Gyroresonance Emission Research Articles

Quiet solar corona: daily images at 8.8–10.7 cm wavelengths

We discuss results of test observations of the 3–6 GHz range array of the Siberian Radio Heliograph (SRH). A method for calibrating brightness temperatures of images was verified using measurements of the brightness temperature of the quiet Sun at a minimum between solar activity cycles 20 and 21 known in the literature. The obtained time dependences of the integral solar flux at 2.8 GHz are similar to those measured at the Dominion Radio Astrophysical Observatory (DRAO), but the absolute values of SRH fluxes are lower relative to the DRAO fluxes by 10–15 %.
 The spectral density of the solar microwave flux at a frequency of 2.8 GHz, the so-called F10.7 index, is one of the main solar activity indices used as input parameters in models of Earth’s ionosphere. The paper considers the relationship between total radio fluxes and changes in the structure of sources on the solar disk during an interval of 50 days. During the period of daily observations from September 1 to October 20, 2021, the number of active regions on the disk changed several times, and the integral flux density at 2.8 GHz changed up to 1.5 times. We determine the relative contributions to the integral flux of bremsstrahlung of near-limb brightenings and plage regions, as well as bremsstrahlung in magnetic fields of active regions. The measured brightness temperatures of SRH radio maps are compared to the model temperatures calculated from observations of extreme ultraviolet emission (EUV) with the AIA/SDO telescope. The results of the analysis can be used to organize regular measurements of the corrected solar activity proxy index F10.7 at SRH, in which the contribution of gyroresonance emission is excluded.

Спокойная корона Солнца: ежедневные изображения на длинах волн 8.8–10.7 см

We discuss results of test observations of the 3–6 GHz range array of the Siberian Radio Heliograph (SRH). A method for calibrating brightness temperatures of images was verified using measurements of the brightness temperature of the quiet Sun at a minimum between solar activity cycles 20 and 21 known in the literature. The obtained time dependences of the integral solar flux at 2.8 GHz are similar to those measured at the Dominion Radio Astrophysical Observatory (DRAO), but the absolute values of SRH fluxes are lower relative to the DRAO fluxes by 10–15 %.
 The spectral density of the solar microwave flux at a frequency of 2.8 GHz, the so-called F10.7 index, is one of the main solar activity indices used as input parameters in models of Earth’s ionosphere. The paper considers the relationship between total radio fluxes and changes in the structure of sources on the solar disk during an interval of 50 days. During the period of daily observations from September 1 to October 20, 2021, the number of active regions on the disk changed several times, and the integral flux density at 2.8 GHz changed up to 1.5 times. We determine the relative contributions to the integral flux of bremsstrahlung of near-limb brightenings and plage regions, as well as bremsstrahlung in magnetic fields of active regions. The measured brightness temperatures of SRH radio maps are compared to the model temperatures calculated from observations of extreme ultraviolet emission (EUV) with the AIA/SDO telescope. The results of the analysis can be used to organize regular measurements of the corrected solar activity proxy index F10.7 at SRH, in which the contribution of gyroresonance emission is excluded.

Understanding the Relationship between Solar Coronal Abundances and F10.7 cm Radio Emission

Sun-as-a-star coronal plasma composition, derived from full-Sun spectra, and the F10.7 radio flux (2.8 GHz) have been shown to be highly correlated (r = 0.88) during solar cycle 24. However, this correlation becomes nonlinear during increased solar magnetic activity. Here we use cotemporal, high spatial resolution, multiwavelength images of the Sun to investigate the underlying causes of the nonlinearity between coronal composition (FIP bias) and F10.7 solar index correlation. Using the Karl G. Jansky Very Large Array, Hinode/EIS (EUV Imaging Spectrometer), and the Solar Dynamics Observatory, we observed a small active region, AR 12759, throughout the solar atmosphere from the photosphere to the corona. The results of this study show that the magnetic field strength (flux density) in active regions plays an important role in the variability of coronal abundances, and it is likely the main contributing factor to this nonlinearity during increased solar activity. Coronal abundances above cool sunspots are lower than in dispersed magnetic plage regions. Strong magnetic concentrations are associated with stronger F10.7 cm gyroresonance emission. Considering that as the solar cycle moves from minimum to maximum, the sizes of sunspots and their field strength increase with the gyroresonance component, the distinctly different tendencies of radio emission and coronal abundances in the vicinity of sunspots is the likely cause of saturation of Sun-as-a-star coronal abundances during solar maximum, while the F10.7 index remains well correlated with the sunspot number and other magnetic field proxies.

Strongest Coronal Magnetic Fields in Solar Cycles 23 and 24: Probing, Statistics, and Implications

A strong coronal magnetic field, when present, manifests itself as bright microwave sources at high frequencies produced by the gyroresonant (GR) emission mechanism in thermal coronal plasma. The highest frequency at which this emission is observed is proportional to the absolute value of the strongest coronal magnetic field on the line of sight. Although no coronal magnetic field larger than roughly 2000 G has been expected, recently a field at least 2 times larger has been reported. Here, we report on a search for and a statistical study of such strong coronal magnetic fields using high-frequency GR emission. A historic record of spatially resolved microwave observations at high frequencies, 17 and 34 GHz, is available from the Nobeyama RadioHeliograph for a period covering more than 20 yr (1995–2018). Here, we employ this data set to identify sources of bright GR emission at 34 GHz and perform a statistical analysis of the identified GR cases to quantify the strongest coronal magnetic fields during two solar cycles. We found that although active regions with a strong magnetic field are relatively rare (less than 1% of all active regions), they appear regularly on the Sun. These active regions are associated with prominent manifestations of solar activity.

Coronal Heating Law Constrained by Microwave Gyroresonant Emission

Abstract The question why the solar corona is much hotter than the visible solar surface still puzzles solar researchers. Most theories of the coronal heating involve a tight coupling between the coronal magnetic field and the associated thermal structure. This coupling is based on two facts: (i) the magnetic field is the main source of the energy in the corona and (ii) the heat transfer preferentially happens along the magnetic field, while is suppressed across it. However, most of the information about the coronal heating is derived from the analysis of extreme ultraviolet or soft X-ray emissions, which are not explicitly sensitive to the magnetic field. This paper employs another electromagnetic channel—the sunspot-associated microwave gyroresonant emission, which is explicitly sensitive to both the magnetic field and thermal plasma. We use nonlinear force-free field reconstructions of the magnetic skeleton dressed with a thermal structure as prescribed by a field-aligned hydrodynamics to constrain the coronal heating model. We demonstrate that the microwave gyroresonant emission is extraordinarily sensitive to details of the coronal heating. We infer heating model parameters consistent with observations.

Goldstone Apple Valley Radio Telescope Observations of 2012 Solar Eclipse: A Multi-wavelength Study of cm-λ Gyroresonance Emission from Active Regions

Goldstone Apple Valley Radio Telescope (GAVRT) is a science education partnership among NASA, the Jet Propulsion Laboratory, and the Lewis Center for Educational Research, offering unique opportunities for K-12 students and their teachers. The GAVRT program operates a 34 m radio telescope with a wide-band, low noise receiver, which is tunable in four independent dual-polarization bands from 3 to 14 GHz. The annular eclipse of the Sun on 2012 May 20 was observed by GAVRT as part of education outreach. In this paper we present the results of this eclipse data and discuss the multi-wavelength strip scan brightness distribution across three active regions. We derive the source brightness temperatures and angular sizes as a function of frequency and interpret the results in terms of the gyroresonance mechanism. We show examples of the increasing brightness and widening of source size (isogauss surface) with wavelength as evidence for gyroresonance emission layers of broader (diverging) isogauss surfaces of the magnetic field geometry in the corona above solar surface. We present an example how the derived frequency–brightness temperature relationship is translated to a magnetic field–brightness temperature relationship under the frame-work of gyroresonance emission. Our results demonstrate the usefulness of GAVRT bands as excellent probes to study the layers of the corona above the active regions (Sun spots), in particular the prevalence of the gyroresonance mechanism. Our results provide a frame-work for multiwavelength cm-λ eclipse observations and illustrate how the GAVRT program and K-12 student/teacher participation can produce science data useful to the scientific community and science missions.

The Slowly Varying Corona. II. The Components of F 10.7 and Their Use in EUV Proxies

Using four years of full-disk-integrated coronal differential emission measures calculated in Schonfeld et al. (2017), we investigate the relative contribution of bremsstrahlung and gyroresonance emission in observations of F$_{10.7}$, the 10.7 cm (2.8 GHz) solar microwave spectral flux density and commonly used activity proxy. We determine that the majority of coronal F$_{10.7}$ is produced by the bremsstrahlung mechanism, but the variability observed over individual solar rotations is often driven by gyroresonance sources rotating across the disk. Our analysis suggests that the chromosphere may contribute significantly to F$_{10.7}$ variability and that coronal bremsstrahlung emission accounts for 14.2 $\pm$ 2.1 sfu ($\sim20\%$) of the observed solar minimum level. The bremsstrahlung emission has a power-law relationship to the total F$_{10.7}$ at high activity levels, and this combined with the observed linearity during low activity yields a continuously differentiable piecewise fit for the bremsstrahlung component as a function of F$_{10.7}$. We find that the bremsstrahlung component fit, along with the Mg II index, correlates better with the observed 5-37 nm spectrum than the common 81 day averaged F$_{10.7}$ proxy. The bremsstrahlung component of F$_{10.7}$ is also well approximated by the moderate-strength photospheric magnetic field parameterization from Henney et al. (2012), suggesting that it could be forecast for use in both atmospheric research and operational models.

Flat-spectrum Radio Continuum Emission Associated with ϵ Eridani

We present Very Large Array observations at 33.0 GHz that detect emission coincident with Eridani to within 007 (0.2 au at the distance of this star), with a positional accuracy of 005. This result strongly supports the suggestion of previous authors that the quiescent centimeter emission comes from the star and not from a proposed giant exoplanet with a semimajor axis of ∼10 (3.4 au). The centimeter emission is remarkably flat and is consistent with optically thin free–free emission. In particular, it can be modeled as a stellar wind with a mass-loss rate of the order of 6.6 × 10−11 yr−1, which is 3300 times the solar value, exceeding other estimates of this star's wind. However, interpretation of the emission in terms of other thermal mechanisms like coronal free–free and gyroresonance emission cannot be discarded.

Detection of thermal radio emission from a single coronal giant

We report the detection of thermal continuum radio emission from the K0 III coronal giant Pollux ($\beta$ Gem) with the Karl G. Jansky Very Large Array (VLA). The star was detected at 21 and 9 GHz with flux density values of $150\pm21$ and $43\pm8\,\mu$Jy, respectively. We also place a $3\sigma_{\mathrm{rms}}$ upper limit of $23\,\mu$Jy for the flux density at 3 GHz. We find the stellar disk-averaged brightness temperatures to be approximately 9500, 15000, and $<71000\,$K, at 21, 9, and 3 GHz, respectively, which are consistent with the values of the quiet Sun. The emission is most likely dominated by optically thick thermal emission from an upper chromosphere at 21 and 9 GHz. We discuss other possible additional sources of emission at all frequencies and show that there may also be a small contribution from gyroresonance emission above active regions, coronal free-free emission and free-free emission from an optically thin stellar wind, particularly at the lower frequencies. We constrain the maximum mass-loss rate from Pollux to be less than $3.7\times 10^{-11}\,M_{\odot}$ yr$^{-1}$ (assuming a wind terminal velocity of 215 km s$^{-1}$), which is about an order of magnitude smaller than previous constraints for coronal giants and is in agreement with existing predictions for the mass-loss rate of Pollux. These are the first detections of thermal radio emission from a single (i.e., non-binary) coronal giant and demonstrate that low activity coronal giants like Pollux have atmospheres at radio frequencies akin to the quiet Sun.

Determination of the Structure of the Coronal Magnetic Field Using Microwave Polarization Measurements

An analysis of the oscillatory motions and wave processes in active regions requires knowledge of the structure of the magnetic fields in the chromosphere and corona. We study the magnetic field structure of active regions at coronal heights, as they are determined by means of multiwave observations of polarized radio emission of active regions in the microwave range. Two methods, a stereoscopic method and the analysis of the radio spectrum are used. The method of stereoscopy rotation allows estimating the height of radio sources in a stable active region relative to the photosphere, based on its apparent motion in the image plane recorded over several days of observation. At various times one-dimensional scans at multiple frequencies spanning the 5.98 – 15.95 GHz frequency range from the RATAN-600 instrument are used. The gyroresonance emission mechanism, which is sensitive to the coronal magnetic field strength, is applied to convert the radio source estimated heights at various frequencies, \(h(f)\), to information as regards magnetic field vs. height, \(B(h)\). Diagrams of longitude – height of some polarized radio sources revealed multiple reversals, suggestive of a spiral magnetic structure. In all cases, the magnetic field strength maintains high values (800 – 1000 G) at the highest altitudes analysed, which reflects a relatively weak divergence in the field of magnetic flux tubes (in the height range 8 – 14 Mm) responsible for the main part of the radio emission of active regions.

Synthetic Radio Views of Simulated Solar Flux Ropes

In this paper, we produce synthetic radio views on simulated flux ropes in the solar corona, where finite-beta magnetohydrodynamic (MHD) simulations serve to mimic the flux rope formation stages, as well as their stable endstates. These endstates represent twisted flux ropes where balancing Lorentz forces, gravity and pressure gradients determine the full thermodynamic variation throughout the flux rope. The obtained models are needed to quantify radiative transfer in radio bands, and allow us to contrast weak to strong magnetic field conditions. Field strengths of up to 100 G in the flux rope yield the radio views dominated by optically thin free-free emission. The forming flux rope shows clear morphological changes in its emission structure as it deforms from an arcade to a flux rope, both on disk and at the limb. For an active region filament channel with a field strength of up to 680 G in the flux rope, gyroresonance emission (from the third-fourth gyrolayers) can be detected and even dominates over free-free emission at the frequencies of up to 7 GHz. Finally, we also show synthetic views on a simulated filament embedded within a (weak-field) flux rope, resulting from an energetically consistent MHD simulation. For this filament, synthetic views at the limb show clear similarities with actual observations, and the transition from optically thick (below 10 GHz) to optically thin emission can be reproduced. On the disk, its dimension and temperature conditions are as yet not realistic enough to yield the observed radio brightness depressions.

CORONAL SOURCES OF THE SOLAR F10.7RADIO FLUX

We present results from the first solar full-disk F$_{10.7}$ (the radio flux at $10.7$ cm, $2.8$ GHz) image taken with the S-band receivers on the recently upgraded Karl G. Jansky Very Large Array (VLA) in order to assess the relationship between the F$_{10.7}$ index and solar extreme ultra-violet (EUV) emission. To identify the sources of the observed $2.8$ GHz emission, we calculate differential emission measures (DEMs) from EUV images collected by the Atmospheric Imaging Assembly (AIA) and use them to predict the bremsstrahlung component of the radio emission. By comparing the bremsstrahlung prediction and radio observation we find that $8.1\pm 0.5\%$ of the variable component of the F$_{10.7}$ flux is associated with the gyroresonance emission mechanism. Additionally, we identify optical depth effects on the radio limb which may complicate the use of F$_{10.7}$ time series as an EUV proxy. Our analysis is consistent with a coronal iron abundance that is $4$ times the photospheric level.

Intersunspot Microwave Sources

We studied a number of solar active regions using two-dimensional spatially resolved microwave observations. Data from the Nobeyama Radioheliograph and the Siberian Solar Radio Telescope together with observations by the Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory (SOHO) have allowed us to identify long-lived intersunspot sources (ISSs) in most of the investigated active regions. Their centers are often located above the line-of-sight magnetic field inversion line that separates the leading and following polarities of a full active region (first type of ISS) or above the inversion line that separates magnetic polarities inside of a complex of sunspots (second type of ISS). ISSs of the first type are extended and, in general, they are sources of bremsstrahlung emission. ISSs of the second type are compact and are, most likely, sources of gyroresonance or gyrosynchrotron emission. We propose a qualitative model involving three types of magnetic connectivity to explain how long-lasting ISSs may be generated.

THE 17 GHz ACTIVE REGION NUMBER

We report the statistics of the number of active regions (NAR) observed at 17 GHz with the Nobeyama Radioheliograph between 1992, near the maximum of cycle 22, and 2013, that also includes the maximum of cycle 24, and we compare with other activity indexes. We find that NAR minima are shorter than those of the sunspot number (SSN) and radio flux at 10.7 cm (F10.7). This shorter NAR minima could reflect the presence of active regions generated by faint magnetic fields or spotless regions, which were a considerable fraction of the counted active regions. The ratio between the solar radio indexes F10.7/NAR shows a similar reduction during the two minima analyzed, which contrasts with the increase of the ratio of both radio indexes in relation to the SSN during the minimum of cycle 23-24. These results indicate that the radio indexes are more sensitive to weaker magnetic fields than those necessary to form sunspots, of the order of 1500 G. The analysis of the monthly averages of the active region brightness temperatures shows that its long term variation mimics the solar cycle, although, due to the gyro-resonance emission, a great number of intense spikes are observed in the maximum temperature study. The decrease, in number, of these spikes is also evident during the current cycle 24, a consequence of the sunspot magnetic field weakening in the last years.

FIRST DETECTION OF THERMAL RADIO EMISSION FROM SOLAR-TYPE STARS WITH THE KARL G. JANSKY VERY LARGE ARRAY

We present the first detections of thermal radio emission from the atmospheres of solar-type stars {\tau} Cet, {\eta} Cas A, and 40 Eri A. These stars all resemble the Sun in age and level of magnetic activity, as indicated by X-ray luminosity and chromospheric emission in calcium-II H and K lines. We observed these stars with the Karl G. Jansky Very Large Array with sensitivities of a few {\mu}Jy at combinations of 10.0, 15.0, and 34.5 GHz. {\tau} Cet, {\eta} Cas A, and 40 Eri A are all detected at 34.5 GHz with signal-to-noise ratios of 6.5, 5.2, and 4.5, respectively. 15.0-GHz upper limits imply a rising spectral index greater than 1.0 for {\tau} Cet and 1.6 for {\eta} Cas A, at the 95% confidence level. The measured 34.5-GHz flux densities correspond to stellar disk-averaged brightness temperatures of roughly 10,000 K, similar to the solar brightness temperature at the same frequency. We explain this emission as optically- thick thermal free-free emission from the chromosphere, with possible contributions from coronal gyroresonance emission above active regions and coronal free-free emission. These and similar quality data on other nearby solar-type stars, when combined with ALMA observations, will enable the construction of temperature profiles of their chromospheres and lower transition regions.

THREE-DIMENSIONAL STRUCTURE OF MICROWAVE SOURCES FROM SOLAR ROTATION STEREOSCOPY VERSUS MAGNETIC EXTRAPOLATIONS

We use rotation stereoscopy to estimate the height of a steady-state solar feature relative to the photosphere, based on its apparent motion in the image plane recorded over several days of observation. The stereoscopy algorithm is adapted to work with either one- or two-dimensional data (i.e. from images or from observations that record the projected position of the source along an arbitrary axis). The accuracy of the algorithm is tested on simulated data, and then the algorithm is used to estimate the coronal radio source heights associated with the active region NOAA 10956, based on multifrequency imaging data over 7 days from the Siberian Solar Radio Telescope near 5.7 GHz, the Nobeyama Radio Heliograph at 17 GHz, as well as one-dimensional scans at multiple frequencies spanning the 5.98--15.95 GHz frequency range from the RATAN-600 instrument. The gyroresonance emission mechanism, which is sensitive to the coronal magnetic field strength, is applied to convert the estimated radio source heights at various frequencies, h(f), to information about magnetic field vs. height B(h), and the results are compared to a magnetic field extrapolation derived from photospheric magnetic field observations obtained by Hinode and MDI. We found that the gyroresonant emission comes from the heights exceeding location of the third gyrolayer irrespectively on the magnetic extrapolation method; implications of this finding for the coronal magnetography and coronal plasma physics are discussed.

THREE-DIMENSIONAL STRUCTURE OF A SOLAR ACTIVE REGION FROM SPATIALLY AND SPECTRALLY RESOLVED MICROWAVE OBSERVATIONS

We report on the structure of the solar atmosphere above active region (AR) 10923, observed on 2006 November 10, as deduced from multi-wavelength studies including combined microwave observations from the Very Large Array (VLA) and the Owens Valley Solar Array (OVSA). The VLA observations provide excellent image quality at a few widely spaced frequencies, while the OVSA data provide information at many intermediate frequencies to fill in the spectral coverage. Images at 25 distinct frequencies are used to provide spatially resolved spectra along many lines of sight in the AR, from which microwave spectral diagnostics are obtained for deducing maps of temperature, magnetic field, and column density. The derived quantities are compared with multiwavelength observations from the Solar and Heliospheric Observatory and Hinode spacecraft, and with a current-free magnetic field extrapolation. We find that a two-component temperature model is required to fit the data, in which a hot (>2 MK) lower corona above the strong-field plage and sunspot regions (emitting via the gyroresonance process) is overlaid with somewhat cooler (∼1 MK) coronal loops that partially absorb the gyroresonance emission through the free–free (Bremsstrahlung) process. We also find that the extrapolated potential magnetic fields can quantitatively account for the observed gyroresonance emission over most of the AR, but in a few areas a higher field strength is required. The results are used to explore the coronal configuration needed to explain the observations. These results show that the bulk of free–free emission in both radio and X-rays emanates from two loop systems, distinguished by the location of their loop footpoints. We discuss the implications of such comparisons for studies of AR structure when better microwave spectral imaging becomes available in the future.

Long period oscillations in sunspots

Long period oscillations of the gyroresonant emission from sunspot atmospheres are studied. Time series data generated from the sequences of images obtained by the Nobeyama Radioheliograph operating at a frequency of 17 GHz for three sunspots have been analysed and are found to contain significant periods in the range of several tens of minutes. Wavelet analysis shows that these periods are persistent throughout the observation periods. The presence of the oscillations is confirmed by several methods (periodogram, wavelets, Fisher randomisation and empirical mode decomposition). Spatial analysis using the techniques of period, power, correlation and time lag mapping reveals regions of enhanced oscillatory power in the umbral regions. Also seen are two regions of coherent oscillation of about 25 pixels in size, that oscillate in anti-phase with each other. Possible interpretation of the observed periodicities is discussed, in terms of the shallow sunspot model and the leakage of the solar g-modes.

Radio Emissions from Solar Active Regions

Solar active region coronae are known for strong magnetic fields permeating tenuous plasma, which makes them an ideal astronomical laboratory for magnetohydrodynamics research. It is, however, relatively less known that this physical condition also permits a very efficient radiation mechanism, gyro-resonant emission, produced by hot electrons gyrating in the coronal magnetic field. As a resonant mechanism, gyro-emission produces high enough opacity to fully reveal the coronal temperature, and is concentrated at a few harmonics of the local gyrofrequency to serve as an excellent indicator of the magnetic field. In addition, the polarization of the ubiquitous free–free emission and a phenomenon of depolarization due to mode coupling extend the magnetic field diagnostic to a wide range of coronal heights. The ability to measure the coronal temperature and magnetic field without the complications that arise in other radiative inversion problems is a particular advantage for the active region radio emissions available only at these wavelengths. This article reviews the efforts to understand these radiative processes, and use them as diagnostic tools to address a number of critical issues involved with active regions.

Sunspot Gyroresonance Emission at 17 GHz: A Statistical Study

Abstract We investigate the sunspot gyroresonance emission at 17 GHz using the synoptic database of the Nobeyama Radioheliograph. Our statistical study is based on full disk observations obtained during the maximum of Cycle 22 (1992–94). We study the center-to-limb variation of the brightness and polarization of the sunspot radio emission and present some cases of polarization reversal. We find that the radio emission is most likely 3rd-harmonic gyroresonance emission arising from 2000 G fields in transition region, or low corona temperatures.