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Silicate dust is found in a wide range of astrophysical environments. Nucleation and growth of silicate dust grains in circumstellar environments likely involves species with diameters ranging from <1 nm (molecular silicates) to a few nanometers (nanosilicates). When fully formed silicate grains with sizes ∼0.1 μm enter the interstellar medium, supernovae shockwaves cause collision-induced shattering which is predicted to redistribute a significant proportion of the silicate dust mass into a huge number of nanosilicates. This presumed population has thus far not been unambiguously confirmed by observation but is one of the main candidates for causing the anomalous microwave emission. By virtue of their extreme small size, nanosilicates and molecular silicates could exhibit significantly different properties to larger silicate grains, which could be of astrochemical and astrophysical importance. Herein, we briefly review the properties of these ultrasmall silicate dust species with a focus on insights arising from bottom-up atomistic computational modelling. Finally, we highlight how such modelling also has the unique potential to predict observationally verifiable spectral features of nanosilicates that may be detectable using the James Webb Space Telescope. [ABSTRACT FROM AUTHOR]

We consider the M1.1-class solar flare occurred on July 5, 2012, at UT. This event is unique in that a helioseismic perturbation was detected in it despite its low hard X-ray flux in the 25–50 keV energy band and its very soft hard X-ray spectrum. As a rule, most of the known sunquakes have been detected in solar flares with large hard X-ray fluxes at high energies (at least up to 100–300 keV). The event under consideration contradicts the popular hypothesis about the generation of sunquakes by beams of accelerated high-energy electrons. An analysis of the available RHESSI X-ray spectra shows that they can be explained in two ways. The X-ray spectrum in the 25–50 keV energy band is explained by a power-law distribution of accelerated electrons with an power-law index of 7–9 or by the presence of a superhot plasma with a temperature –60 MK. In both cases, we are dealing with electrons of relatively low energies that either were responsible for the sunquake generation or should be considered as a secondary (accompanying) phenomenon with respect to the true cause of the photospheric perturbation. The results of a joint analysis of the X-ray and microwave spectra are presented for the first time for a helioseismically active solar flare. Our analysis shows that the spectra in both ranges can be well explained by the emission of a superhot magnetized plasma and not by accelerated electrons with a soft spectrum. However, the explanation of the spectra when considering partially magnetically trapped accelerated electrons is also possible. We have estimated the parameters of the thermal plasma, accelerated electrons, and energy fluxes of various types. We analyze the dynamics of ultraviolet and X-ray emission sources. We also present an analysis of the magnetic field structure based on vector magnetograms and the nonlinear force-free coronal magnetic field extrapolation. We discuss the mechanisms for the generation of the helioseismic perturbation during the solar flare under consideration. An eruptive process could probably be both primary and secondary causes of the sunquake. The appearance of a superhot plasma in the corona could give rise to propagating thermal fronts into the lower layers of the solar atmosphere, where helioseismic waves are excited. Our analysis does not allow the possibility of the sunquake generation by accelerated electrons with a soft spectrum to be ruled out either. [ABSTRACT FROM AUTHOR]

Numerous jets were observed in the extreme ultraviolet images of the SDO/AIA space observatory in early March 2015 in the developing active region of a complex magnetic configuration located near the eastern limb of the Sun. The analysis of the observations of several jets showed that the place of their origin and the direction of motion of the material was such that it absorbed the radiation of the regions of the quiet Sun with an area of less than 1% of the area of the solar disk. There was also a partial screening of the flare radio source located to the north of the stationary one. Only in one case, the appearance of jet led to a response in the microwave range in the form of a small depression of radio emissions (negative burst), registered on the records of the Nobeyama radio polarimeters and the Ussuriysk radiometer. Distinguishing features of this event from other jets are revealed. Simultaneously with the studied jet, whose material moved to the east of the active region, small jets were observed to the northwest of it. This led to an increase in the total screening area of the quiet Sun regions, which exceeded 1% of the area of the solar disk. Another jet arose immediately before the jet, as a result of which the material returned to the solar surface probably had a large mass compared to other jets, which manifested itself in the maximum absorption value compared to other cases. The radio burst before the depression of radio emissions was impulsive and there was no activity in other regions of the Sun. The combination of these factors was the cause of the negative burst of small depth. In other cases, the possible depression of radio emissions could be compensated by an increased level of radio emissions due to additional activity, or the mass of the jet material was insufficient for the occurrence of negative bursts. [ABSTRACT FROM AUTHOR]

In passive microwave remote sensing of the Earth's surface, it is essential to relate the emission to geophysical parameters. The emissivity ranges between 0 and 1. Hence, a slight emissivity variation leads to a significant change in brightness temperature. Many sources of error contribute to such tiny variations in emission. This paper quantifies microwave emission variability from a rough surface through model simulation due to the non-Gaussianity in height probability density (HPD) and power spectrum density (PSD). We considered Gaussian and exponential distributions for surface height and correlation functions, representing two extremes of asperity and skewness. Additionally, the surface under consideration contains multiscale roughness. The impact of the HPD and multiscale roughness on the polarization index of the emissivity is evaluated as a function of frequency and roughness. In general, assuming that Gaussian-distributed height leads to an underestimation of the emissivity, with V polarization being less sensitive to the non-Gaussian HPD and PSD effects than H polarization, the emissions are enhanced at high roughness with small look angles but are reduced for smooth surfaces at large look angles under non-Gaussian PSD. In a specific scenario, the dynamic range of the difference between exponential and Gaussian HPD is 0~10%, and the difference in emissivity caused by non-Gaussian PSD ranges from −2% to 16%. These results should be helpful in interpreting the radiometric measurements that exhibit fluctuations and differences with model predictions. [ABSTRACT FROM AUTHOR]

Radio continuum, microwave and gamma-ray images of the Milky Way reveal a conspicuous, loop-like structure that fills almost half of the northern Galactic hemisphere, called Loop I. The interior of Loop I is the most conspicuous region shining in soft X-rays, whose eastern base is a remarkably bright, elongated structure seeming to emerge from the Galactic plane, dubbed the North Polar Spur (NPS). After 40 years of debates, two very different, contradictory views of Loop I/NPS are still defended: on the one hand, the NPS is a gigantic volume of expanding hot gas that envelops and extends the northern “Fermi Bubble” (FB) known to be blown by the Galactic center, and Loop I marks the shock front; on the other hand, the NPS is totally independent of the northern FB, it is a nearby, ordinary cavity of hot gas blown by supernovae, Loop I is its shock front and both are coincidentally located in the direction of the FB. To an observer at the Sun, both can produce the same perspective view, although the former has a size comparable to the Milky Way itself, and the latter a diameter of a few hundreds parsecs. The energy involved varies by 3-4 orders of magnitude, and the solution has important consequences on the structure and history of our Galactic neighborhood, on the age of the North and South FBs and the activity at the Galactic center. Moreover, whatever are the actual shape and distance of Loop I/NPS, accurate modeling of the polarized emission associated with Loop I is important for CMB foreground removal. After a short review, I discuss recent results which have a connection with Loop I/NPS. Some of them have been used as arguments in the two opposite ways, while for others the connections with LoopI/NPS have been overlooked. They involve very different spacecraft, from a 12 Kg Cubesat (HaloSat) to major space-borne observatories (HST, Gaia, and Spektr-RG). I make use of updated 3D maps of dust and a recent massive star catalog. I distinguish arguments based on geometric similarity or dissimilarity from those derived from measurements and physical models. Considering all past and recent constraints, it is clear that there is no entirely local or entirely distant scenario that is free from contradictions with some of the data analyses or from improbable coincidences. I discuss a speculative scenario, partially inspired by recent BF and Milky Way gas models, combining near and far aspects and seeming to be able to meet the various constraints. However, new data and models are needed to bring the controversy to a close and we can still expect new episodes of this long story.

Natural rough surfaces have inherent multiscale roughness. This article presents the modeling and analysis of microwave emission from a multiscale soil surface. Unlike the linear superposition of different correlation functions with various correlation lengths, we applied the frequency modulation concept to characterize the multiscale roughness, in which the modulation does not destroy the surface's curvature but only modifies it. The multiscale effect on emission under different observation geometries and surface parameters was examined using an AIEM model. The paper provides new insights into the dependence of polarized emissivity on multiscale roughness: V-polarized emissivity is much less sensitive to multiscale roughness across the moisture content from dry to wet (5–30%). The H-polarized is sensitive to multiscale roughness, especially at higher moisture content. The predicted emissivity will have considerable uncertainty, even for the same baseline correlation length, without accounting for the multiscale roughness effect. V-polarized emissivity is less sensitive to the multiscale effect than H-polarized and the higher modulation ratio indicates larger emissivity. The higher modulation ratio indicates larger emissivity. Multiscale roughness weakens the polarization difference, particularly in higher moisture conditions. In addition, ignoring the multiscale effect leads to underestimated emissivity to a certain extent, particularly at the larger RMS height region. Finally, when accounting for multiscale roughness, model predictions of emission from a soil surface are in good agreement with two independently measured data sets. [ABSTRACT FROM AUTHOR]

We analyze the presence of a microwave neutral-line-associated source (NLS) in a super-active region NOAA 12673, which produced a number of geo-effective events in September 2017. To estimate the NLS position, we use data from the Siberian Radioheliograph in a range 4–8 GHz and from the Nobeyama Radioheliograph at 17 GHz. Calculation of the coronal magnetic field in a non-linear force-free approximation has revealed an extended structure consisting of interconnected magnetic flux ropes, located practically along the entire length of the main polarity separation line of the photospheric magnetic field. NLS is projected into the region of the strongest horizontal magnetic field, where the main energy of this structure is concentrated. During each X-class flare, the active region lost magnetic helicity and became a CME source.

Good understanding the emission in the presence of foam layer is essential for effective retrieval of oceanic parameter from microwave measurements. In general, microwave emissivity is enhanced by the foam layer, depending not only on the macroscopic dielectric properties and the foam layer thickness, but also on the absorption and scattering of the bubbles. In this paper, a numerical emission model is developed based on a matrix doubling method by which the volume scattering effects in foam layer are accounted for. We explore the effects of the water fraction and foam layer thickness on total emission from foam-covered ocean surface, by analyzing the contributions from both the up-welling and the down-welling emissions from foam layer, and the emission from the seawater. In particular, the dependence of emission, consisting of the three effective emission sources, on the foam layer thickness and water fraction at L-band to Ka-band are investigated. Results are compared with predictions by the incoherent method. Physical reasons given rise to the depolarized emissivity at high frequency bands, and the dry sub-layers at the top of foam layer are discussed.

This paper is a study of the spatial dynamics of microwave emission (data from the Nobeyama Radioheliograph) from two active regions NOAA 12673 and NOAA 11261 for several hours prior to powerful class M x-ray flares. The spatial distribution of the radio brightness and emission intensity in certain parts of the active region is found to be highly nonstationary. Data in the ultraviolet (AIA/SDO) are also used for studying the fine structure of the active emission sources. It is shown that the spatial dynamics of the microwave sources correlates well with the position and appearance of bright ultraviolet (UV) loops in the 94 Å line, i.e., with magnetic loops where the plasma is heated to high temperatures ~107 K. A combined analysis of the UV and radio emission charts with magnetic field charts (HMI/SDO) for NOAA 12673 shows that the highly nonstationary behavior of the microwave sources develops simultaneously with the ignition of a system of intersecting UV loops (X-structures). It is also shown that the bright UV loops and microwave sources arise in the area where new magnetic fluxes emerge. [ABSTRACT FROM AUTHOR]

This study monitored thermal denaturation of albumin using microwave radiometry. Brightness Temperature, derived from Microwave Emission (BTME) of an aqueous solution of bovine serum albumin (0.1 mM) was monitored in the microwave frequency range 3.8–4.2 GHz during denaturation of this protein at a temperature of 56°C in a conical polypropylene cuvette. This method does not require fluorescent or radioactive labels. A microwave emission change of 1.5–2°C in the BTME of aqueous albumin solution was found during its denaturation, without a corresponding change in the water temperature. Radio thermometry makes it possible to monitor protein denaturation kinetics, and the resulting rate constant for albumin denaturation was 0.2 ± 0.1 min−1, which corresponds well to rate constants obtained by other methods.

Abstract: This paper presents an analysis of the experimental measurements of the azimuthal anisotropy of the microwave emissions of a rough water surface. The experimental data were obtained using microwave radiometers with operating frequencies of ~37 GHz (a wavelength of ~8 mm) on the oceanographic platform of the Black Sea Hydrophysical Experimental Facility of the Russian Academy of Sciences in the period from 2005 to 2016. This paper describes the azimuthal anisotropy effect, experimental studies, measurement methods, and data processing. A qualitative comparison of the measurement results with the model calculations and other experimental studies of anisotropy was performed. It was shown that there is a strong sensitivity of the variation in the radio emissions of a rough water surface, both to the wind speed and to its direction at different incidence observation angles. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the functional dependence of the microwave emission of a rough water surface at a frequency of 37.5 GHz (wavelength of ~8 mm). The MiROSE model (Microwave Rough Ocean Surface Emission model) is based on the experimental studies of the own thermal radio emission of the water surface, which were carried out in 2005 and 2007 on the oceanographic platform of the Black Sea Hydrophysical Experimental Facility of the Russian Academy of Sciences. This paper demonstrates the steps of the simulation for selecting the optimal functions to describe the incident angle, wind, and temperature dependences of the increment of the own water surface emission. The following parameters can be calculated on the basis of the proposed model: the increment of the brightness temperature, radio-brightness contrast, and radio-brightness temperature for horizontal and vertical polarizations of the received emission. The model is applicable to water temperatures ranging from 12.5 to 25°C, wind speeds of 3-13 m/s, and incident angles of 30°-80° measured from the nadir. [ABSTRACT FROM AUTHOR]

The matrix doubling (MD) model is a numerical solution to the radiative transfer equation. It can achieve better accuracy in simulating microwave signals from vegetated terrain by considering multiple-scattering effects. However, it is difficult to apply the MD model to retrieving work due to its high complexity. This letter presents a case study performed on corn to demonstrate a multiangular (5°–65°), multiband (1.4/6.925/10.65 GHz) microwave emission model considering multiple-scattering effects by parameterizing the MD model. The simulated emissivity differences between the theoretical model and parameterized model are small. The mean absolute percent errors are all less than 1%, and the root mean square errors (RMSEs) are all within the range of $10^{-3}$. Validations using airborne polarimetric L-band microwave radiometer data and ground-based trunk-mounted multifrequency microwave radiometer data indicate that the parameterized model achieves good accuracy with overall RMSEs within 8K at all three bands. [ABSTRACT FROM AUTHOR]

This study monitored thermal denaturation of albumin using microwave radiometry. Brightness Temperature, derived from Microwave Emission (BTME) of an aqueous solution of bovine serum albumin (0.1 mM) was monitored in the microwave frequency range 3.8–4.2 GHz during denaturation of this protein at a temperature of 56°C in a conical polypropylene cuvette. This method does not require fluorescent or radioactive labels. A microwave emission change of 1.5–2°C in the BTME of aqueous albumin solution was found during its denaturation, without a corresponding change in the water temperature. Radio thermometry makes it possible to monitor protein denaturation kinetics, and the resulting rate constant for albumin denaturation was 0.2 ± 0.1 min−1, which corresponds well to rate constants obtained by other methods. [ABSTRACT FROM AUTHOR]

A correlation analysis of the model calculations and experimental measurements of wind-speed sensitivity of a rough sea-surface microwave emission at a frequency of 37.5 GHz are presented. The field data used in the research were collected over 3 years in the summer and autumn periods at the oceanographic platform of the Marine Hydrophysical Institute, Russian Academy of Sciences (RAS). A hypothesis about a significant correlation between the model calculations and experimentally measured sea-surface emission ability caused by wind forcing was formulated and tested to reveal this correlation. An evaluation of the discrepancy between the model and experimental data has been performed by an analysis of residuals. Our studies have shown that among the selected models not a single one adequately describes the experimental data. [ABSTRACT FROM AUTHOR]

In this study, two retrieval algorithms for estimating the water content of vegetation (VWC) in the range 0–8 kg/m2 from multifrequency microwave radiometric data have been implemented, with the purpose of contributing to the development of an all-weather VWC product for the satellite AMSR2 radiometer of the JAXA (Japan Aerospace Exploration Agency) GCOM-W mission. The first algorithm estimates VWC through a semi-empirical combination of the polarization index (PI) acquired at X and Ku bands, while the second one endeavors to improve the retrieval accuracy adding C-band data and using an artificial neural network (ANN) method. The sensitivity to vegetation biomass of multifrequency PIs at various frequencies, which was already pointed out in the previous literature, has been first further evaluated with the support of the well-known tau-omega solution of the radiative transfer model and experimental data. Two years of AMSR2 data collected on a wide portion of Africa, which includes a large variety of vegetation types and biomasses, have been considered for implementing and testing both algorithms. VWC reference data with the same temporal and spatial coverage of AMSR2, needed for validating the algorithm outputs, have been derived from SPOT4 normalized difference vegetation index (NDVI), downsampled to the AMSR2 ground resolution. The test results provided a correlation coefficient R > 0.88 with root mean square error (RMSE) < 1.4 kg/m 2 for the semi-empirical algorithm, and R = 0.98 with RMSE < 0.5 kg/m2 for ANN algorithm, thus demonstrating that, although both approaches are able to estimate VWC, the ANN algorithm is able to obtain better results. An independent validation of the two algorithms was then carried out on the entire Australian continent, considering 4 periods of 16 days each in different seasons of 2013. In this case, the algorithm outputs have been compared with VWC derived from MODIS 16 days averaged NDVI. The independent validation, as expected, showed slightly worst results, with R = 0.82 and RMSE around 1.5 kg/m2 for the semi-empirical algorithm, and R = 0.88 and RMSE around 0.86 kg/m2 for the ANN. This study demonstrated that microwave data from AMSR2 and polarization indices can be legitimately used to produce vegetation maps on a global scale by separating several levels of biomass, without any need of further information from other sensors. [ABSTRACT FROM PUBLISHER]

ABSTRACT The goal of this study was to apply the concept of passive microwave remote sensing to find the data set of microwave brightness temperatures of the dry soil surface as a function of actual salinity factors that influence the dielectric properties of the soil. From the agricultural point of view, salinity factor refers to the state of accumulation of the soluble salts in the soil. In many area, this factor limiting plant growth (such as sodium chloride). The measurements of the dielectric permittivity were conducted using shorted-circuited-waveguide technique at L-and C-band microwave frequencies (i.e. 1.4GHz and 5GHz, respectively). The dielectric properties of the salted soil samples were investigated at a thermodynamic temperature of 20.4 c . The results show that the frequency of 1.4GHz (L-band) is more effective frequency for study of the penetration depth of the soil surface. Also the vertical polarized microwave signals is preferred to study the variation of the emission characteristics of the dry soil with respect to the salinity by using the microwave radiometric remote sensing techniques especially at low microwave frequency.

A comparison between two algorithms for estimating soil moisture with microwave satellite data was carried out by using the datasets collected on the four Agricultural Research Service (ARS) watershed sites in the US from 2002 to 2009. These sites collectively represent a wide range of ground conditions and precipitation regimes (from natural to agricultural surfaces and from desert to humid regions) and provide long-term in-situ data. One of the algorithms is the artificial neural network-based algorithm developed by the Institute of Applied Physics of the National Research Council (IFAC-CNR) (HydroAlgo) and the second one is the Single Channel Algorithm (SCA) developed by USDA-ARS (US Department of Agriculture-Agricultural Research Service). Both algorithms are based on the same radiative transfer equations but are implemented very differently. Both made use of datasets provided by the Japanese Aerospace Exploration Agency (JAXA), within the framework of Advanced Microwave Scanning Radiometer–Earth Observing System (AMSR-E) and Global Change Observation Mission–Water GCOM/AMSR-2 programs. Results demonstrated that both algorithms perform better than the mission specified accuracy, with Root Mean Square Error (RMSE) ≤0.06 m3/m3 and Bias

We study analytically the generation process of the first harmonics of the pure electron weakly oblique Bernstein modes. This mode can appear as a result of the rise and development of a corresponding instability in a solar active region. We assume that this wave mode is modified by the influence of pair Coulomb collisions and a weak large-scale sub-Dreicer electric field in the pre-flare chromosphere near the footpoints of a flare loop. To describe the pre-flare plasma we used the model of the solar atmosphere developed by Fontenla, Avrett, and Loeser ( Astrophys. J. 406, 319, ). We show that the generated first harmonic is close to the upper-hybrid frequency. This generation process begins at the very low threshold values of the sub-Dreicer electric field and well before the beginning of the preheating phase of a flare. We investigate the necessary conditions for the existence of non-damped first harmonics of oblique Bernstein waves with small amplitudes in the flare area. [ABSTRACT FROM AUTHOR]

It was shown that composite materials based on Pb(Zr1-xTix)O3 (PZT) ferroelectric particles, covered with graphene nanosheets, can be used for wireless detection of force applied to those composites. Ferroelectric materials with their enhanced piezoelectric and emission properties are used in this case as electron sources. During the mechanical loading of composites so called ferroelectric emission (FEE) appears in form of shot impulses of electrons. The short electron impulses generate the intensive microwave impulses that can be detected by means of antenna. This composite could be used as sensor for wireless measurement of machinery components response on applied stress in the locations where the connection with cables is restricted. [ABSTRACT FROM AUTHOR]

Forests play an important role in the global carbon cycle and have a potential impact on global climatic change. Monitoring forest biomass is of considerable importance in understanding the hydrological cycle. Because of the problem of dense forest cover, no reliable method with which to retrieve soil moisture in forest areas from the microwave emission signature has been established. All of these issues relate to the microwave emissivity and transmissivity characteristics of a forest. The microwave emission contribution received by a sensor above a forest canopy comes from both the soil surface and the vegetation layer. To analyze the relationship of forest biomass and forest emission and transmissivity, a high-order emission model, the matrix-doubling model, which consists of both soil and vegetation models, was developed and then validated for a young deciduous forest stand in a field experiment. To simulate the emissivity and transmissivity of a deciduous forest in the L and X bands using the matrix-doubling model, the parameters of components of deciduous trees when the leaf area index varies from 1 to 10 were generated by an L-system and a forest growth model. The emissivity and transmissivity of a forest and the relationships of these parameters to forest biomass are presented and analyzed in this paper. Emissivity in the L band when the leaf area index is less than 6 and at viewing angles less than 40°, and transmissivity in the L band are the most sensitive parameters in deciduous forest biomass estimation. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes a field test to verify a newly discovered phenomenon of microwave emission due to rock fracture in a volcano. The field test was carried out on Miyake Island, 150km south of Tokyo. The main objective of the test was to investigate the applicability of the phenomenon to the study of geophysics, volcanology, and seismology by extending observations of this phenomenological occurrence from the laboratory to the natural field. We installed measuring systems for 300MHz, 2GHz, and 18GHz-bands on the mountain top and mountain foot in order to discriminate local events from regional and global events. The systems include deliberate data subsystems that store slowly sampled data in the long term, and fast sampled data when triggered. We successfully obtained data from January to February 2008. During this period, characteristic microwave pulses were intermittently detected at 300MHz. Two photographs taken before and after this period revealed that a considerably large-scale collapse occurred on the crater cliff. Moreover, seismograms obtained by nearby observatories strongly suggest that the crater subsidence occurred simultaneously with microwave signals on the same day during the observation period. For confirmation of the microwave emission caused by rock fracture, these microwave signals must be clearly discriminated from noise, interferences, and other disturbances. We carefully discriminated the microwave data taken at the mountaintop and foot, checked the lightning strike data around the island, and consequently concluded that these microwave signals could not be attributed to lightning. Artificial interferences were discriminated by the nature of their waveforms. Thus, we inferred that the signals detected at 300MHz were due to rock fractures during cliff collapses. This result may provide a useful new tool for geoscientists and for the mitigation of natural hazards. [Copyright &y& Elsevier]

Abstract: Hypervelocity impacts are accompanied by electromagnetic emissions originating from the impact-induced plasma. Most studies in the past have been devoted to analyzing and interpreting spectra in the visible regime. Only little effort has been spent on understanding emissions in the radio frequency spectrum and giving explanations of their origins from microscale processes. This publication compiles and reviews most relevant processes that are accompanied by radio frequency emissions and ranks them in terms of their dominance in hypervelocity impact plasmas. [Copyright &y& Elsevier]

Regional soil moisture can be estimated by assimilating satellite microwave brightness temperature into a land surface model (LSM). This paper explores how to improve soil moisture estimation based on sensitivity analysis when assimilating Advanced Microwave Scanning Radiometer for the Earth Observing System brightness temperatures. By assimilating a lower and a higher frequency combination, the land data assimilation system (LDAS) used in this paper estimates first model parameters in a calibration pass and then estimates soil moisture in an assimilation pass. The ground truth of soil moisture was collected at a soil moisture network deployed in a semiarid area of Mongolia. Analyzed are the effects of assimilating different polarizations, frequencies, and satellite overpass times on the accuracy of the estimated soil moisture. The results indicate that assimilating the horizontal polarization signal underestimates soil moisture and assimilating the daytime signal overestimates soil moisture. The former is due to improper parameter estimation perhaps caused by high sensitivity of the horizontal polarization to land surface heterogeneity, and the latter is due to the effective soil temperature for microwave emission in the daytime being close to the one at a soil depth of several centimeters but not to the surface skin temperature simulated in the LSM. Therefore, assimilating the nighttime vertical polarizations in the LDAS is recommended. A further analysis shows that assimilating different frequency combinations produces different soil moisture estimates, and none is always superior to the others, because different frequency signals may be contaminated by varying clouds and/or water vapor with different degrees. Thus, an ensemble estimation based on frequency combinations was proposed to filter off, to some extent, the stochastic frequency-dependent biases. The ensemble estimation performs more robust when driven by different forcing data. [ABSTRACT FROM PUBLISHER]

The analysis of narrowband drifting of type III-like structures in radio bursts dynamic spectra allows one to obtain unique information about the primary energy release mechanisms in solar flares. The SSRT (Siberian Solar Radio Telescope) spatially resolved images and its high spectral and temporal resolution allow for direct determination not only of the source positions but also of the exciter velocities along the flare loop. Practically, such measurements are possible during some special time intervals when SSRT is observing the flare region in two high-order fringes near 5.7 GHz; thus, two 1D brightness distributions are recorded simultaneously at two frequency bands. The analysis of type III-like bursts recorded during the flare 14 April 2002 is presented. Using multiwavelength radio observations recorded by the SSRT, the Huairou Solar Broadband Radio Spectrometer (SBRS), the Nobeyama Radio Polarimeters (NoRP), and the Radio Solar Telescope Network (RSTN), we study an event with series of several tens of drifting microwave pulses with drift rates in the range from −7 to 13 GHz s. The sources of the fast-drifting bursts were located near the top of a flare loop in a volume of a few Mm in size. The slow drift of the exciters along the flare loop suggests a high pitch anisotropy of the emitting electrons. [ABSTRACT FROM AUTHOR]

To use the 0th-order τ- ω model to retrieve soil moisture from radiometric data at frequencies higher than the C band, the characteristics of the effective single scattering albedo ω and the opacity τ of vegetation must be studied. In this paper, the ω and τ values of corn for the C, X, and Ku bands were retrieved by matching the simulations of a high-order matrix-doubling model to the τ- ω model. First, the brightness temperature of the matrix-doubling was validated by a truck-mounted radiometer in a field experiment, where the vegetation emission contributions were validated with aluminum foil to mask the soil emission. Then an emissivity database of corn fields for different growing seasons was established for a variety of soil conditions. With the transmissivity of corn determined from the database, the effective single scattering albedos of corn for different heights at the C, X, and Ku bands and at a 55° viewing angle were derived. To verify the accuracy of the derived ω and τ values, we used SMEX02/PSR aircraft data and the Q model to retrieve the soil moisture; the RMSE between the retrieval and the measurements was 4.76% at the C band and 5.36% at the X band. [ABSTRACT FROM AUTHOR]

This study reports on the utilization of microwave modeling, together with ground truth, and L-band (1.4-GHz) brightness temperatures to investigate the passive microwave characteristics of a conifer forest floor. The microwave data were acquired over a natural Virginia Pine forest in Maryland by a ground-based microwave active/passive instrument system in 2008/2009. Ground measurements of the tree biophysical parameters and forest floor characteristics were obtained during the field campaign. The test site consisted of medium-sized evergreen conifers with an average height of 12 m and average diameters at breast height of 12.6 cm. The site is a typical pine forest site in that there is a surface layer of loose debris/needles and an organic transition layer above the mineral soil. In an effort to characterize and model the impact of the surface litter layer, an experiment was conducted on a day with wet soil conditions, which involved removal of the surface litter layer from one half of the test site while keeping the other half undisturbed. The observations showed detectable decrease in emissivity for both polarizations after the surface litter layer was removed. A first-order radiative transfer model of the forest stands including the multilayer nature of the forest floor in conjunction with the ground truth data are used to compute forest emission. The model calculations reproduced the major features of the experimental data over the entire duration, which included the effects of surface litter and ground moisture content on overall emission. Both theory and experimental results confirm that the litter layer increases the observed canopy brightness temperature and obscure the soil emission. [ABSTRACT FROM AUTHOR]

This paper analyzes the magnetic field structure of active regions at coronal heights determined by means of multi-wavelength observations of polarized radio emission in the microwave range, and compares it with the force-free magnetic field extrapolation into the corona from the photospheric magnetograms. Our method of one-dimensional radio stereoscopy indicates higher magnetic field strength compared with the field reconstructed from photospheric magnetograms. It is shown that the sense of inclinations of the field lines we obtained from the radio data matches the shape of the reconstructed magnetic field lines, although the degree of the inclinations is very different. [ABSTRACT FROM AUTHOR]

The so-called zebra structures in radio dynamic spectra, specifically their frequencies and frequency drifts of emission stripes, contain information on the plasma parameters in the coronal part of flare loops. This paper presents observations of zebra structures in a microwave range. Dynamic spectra were recorded by Chinese spectro-polarimeters in the frequency band close to the working frequencies of the Siberian Solar Radio Telescope. The emission sources are localized in the flare regions, and we are able to estimate the plasma parameters in the generation sites using X-ray data. The interpretation of the zebra structures in terms of existing theories is discussed. The conclusion has been arrived at that the preferred generation mechanism of zebra structures in the microwave range is the conversion of plasma waves to electromagnetic emission on the double plasma resonance surfaces distributed across a flare loop. [ABSTRACT FROM AUTHOR]

Based on the observations of solar eclipses performed on the RT-22 radio telescope at CrAO in the wavelength range 2.0-3.5 cm, we consider the fine spatial structure of the microwave emission from the quiet Sun. We have established that the positions of compact radio sources with a typical size of about 7″.0 and coronal bright points coincide. The mean radio flux exceeds the level of the quiet Sun by 0.28 sfu. The brightness temperatures increase with wavelength and lie within the range 0.3-2.7 MK. Evidence for a nonthermal nature of the emission from compact radio sources has been obtained. [ABSTRACT FROM AUTHOR]

In this paper, we report the results of an experimental study aimed toward investigating microwave emission from forests. The experiment was carried out in 2006 on two forest stands of poplar (Populus alba) and pine (Pinus italica), using ground-based microwave radiometers at the L-, C-, X-, Ku-, and Ka-bands, in H and V polarizations. Measurements on poplar were performed on different dates and at different incidence and azimuth angles, looking downward (from the top of trees and from below the crown) and upward (from the soil level). Only one downward-looking measurement was carried out over a pine plot with dry soil in April. All the remote sensing measurements were complemented with "ground-truth" data. The collected experimental data made it possible to quantify the spectral signatures of poplar, as well as the variation of angular trends of brightness temperature in different seasons of the year. The sensitivity of L-band emission to soil properties and leaf biomass was also investigated. Moreover, the measurements on poplar, combined with a simple radiative transfer model (the so-called omega-tau equation), allowed estimating the transmissivity of the canopy with and without leaves. The analysis of data has shown that for the observed forest type, the sensitivity to soil moisture under defoliated trees can be noted at both the L- and C-bands. [ABSTRACT FROM AUTHOR]

Zebra pattern is observed as a number of almost parallel bright and dark stripes in the dynamic spectrum of solar radio emission. Recent observations show that zebra patterns in the microwave range often have superfine temporal structure, when the zebra stripes consist of individual short pulses similar to millisecond spikes. In this article, the burst on 21 April 2002 is investigated. The burst with a distinct superfine structure was detected at the Huairou Station (China) in 2.6 – 3.8 GHz frequency range. It is found that the emission pulses are quasi-periodic, the pulse period is about 25 – 40 ms and decreases with an increase of the emission frequency. The degree of circular polarization of zebra pattern increases with an increase of the emission frequency, it varies from moderate (about 20%) to relatively high (>60%) values. The temporal delay between the signals with left- and right-handed polarization is not found. The conclusion is made that the emission is generated by plasma mechanism at the fundamental plasma frequency in a relatively weak magnetic field. The observed polarization of the emission is formed during its propagation due to depolarization effects. A model is proposed in which the superfine temporal structure is formed due to modulation of the emission mechanism by downward propagating MHD oscillations; this model allows us to explain the observed variation of the pulse period with the emission frequency. [ABSTRACT FROM AUTHOR]

Abstract: The goal is to study parameters of drifting type III bursts, and find out the emission mechanism of these bursts and understand what factors affect instantaneous spectral bandwidth of these bursts. We used simultaneous observations of microwave type III bursts with high temporal, spatial and spectral resolution from the Siberian Solar Radio Telescope (5.7GHz, 14ms resolution) and from the spectropolarimeters (5.2–7.6GHz, 5ms) of the National Astronomical Observatories. Characteristic parameters such as starting frequency, total and instantaneous bandwidth, polarization degree, total duration, and rate of frequency drift were determined for the radio bursts. We analyzed and estimated parameters, which determine instantaneous bandwidth. [Copyright &y& Elsevier]

In this study we examine the critical linkages between thermophysical properties and microwave emissions of landfast snow-covered first-year sea ice during spring melt. For this we analyzed the temporal evolution of radiation fluxes, electro-thermophysical properties and microwave emissions, and perform model simulations to evaluate the observations. The results show five major microwave signature events: brine-rich, blowing snow, melt onset, the onset of funicular regime, and freezing. A brine-rich snow basal layer can considerably increase the snow wetness in the upper and mid layers, resulting in a significant increase in complex permittivity that in turn increases in polarization difference (δp) at 19 and 37 GHz. A dense (∼0.40 g cm−3) wind-packed snow surface layer, during a blowing snow event, was found to increase the permittivity (i.e., surface reflectivity) that in turn increases δp in microwave emissions. Melt onset caused by sustained warming (above −5 °C) corresponded to increased δp of ∼9 K at 19 GHz. The most dramatic increase in δp (up to 17 K at 19 GHz) coincided with the occurrence of a rainstorm. During a freezing, melt-freeze events enlarged snow grains and led to formation of ice lenses and layers within the snow, thereby significantly decreasing microwave emissions. We found that these five factors state above were critical to the melt indicators (i.e., ΔT B(H) (T B(19H)− T B(37H)) and XPGR ([T B(19H)− T B(37V)]/[T B(19H)+ T B(37V)])) commonly used in the satellite melt detection algorithms. The results suggests that the absolute value of T B(19H) (brightness temperature of horizontal polarization at 19 GHz) would be a good indicator along with ΔT B(H) (or XPGR) to delineate the melt onset from ambiguous factors (i.e., a brine-rich slush layer or wind-packed layer), and that the funicular stage of snow melt on sea ice could be unambiguously detected by either ΔT B(H) or XPGR. [Copyright &y& Elsevier]

We develop composition microwave electromagnetic models to calculate the emissivity of sea foam and spray. To describe foam nonuniformities, we suggest incorporating vertical profiles of an effective complex dielectric constant associated with two-phase (air-water) stratifications. We consider a certain structural hierarchy: Wet foam is modeled by mono- and polydispersed systems of bubbles; dry foam is represented by a statistical two-phase mixture containing thin water films and air; and spray is modeled by a discrete system of spherical water droplets. The impact of effective profiles on the spectral and polarization dependencies of the emissivity (signatures) is analyzed numerically using a multilayer computer-simulation technique. This technique allows us to provide a comprehensive model for variable configurations of complex effective profiles and different incidence angles. New results are generated for three basic compositions: ‘water + foam + spray,’ ‘water + spray,’ and ‘foam + spray.’ We intend this modeling for use in advanced ocean remote sensing studies, including monitoring of gales, detection of two-phase wakes, estimations of air-sea fluxes and evaporation, and for retrieval purposes. [ABSTRACT FROM AUTHOR]

The ability of electromagnetic models to accurately predict microwave emission of a snowpack is complicated by the need to account for, among other things, nonindependent scattering by closely packed snow grains, stratigraphic variations, and the occurrence of wet snow. A multilayer dense medium model can account for the first two effects. While microwave remote sensing is well known to be capable of binary wet/dry discrimination, the ability to model brightness as a function of wetness opens up the possibility of ultimately retrieving a percentage wetness value during such hydrologically significant melting conditions. In this paper, the first application of a multilayer dense medium radiative transfer theory (DMRT) model is proposed to simulate emission from both wet and dry snow during melting and refreezing cycles. Wet snow is modeled as a mixture of ice particles surrounded by a thin film of water embedded in an air background. Melting/refreezing cycles are studied by means of brightness temperatures at 6.7, 19, and 37 GHz recorded by the University of Michigan Truck-Mounted Radiometer System at the Local Scale Observation Site during the Cold Land Processes Experiment-1 in March 2003. Input parameters to the DMRT model are obtained from snow pit measurements carried out in conjunction with the microwave observations. The comparisons between simulated and measured brightness temperatures show that the electromagnetic model is able to reproduce the brightness temperatures with an average percentage error of 3% (∼8 K) and a maximum relative percentage error of around 8% (∼2O K). [ABSTRACT FROM AUTHOR]

Sand seas (ergs) of the Sahara are the most dynamic parts of the desert. Aeolian erosion, transportation, and deposition continue to reshape the surface of the ergs. The large-scale features (dunes) of these bedforms reflect the characteristics of the sand and the long-term wind. Radiometric emissions from the ergs have strong dependence on the surface geometry. We model the erg surface as composed. of tilted rough facets. Each facet is characterized by a tilt distribution dependent upon the surface roughness of the facet. The radiometric temperature (Tb) of ergs is then the weighted sum of the Tb from all the facets. We use dual-polarization Tb measurements at 19 and 37 GHz from the Special Sensor Microwave Imager aboard the Defense Meteorological Satellite Program and the Tropical Rainfall Measuring Mission Microwave Imager to analyze the radiometric response of erg surfaces and compare them to the model results. The azimuth angle (φ) modulation of Tb is caused by the surface geometrical characteristics. It is found that longitudinal and transverse dune fields are differentiable based on their polarization difference (ΔTb) φ-modulation, which reflects type and orientation of dune facets. ΔTb measurements at 19 and 37 GHz provide consistent results. The magnitude of ΔTb at 37 GHz is lower than at 19 GHz due to higher attenuation. The analysis of ΔTb over dry sand provides a unique insight into radiometric emission over ergs. [ABSTRACT FROM AUTHOR]

Abstract: A study is made of the initial stage of the 25 August 1999, event using high time-resolution microwave observations, as well as the data from the Siberian Solar Radio Telescope and Yohkoh/SXT/HXT. Microwave emission revealed fine structures with a typical time scale of about 1s with a frequency drift ∼1GHz/s, and structures with a typical time scale of less than 0.1s with a frequency drift ∼10GHz/s. Emission bands with a slower frequency drift are generated by energetic electrons subjected to the cyclotron resonance condition. This slower frequency drift is a consequence of a change in plasma density caused by the coalescence of magnetic structures with a typical size of the order of ∼108 cm, while the emission with a faster drift is generated by beams of energetic electrons generating Cherenkov resonance emission. The energetic electron beams are produced as a result of the interaction of magnetic structures ∼106–107 cm in size. [Copyright &y& Elsevier]

Abstract: A new technique is described that detects when and where new snow falls on ice sheets and then determines the thickness of new accumulation. Measurements of vertically polarized passive emission at 85 GHz are filtered with the Hilbert–Huang Transform to identify periods where the surface snow has changed significantly. These are shown to be commonly the result of new snow by comparison with both field observations and in situ instrumentation. Temperature, atmospheric emission and clouds all affect the passive microwave signal but each is examined and shown not to prevent the identification of new snow events. The magnitude of the brightness temperature change is not strongly correlated with snowfall amount. To quantify the amount of new snow, the spatial extent and timing of new snowfalls are examined with ICESat/GLAS laser altimetry data. Crossover differences between altimetric profiles taken before, during, and after the snowfall event provide a measure of the thickness of new snow. Specific cases are presented where 11 and 13 cm of new snow were detected over large regions. [Copyright &y& Elsevier]

Abstract: We present the results of the first observations of the solar microwave burst with fine spectral structure of zebra type at the frequency about 5.7GHz. The burst has been detected simultaneously by the Siberian Solar Radio Telescope and by the spectropolarimeter of the National Astronomical Observatory of China. Zebra pattern consisted of three parallel stripes with complex frequency drift. The degree of circular polarization of emission reached 100%, the polarization sense corresponded to the extraordinary wave (X-mode). We have determined the plasma parameters in the emission source: plasma density about 1011 cm−3, magnetic field strength 60–80G. We argue that in the given event the most probable mechanism of the zebra pattern generation is non-linear coupling of harmonics of Bernstein modes. [Copyright &y& Elsevier]

Abstract: Due to the narrow bandwidth and the small size of the subsecond microwave pulses, we can use them as probe sources for study of propagation effects in the low corona. More than 160 microwave bursts with subsecond pulses (SSP) have been observed with the Siberian Solar Radio Telescope at 5.7CHz for the period 2000–2004. Working with a large dataset of homogeneous observational material (spatial resolution from 15″ to 20″, temporal resolution 14mc), we estimated sizes of SSP and studied relation between SSP sizes and sense of the polarization and their position on the Sun. Our results are in accordance with those obtained during the 22nd solar cycle. The apparent sizes of SSP increase toward the solar limb. The obtained dependence is in agreement with Bastian‘s (Bastian, T.S. Angular scattering of solar radio emission by coronal turbulence. ApJ 426, 774, 1994.) model calculations. The center-to-limb variation of the source size is explained by scattering on plasma turbulence along the ray path in the solar corona. The most events with high polarization occur near the central meridian (±30°). The polarization sense corresponds mainly to the ordinary mode. [Copyright &y& Elsevier]

Based on observational data obtained with the RT-22 Crimean Astrophysical Observatory radio telescope at frequencies of 8.6 and 15.4 GHz, we investigate the quasi-periodic variations of microwave emission from solar active regions with periods Tp < 10 min. As follows from our wavelet analysis, the oscillations with periods of 3–5 min and 10–40 s have the largest amplitudes in the dynamic power spectra, while there are virtually no oscillations with Tp < 10 s. Our analysis shows that acoustic modes with Tp ≲ 1 min strongly dissipate in the lower solar corona due to thermal conduction losses. The oscillations with Tp = 10–40 s are associated with Alfvén disturbances. We analyze the influence of acoustic and Alfvén oscillations on the thermal mechanisms of microwave emission in terms of the homogeneous model. We discuss the probable coronal heating sources. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]