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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]

Microwave emission, in the x-band frequency range (8.2-12.4 GHz), from a thin, large, rectangular sheet plasma has been measured. The plasma electron density was such that the plasma frequency was within or just above this frequency range. The plasma was immersed in an external magnetic field from a set of Helmholz coils. The magnetic field was oriented parallel to the electric field between the anode ground plane and a cylindrical, hollow cathode. The spectrum of the emitted noise was measured for both ordinary mode (P to B) and extraordinary mode (⊥ to B) polarization in the x-band. The emission was strongest at high harmonics of the electron cyclotron frequency. Mechanisms that might produce this noise and its potential use as a diagnostic tool are discussed [ABSTRACT FROM PUBLISHER]

Hybrid architectures integrating mesoscopic electronic conductors with resonant microwave cavities have a great potential for investigating unexplored regimes of electron–photon coupling. In this context, producing nonclassical squeezed light is a key step towards quantum communication with scalable solid-state devices. Here we show that parametric driving of the electronic conductor induces a squeezed steady state in the cavity. We find that squeezing properties of the cavity are essentially determined by the electronic noise correlators of the quantum conductor. In the case of a tunnel junction, we predict that squeezing is optimized by applying a time-periodic series of quantized δ —peaks in the bias voltage. For an asymmetric quantum dot, we show that a sharp Leviton pulse is able to achieve perfect cavity squeezing.