Brightness temperature and size of the quiet Sun at 34.5 MHz

Subramanian, K. R.

doi:10.1051/0004-6361:20047120

Cited by 17 publications

(18 citation statements)

References 16 publications

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“…Such interferometers provide the opportunity to define the location of radio emission sources and their sizes at decameter wavelengths if they have sizes from some minutes to tens of minutes. Such sizes are close to those of the quiet Sun (Aubier, Leblanc, and Boischot, 1971;Kundu, Erickson, and Gergely, 1977;Erickson et al, 1977;Bhonsle, Sawant, and Degaonkar, 1979;Abranin and Bazelian, 1986;Thejappa and Kundu, 1992;Sastry, 1994;Subramanian, 2004;Ramesh et al, 2006;Brazhenko et al, 2012;Stanislavsky, Koval, and Konovalenko, 2013) and the sources of Type III bursts (Abranin et al, 1976;Chen and Shawhan, 1978;Bhonsle, Sawant, and Degaonkar, 1979;Abranin et al, 1980;Suzuki and Dulk, 1985;Melnik et al, 2017), Type II bursts (Chen and Shawhan, 1978;Bhonsle, Sawant, and Degaonkar, 1979;Nelson and Melrose, 1985), and Type IV bursts (Gergely and Kundu, 1974;Chen and Shawhan, 1978) in the decameter range. Previously the sizes of these bursts were measured only occasionally at some discrete frequencies, but now we have the opportunity to observe radio emission of different bursts in the whole frequency band from 8 to 33 MHz on a regular basis.…”

Section: Introductionsupporting

confidence: 54%

“…These sizes are close to those obtained by Kundu, Erickson, and Gergely (1977) in interferometric observations by the radio telescope at Clark Lake, although those observations were carried out in the years of minimum solar activity. At the same time heliograph measurements (Abranin and Bazelian, 1986;Stanislavsky, Koval, and Konovalenko, 2013) the sizes by the method in which the radio source is passing through the antenna beam as was reported by Aubier, Leblanc, and Boischot (1971), Sastry (1994), Subramanian (2004), and Ramesh et al (2006), gave probably larger sizes. A distinction of sizes can be connected with their dependences on the activity of the Sun.…”

Section: Observationsmentioning

confidence: 56%

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Interferometric Observations of the Quiet Sun at 20 and 25 MHz in May 2014

et al. 2018

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Results of solar observations at 20 and 25 MHz by the UTR-2 (Ukrainian T-shaped Radio telescope of the second modification) radio telescope in the interferometric session from 27 May to 2 June 2014 are presented. In such a case the different baselines 225, 450, and 675 m between sections of East-West and North-South arms of the radio telescope UTR-2 were used. On 29 May 2014, strong sporadic radio emission consisting of Type III, a Type II and a Type IV bursts was observed. On other days there was no solar radio activity in the decameter range. We discuss the results of observations of such the quiet Sun. Fluxes and sizes of the Sun in East-West and North-South directions were measured. The average fluxes were 1050-1100 Jy and 1480-1570 Jy at 20 and 25 MHz, respectively. Angular sizes of the quiet Sun in equatorial and polar directions were 55 ′ and 49 ′ at 20 MHz and 50 ′ and 42 ′ at 25 MHz. Brightness temperatures of radio emission were T b = 5.1 × 10 5 K and T b = 5.7 × 10 5 K at 20 and 25 MHz, respectively.

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Section: Introductionsupporting

confidence: 54%

Section: Observationsmentioning

confidence: 56%

Interferometric Observations of the Quiet Sun at 20 and 25 MHz in May 2014

et al. 2018

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“…The most comprehensive and detailed studies of solar continuum emission have been fulfilled in the centimetermillimeter range of wavelengths. A small number of measurements of the flux densities and spectra of the undisturbed Sun radio emission have been made in lowerfrequency range (Aubier, Leblanc & Boischot 1971;Erickson et al 1977;Sheridan 1978;Abranin & Bazelyan 1986;Thejappa & Kundu 1992;Lantos & Alissandrakis 1999;Ramesh 2000;Subramanian 2004;Thejappa & MacDowall 2008), especially at decameter wavelengths (10-30 MHz) because of technical and observational difficulties. To extract a contribution of the quiet Sun on background of a strong radio emission of sky (Galactic noise) in this frequency range, it is necessary to use giant radio telescopes (with total collecting area > 10 5 m 2 ).…”

Section: Introductionmentioning

confidence: 99%

Low‐frequency heliographic observations of the quiet Sun corona

2013

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We present new results of heliographic observations of quiet-Sun radio emission fulfilled by the UTR-2 radio telescope. The solar corona investigations have been made close to the last solar minimum (Cycle 23) in the late August and early September of 2010 by means of the two-dimensional heliograph within 16.5-33 MHz. Moreover, the UTR-2 radio telescope was used also as an 1-D heliograph for one-dimensional scanning of the Sun at the beginning of September 2010 as well as in short-time observational campaigns in April and August of 2012. The average values of integral flux density of the undisturbed Sun continuum emission at different frequencies have been found. Using the data, we have determined the spectral index of quiet-Sun radio emission in the range 16.5-200 MHz. It is equal to -2.1±0.1. The brightness distribution maps of outer solar corona at frequencies 20.0 MHz and 26.0 MHz have been obtained. The angular sizes of radio Sun were estimated. It is found that the solar corona at these frequencies is stretched-out along equatorial direction. The coefficient of corona ellipticity varies slightly during above period. Its mean magnitudes are equal to ≈ 0.75 and ≈ 0.73 at 20.0 MHz and 26.0 MHz, respectively. The presented results for continuum emission of solar corona conform with being ones at higher frequencies.

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“…Thus, these studies have clearly demonstrated that to a large extent, the scattering is responsible for the unusual behavior of the quiet-Sun radio emission, even though very idealized spherical symmetric density models for the solar corona, and Gaussian spectrum for the electron density fluctuations with arbitrary values for the relative level of density fluctuations and spatial scales l were used in these studies. However, some authors (Sastry 1994;Subramanian 2004;Ramesh et al 2006) argue against this scenario because the higher -values required by the scattering to lower the brightness temperatures to the observed values may cause an abnormal increase in the sizes of the radio Sun.…”

Section: Introductionmentioning

confidence: 99%

Effects of Scattering on Radio Emission from the Quiet Sun at Low Frequencies

Thejappa¹,

MacDowall²

2008

ApJ

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The observations of the quiet Sun at meter and decameter wavelengths show that its brightness temperatures can be $1 order of magnitude lower than the expected values of 10 6 K and the apparent diameters can be very large. We examine whether this unusual behavior is due to refraction in the smoothly varying coronal plasma and scattering by random density fluctuations using an improved Monte Carlo simulation technique. We use a three-dimensional model for the electron density, N e , and a power-law spectrum for the density fluctuations with an exponential index of . We consider two cases: (1) ¼ 11/3 (Kolmogorov spectrum), relative level of density fluctuations ¼ ÁN e /N e ¼ 0:1, and scale heights ranging from the inner scale l i ¼ 684 (N e cm À3 ) À1/2 to outer scale l o ¼ 10 6 l i , and (2) ¼ 3 (flat spectrum), ¼ 0:02, and scale heights ranging from 50 to 75 km. We consider the fluctuations to be anisotropic with axial ratios of $10. The Kolmogorov type of fluctuations yield slightly lower brightness temperatures in comparison with those of a flat spectrum. The brightness temperature distributions, east-west diameters, and central brightness temperatures at various frequencies indicate that the refraction and scattering are probably the underlying reasons for the unusual behavior of the quiet-Sun radio emission. This study clearly demonstrates that Monte Carlo techniques can be very effective in extracting the coronal electron temperatures accurately from the radio data, provided the information about the density distributions and density fluctuations is known by some independent methods.

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Brightness temperature and size of the quiet Sun at 34.5 MHz

Cited by 17 publications

References 16 publications

Interferometric Observations of the Quiet Sun at 20 and 25 MHz in May 2014

Interferometric Observations of the Quiet Sun at 20 and 25 MHz in May 2014

Low‐frequency heliographic observations of the quiet Sun corona

Effects of Scattering on Radio Emission from the Quiet Sun at Low Frequencies

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