Polarization of the Thermal Radio Emission From Outer Solar Corona

Sastry, Ch. V.

doi:10.1088/0004-637x/697/2/1934

Cited by 29 publications

(31 citation statements)

References 36 publications

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“…We choose a slightly larger threshold of 0.5 % because we wanted to implement our procedure uniformly, and the noise level in some of our observations makes a lower threshold impractical. This value is also consistent with the predictions of Sastry (2009), and the effect of varying the threshold is folded into V /I uncertainty estimates presented in Section 5. Our algorithm therefore assumes that most of the pixels in our images of the quiescent corona should exhibit polarization fractions of less than 0.5 % and determines the leakage fraction that minimizes the number of pixels with V /I values greater than 0.005.…”

Section: An Algorithm To Mitigate the Leakage Of Stokes I Into Vsupporting

confidence: 88%

“…First, low-frequency radio emission is heavily influenced by propagation effects, namely refraction, scattering, and mode coupling, that can influence the polarization sign and fraction, and these effects become more pronounced near the limb (Shibasaki, Alissandrakis, and Pohjolainen, 2011). Second, although the polarization fraction is expected to be highest off the limb (Sastry, 2009), the intensity is much lower there. The third column of Figure 12 shows that we often do find relatively high polarization fractions toward the radio limb, but these pixels are very close to the noise level in Stokes V and we do not regard them as reliable.…”

Section: The Large-scale Quiescent Structurementioning

confidence: 99%

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The Low-Frequency Solar Corona in Circular Polarization

et al. 2019

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We present spectropolarimetric imaging observations of the solar corona at low frequencies (80 -240 MHz) using the Murchison Widefield Array (MWA). These images are the first of their kind, and we introduce an algorithm to mitigate an instrumental artefact by which the total intensity signal contaminates the polarimetric images due to calibration errors. We then survey the range of circular polarization (Stokes V ) features detected in over 100 observing runs near solar maximum during quiescent periods. First, we detect around 700 compact polarized sources across our dataset with polarization fractions ranging from less than 0.5 % to nearly 100 %. These sources exhibit a positive correlation between polarization fraction and total intensity, and we interpret them as a continuum of plasma emission noise storm (Type I burst) continua sources associated with active regions. Second, we report a characteristic "bullseye" structure observed for many low-latitude coronal holes in which a central polarized component is surrounded by a ring of the opposite sense. The central component does not match the sign expected from thermal bremsstrahlung emission, and we speculate that propagation effects or an alternative emission mechanism may be responsible. Third, we show that the large-scale polarimetric structure at our lowest frequencies is reasonably well-correlated with the line-of-sight (LOS) magnetic field component inferred from a global potential field source surface (PFSS) model. The boundaries between opposite circular polarization signs are generally aligned with polarity inversion lines in the model at a height roughly corresponding to that of the radio limb. This is not true at our highest frequencies, however, where the LOS magnetic field direction and polarization sign are often not straightforwardly correlated.

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Section: An Algorithm To Mitigate the Leakage Of Stokes I Into Vsupporting

confidence: 88%

Section: The Large-scale Quiescent Structurementioning

confidence: 99%

The Low-Frequency Solar Corona in Circular Polarization

et al. 2019

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“…The combination of SKA1-MID's unprecedented sensitivity and high frequency resolution will allow, for the first time, to constrain the longitudinal component of the coronal magnetic field to a few Gauss. An extension of the above method could be used to provide estimates of the magnetic field strength of undisturbed streamers at higher altitudes (heliocentric distances of 1.5R ⊙ to 2.5R ⊙ ) by measuring the degree of circular polarization of the associated free-free emission using SKA1-LOW observations (see Sastry, 2009;Ramesh et al, 2010b, for details).…”

Section: Magnetic Field From Free-free Emissionmentioning

confidence: 99%

Solar physics with the Square Kilometre Array

Nindos

Kontar

Oberoi

2019

Advances in Space Research

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The Square Kilometre Array (SKA) will be the largest radio telescope ever built, aiming to provide collecting area larger than 1 km 2 . The SKA will have two independent instruments, SKA-LOW comprising of dipoles organized as aperture arrays in Australia and SKA-MID comprising of dishes in South Africa. Currently the phase-1 of SKA, referred to as SKA1, is in its late design stage and construction is expected to start in 2020. Both SKA1-LOW (frequency range of 50-350 MHz) and SKA1-MID Bands 1, 2, and 5 (frequency ranges of 350-1050, 950-1760, and 4600-15300 MHz, respectively) are important for solar observations. In this paper we present SKA's unique capabilities in terms of spatial, spectral, and temporal resolution, as well as sensitivity and show that they have the potential to provide major new insights in solar physics topics of capital importance including (i) the structure and evolution of the solar corona, (ii) coronal heating, (iii) solar flare dynamics including particle acceleration and transport, (iv) the dynamics and structure of coronal mass ejections, and (v) the solar aspects of space weather. Observations of the Sun jointly with the new generation of groundbased and space-borne instruments promise unprecedented discoveries.

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“…Solar coronal magnetic field can be effectively estimated using low frequency radio observations (< 150 MHz or so) in the cross-correlation mode between signals received by two mutually orthogonal dipole antennas (Sastry, 2009). It is only the circularly polarized radio emission that is observed since linearly polarized radio emission averages to zero over typical observing bandwidths at low frequencies.…”

Section: Radio Observations Of Solar Magnetic Fieldsmentioning

confidence: 99%

Probing Magnetic Fields with Square Kilometre Array and its Precursors

et al. 2016

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Origin of magnetic fields, its structure and effects on dynamical processes in stars to galaxies are not well understood. Lack of a direct probe has remained a problem for its study. The first phase of Square Kilometre Array (SKA-I), will have almost an order of magnitude higher sensitivity than the best existing radio telescope at GHz frequencies. In this contribution, we discuss specific science cases that are of interest to the Indian community concerned with astrophysical turbulence and magnetic fields. The SKA-I will allow observations of a large number of background sources with detectable polarization and measure their Faraday depths (FDs) through the Milky Way, other galaxies and their circum-galactic mediums. This will probe line-of-sight magnetic fields in these objects well and provide field configurations. Detailed comparison of observational data (e.g., pitch angles in spirals) with models which consider various processes giving rise to field amplification and maintenance (e.g., various types of dynamo models) will then be possible. Such observations will also provide the coherence scale of the fields and its random component through RM structure function. Measuring the random component is important to characterise turbulence in the medium. Observations of FDs with redshift will provide important information on magnetic field evolution as a function of redshift. The background sources could also be used to probe magnetic fields and its coherent scale in galaxy clusters and in bridges formed between interacting galaxies. Other than FDs, sensitive observations of synchrotron emission from galaxies will provide complimentary information on their magnetic field strengths in the sky plane. The core shift measurements of AGNs can provide more precise measurements of magnetic field in the sub parsec region near the black hole and its evolution. The low band of SKA-I will also be useful to study circularly polarized emission from Sun and comparing various models of field configurations with observations.

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Polarization of the Thermal Radio Emission From Outer Solar Corona

Cited by 29 publications

References 36 publications

The Low-Frequency Solar Corona in Circular Polarization

The Low-Frequency Solar Corona in Circular Polarization

Solar physics with the Square Kilometre Array

Probing Magnetic Fields with Square Kilometre Array and its Precursors

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