Gyrosynchrotron Emission Research Articles

Simultaneous imaging and spectral observations in microwaves and hard X-rays of the impulsive phase of a solar limb flare

We have observed the impulsive phase of a solar flare at microwave wavelengths and in hard X-rays, and deduced the strength of the magnetic field and the number of energetic electrons producing the burst. The microwave observations, using the Very Large Array at 6 cm, had spatial resolution of 8″×8″, close to the resolution of the Hard X-ray Imaging Spectrometer on SMM which also imaged this flare. The Hard X-ray Burst Spectrometer determined the spectrum of the burst in the range 25–512 keV, and several patrol telescopes recorded the microwave time profile at frequencies from 2.8 to 19.6 GHz. The combined data show that the derived number of microwave emitting electrons is at least three orders of magnitude fewer than the number of thick target electrons producing the hard X-rays. We propose that the reason for this discrepancy in numbers is that the fast electrons are highly beamed, and radiate gyrosynchrotron emission less efficiently than isotropically distributed electrons. However, we cannot rule out the alternate explanation: that the electron energy spectrum steepens at moderate energies (> 150 keV), and then flattens at higher energies (> 200 keV) where gyrosynchrotron processes become important.

Properties of Stellar Magnetic Fields in Close Binaries Deduced from Non-Thermal Radio Observations

AbstractAnalysis of VLA and VLBI observations of non-thermal radio emission from close binary stellar systems can produce reliable estimates of magnetic field strength and geometry in the source emission region. Assuming a gyrosynchrotron emission model for the non-flare emission, one can model the magnetic field as a relatively small number of stable magnetic loops of overall size R ⋍ (1-3)×1012 cm. with an average field strength of 101 < B < 102 gauss. A correlation of mean fractional circular polarization with inclination angle along with an absence of any clear phase dependence, indicates that the loops are azimuthally symmetric with poles nearly orthogonal to the orbital plane, i.e. axisymmetric dipolar. The loops are probably the same as the loops derived from observations of x-ray emission of ‘hot’ (T ⋍ 107K) coronal gas.

Microwave imaging of a solar limb flare - Comparison of spectra and spatial geometry with hard X-rays

A solar limb flare was mapped using the Very Large Array (VLA) together with hard X-ray (HXR) spectral and spatial observations of the Solar Maximum Mission satellite. Microwave flux records from 2.8 to 19.6 GHz were instrumental in determining the burst spectrum, which has a maximum at 10 GHz. The flux spectrum and area of the burst sources were used to determine the number of electrons producing gyrosynchrotron emission, magnetic field strength, and the energy distribution of gyrosynchrotron-emitting electrons. Applying the thick target model to the HXR spectrum, the number of high energy electrons responsible for the X-ray bursts was found to be 10 to the 36th, and the electron energy distribution was approximately E exp -5, significantly different from the parameters derived from the microwave observations. The HXR imaging observations exhibit some similiarities in size and structure o the first two burst sources mapped with the VLA. However, during the initial burst, the HXR source was single and lower in the corona than the double 6 cm source. The observations are explained in terms of a single loop with an isotropic high-energy electron distribution which produced the microwaves, and a larger beamed component which produced the HXR at the feet of the loop.

Gyrosynchrotron emission - Generalizations of Petrosian's method

Gyrosynchrotron emission - Generalizations of Petrosian's method

The numbers of fast electrons in solar flares as deduced from hard X-ray and microwave spectral data

The previously reported discrepancy between the number of fast solar flare electrons deduced from hard X-ray burst observations and microwave observations of the same events is reexamined. The thin and thick-target models for hard X-ray production and their consequences for gyrosynchrotron emission of microwaves are discussed. Previous work that led to the perceived discrepancy is reviewed, and it is suggested that the discrepancy can be reduced when thick-target hard X-ray emission is assumed. The number of electrons in microwaves and hard X-rays are calculated assuming a homogeneous source for a sample of flares given by Wiehl et al. (1983). The numbers are given separately under the two assumptions of thin and thick-target hard X-ray emission. It is found that when the latter emission is assumed, the number of electrons deduced from the microwaves is very similar to the number deduced from the hard X-rays. 36 references.

Radio Emission from the Sun and Stars

Radio astronomy of the sun has reached a high level of maturity, while radio astronomy of the stars is now a burgeoning new field of study. The present review is mainly concerned with radiation which is emitted by 'normal' stars, defined here to include those appearing on classical Hertzsprung-Russell diagrams. The mechanisms for both steady, quiescent emission and intense, strongly varying outbursts are discussed. Included are discussions of bremsstrahlung, gyrosynchrotron emission, electron-cyclotron masers, and plasma radiation. The manifestation of these mechanisms in various kinds of solar radiation are considered along with stellar manifestations.

Radio outbursts in RS Canum Venaticorum stars - Coronal heating and electron runaway

view Abstract Citations (12) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radio outbursts in RS Canum Venaticorum stars : Coronal heating and electron runaway. Mullan, D. J. Abstract Radio outbursts of RS CVn stars are sometimes thought of as analogs of flares in red dwarf stars. The author examines the possibility that the outbursts are not due to flares, but rather to a highly efficient case of coronal heating: mechanical energy reaching the corona from the convection zone of an RS CVn star gives rise to induced electric fields which may be so large that electron runaway becomes possible. It is proposed that gyrosynchrotron emission from fast electrons which are produced by the process are the source of radio outbursts from RS CVn stars. Publication: The Astrophysical Journal Pub Date: August 1985 DOI: 10.1086/163405 Bibcode: 1985ApJ...295..628M Keywords: Coronal Loops; Eclipsing Binary Stars; Electron Runaway (Plasma Physics); Radio Bursts; Radio Stars; Stellar Coronas; Electric Field Strength; Electron Acceleration; Magnetohydrodynamic Turbulence; Plasma Heating; Stellar Magnetic Fields; Synchrotron Radiation; Astrophysics full text sources ADS | data products SIMBAD (1)

Detection of a sub-milliarcsecond radio component in the RS CVn system HR 1099

The RS CVn system HR 1099 was observed with a five station VLBI array at a frequency of 8.4 GHz during a strong radio outburst of approximately 400 mJy. The data are consistent with a circular Gaussian source of 0.8 + or - 0.12 milli-arcsec (FWHM), corresponding to a linear size of 4 + or - 0.6 x 10 to the 11th cm. This is comparable to the distance between the surfaces of the two stars, or to 75 percent of the diameter of the chromospherically active K star. Extrapolation of published photometric data shows that the starspot formation of HR 1099 was facing toward the observers at the times of observations. The high equivalent brightness temperature, approximately 10 billion K, is consistent with gyrosynchrotron emission from a power-law energy distribution of electrons in a magnetic field of strength about 30 gauss.

Decimetric gyrosynchrotron emission during a solar flare

A decimetric, microwave, and hard X-ray burst was observed during a solar flare in which the radio spectrum below peak flux fits an f+2 power law over more than a decade in frequency. The spectrum is interpreted to mean that the radio emission originated in a homogeneous, thermal, gyrosynchrotron source. This is the first time that gyrosynchrotron radiation has been identified at such low decimetric frequencies (900-998) MHz). The radio emission was cotemporal with the largest single hard X-ray spike burst ever reported. The spectrum of the hard X-ray burst can be well represented by a thermal bremsstrahlung function over the energy range from 30 to 463 keV at the time of maximum flux. The temporal coincidence and thermal form of both the X-ray and radio spectra suggest a common source electron distribution. The unusual low-frequency extent of the single-temperature thermal radio spectrum and its association with the hard X-ray burst imply that the source had an area approx. 10(18) sq cm a temperature approx 5x10(8) K, an electron density approx. 7.10(9) cu cm and a magnetic field of approx. 120 G. H(alpha) and 400-800 MHz evidence suggest that a loop structure of length 10,000 km existed in the flare active region which could have been the common, thermal source of the observed impulsive emissions.

The spatial distribution of 6 centimeter gyroresonance emission from a flaring X-ray loop

We compare simultaneous high resolution soft X-ray and 6 cm images of the decay phase of an M3 X-ray flare in Hale Region 16413. The photographic X-ray images were obtained on an AS & E sounding rocket flown 7 November, 1979, and the 6 cm observations were made with the VLA. The X-ray images were converted to arrays of line-of-sight emission integrals and average temperature throughout the region. The X-ray flare structure consisted of a large loop system of length ∼ 1.3 arc min and average temperature ∼8 × 106 K. The peak 6 cm emission appeared to come from a region below the X-ray loop. The predicted 6 cm flux due to thermal bremsstrahlung calculated on the basis of the X-ray parameters along the loop was about an order of magnitude less than observed. We model the loop geometry to examine the expected gyroresonance absorption along the loop. We find that thermal gyroresonance emission requiring rather large azimuthal or radial field components, or nonthermal gyrosynchrotron emission involving continual acceleration of electrons can explain the observations. However, we cannot choose between these possibilities because of our poor knowledge of the loop magnetic field.

VLBI observations of the RS Canum Venaticorum binary systems UX ARIETIS and HR 1099 at 1.65 GHz

VLBI observations of the RS CVn binaries UX Arietis and HR 1099 have been made at 1.65 GHz using a three-element array with a minimum fringe spacing of 11.5 milli-arcsec. Both sources were found to be unresolved within measurement uncertainties. In both cases, the derived upper limit to the source size was comparable to the overall size of each binary system. The lower limits to the brightness temperature were 1.4 x 10 to the 10th K for UX Arietis and 2.9 x 10 to the 10th K for HR 1099. Simultaneous polarization measurements at the VLA showed 4-8 percent circular polarization and less than 2 percent linear polarization. It is found that the data are consistent with gyrosynchrotron emission from a power-law energy distribution of electrons in a magnetic field B less than or approximately equal to 6 gauss.

VLA observations of the eclipsing binary system AR Lacertae

view Abstract Citations (23) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLA observations of the eclipsing binary system AR Lacertae. Doiron, D. J. ; Mutel, R. L. Abstract Dual-frequency radio observations of the RS CVn binary system AR Lacertae are reported during two secondary eclipses. No clearly defined eclipse signature was observed during either eclipse at either 1.5 or 4.9 GHz. This implies a lower limit of 5×1011 cm for the source diameter if the source is located in the orbital plane. Significant circular polarization (2% - 8%) was observed on both days, with a helicity reversal between 1.5 and 4.9 GHz. This polarization signature is consistent with a gyro-synchrotron emission mechanism if the source becomes optically thick between 1.5 and 4.9 GHz. Using this emission model, the derived range of magnetic field strength is 5 ⪉ B ⪉ 80 G. Publication: The Astronomical Journal Pub Date: March 1984 DOI: 10.1086/113533 Bibcode: 1984AJ.....89..430D Keywords: Bremsstrahlung; Circular Polarization; Eclipsing Binary Stars; Light Curve; Radio Astronomy; Stellar Radiation; Synchrotron Radiation; X Ray Astronomy; Astrophysics full text sources ADS | data products SIMBAD (1)

Microwave emission from the coronae of late-type dwarf stars

VLA microwave observations of 14 late-type dwarf and subgiant stars and binary systems are examined. In this extensive set of observations, four sources at 6 cm (Chi-1 Ori, UV Cet, YY Gem, and Wolf 630AB) were detected and low upper limits for the remaining stars were found. The microwave luminosities of the nondetected F-K dwarfs are as small as 0.01 those of the dMe stars. The detected emission is slowly variable in all cases and is consistent with gyroresonant emission from thermal electrons spiraling in magnetic fields of about 300 gauss if the source sizes are as large as R/R(asterisk) = 3-4. This would correspond to magnetic fields that are probably in the range 0.001-0.0001 gauss at the photospheric level. An alternative mechanism is gyrosynchrotron emission from a relatively small number of electrons with effective temperature.

The gyrosynchrotron emission from quasi-thermal electrons and applications to solar flares

view Abstract Citations (60) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The gyrosynchrotron emission from quasi-thermal electrons and applications to solar flares Dulk, G. A. ; Melrose, D. B. ; White, S. M. Abstract We present theoretical results on the gyrosynchrotron radiation from electrons with a Maxwellian energy distribution. We review the analytical expressions for the gyromagnetic absorption coefficient and find two which cover the range of interest for microwave emission from solar flares, i.e., frequencies ω to ∼100Ωe and temperatures Te to ∼109 K. Numerical calculations are used to check the analytic expressions and to derive simplified empirical formulae which relate the observable characteristics of the radiation to the temperature and magnetic field in the source. We apply the results to the sources of impulsive microwave and hard X-ray bursts from solar flares. For an isothermal source the theory predicts a microwave spectrum where the flux density rises as f2 at low frequencies, maximizes as some frequency fpeak, and falls very rapidly thereafter; this shape fits the observed spectra qualitatively. The optical depth τ of the source varies rapidly with f, with τ = 1 at f ≈ fpeak. For Te ≳ 108 K we derive the relation fpeak ∝ Te0.7B, which allows a direct estimate of the magnetic field B in the impulsive burst source if the temperature is known-for instance, from hard X-ray observations. For the impulsive burst of 1972 May 18, reported by Hoyng and Stevens, we find that the microwave and hard X-ray data are well fitted by a model source with Te ≈ 2.3 × 108 K, B ≈ 370 gauss, ne 2 × 109 cm-3, and scale length L ≈ 8600 km. Publication: The Astrophysical Journal Pub Date: December 1979 DOI: 10.1086/157597 Bibcode: 1979ApJ...234.1137D Keywords: Electron Energy; Solar Flares; Solar Radio Emission; Synchrotron Radiation; Energy Distribution; Maxwell-Boltzmann Density Function; Solar Electrons; X Ray Spectra; Solar Physics full text sources ADS |

A comparison of the optical and microwave emissions of some major solar flares

In the first part of the paper, we study the relations between the frequency of maximum radio flux fmax and the magnetic field strength at the photosphere Bp and between the maximum radio flux Fmax and the field and its scale L for two differing flares occurring above very different photospheric conditions. It is shown that the simple relations predicted by the gyro-synchrotron emission mechanism fmax ∼ Bp and Fmax ∼ B2L2 account for the fact that the flares produced microwave bursts of about the same Fmax, but of differing fmax.

Heliograph Observations of both Fast Drift (Type III) and Continuum (Type IV) Solar Emission from the one Source high above the Solar Limb

Many authors (e.g. Magun et al. 1975) have debated the question ‘does solar type IV continuum emission arise through Langmuir wave conversion or through gyro-synchrotron emission?’ Culgoora radio spectrograph and heliograph observations of the late stages of a recent (7 September 1977) type II, IV event showed both type IV* (continuum) and transient type III (electron-beam) emission coming from the same source, high above the solar limb. These observations suggest that this particular type IV continuum arose through a mechanism similar to that of the type III, probably fundamental Langmuir-wave emission, in a corona with a tenfold enhancement of plasma density.

Observations of High Brightness Temperatures in Moving Type IV Solar Radio Bursts

It has generally been accepted that moving type IV bursts are generated as synchrotron radiation from energetic electrons high in the solar corona (Boischot and Denisse 1957). At 80 MHz the peak brightness temperature is usually ~ 108 K and the radiation becomes highly circularly polarized as the burst decays. This has led several authors (Kai 1969; Dulk 1970, 1973; Schmahl 1972; Robinson 1974, 1977; Nelson 1977) to the conclusion that the radiation comes from mildly relativistic (~ 100 keV) electrons and occurs at low harmonics of the gyro-frequency (gyro-synchrotron radiation). We present evidence of moving type IV bursts at 43, 80 and 160 MHz with brightness temperatures of ~ 109 K, and one at 43 MHz as high as 1010 K. The number (~ 1033) of energetic (≥ 1 MeV) electrons which is required in order to explain such high brightness temperatures by incoherent gyro-synchrotron emission is very large and near the upper limit for the number of fast electrons accelerated in the second phase of a solar flare. If amplification takes place a smaller number of electrons with energies ~ 100 keV would be required.

A two-component model of impulsive microwave burst emission consistent with soft and hard X-rays

A two-component (core-halo) emission model has been applied reconciling hard and soft X-ray burst emissions with the microwave burst radiation. The core region is represented by a nonthermal energy distribution (Maxwellian+power law tail) and assumed to be surrounded by a thermal halo. Parameters characterizing the energy distribution and emission measures have been derived numerically from soft and hard X-ray measurements. Using an artificial magnetic field model the microwave flux spectrum has been calculated on the basis of gyro-synchrotron emission and absorption by solving the equation of radiation transfer along the ray trajectories. Open parameters were used to adapt the spectrum to the radio measurements. Thus probable informations about the most appropriate magnetic field parameters as well as about the time- and frequency- dependent source diameters (yielding growth velocities of the core region during the impulsive phase) are deduced for the burst of 1972 May 18 as an example. A fit of the observed spectrum at the burst maximum is consistent with a magnetic field of 150O G at the core centre decreasing up to about 40 G at the top of the halo at a height of 50 000 km above the centre, a core density of 1010 cm−3 decreasing to 109 cm−3 at the outer halo boundary, and a core diameter of 15 000 km (∼]20″). Due to the simple geometry and emission process adopted,- the model refers primarily to special impulsive bursts. For the representation of broad band microwave bursts, e.g. type IV μ, events, a more complex source geometry and/or other variants of the emission mechanism must be invoked.

Mechanisms for Flash Phase Phenomena in Solar Flares

Mechanisms for explaining the various forms of particles and radiation observed during the flash phase of solar flares are reviewed under the working hypothesis that the flash phase is the time in which electrons and to a lesser degree protons are accelerated in less than one second. A succession of such accelerations is allowed to explain longer lasting or quasi-periodic phenomena. Mechanisms capable of such acceleration are reviewed and it is concluded that first-order Fermi acceleration in a reconnecting current sheet is the most likely basic process. Such acceleration, however, gives rise to a rather narrow distribution of particle velocities along a given field line which is unstable to the production of electron plasma and ion-acoustic waves. This plasma turbulence can heat the plasma to produce soft X-rays and filter the initially narrow velocity distribution to produce a power law energy distribution. Electrons travelling inward from the acceleration region produce hard X-rays by bremsstrahlung and microwave bursts by gyro-synchrotron emission. Whereas the interpretation of X-ray spectra is relatively straightforward, the interpretation of microwave spectra is difficult because the source at low frequencies can be made optically thick by several different mechanisms.Electrons travelling further inward presumably thermalize and produce impulsive EUV and Hα emission. The theory for these emissions, although amenable to present techniques in radiative transfer, has not been worked out. Electrons travelling outward give rise to type III radio bursts by excitation of electron plasma waves and the electrons observed at the Earth. Study of the interaction of a stream of electrons with the ambient plasma shows that the electron spectra observed at the Earth do not necessarily reflect their spectrum at the acceleration region since they interact via plasma waves as well as through Coulomb collisions. The mechanisms for the conversion of plasma waves into radiation and the propagation of the radiation from its source to the observer are reviewed.

The Gyro-Synchrotron Radiation from Moving Type IV Sources in the Solar Corona

(Solar Phys.). Calculations of the gyro-synchrotron emission are made for conditions which might be expected in moving type IV sources in the solar corona. Two simple models for an evolving source are treated: a uniform cube and an inhomogeneous sphere. The results suggest that most moving sources have the following features: (1) A rather strong magnetic field, ≈ 10 G, is carried out within the source. This is required to achieve the high degree of circular polarization often observed. (2) Synchrotron self-absorption causes the source to be optically thick at frequencies less than about 100 MHz, thus restricting the bandwidth of the radiation. The self-absorption decreases as the source moves outward and expands. The turnover frequency, which separates the optically thick and thin spectral regimes, moves rapidly to lower frequencies, accompanied by a change from low to high circular polarization. In the case of an inhomogeneous source, the source appears to be larger at the lower frequencies. (3) Razin-Tsytovich suppression cannot be an important factor in determining the characteristics of most sources.