Microwave Emission Spectrum Research Articles

Microwave diagnostics of flare plasma by the direct fitting method based on data from the Siberian Radioheliograph

In this paper, we analyze images and the frequency spectrum of microwave emission in the maximum of brightness distribution in the January 20, 2022 and July 16, 2023 flares recorded by the Siberian Radioheliograph in the 3–6 GHz and 6–12 GHz ranges. We use the obtained spectrum data for radio diagnostics of magnetic field strength and orientation, plasma density, and parameters of accelerated particles in a radio source. The radio diagnostics is carried out by a method based on minimizing the functional containing the intensities of theoretically calculated and observed frequency spectra of left-polarized and right-polarized emission. Since the form of such a multidimensional functional is quite complex, and it is not possible to minimize it by standard approaches, we employ a genetic minimization method. The radio diagnostics allows us to determine features of the dynamics of the magnetic field intensity and orientation, as well as the density and the energy spectral index of non-thermal electrons in the region of maximum brightness of the radio source. We have found that during the growth phase of the main radiation peaks the magnetic field decreases, whereas during the decay phase, on the contrary, it increases. The rate of these changes varies from a few G/s to 11 G/s for the January 20, 2022 flare and is about 1 G/s for the July 16, 2023 flare.

Микроволновая диагностика вспышечной плазмы методом фитирования по данным Cибирского радиогелиографа

In this paper, we analyze images and the frequency spectrum of microwave emission in the maximum of brightness distribution in the January 20, 2022 and July 16, 2023 flares recorded by the Siberian Radioheliograph in the 3–6 GHz and 6–12 GHz ranges. We use the obtained spectrum data for radio diagnostics of magnetic field strength and orientation, plasma density, and parameters of accelerated particles in a radio source. The radio diagnostics is carried out by a method based on minimizing the functional containing the intensities of theoretically calculated and observed frequency spectra of left-polarized and right-polarized emission. Since the form of such a multidimensional functional is quite complex, and it is not possible to minimize it by standard approaches, we employ a genetic minimization method. The radio diagnostics allows us to determine features of the dynamics of the magnetic field intensity and orientation, as well as the density and the energy spectral index of non-thermal electrons in the region of maximum brightness of the radio source. We have found that during the growth phase of the main radiation peaks the magnetic field decreases, whereas during the decay phase, on the contrary, it increases. The rate of these changes varies from a few G/s to 11 G/s for the January 20, 2022 flare and is about 1 G/s for the July 16, 2023 flare.

On Detecting the Fourth Gyrofrequency Harmonic in Microwave Emission Spectra above Sunspots

Spectral polarization observations of radio sources above sunspots are regularly carried out with the RATAN-600 radio telescope (RATAN is a Russian acronym for the Radio Astronomical Telescope of the Academy of Sciences). The in-depth studies of the spectra reveal new effects. In this paper, we analyze the manifestation of radio emission of the fourth gyrofrequency harmonic in microwave spectra obtained with 1-percent frequency resolution in a range of 3–18 GHz. Registration of the extraordinary mode in the short-wavelength range of the spectrum is compared to the model calculations of the second-to-fifth gyrofrequency harmonics against a background of the thermal bremsstrahlung emission of flocculi, surrounding the spot structure of an active region. The brightening of the extraordinary mode in the short-wavelength spectral range and the kinks in the intensity spectra of emission are analyzed. The interpretation of the RATAN-600 observational data with probable diagnostics of the emission of the fourth gyrofrequency harmonic is considered as examples.

Broadband microwave emission spectrum associated with kinetic instabilities in minimum-B ECR plasmas

Plasmas of electron cyclotron resonance ion sources (ECRISs) are prone to kinetic instabilities due to the resonant heating mechanism resulting in anisotropic electron velocity distribution. Frequently observed periodic oscillations of extracted ion beam current in the case of high plasma heating power and/or strong magnetic field have been proven to be caused by cyclotron-type instabilities leading to a notable reduction and temporal variation of highly charged ion production. Thus, investigations of such instabilities and techniques for their suppression have become important topics in ECRIS research. The microwave emission caused by the instabilities contains information on the electron energy distribution and growth mechanism of the instability. The emission has been studied earlier in the frequency range of 8–14 GHz allowing us to deduce the probable excited mode. A more detailed study of the microwave emission spectrum of a minimum-B ECR plasma, sustained by 14 GHz microwave radiation, is presented in this work. It was found that the frequencies of the microwaves emitted by the plasma consist of several harmonics of the main band and extend from 6 GHz up to 25 GHz, being independent of the plasma parameters.

First rate coefficients for an interstellar anion: application to the CN−-H2 collisional system

Recently, several molecular anions have been detected in the interstellar medium. Accurate modelling of their microwave emission spectra requires the calculation of rates for rotational excitations by collisions with the most abundant species. We report the first collisional data for an interstellar anion, the rate coefficients for the rotational excitation of CN− by H2. State-to-state rate coefficients between the 11 lowest rotational levels of CN− were calculated for temperatures ranging from 5 to 100 K. Collisions with para- and ortho-H2 are taken into account. The CN− rate coefficients are compared with those of the neutral CO molecule, and, as expected, it is found that the rate coefficients for the anion are significantly larger than those of the neutral species. The new set of rate coefficients will help significantly in the interpretation of the CN− emission lines observed with current and future telescopes, and enable this molecule to become a powerful astrophysical tool.

Analysis of coronal magnetic fields in active regions based on detailed radio spectral data

Here we present an analysis of microwave emission spectra from flare-productive active regions (FPAR). For that we used regular observations in a wide frequency range with high spectral resolution and polarization sensitivity made with the RATAN-600 radio telescope. The multi-frequency observations make possible to detect small changes of magnetic field structures at different heights of the solar atmosphere. Observations with RATAN-600 and other large radio heliographs revealed an existence of narrow-band irregularities in the circular polarization spectra of FPARs radio emission. The new receiver complex, recently installed at RATAN-600, significantly improves its technical capabilities for the FPARs further study. Now it provides 1% (or 100 MHz) resolution in the frequency range from 6 GHz to 18 GHz in simultaneous registration. We present some observational results obtained with this new equipment.

Validation of ground‐based observations of stratomesospheric ozone

Since January 1995, an ozone line at 110 GHz is observed with a ground‐based microwave radiometer at the Bordeaux Observatory, France (45°N), belonging to the Network for the Detection of Stratospheric Change (NDSC). Ozone profiles from 25 to 75 km are retrieved from the microwave emission spectra using the Optimal Estimation Method. Improvements on the data acquisition process and calibration procedure are presented. A comprehensive comparison with satellite (Halogen Occultation Experiment (HALOE), Microwave Limb Sounder (MLS), and Stratospheric Aerosol and Gas Experiment II (SAGE II)) and ground‐based instrument data (microwave radiometer in Bern, Switzerland, lidar at the Observatoire de Haute‐Provence, France) is given. The average profiles, using the statistically most significant data sets, deviate not more than 15% in the stratosphere and 30% in the mesosphere. We can thus assert that the Bordeaux radiometer is a well‐validated instrument providing consistent and high‐quality data.

Laboratory Measurements of the Microwave Opacity of Phosphine: Opacity Formalism and Application to the Atmospheres of the Outer Planets

Preliminary room-temperature measurements of the microwave opacity of phosphine (Hoffman and Steffes 1999). Icarus 140, 235–238, suggested that phosphine (PH 3) may contribute significantly to the microwave emission spectrum of Neptune, and to the centimeter-wavelength opacity measured by the Voyager 2 spacecraft at Neptune and Saturn. As a result, new laboratory measurements of the microwave opacity of PH 3 in an H 2/He atmosphere have been conducted at frequencies (wavelengths) of 1.5 GHz (20 cm), 2.2 GHz (13.3 cm) 8.3 GHz (3.6 cm), 13.3, GHz (2.3 cm), 21.6 GHz (1.4 cm), and 27 GHz (1.1 cm) over a range of temperatures (175–298 K) and pressures (1–6 bars). A measurement-based centimeter-wavelength opacity formalism for phosphine has been developed and provides an order of magnitude improvement over previous models. The resulting model is applied to results from the Voyager 2 radio science experiments at Saturn and Neptune and to disk-averaged, centimeter-wavelength observations of Neptune and Saturn. Future uses will include application to spatially resolved microwave images of the outer planets from radio telescopes and spacecraft radiometry and to future radio occultation measurements, including Cassini–Saturn.

Laboratory Measurements of Phosphine's Microwave Opacity: Implications for Planetary Radio Science

It has been suggested that phosphine (PH3) may contribute significantly to the microwave emission spectrum of Neptune, and to the centimeter-wavelength opacity measured by the Voyager 2 spacecraft at Neptune (D. R. DeBoer and P. G. Steffes 1996a, Icarus123, 324–335). As a result, laboratory measurements of the microwave opacity and refractivity of PH3 in an H2/He atmosphere have been conducted at 1.5 GHz (20 cm), 2.2 GHz (13.3 cm), 8.3 GHz (3.6 cm), 13.3 GHz (2.3 cm), and 21.6 GHz (1.4 cm) at pressures of 1, 3, and 5 bars at room temperature. Results indicate that current theories significantly understate the centimeter-wavelength opacity of PH3. This higher-than-expected opacity is likely due to a molecular inversion spectrum, of which no measurements have been reported. These results will elucidate the interpretation of centimeter-wavelength microwave observations from radio telescopes and spacecraft radio occultation measurements of the atmospheres of all four jovian planets.

Laboratory measurement of the temperature dependence of gaseous sulfur dioxide (SO2) microwave absorption with application to the Venus atmosphere

High‐accuracy laboratory measurements of the temperature dependence of the opacity from gaseous sulfur dioxide (SO2) in a carbon dioxide (CO2) atmosphere at temperatures from 290 to 505 K and at pressures from 1 to 4 atm have been conducted at frequencies of 2.25 GHz (13.3 cm), 8.5 GHz (3.5 cm), and 21.7 GHz (1.4 cm). Based on these absorptivity measurements, a Ben‐Reuven (BR) line shape model has been developed that provides a more accurate characterization of the microwave absorption of gaseous SO2 in the Venus atmosphere as compared with other formalisms. The developed BR formalism is incorporated into a radiative transfer model. The resulting microwave emission spectrum of Venus is then used to set an upper limit on the disk‐averaged abundance of gaseous SO2 below the main cloud layer. It is found that gaseous SO2 has an upper limit of 150 ppm, which compares well with previous spacecraft in situ measurements and Earth‐based radio astronomical observations.

Evaluation of the microwave spectrum of Venus in the 1.2-22 centimeter wavelength range based on laboratory measurements of constituent gas opacities

Laboratory measurements of the microwave (1.2-22.3 cm) spectrum properties of Venus' gaseous atmospheric constituents were performed using an apparatus for simulating conditions of the middle atmosphere of Venus (gaseous H2SO4 + CO2 at 1 to 6 atm). The results have shown that at wavelengths longer than 1.8 cm, gaseous H2SO4 and CO2 are the predominant microwave absorbers, while at wavelengths from 1.2 to 1.8 cm, SO2 and CO2 are the predominant absorbers. These results were used to develop a model for the microwave emission spectrum of Venus, which correlated well with microwave observations of this planet.

Josephson junction fluxon oscillators of in-line geometry

Resonant fluxon propagation on an in-line geometry Josephson tunnel junction of length 5λ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</inf> having a McCumber β <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> =5π is studied by numerical integration of the circuit equations of a 50-section RSJ-type model. Dc current-voltage characteristics and microwave emission spectra are calculated and shown to be qualitatively similar to, although quantitatively somewhat different from, corresponding results for an overlap geometry junction.

Detection of HC11N in IRC + 10°216

The observations of three rotational transitions (J = 70 → 69, 71 → 70, and 72 → 71) of HC11N (cyano-deca-penta-yne) in the microwave emission spectrum of the circumstellar envelope of the cool carbon star IRC + 10°216 are reported here. The abundances relative to molecular hydrogen and HC7N are estimated to be ∼7×10−8 and ∼0.7, respectively. With these observations, taken during March and May 1981 at the Haystack Observatory, HC11N becomes the largest and heaviest molecule yet detected outside the Earth's atmosphere.

Two-photon spectroscopy of a resonant three-level spin system. II. Resonance shift

For pt.I see ibid., vol.11, p.365 (1978). A resonance shift, orders of magnitude larger than the Bloch-Siegert shift, has been measured in the second-harmonic microwave emission spectrum of a dilute ruby sample (Cr3+:Al2O3) near a three-level resonance. The observed shift is ascribed to the interference between the direct and the resonant second-harmonic generating processes.

Reduction of the Baseline Ripple on Spectra Recorded with the Parkes Radio Telescope

One of the factors determining system sensitivity in radio astronomy measurements of microwave emission and absorption spectra is the flatness of the spectrometer response in the absence of any spectral features. Ripple appears as a quasi-sinusoidal variation of this baseline and occurs whenever two or more components of the same signal reach the receiver by different paths and interfere.

Possible effect of subsurface inhomogeneities on the lunar microwave spectrum

Inhomogeneities beneath the lunar surface could alter the average microwave emission spectrum of the Moon in a fashion generally consistent with observations, even in the absence of an average heat flux or density gradients with depth. The lunar subsurface was modeled as an inhomogeneous lossy dielectric with three-dimensional refractive index fluctuations characterized by independent horizontal and vertical correlation lengths. The model suggests that attempts to infer the physical properties of the Moon from the lunar microwave spectrum could be significantly inaccurate if subsurface scattering were neglected.

Theory for passive microwave remote sensing of near-surface soil moisture

The theory of microwave thermal emission from a nonscattering half-space medium is developed for application to regions with nonuniform subsurface soil-moisture and temperature variations. A coherent stratified model is presented which is valid for nonuniform temperature profiles and rapidly varying moisture profiles, under which conditions the commonly used emissivity and radiative-transfer approaches become inaccurate. For naturally occurring profiles the stratified model gives more accurate results than the other approaches at frequencies below about 4 GHz. Experimental results from ground-based radiometric observations of a controlled target area compare systematically with brightness temperatures predicted from the theoretical model to within approximately 10 K. Results of dielectric-constant measurements of the sand are given at seven frequencies in the microwave range and for moisture contents in the range from 0% to 30% by volume. By using this model, the thermal microwave emission spectrum is computed for a number of representative moisture and temperature profiles in the frequency range from 0.25 to 25 GHz.

The ammonia profile in the atmosphere of Saturn from inversion of its microwave emission spectrum

The ammonia profile in the atmosphere of Saturn is inferred directly from inversion of the planet's microwave emission spectrum between 1 and 20 cm wavelength. The inferred profile is characterized by an ammonia mixing ratio of about 1 x 10/sup -4/ and the presence of a saturated layer with a base temperature and pressure of about 154 K and 4 atm, respectively. (AIP)

The microwave emission spectrum from sparking water drops

The microwave emission spectrum is obtained for the discharge during collision of two oppositely charged water drops of equal size in the absence of an external electric field. The power spectra obtained for drops 1.00 and 1.50 mm in diameter covering the frequency range 3291–10,940 MHz show pronounced maximums and minimums, unlike blackbody radiation, which follows an ƒ2 law, where ƒ is the frequency. These results imply that radiation from sparking drops is separate from the thermal radiation from clouds. The observed emission spectra agree well with the theoretical results of Atkinson and Paluch if the duration of the spark is taken to be 2×10−9.8 s. The variation of radiated energy E with drop size d and charge Q is found at 9210 MHz to be E ∝ Q6.6/101.96d. For colliding drops of equal size the radiated intensity is dependent on the total charge neutralized between the drops and not on how the charge is initially distributed, even when one drop is initially uncharged. On the basis of the experimental results a simple cloud model is used to calculate contributions to the antenna temperature of a radiometer from (1) the electrical activity due to sparking drops and (2) the cloud liquid water present in the antenna beam; It appears feasible to utilize a dual-wavelength radiometer as a remote sensing probe to determine simultaneously the liquid water content and the electrical activity in certain types of storms.

Microwave Emission Spectrum of the Moon: Mean Global Heat Flow and Average Depth of the Regolith

Earth-based observations of the lunar microwave brightness temperature spectrum at wavelengths between 5 and 500 centimeters, when reexamined in the light of physical property data derived from the Apollo program, tentatively support the high heat flows measured in situ and indicate that a regolith thickness between 10 and 30 meters may characterize a large portion of the lunar near side.