The ground-based radiometer-scatterometer system of the University of Bern, RASAM, measures brightness temperatures and backscattering coefficients at frequencies between 2 and 12 GHz. Between 1984 and 1992 many campaigns were carried out on agricultural fields. Data from more than 70 bare agricultural soils have now been organized in a bare soil signature catalogue. In addition to the microwave data, detailed ground-truth information was collected including highly accurate surface roughness profiles. Information on the data base is given, together with short descriptions of the instruments and measurement procedures. >

Since fast fine structures (FFS) superimposed on microwave bursts were found with high time resolution observations, they have been observed in extensive frequency range (Slottje 1978; Fu, et al. 1986; Stähli and Magun 1986; and Stepanov and Yurovsky 1991), and these results make understanding of the emission deepening. But, at the same time, the puzzling problem, these phenomena originate from sun or are only artificial, is often concerned and disputed (Benz and F ü rst 1987). As it was pointed out by Benz and F ü rst (1987), “the only really reliable way to study solar microwave fluctuation is to use two or more widely separated radio telescopes”. In this paper, some of FFS events superimposed on microwave bursts simultaneously obtained at Beijing Astronomical Observatory (BAO), Crimea Astrophysical Observatory (CAO) and Institute of Applied Physics, University of Bern (IAP), are presented at first time. It is a conclusive evidence of rapid radio fluctuation originating from sun and associated with flare appearance.

Active and passive microwave signatures of bare soil were measured simultaneously with soil parameters as temperature, soil moisture, surface roughness, soil texture and permittivity. The results of three special situations, where only one soil parameter changed, are presented. In the first situation of freezing and thawing cycles only the permittivity is different. In the second situation four measurements of a bare soil are compared. At all four days the soil moisture is approximately the same, but the surface roughness changes due to erosion by rain. Third we measured a bare soil before and after a thunderstorm with hail. Again erosion smoothed the soil surface. Two days with the same soil moisture were selected. These measurements can now be used to test and create models. Microwave signatures of a frozen soil and of a soil after a thunderstorm are shown.

Hα observations with high temporal resolution allow a detailed analysis of the chromospheric response to thre primary energy release. The timing; relationship between Hα and hard X-rays gives information about the energy transport mechanism. Sites of massive injection of nonthermal electrons are identified by characteristic features in the the line profile (Canfield et al. 1984;. Canfield and Gayley 1987).

Etude de l'interaction parametrique des ondes electromagnetiques dans un plasma magnetise froid, consideree comme une source possible de transfert de l'energie entre les spectres fondamental et harmonique des ondes electromagnetiques incoherentes

First observational evidence of harmonic radiation at microwave frequencies during solar bursts is presented for the event of April 28, 1983. The recordings between 3.1 and 19.6 GHz show a typical continuum with a spectral maximum near 5.2 GHz. Superimposed fine structures with durations in the order of some seconds exhibit a very unusual spectral behaviour. Narrow-banded intensity peaks appeared at 5.2 and 11.8 GHz which were barely visible at 3.1, 8.4 and 19.6 GHz. These structures can be interpreted as harmonic emission. Harmonic radio emission can be generated either by plasma radiation, gyroradiation, electron-cyclotron maser or by nonlinear conversion processes. However, all of those mechanisms require extreme assumptions on the source and the ambient plasma in order to account for the observations.

Extensive theoretical studies as well as laboratory and field measurements of rough-surface scattering and emission from agricultural soil and sea surface and of volume scattering and emission from snow and vegetation yield a considerable comprehension of the interaction of microwaves with the molecular properties and the geometrical features of these media. Hence algorithms have been derived which allow the interpretation of air- and space-borne radiometer- and radar-data in terms of natural parameters. The absorption lines of atmospheric constituents in the millimeter wave range can be utilized to measure with radiometers various trace gases and other parameters, like temperature, throughout the strato-and mesosphere. Knowledge of spectroscopic parameters of the considered molecules, high spectral resolution of the measured lines and special inversion algorithms of the radiation transfer allow the determination of the height distribution of constituents and temperature with a resolution of less than 10 km.

The particle-size dependence of microwave scattering from snow is investigated. Frozen crusts have been analysed, and the microslruclure is described by correlation functions. Relations between the correlation length and various quantities such as ice-intercept lengths or volume-to-surface ratios are checked. The scattering is calculated using the Bom approximation and is linked to the emissivity through the albedo. The model yields good agreement with the microwave data from C to K bands, and the predicted particle-size dependence is verified. The claims that the variance and the correlation length of the permittivity fluctuations are the characteristic parameters are confirmed. Initial uncertainties with respect to the validity of the Born approximation for an air-ice mixture are clarified.

The contribution of volume scattering from the subsurface stratum of soil or snow to the backscattered radar signal decreases rapidly with increasing moisture content as compared to the contribution by rough-surface scattering. In order to obtain an evaluation of the relative importance of volume scattering of moist strata, a heterogeneous two-component mixture of water inclusions in a dry background medium is assumed and the formulation of the distorted Born approximation according to Tsang et al. is applied. Computed permittivities and backscatter cross-sections are compared with experimental data of situations with modest moisture content. From there, conclusions on the relative portion of volume scattering and on shape and size of the effectively-scattering water inclusions can be drawn.

Microwave scattering and emission behavior of snow and soil with locally and temporally varying parameters, such as moisture content, surface roughness and subsurface structure, is determined by absorption, surface scattering and volume scattering. Various concepts in either of these regimes have been put forward during recent years. Some of them are well-supported by experimental results. More emphasis has recently been put to investigations of the dielectric properties and its relation to volume scattering. A few highlights of the recent developments in modelling of microwave scatterers, and emission properties of snow and soil are summarized in this presentation and some new investigations are discussed.

Determining the three-dimensional distribution of minor constituents and the temperature throughout the middle atmosphere (approximately 10 to 100 km) on a global scale is a fundamental requirement to understand the complex chemistry and dynamics of this region. Millimeter wave spectroradiometry using limb sounding technique from a spacecraft in low orbit offers the opportunity to obtain height profiles with near global coverage. The Millimeter Wave Atmospheric Sounder (MAS) to be flown on Space Shuttle will measure ozone, water vapour, chlorine monoxide, temperature and pressure over an altitude range of 15 to 100 km. MAS is scheduled for the Atmospheric Laboratory for Application and Science (ATLAS) Missions 1, 2 and 3 at roughly one year intervals with a first start in 1989. Scientific fundamentals, design rationales and technical solutions are presented.

First results from a recently developed surface sensor for measuring the dielectric constant of snow at about 1 GHz are reported together with ground-based measurements of brightness temperatures between 4.9 and 94 GHz. The data are used to derive spectra of complex dielectric constants of wet snow for frequencies between 1 and 100 GHz. The result is simple: Debye relaxation spectra with a constan...

Hydrological interest in mapping snow concentrates on the phase of snow depletion when at least part of the snow cover is wet. In this situation, snow has a very low backscatter coefficient, smaller than almost any land surface at X-band. Together with the independence of cloud cover and time, this unique signature of snow enables frequent and regular mapping of snow even in rugged terrain. First results from a synthetic aperture radar experiment made during the melting season—even under unfavourable conditions—clearly indicate this potential. The backscatter data used in this work are based on four seasons of scatterometer-measurements made on the alpine test site Weissfluhjoch, Davos, and on a comparison with additional backscatter data from groups in Europe and the U.S.A

The extent of the snow cover, the stored amount of water and the state of the snow metamorphism, in particular the date of the snow-melt and the intensity of the run-off, are of prime importance to hydrology and water management in large regions as well as to weather and climate. Regular observations by satellite-borne sensors can substantially improve the information on the status and the dynamic behaviour of these hydrologic parameters on a global scale. Microwave radiometers or scatterometers as all-weather and day and night sensors are excellent tools for reliable observations of cloud-covered regions of the world. The interaction of microwaves with snow strongly depends on snow wetness and size and structure of snow grains. Multifrequency observations can be used to classify snow conditions, to estimate the water equivalent or dry snow and to determine the start of the melting period. Results of several years of ground-based microwave observations along with classical hydrologic measurements are discussed and experience with satellite-borne microwave radiometers for the determination of the extent and state of the northern hemispheric snow cover is reported. Conclusions are drawn to define optimum sensor specifications for satellite-borne microwave remote sensing of the snow cover.