Gyroresonance Radiation Research Articles

Detection of 2–4 GHz Continuum Emission from ϵ Eridani

Abstract The nearby star ϵ Eridani has been a frequent target of radio surveys for stellar emission and extraterrestrial intelligence. Using deep 2–4 GHz observations with the Very Large Array, we have uncovered a 29 μJy compact, steady continuum radio source coincident with ϵ Eridani to within 0.″06 (≲2σ; 0.2 au at the distance of the star). Combining our data with previous high-frequency continuum detections of ϵ Eridani, our observations reveal a spectral turnover at 6 GHz. We ascribe the 2–6 GHz emission to optically thick, thermal gyroresonance radiation from the stellar corona, with thermal free–free opacity likely becoming relevant at frequencies below 1 GHz. The steep spectral index (α ≃ 2) of the 2–6 GHz spectrum strongly disfavors its interpretation as stellar-wind-associated thermal bremsstrahlung (α ≃ 0.6). Attributing the entire observed 2–4 GHz flux density to thermal free–free wind emission, we thus derive a stringent upper limit of 3 × 10−11 M ⊙ yr−1 on the mass-loss rate from ϵ Eridani. Finally, we report the nondetection of flares in our data above a 5σ threshold of 95 μJy. Together with the optical nondetection of the most recent stellar maximum expected in 2019, our observations postulate a likely evolution of the internal dynamo of ϵ Eridani.

Thermal Bremsstrahlung of Local Sources Over Solar Spots Based on Microwave Observations

The paper presents the results of a study of microwave thermal bremsstrahlung of local sources above sunspots. Special attention is paid to the transition from thermal bremsstrahlung to the dominant gyroresonance radiation in the high-frequency part of the spectrum of 12–20 GHz (1.5–2.5 cm). At high frequencies, for which the magnitude of the magnetic field is not enough to generate gyroresonance radiation, thermal bremsstrahlung is the main factor. The presented calculations demonstrate an increase in brightness in an extraordinary mode and a decrease in its intensity in an ordinary mode, as well as corresponding examples of observations. The possibilities and advantages of determination of the field with one polarization corresponding to ordinary radiation are revealed. The studied spectral-polarization features of the radiation, estimates of the effective radiation height, and the characteristic features of the change in the sizes of sources with the wave frequency will make it possible to correctly interpret the observations and to improve the accuracy of magnetic field measurement and the diagnostics of the transition region temperature.

A microflare with hard X-ray-correlated gyroresonance line emission at 314 MHz

Context. Small energy release events in the solar corona can give insights into the flare process which are regularly hidden in the complex morphology of larger events. For one case we find a narrowband radio signal well correlated with the hard X-ray flare. We investigate wether these signals are probes for the flare current sheet. Aims. We aim to establish the relation between narrowband and short-duration features (<1% of the observing frequency in the spectral range 250–340 MHz, and some 5 s until 2 min, respectively) in dynamic radio spectral diagrams and simultaneously occuring HXR bursts. Methods. We use dynamic radio spectra from the Astrophysical Institute Potsdam, HXR images of RHESSI, TRACE coronal and chromospheric images, SOHO-MDI high resolution magnetogram data, and its potential field extrapolation for the analysis of one small flare event in AR10465 on September 26, 2003. We point to similar effects in e.g. the X-class flare on November 03, 2003 to demonstrate that we are not dealing with a singular phenomenon. Results. We confirm the solar origin of the extremely narrowband radio emission. From RHESSI images and the magnetic field data we identify the probable site of the radio source as well as the HXR footpoint and the SXR flare loop emission. The flare loop is included in an ongoing change of magnetic connectivity as confirmed by TRACE images of hot coronal loops. The flare energy is stored in the nonpotential magnetic field substructure around the microflare site which is relaxed to a potential one. Conclusions. We conclude that the correlated HXR footpoint/narrowband radio emission, and the transition to a second energy release in HXR without associated radio emission are direct probes of changing magnetic connectivity during the flare. We suppose that the narrowband radio emission is due to gyroresonance radiation at the second harmonic of the local electron cyclotron frequency. It follows an upper limit of the magnetic field in the radio source volume of less than 50% of the mean potential field in the same height range. This supports the idea that the narrowband radio source is situated in the immediate surroundings of the flare current sheet.

Coronal Currents, Magnetic Fields, and Heating in a Solar Active Region

We compare microwave images of a solar active region with state-of-the-art fully nonlinear force-free extrapolations of the photospheric fields in order to study the link between coronal currents and heating of the corona. This extrapolation fully takes into account the nonuniform distribution of electric currents observed in the photosphere and its role in the coronal magnetic structure. We carry out the comparison for AR 6615, a complex region observed with the VLA on 1991 May 7. Under the assumption that the microwave emission is dominated by optically thick gyroresonance radiation, we may use the radio images to infer the temperature of the corona at different heights and locations. This is then compared with heating models based on the observed current distribution. We are able to reproduce the radio images remarkably well with a model in which temperature is structured along magnetic field lines, depends on the current on the field line, and increases with height in a manner similar to that inferred from static heated loop models. This result implies a direct link between electric currents and coronal heating.

Magnetospheres of Solar Active Regions Inferred from Spectral-Polarization Observations with High Spatial Resolution

The strong magnetic fields of active regions organize both the plasma structures and energy processes in the chromosphere and corona. Recent radio observations with high spatial resolution permit measurements of the magnetic fields in these regions and also localize regions of thermal and nonthermal energy release. They can additionally be used to determine temperatures and electron densities in these regions. The results of such diagnostics suggest the term magnetosphere for the space surrounding an active region in the solar atmosphere where the basic structures and physical processes are controlled by the magnetic fields/electric currents of the particular region. The physical parameters of quiescent, or nonflaring, structures in the low solar corona and upper chromosphere have been inferred from nearly simultaneous spectral polarization observations (RATAN 600) and high-spatial-resolution radio observations (VLA). They have been compared with images from the Soft X-ray Telescope (SXT) aboard the Yohkoh satellite, indicating that bright radio (20 cm) and X-ray structures coincide, but that there is radio emission that is not detectable at X-ray wavelengths. Variable soft X-ray emission on time scales of hours suggests continued, varying, low-level heating or particle acceleration in localized areas of active regions. The RATAN-600 observations have been combined with the theory of thermal cyclotron emission to infer magnetic field strengths in the low corona above practically all large sunspots with an accuracy of 2 or 3 %. They indicate that the magnetic field strength of the thermal plasma at the million-degree level above large sunspots is 75%-80% of the magnetic field strength in the underlying photospheric sunspots. The evolution of the magnetic structures is specified. Coronal potential field extrapolations are also provided, suggesting that the magnetic fields in the corona diverge more slowly than expected from a simple dipole located below the surface. Theoretical models are compared with multiple-wavelength VLA observations and potential field extrapolations, indicating that the radio emission from one active region can be explained by thermal gyroresonance radiation in a conductive flux model. However, the high brightness temperature and steep spectrum of the radio emission of another active region cannot be explained by conventional thermal models, and instead suggest long-lasting nonthermal heating in localized coronal sources above the magnetic neutral line in the underlying photosphere Gyrosynchrotron radiation of nonthermal electrons cannot explain the observations of one such source, but heating within a localized neutral current sheet might account for them. Long-lasting radio sources with high brightness temperatures ≥ 10<SUP>7</SUP> K and steep radiation spectra are often associated with active regions with a multipolar δ configuration of the photospheric sunspots. These "peculiar" coronal radio sources appear above the magnetic neutral line in the photosphere, and appear to require nearly continuous acceleration of energetic nonthermal electrons by a yet unknown process.

The magnetic field in the corona above sunspots at the eclipse of 1991 July 11

view Abstract Citations (12) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Magnetic Field in the Corona above Sunspots at the Eclipse of 1991 July 11 Gary, D. E. ; Leblanc, Y. ; Dulk, G. A. ; Golub, L. Abstract A partial solar eclipse and an X-ray image are used to study the magnetic field as a function of height in the corona above an active region during the solar eclipse of July 11, 1991. The dominant features of AR 6718 are two leading spots of positive polarity followed by two spots of negative polarity about 3 arcmin to the east. Bright radio emission coincides with the positions of the sunspots, attributable to a gyroresonance radiation from ambient electrons above the spots. A simplified model of the source as a function of frequency based on the interferometer fringe amplitudes is used to obtain brightness temperature spectra for the emission associated with the sunspots. It is deduced that the magnetic field strength at the base of the corona above the leading spots was 1200 G, and about 1100 G above the following spots. The soft X-ray brightness above the sunspots was very low, about 30 times lower than that of the adjacent plage-associated emission. Publication: The Astrophysical Journal Pub Date: July 1993 DOI: 10.1086/172931 Bibcode: 1993ApJ...412..421G Keywords: Solar Corona; Solar Eclipses; Solar Magnetic Field; Sunspots; Brightness Temperature; Emission Spectra; Field Strength; Photosphere; Radio Spectra; Solar Arrays; X Ray Imagery; Solar Physics; ECLIPSES; SUN: CORONA; SUN: MAGNETIC FIELDS; SUN: SUNSPOTS full text sources ADS |

Combined radiation mechanism in the sun's active region No. 75 during the eclipse of 16 February, 1980

We assume that the physical conditions above an active solar region vary continuously from the centers of sunspots to the adjacent quiet region and try to take into account the influence of the magnetic field. Thus we calculate the three-dimensional distribution of the electron temperature and density based on the radio spectrum of active region No. 75 obtained from the solar eclipse observation of 16 February, 1980. If we assume a potential field, we calculated the magnetic field above the active region in terms of the solar photospheric magnetic field. Using the electron temperature, density, and the magnetic field as described above, and assuming the slowly varying radiation mechanism to consist of bremsstrahlung and gyro-resonance radiation, we obtained the flux density spectrum and the brightness temperature spectrum. The calculated results are essentially consistent with the observations.

Solar Radio S-Component Variation with the Magnetic Field of Active Regions

AbstractAccording to an active region model proposed in the present paper, i.e. three-dimensional and continuous distributions of the three plasma parameters (electron temperature, electron density, and magnetic field) from the active region to the quiet region, and using the combined mechanisms of gyroresonance radiation and bremsstrahlung, we have researched the solar radio S-component (i.e. the slowly varying component or SVC) variation with the magnetic field of active regions.

Very Large Array-RATAN 600 observations of a solar active region

The Very Large Array (VLA) and the RATAN 600 were used to observe a solar active region on two consecutive days around the time of a partial solar eclipse in July 1990. VLA synthesis maps at 2.0, 3.5, and 6.2 cm wavelength reveal bright (Tb = 0.2 − 2.2 × 106 K), compact (θ = 10″–40″) sources above the penumbra of the leading sunspot while maps at 20 cm wavelength reveal an extended (θ ≈ 4.5′) looplike structure (Tb ≈ 106 K) between the dominant spots. Total flux and brightness temperature spectra of both components were obtained by the RATAN at nine wavelengths between 1.7 and 21 cm. The relatively-flat spectrum of the extended emission is attributed to the optically thin thermal brems Strahlung of electrons trapped in a magnetic loop at coronal temperatures. Step-spectrum sunspot-associated emission is attributed to thermal gyroresonance radiation at different heights along the leg of a loop joining regions of opposite magnetic polarity. Comparisons with predicted distributions of gyroresonance radiation indicate that the compact sunspot-associated sources lie at heights of h = 2500–17500 km above the photosphere. Although potential fields of sufficient strength appear to exist at coronal heights, differences n the observed and predicted brightness distributions suggest some role for non-potential fields or for an inhomogeneous distribution of electron density or temperature above the sunspot.

A model of SVC radiation in magnetic dipole field and radiation varying with magnetic field. I - Active region model and SVC radiation source model

In our model, the latest EUV data on the chromosphere-corona transition region havebeen used. We have assumed that the plasma conditions above an active region are varyingcontinuously from the centre of the region to the adjacent quiet region. We have taken thethree physical parameters of the plasma (electron temperature and density, and magneticfield) to be functions of both the height above the photosphere and the distance from thecentral axis of the active region. According to the combined mechanism including both the gyroresonance radiation and thebremsstrahlung, using a dipole magnetic field and above-mentioned model of the electrontemperature and density, we have calculated the radio radiation of the slowly varying compo-nent (SVC) and emphatically studied the radiative characteristics of the SVC radiation (suchas the spectra of flux density, polarization degree, and brightness temperaturc, and thebrightness distribution) and the geometrical features of the SVC radiation source (such asheight and radius), and their variations with the magnetic field. Some very interesting resultshave been obtained and they are considerably in agreement with the observations.

VLA Observations of the Coronal Plasma

VLA observations at 20-cm wavelength specify the brightness temperature and magnetic structure of plasma constrained within coronal loops in solar active regions. Comparisons with simultaneous SMM observations at soft X-ray wavelengths lead to measurements of physical parameters like electron density, electron temperature and magnetic field strength. Such comparisons also indicate coronal loops can be detected at either radio or X-ray wavelengths while remaining invisible in the other spectral domain, and that the dominant radiation mechanisms can be thermal bremsstrahlung or thermal gyroresonance radiation. VLA observations at the longer 90-cm wavelength reveal the thermal emission of a hot transition sheath enveloping a cooler, underlying Hα filament seen in absorption. The 20-cm VLA observations indicate that the precursor, impulsive and post-flare components of solar flares originate in spatially separated and resolved sources.

Flare stars at radio wavelengths

The radio emission from dMe flare stars is discussed using Very Large Array and Arecibo observations as examples. Active flare stars emit weak, unpolarized, quiescent radio radiation that may be always present. Although thermal bremsstrahlung and/or thermal gyroresonance radiation account for the slowly-varying, quiescent radio radiation of solar active regions, these processes cannot account for the long-wavelength quiescent radiation observed from nearby dMe flare stars. It has been attributed to nonthermal gyrosynchrotron radiation, but some as yet unexplained mechanism must be continually producing the energetic electrons. Long-duration (hours), narrow-band (Δv/v &lt; 0.1) radiation is also emitted from some nearby dMe stars at 20 cm wavelength. Such radiation may be attributed to coherent plasma radiation or to coherent electron-cyclotron masers. Impulsive stellar flares exhibit rapid variations (&lt; 100 msec) that require radio sources that are smaller than the star in size, and high brightness temperatures TB &gt; 1015 K that are also explained by coherent radiation processes. Quasi-periodic temporal fluctuations suggest pulsations during some radio flares. Evidence for frequency structure and positive or negative frequency drifts during radio flares from dMe stars is also presented.

Solar active region physical parameters inferred from a thermal cyclotron line and soft X-ray spectral lines

Simultaneous high-resolution observations of coronal loops were made at the 20-cm wavelength with the VLA and at soft X-ray wavelengths with the SMM FCS. The images obtained at both wavelengths have nearly identical sizes and ellipsoidal shapes, with the emission stretching between and across regions of opposite magnetic polarity in the underlying photosphere. The results indicate that the radiation at 20 cm and soft X-ray wavelengths originates from the same region, and that 20 cm maps can image X-ray coronal loops. The X-ray spectral lines were used to obtain values of electron temperature, T(e), of about 2.6 x 10 to the 6th K and electron density, N(e), of about 3.1 x 10 to the 9th/cu cm. These parameters were used to show that the layers emitting 20-cm radiation can be optically thick to either thermal bremsstrahlung or thermal gyroresonance radiation, depending upon unknown but plausible values of loop thickness, magnetic scale height, and magnetic field strength.

Simultaneous SMM flat crystal spectrometer and Very Large Array observations of solar active regions

High-resolution images of the quiescent emission from two solar active regions at 20 cm (VLA) and soft X-ray (SMM FCS) wavelengths are compared. There are regions where the X-ray coronal loops have been completely imaged at 20 cm wavelength. In other regions, the X-ray radiation was detected without detectable 20 cm radiation, and vice versa. The X-ray data were used to infer average electron temperatures of about 3-million K and average electron densities of about 2.5 x 10 to the 9th/cu cm for the X-ray emitting plasma in the two active regions. The thermal bremsstrahlung of the X-ray emitting plasma is optically thin at 20 cm wavelength. The 20 cm brightness temperatures were always less than T(e), which is consistent with optically thin bremsstrahlung. The low T(B) can be explained if a higher, cooler plasma covers the hotter X-ray emitting plasma. Thermal gyroresonance radiation must account for the intense 20 cm radiation near and above sunspots where no X-ray radiation is detected.

VLA observations of a radio plage at centimeter wavelengths

VLA observations of a solar plage region at 6 and 20 cm wavelengths are presented. The high frequency 6 cm emission correlates well with the associated sunspots, whereas 20 cm emission shows good correlation with the Hα plage. Large temperature variations over a period of one day are observed in the plage associated component without any significant changes in the sunspots. The dominant emission mechanisms at 6 and 20 cm are found to be gyroresonance radiation and bremstrahlung respectively. It is concluded that the coronal condensation above the chromospheric Hα plage has an electron density of ∼5 × 109 cm−3 and it extends to a height of ∼5 × 104 km.

Three-dimensional structures of two solar active regions from VLA observations at 2, 6, and 20 centimeter wavelengths

view Abstract Citations (36) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Three-dimensional structures of two solar active regions from VLA observations at 2, 6, and 20 centimeter wavelengths Shevgaonkar, R. K. ; Kundu, M. R. Abstract Three-dimensional structures of two active region groups are determined from observations with the Very Large Array (VLA) at 2, 6, and 20 cm. One of the groups exhibits a single magnetic loop of length approximately 10 to the 10th cm. The 2 cm radiation is mostly thermal bremsstrahlung and originates from the footprints of the loop. The 6 and 20 cm radiation is dominated by low-harmonic gyroresonance radiation and originates from the upper portion of the legs or the top of the loop. The loop broadens toward the apex. The top of the loop is not found to be the hottest point, but two temperature maxima on either side of the loop apex are observed, which is consistent with the model proposed for long loops. From 2 and 6 cm observations it can be concluded that the electron density and temperature cannot be uniform in a plane perpendicular to the axis of the loop; the density should decrease away from the axis of the loop. Publication: The Astrophysical Journal Pub Date: August 1984 DOI: 10.1086/162320 Bibcode: 1984ApJ...283..413S Keywords: Centimeter Waves; Coronal Loops; Solar Activity; Solar Magnetic Field; Solar Radio Emission; Very Long Base Interferometry; Astronomical Maps; Bremsstrahlung; Circular Polarization; Gyrofrequency; Solar Temperature; Very Large Array (Vla); Solar Physics full text sources ADS |

Radio emission from the outer parts of stellar atmospheres (mantles) at centimeter wavelengths and the winds of early-type stars

It is noted that when magnetic lines of force are present in the mantle of an early-type star, gyroresonance radiation at radio wavelengths may occur at detectable levels in addition to bremsstrahlung. An expression is derived for the effective value of the ratio of the emissivity in gyroresonance radiation to that in bremsstrahlung, and some numerical values are estimated using values of the parameters which may be typical for conditions in the mantles of early-type stars. The volume emissivity in gyroresonance radiation may exceed that for bremsstrahlung by a large factor. The implications of this result for the interpretation of the radio fluxes from early-type stars are discussed. It is shown that the radio-frequency radiation emerging from a hot star probably consists of gyroresonance radiation from an array of magnetic loops as well as bremsstrahlung from a low-density wind. It is argued that it is probable that the estimates of rate of mass loss made ignoring the possibility of gyroresonance radiation may be too large by a factor of at least 10.

The role of gyro-resonance radiation in the slowly varying component of the solar radio emission

In this paper we investigate the contribution of gyro-resonance to the SVC of the solar radio emission. We calculated the spatial distributions of the gyroresonance radiation over uinpolar and bipolar active regions at a number of wavelengths and we found some interesting features. We also calculated the spectra of flux and polarization and our results were in general agreement with observations. This suggests that the gyro-resonance radiation is indeed an Important mechanism for the solar SVC.

A model of the solar radio slowly varying component

Our model uses the latest EUV data on the chromosphere-corona transition region. We take the three physical parameters (electron temperature and density and magnetic field) to be functions both of the height above the photosphere and the distance from the axis of our unipolar model. We consider both the gyroresonance radiation and the bremsstrahlung. Our calculated flux and polarization spectra peak at λ = 6 cm and 3 cm respectively, in general agreement with observations. Some features in the spatial distribution of the flux and polarization were obtained, which may be compared with future high-resolution data. We found that in the SVC radiation, the gyroresonance emission is all-important, while the contribution from the reflection of the extraordinary wave is almost nil. These results are directly opposite to the conclusions by Shimabuburo et al./1/.

On the type of spectra of S-component sources and their correlation with flare occurrence

From solar maps at 8.6 mm wavelength and total flux measurements at wavelengths of 1.7 cm to 122 cm, various spectra of the slowly varying component have been studied. The main distinction between these various types of spectra is the slope of the spectra toward wavelengths of less than 2 cm. It has been shown that the probability of flare occurrence is correlated with the type of the source spectra. It is proposed that enhanced flare production occurs from a source of SVC whose spectrum has a peak around 6–10 cm wavelength but whose slope is flatter around λ ≃ 8 mm, but steeper toward longer centimeter wavelengths, than in the case of ‘normal’ SVC-spectra attributed to gyro resonance radiation. The implications of such spectra in terms of changes in magnetic field structure before the occurrence of a flare are discussed.