Abstract. Errors in radio occultation experiments associated with departures from sphericity can be analyzed on the basis of straight-ray tomography in which (1) only one parallel projection of the object is available, (2) diffraction is neglected, and (3) inversion is achieved by assuming the object to be circularly symmetric. The straight-line tomographic case and the actual curved ray paths in a spherically symmetric atmosphere are related by an analytic transformation. A previously unknown kernel function transforms the two-dimensional (2-D) atmospheric refractivity map from the plane of propagation to a 1-D radial profile along the locus of ray periapsides. This function consists of a discrete, positive singularity plus an extended negative branch; the negative branch rises smoothly from negative infinity in the neighborhood of the positive singularity toward zero. Knowledge of the resolving kernel allows study of horizontal and vertical resolution in a rigorous manner. Spherically symmetric structures are reproduced exactly with the kernel; processing of band-limited projection data results in a modified kernel of finite vertical resolving power. In general, the horizontal extent of significant contributions, including levels of 10% of the peak, is of the order 2X/4.6rH centered on the locus of periapsides, where r is the radius of the point in consideration and H is the refractivity scale height; the vertical extent of significant contributions extends from approximately one sampling distance below r to a few sampling distances above it. We confirm the expected result that a refractivity structure of radial thickness Ar will be retrieved properly only if its circumferential extent is approximately 2X/2rAr or greater centered on the periapsis point. In addition, a positive refractivity structure limited in circumferential extent introduces artifacts of negative refractivity in the retrieved profile. The artifacts first appear at the projected altitude onto the vertical of the structure's circumferential edges and continue downward. The error represented by such negative refractivity artifacts is diminished somewhat by the general exponential increase in refractivity with decreasing altitude of a real atmosphere. Thus the relative error depends on the local scale height and the circumferential extent of the refractivity structure.
Abstract. Reduction of radio occultation data to retrieve atmospheric profiles (T-p(r)) requires knowledge or assumption of the horizontal structure of the atmosphere. In the case of terrestrial planets the atmosphere in the vicinity of ray periapsides usually is assumed to be spherically symmetric. This assumption leads to an integral transform relationship between the profiles of refractivity versus radius and the total bending angle versus the asymptotic closest approach of rays, where the latter is directly obtainable from occultation frequency data and trajectory information. Occultation studies of the giant planets have demonstrated that departures from spherical symmetry, if not accounted for, can result in serious errors in derived T-p(r) profiles. We analyze errors in atmospheric profiles due to large-scale departures from spherical symmetry. For analytic convenience we represent departures from spherical symmetry as locally spherical structures with center of curvature offset in three dimensions from the center of mass, from which follow analytic expressions for errors in bending angle and impact parameter as functions of the offset and trajectory parameters. Since these expressions are not restricted to any specific occultation type, it is easy to identify the geometrical configurations and the specific trajectory parameters that enhance or suppress these errors. Errors in bending angle and impact parameter carry over into the refractivity and radius profiles, while at the same time, new errors are introduced because the bending angle versus impact parameter profile is integrated along a nonvertical path in the presence of large-scale departures from spherical symmetry, to obtain refractivity and radius profiles. Similarly, refractivity and radius errors propagate into the temperature and pressure profiles, while a nonvertical path of integration in the presence of horizontal gradients provides another opportunity for new errors to be introduced. We estimate that fractional errors in temperature profiles can be as large as a few percent for the Martian atmosphere above 20 km, decreasing in magnitude closer to the surface. For Earth, such errors are estimated to be less than 1% above 30 km. In the lower parts of Earth's atmosphere, however, and especially in the lower troposphere, these errors can be very sensitive to horizontal gradients and hence highly variable; typically, the error magnitude remains less than 2% for the dry regions of Earth's troposphere. We have not addressed the effect on errors of water vapor gradients, or of more extreme structures such as sharp weather fronts. A small variation on this approach can incorporate errors due to imprecise knowledge of the transmitter and receiver trajectories. IntroductionRadio occultation is a remote sensing method for "sounding" planetary atmospheres. Since its inception in the early 1960s, this methodology has been used to obtain high vertical resolution profiles of atmospheric temperature and pressure for most of the planets of the solar system...
Timing of activity along the Yammuneh segment (Lebanon–Syria) of the Dead Sea Fault Zone and its northward continuation is still a subject of controversy. Our field structural analysis and observations on radar, Landsat and digital elevation model imagery of the Homs region (Syria) are of concern for Plio-Quaternary tectonics of the whole northern part of the Dead Sea Fault Zone. We show in this paper that the northern Dead Sea Fault Zone has remained active since the onset of the Homs basalts at c . 6 Ma. Continuing movement in Recent times is indicated by the occurrence of Quaternary pull-apart structures and offset of active ravines along the fault. The Homs basalts are related to the distinct oval-shaped Shin volcanic edifice, of which the long axis trends NW. The volcano was fed through NW-striking tension fractures, which now form dykes and volcanic ridges. These patterns are consistent with a NE–SW extension that occurred c . 6 Ma ago. The northwestern end of the Shin volcano is left-laterally displaced c . 20 km, yielding a c . 3.3 mm a −1 mean rate of relative movement between the Arabian and African plates. In the northern part of the Dead Sea Fault Zone, the overall trace of the main active Dead Sea Fault Zone is not a single transform but forms an irregular plate boundary composed of transform fault and collision belt segments. Fine-grained mylonite developed in the fault corridor may have favoured aseismic deformation in the Shin volcano area.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.