship with the variation of the spatial structure of the oscillations. Methods. High-resolution data obtained with the Nobeyama Radioheliograph, TRACE and SDO/AIA were analysed with pixelised wavelet filtering (PWF) and wavelet skeleton techniques. Results. Three-minute oscillations in sunspots appear in the form of recurring trains of 8-20 min duration (13 min in average). The typical interval between the trains is 30-50 min. The oscillation trains are transient in frequency and power. The relative amplitude of 3-min oscillations was about 3-8% and sometimes reached 17%. Recurring frequency drifts of 3-min oscillations were detected during the development of individual trains, with the period varying in the range 90-240 s. A wavelet analysis showed that there are three types of oscillation trains: with positive drifts (to high frequencies), negative drifts, and without a drift. Negative drifts, i.e., when the 3-min oscillation period gradually increases, were found to occur more often. The start and end of the drifts coincides with the start time and end of the train. Sometimes two drifts co-exist, i.e. during the end of the previous drift, a new drift appears near 160 s, when the frequency is in the low-frequency part of the 3-min spectrum, near 200 s. This behaviour is seen at all levels of the sunspot atmosphere. The speed of the drift is 4-5 mHz/h in the photosphere, 5-8 mHz/h in the chromosphere, and 11-13 mHz/h in the corona. There were also low-frequency peaks in the spectrum, corresponding to the periods of 10-20 min, and 30-60 min. The comparative study of the spatial structure of 3-min oscillations in microwave and EUV shows the appearance of new sources of the sunspot oscillations during the development of the trains. Conclusions. These structures can be interpreted as waveguides that channel upward propagating waves, which in turn are responsible for the 3-min oscillations. A possible explanation of the observed properties are two simultaneously operating factors: dispersive evolution of the upward propagating wave pulses and the non-uniformity of the oscillation power distribution over the sunspot umbra with different wave sources that correspond to different magnetic flux tubes with different physical conditions and line-of-sight angles.

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Section: Introductionmentioning

confidence: 99%

Frequency drifts of 3-min oscillations in microwave and EUV emission above sunspots

Сыч

Zaqarashvili

Nakariakov

et al. 2012

A&A

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“…Gopalswamy (1994, see also Zhang et al 1998 found radio transient brightenings located near a sunspot umbra, perhaps at the footpoints of an X-ray transient loop observed with Yohkoh, which may indicate small-scale heating. Nindos et al (2002) used VLA observations to detect spatially resolved oscillations in the total intensity and circular polarization emission of a sunspot-associated gyroresonant source and found that the oscillations are intermittent in time and patchy in space. They interpreted the radio oscillations as due to variations of the location of gyroresonant surfaces with respect to the base of the chromosphere-corona transition region.…”

Section: Other Sciences Addressed With the Vlamentioning

confidence: 99%

Sunspots at centimeter wavelengths

Kundu

Lee²

2010

Proc. IAU

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Abstract. The early solar observations of Covington (1947) established a good relation between 10.7 cm solar flux and the presence of sunspots on solar disk. The first spatially resolved observation with a two-element interferometer at arc min resolution by Kundu (1959) found that the radio source at 3 cm has a core-halo structure; the core is highly polarized and corresponds to the umbra of a sunspot with magnetic fields of several hundred gauss, and the halo corresponds to the diffuse penumbra or plage region. The coronal temperature of the core was interpreted as due to gyroresonance opacity produced by acceleration of electrons gyrating in a magnetic field. Since the opacity is produced at resonant layers where the frequency matches harmonics of the gyrofrequency, the radio observation could be utilized to measure the coronal magnetic field. Since this simple interferometric observation, the next step for solar astronomers was to use arc second resolution offered by large arrays at cm wavelengths such as Westerbrock Synthesis Radio Telescope and the Very Large Array, which were primarily built for cosmic radio research. Currently, the Owens Valley Solar Array operating in the range 1-18 GHz and the Nobeyama Radio Heliograph at 17 and 34 GHz are the only solar dedicated radio telescopes. Using these telescopes at multiple wavelengths it is now possible to explore three dimensional structure of sunspot associated radio sources and therefore of coronal magnetic fields. We shall present these measurements at wavelengths ranging from 1.7 cm to 90 cm and associated theoretical developments.

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“…Shibasaki (2001) detected 3-min oscillations in the 17 GHz emission of a sunspot for which he applied the values of density and temperature fluctuations deduced from Solar Ultraviolet Measurements of Emitted Radiation (SUMER) onboard SOHO to find a good agreement with the detected radio oscillation and attribute the 3-min oscillation to the resonant excitation of the cut-off frequency mode of the temperature plateau around the temperature minimum. Nindos et al (2002) used VLA observations to detect spatially resolved oscillations in the total intensity and circular polarization emission of a sunspot-associated gyro-resonant source and found that the oscillations are intermittent in time and patchy in space. They interpreted the radio oscillations as due to variations of the location of gyro-resonant surfaces with respect to the base of the chromosphere-corona transition region, that is, either the magnetic field strength or/and the height of the base of the TR oscillates.…”

Section: Coronal Oscillationsmentioning

confidence: 99%

Radio Emissions from Solar Active Regions

Lee

2007

Space Sci Rev

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Solar active region coronae are known for strong magnetic fields permeating tenuous plasma, which makes them an ideal astronomical laboratory for magnetohydrodynamics research. It is, however, relatively less known that this physical condition also permits a very efficient radiation mechanism, gyro-resonant emission, produced by hot electrons gyrating in the coronal magnetic field. As a resonant mechanism, gyro-emission produces high enough opacity to fully reveal the coronal temperature, and is concentrated at a few harmonics of the local gyrofrequency to serve as an excellent indicator of the magnetic field. In addition, the polarization of the ubiquitous free-free emission and a phenomenon of depolarization due to mode coupling extend the magnetic field diagnostic to a wide range of coronal heights. The ability to measure the coronal temperature and magnetic field without the complications that arise in other radiative inversion problems is a particular advantage for the active region radio emissions available only at these wavelengths. This article reviews the efforts to understand these radiative processes, and use them as diagnostic tools to address a number of critical issues involved with active regions.

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Spatially resolved microwave oscillations above a sunspot

Cited by 45 publications

References 21 publications

Frequency drifts of 3-min oscillations in microwave and EUV emission above sunspots

Frequency drifts of 3-min oscillations in microwave and EUV emission above sunspots

Sunspots at centimeter wavelengths

Radio Emissions from Solar Active Regions

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