The interaction of magnetic fields and convection is investigated in the context of the coronal heating problem. We study the motions of photospheric magnetic elements using filtergrams obtained at the Swedish Vacuum Solar Telescope at La Palma. We use potential-field modeling to extrapolate the magnetic and velocity fields to larger height. We find that the velocity in the chromosphere can be locally enhanced at the separatrix surfaces between neighboring flux tubes. The predicted velocities are several km/s, significantly larger than those of the photospheric flux tubes, which may have important implications for coronal heating. sComment: submitted to ApJ, 21 pages, 10 figure
Abstract.A new spectroscopic method, aimed to derive the plasma electron density and outflow velocity in expanding solar coronal regions, is discussed in this paper. The method is based on the analysis of a pair of coronal lines emitted via collisional and radiative excitation by the same ion, such as the O VI 1032, 1037 Å doublet. The merit of this technique consists in allowing us to derive at the same time electron density and outflow velocity of the coronal plasma from nearby lines detectable with the same instrument, provided the constraint on mass flux conservation along the flow tube connecting solar corona and heliosphere is taken into account. The results obtained from the analysis of the OVI emission imply that the physical conditions of a polar coronal hole plasma, observed during minimum activity, are the following. The electron density decreases from 4 × 10 5 cm −3 at 1.7 R to 2−4 × 10 4 cm −3 at 3.1 R , whereas the outflow velocity of the oxygen ions is monotonically increasing to reach 350-500 km s −1 at 3.1 R , depending on the assumptions on the degree of anisotropy of the velocity distribution of the ions. These results of the velocity of expansion of the fast wind confirm those obtained with Doppler dimming techniques when assuming the lowest observed density values for the coronal hole plasma. This implies that, for a rarified corona, the outflow velocity of the fast solar wind in polar holes can be traced by the motion of the O VI ions at least up to 2.4 R . The analysis also shows that the degree of anisotropy of the oxygen ions, due to the acceleration of the ions across the magnetic field in a coronal hole, exhibits a steep increase and that the geometry of the flow tube diverges very rapidly low down in the inner corona/transition region. The observations of the extended corona analysed in this paper are obtained with the Ultraviolet Coronagraph Spectrometer of the SOHO space mission.
In this paper, persistent scatterer interferometry and Synthetic Aperture Radar (SAR) tomography have been applied to Sentinel-1 data for urban monitoring. The paper analyses the applicability of SAR tomography to Sentinel-1 data, which is not granted, due to the reduced range and azimuth resolutions and the low resolution in elevation. In a first part of the paper, two implementations of the two techniques are described. In the experimental part, the two techniques are used in parallel to process the same Sentinel-1 data over two test areas. An intercomparison of the results from persistent scatterer interferometry and SAR tomography is carried out, comparing the main parameters estimated by the two techniques. Finally, the paper addresses the complementarity of the two techniques, and in particular it assesses the increase of measurement density that can be achieved by adding the double scatterers from SAR tomography to the persistent scatterer interferometry measurements.scatterer per resolution cell [4][5][6][7][8][9][10][11][12]. This assumption can be invalid when observing ground scenes with a pronounced extension in the elevation direction, for which more than one scatterer can fall in the same range-azimuth resolution cell. This, for instance, occurs in the presence of buildings of different heights, whose backscattered signals interfere in the same resolution cell, or in urban layover areas [13]. In such areas, the PSI techniques that assume one dominant scatterer usually experience a loss of deformation measurements. This potential limitation can be overcome by using the TomoSAR [14] techniques. In fact, in such techniques, the use of a stack of complex-valued interferometric images makes the separation of the scatterers interfering within the same range-azimuth resolution cell possible. This is achieved by synthesizing apertures along the elevation direction, which results in an elevation resolution, so as to provide the full scene reflectivity profile along azimuth, range, elevation, and average deformation velocity [15]. With respect to PSI techniques, TomoSAR, in addition the scatterers' position in 3D space and their average deformation velocity, also provides their intensity distribution in 3D space, which is additional information that can be conveniently used for selecting the most reliable scatterers in the reconstructed scene. Tomographic processing techniques exploit both the phase and amplitude of the backscattered signal and consist of resolving an inversion problem [16].The aim of this paper is to show the applicability of TomoSAR to Sentinel-1 data in performing deformation monitoring. Then, an intercomparison of the results from TomoSAR and the more mature and experimented technique of persistent scatterer interferometry are carried out. For this purpose, two study areas were analyzed using Sentinel-1 data, which include a portion of the Port of Barcelona (Spain) and a part of the city center. The contributions of the paper can be summarized as follows. Firstly, the paper presents the...
Slope failures pose a substantial threat to mining activity due to their destructive potential and high probability of occurrence on steep slopes close to limit equilibrium conditions, which are often found both in open pits and in waste and tailing disposal facilities. The development of slope monitoring and modeling programs usually entails the exploitation of in situ and remote sensing data, together with the application of numerical modeling, and it plays an important role in the definition of prevention and mitigation measures aimed at minimizing the impact of slope failures in mining areas. In this paper, a new methodology is presented; one that combines satellite radar interferometry and 2D finite element modeling for slope stability analysis at a regional scale, and applied within slope unit polygons. Although the literature includes many studies applying radar interferometry and modeling for slope stability analysis, the addition of slope units as input data for radar interferometry and modeling purposes has, to our knowledge, not previously been reported. A former mining area in southeast Spain was studied, and the method proved useful for detecting and characterizing a large number of unstable slopes. Out of the 1959 slope units used for the spatial analysis of the radar interferometry data, 43 were unstable, with varying values of safety factor and landslide size. Out of the 43 active slope units, 21 exhibited line of sight velocities greater than the maximum error obtained through validation analysis (2.5 cm/year). Finally, this work discusses the possibility of using the results of the proposed approach to devise a proxy for landslide hazard. The proposed methodology can help to provide non-expert final users with intelligible, clear, and easily comparable information to analyze slope instabilities in different settings, and not limited to mining areas.
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