Incoherent Solar Radio Emission

Nindos, A.

doi:10.3389/fspas.2020.00057

Cited by 39 publications

(23 citation statements)

References 191 publications

(239 reference statements)

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“…Free-free (f-f, bremmstrahlung, see review by Nindos, 2020 in this special research topic collection; see also Gelfreikh, 2004) is the principal emission mechanism for thermal plasma in the absence of gyroresonance emission ( √ u = 1, 1/2, 1/3, 1/4,...). The absorption coefficient, k j , is slightly different for the two wave modes and, in the QL approximation, is given by:…”

Section: Circular Polarization Measurementsmentioning

confidence: 99%

“…The characteristics of gyrosynchrotron (g-s) emission from mildly relativistic electrons, trapped in flaring loops, depend strongly on the magnetic field (see the review by Nindos, 2020 in this special research topic collection and the reviews by Nindos et al, 2008). The emission has a quasi-continuous spectrum with maximum in the low harmonics of the gyrofrequency.…”

Section: Gyrosynchrotron Emissionmentioning

confidence: 99%

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Radio Measurements of the Magnetic Field in the Solar Chromosphere and the Corona

Alissandrakis

Gary

2021

Front. Astron. Space Sci.

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The structure of the upper solar atmosphere, on all observable scales, is intimately governed by the magnetic field. The same holds for a variety of solar phenomena that constitute solar activity, from tiny transient brightening to huge Coronal Mass Ejections. Due to inherent difficulties in measuring magnetic field effects on atoms (Zeeman and Hanle effects) in the corona, radio methods sensitive to electrons are of primary importance in obtaining quantitative information about its magnetic field. In this review we explore these methods and point out their advantages and limitations. After a brief presentation of the magneto-ionic theory of wave propagation in cold, collisionless plasmas, we discuss how the magnetic field affects the radio emission produced by incoherent emission mechanisms (free-free, gyroresonance, and gyrosynchrotron processes) and give examples of measurements of magnetic filed parameters in the quiet sun, active regions and radio CMEs. We proceed by discussing how the inversion of the sense of circular polarization can be used to measure the field above active regions. Subsequently we pass to coherent emission mechanisms and present results of measurements from fiber bursts, zebra patterns, and type II burst emission. We close this review with a discussion of the variation of the magnetic field, deduced by radio measurements, from the low corona up to ~ 10 solar radii and with some thoughts about future work.

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Section: Circular Polarization Measurementsmentioning

confidence: 99%

Section: Gyrosynchrotron Emissionmentioning

confidence: 99%

Radio Measurements of the Magnetic Field in the Solar Chromosphere and the Corona

Alissandrakis

Gary

2021

Front. Astron. Space Sci.

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“…Nindos et al 2000;Vourlidas et al 2006). The highest frequency at which gyro-resonance emission has been conclusively detected so far from solar active regions is 17 GHz (Nindos 2020). In this regard, it should be noted that gyro-resonance or gyro-synchrotron emission sources are observed less frequently at ν > 10 GHz than in the ν ∼ 1 -5 GHz range, which is more sensitive to such emission.…”

Section: Thermal Versus Non-thermal Emission In the Mm-cm Rangementioning

confidence: 91%

EMISSA -- Exploring Millimeter Indicators of Solar-Stellar Activity I. The Initial mm-cm Main Sequence Star Sample

Mohan,

Wedemeyer,

Pandit

et al. 2021

Preprint

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Context. Due to their wide wavelength coverage across the millimeter to centimeter (mm -cm) range and their increased sensitivity, modern interferometric arrays facilitate observations of the thermal and non-thermal radiation that is emitted from different layers in the outer atmospheres of stars. Aims. We study the spectral energy distribution (S obs (ν)) of main-sequence stars based on archival observations in the mm -cm range with the aim to study their atmospheric stratification as a function of stellar type. Methods. The main-sequence stars with significant detection in mm bands were identified in the ALMA Science Archive. These data were then complemented with spectral flux data in the extreme ultraviolet (EUV) to cm range as compiled from various catalogues and observatory archives. We compared the resultant S obs (ν) of each star with a photospheric emission model (S mod (ν)) calculated with the PHOENIX code. The departures of S obs (ν) from S mod (ν) were quantified in terms of a spectral flux excess parameter (∆S /S mod ) and studied as a function of stellar type.Results. The initial sample consists of 12 main-sequence stars across a broad range of spectral types from A1 to M3.5 and the Sunas-a-star as reference. The stars with T eff = 3000 − 7000 K (F -M type) showed a systematically higher S obs (ν) than S mod (ν) in the mm -cm range. Their ∆S /S mod exhibits a monotonic rise with decreasing frequency. The steepness of this rise is higher for cooler stars in the T eff = 3000 -7000 K range, although the single fully convective star (T eff ∼ 3000 K) in the sample deviates from this trend. Meanwhile, S obs (ν) of the A-type stars agrees with S mod (ν) within errors. Conclusions. The systematically high ∆S /S mod in F -M stars indicates hotter upper atmospheric layers, that is, a chromosphere and corona, in these stars like for the Sun. The mm -cm ∆S /S mod spectrum offers a way to estimate the efficiency of the heating mechanisms across various outer atmospheric layers in main-sequence stars, and thereby to understand their structure and activity. We emphasise the need for dedicated surveys of main-sequence stars in the mm -cm range.

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“…Nindos et al 2000;Vourlidas et al 2006). The highest frequency at which gyroresonance emission has been conclusively detected so far from solar active regions is 17 GHz (Nindos 2020). In this regard, it should be noted that gyro-resonance or gyro-synchrotron emission sources are observed less frequently at ν > 10 GHz than in the ν ∼ 1-5 GHz range, which is more sensitive to such emission.…”

Section: Thermal Versus Non-thermal Emission In the Mm-cm Rangementioning

confidence: 92%

EMISSA (Exploring Millimeter Indicators of Solar-Stellar Activity)

Mohan

Wedemeyer

Pandit

et al. 2021

A&A

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Context. Due to their wide wavelength coverage across the millimeter to centimeter (mm–cm) range and their increased sensitivity, modern interferometric arrays facilitate observations of the thermal and non-thermal radiation that is emitted from different layers in the outer atmospheres of stars. Aims. We study the spectral energy distribution (Sobs(ν)) of main-sequence stars based on archival observations in the mm–cm range with the aim to study their atmospheric stratification as a function of stellar type. Methods. The main-sequence stars with significant detection in mm bands were identified in the ALMA Science Archive. These data were then complemented with spectral flux data in the extreme ultraviolet to cm range as compiled from various catalogues and observatory archives. We compared the resultant Sobs(ν) of each star with a photospheric emission model (Smod(ν)) calculated with the PHOENIX code. The departures of Sobs(ν) from Smod(ν) were quantified in terms of a spectral flux excess parameter (ΔS∕Smod) and studied as a function of stellar type. Results. The initial sample consists of 12 main-sequence stars across a broad range of spectral types from A1 to M3.5 and the Sun-as-a-star as reference. The stars with Teff = 3000–7000 K (F–M type) showed a systematically higher Sobs(ν) than Smod(ν) in the mm–cm range. Their ΔS∕Smod exhibits a monotonic rise with decreasing frequency. The steepness of this rise is higher for cooler stars in the Teff = 3000–7000 K range, although the single fully convective star (Teff ~ 3000 K) in the sample deviates from this trend. Meanwhile, Sobs(ν) of the A-type stars agrees with Smod(ν) within errors. Conclusions. The systematically high ΔS∕Smod in F–M stars indicates hotter upper atmospheric layers, that is, a chromosphere and corona in these stars, like for the Sun. The mm–cm ΔS∕Smod spectrum offers a way to estimate the efficiency of the heating mechanisms across various outer atmospheric layers in main-sequence stars, and thereby to understand their structure and activity. We emphasise the need for dedicated surveys of main-sequence stars in the mm–cm range.

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Incoherent Solar Radio Emission

Cited by 39 publications

References 191 publications

Radio Measurements of the Magnetic Field in the Solar Chromosphere and the Corona

Radio Measurements of the Magnetic Field in the Solar Chromosphere and the Corona

EMISSA -- Exploring Millimeter Indicators of Solar-Stellar Activity I. The Initial mm-cm Main Sequence Star Sample

EMISSA (Exploring Millimeter Indicators of Solar-Stellar Activity)

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