Deep radio occultations and “evolute flashes”; their characteristics and utility for planetary studies
Abstract
Radio occultation studies of the structure of planetary atmospheres have generally involved relatively shallow penetration of the spacecraft behind the limb of the planet in the plane of the sky. Current radio link sensitivities allow detection of the radio signals at all occultation depths, whenever the planet-spacecraft distance is sufficiently large for the refraction to occur at atmospheric heights where microwave absorption is not too large. Voyager 1 at Jupiter and Voyager 2 at Saturn will pass almost directly behind the planets as viewed from the Earth. Thus they will pass through the caustics that corresponds to the focal line of a spherical planet, expanded by oblateness into a surface approximating a four-cusp cylinder. In the plane of the sky, the projection of this surface approximates the evolute of the planet's limb. As the spacecraft passes behind the planet with its antenna tracking the occulting limb, the strength of the radio signals received on Earth will at first decrease due to defocusing in the atmosphere, but then increase as the evolute is approached, because of the focusing caused by limb curvature. Inside the evolute there are four simultaneous signal paths over four limb positions. If we neglect absorption, focused signals for an instant could become orders of magnitude stronger than for the unocculted spacecraft. Measurements of the frequency and intensity of deep occultation signals, and of the timing and character of these “evolute flashes”, could provide information on atmospheric absorption, turbulence, and structure, and on details of the shape of the atmosphere at the focusing limbs as affected, for example, by planetary gravitational moments, rotation, and zonal winds. Such observations will be attempted with Voyager and potentially could be very fruitful in the Pioneer Venus and Galileo (Jupiter) orbiting missions.
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Cited by (21)
Quick-look estimates of ionospheric properties from radio occultation data
2021, Advances in Space ResearchRadio occultations are commonly used to determine vertical profiles of ionospheric electron density from measurements of frequency. “Frequency residuals” are important intermediate data products in radio occultation experiments. Frequency residuals are defined as the difference between observed and predicted values of the received frequency, where the predicted frequency includes all effects except refraction at the target object. However, relationships between properties of a vertical profile of ionospheric electron density and properties of time series of frequency residuals are not widely known by the broad community of scientists who may work with frequency residuals. Here we illustrate and explain how properties of a vertical profile of ionospheric electron density affect time series of frequency residuals. We also develop four quantitative relationships between electron density values and frequency residuals. These provide predictions for the topside plasma scale height, topside electron densities, peak altitude, and peak electron density that can be generated directly from a set of frequency residuals without the need to implement the full Abel transform analysis that is usually required to determine electron densities from frequency residuals. They can also be generated prior to the implementation of a high-quality baseline correction, to which the Abel transform analysis is sensitive. The development of these predictions assumed that the ionosphere has an exponential topside and a Chapman-like peak. These predictions are successfully tested on two-way radio occultation observations from the MAVEN Radio Occultation Science Experiment (ROSE) at Mars.
Prediction of uncertainties in atmospheric properties measured by radio occultation experiments
2010, Advances in Space ResearchRefraction due to gradients in ionospheric electron density, , and neutral number density, , can shift the frequency of radio signals propagating through a planetary atmosphere. Radio occultation experiments measure time series of these frequency shifts, from which and can be determined. Major contributors to uncertainties in frequency shift are phase noise, which is controlled by the Allan Deviation of the experiment, and thermal noise, which is controlled by the signal-to-noise ratio of the experiment. We derive expressions relating uncertainties in atmospheric properties to uncertainties in frequency shift. Uncertainty in is approximately where is uncertainty in frequency shift, f is the carrier frequency, c is the speed of light, is the electron mass, is the permittivity of free space, V is speed, e is the elementary charge, is a plasma scale height and R is planetary radius. Uncertainty in is approximately where and are the refractive volume and scale height of the neutral atmosphere. Predictions from these expressions are consistent with the uncertainties of the radio occultation experiment on Mars Global Surveyor. These expressions can be used to interpret results from past radio occultation experiments and to perform preliminary design studies of future radio occultation experiments.
We present observations of Saturn's central flash obtained from Palomar and McDonald Observatories during the 3 July 1989 occultation of 28 Sgr. As the star passed close to the geometric center of Saturn's shadow, the focusing of the incident starlight by the planet's atmosphere formed multiple stellar images along the limb which were detected in infrared images obtained at wavelengths of 3.9 (Palomar) and 2.1 μm (McDonald). These are the first reported observations of a central flash due to Saturn, and the first for any planet in which the signal from each stellar image could be determined separately, permitting a comparison of both intensity and position for each image with model predictions. The starlight forming the central flash was strongly modulated by passage through Saturn's rings. Four separate flashes were observed from each observatory, corresponding to the points on the limb where the starlight passed through the Cassini Division and the relatively transparent C Ring, with maximum brightnesses reaching 1-2% of the unocculted stellar intensity. The more usual brightening seen during previous central flashes as the observer crossed the caustic formed by the planet's oblate limb was not detectable in this event due to strong attenuation by the B Ring. The timing of the flashes is quite sensitive to the shape of Saturn's limb, which depends in turn on the planet's zonal gravity harmonics and on the zonal wind profile in the lower stratosphere, near the 2.5-mbar pressure level. The locations of the images along the limb, as well as the timing, shapes, and amplitudes of the individual flash light curves, are well matched by a smoothed model based on a tropospheric zonal wind profile obtained from tracking cloud features in Voyager images and the Saturn ring optical depth profile obtained from the Voyager Photopolarimeter experiment. The smoothing required to give the best match to the data exceeds that attributable to the finite angular extent of the occulted star and may be due to refractive scattering by turbulence or wave structure in Saturn's atmosphere. The detailed agreement of the models and observations is improved by assuming a uniform zonal wind speed of 40 msec-1 at 25-70° north latitude in the stratosphere, suggesting that midlatitude zonal winds on Saturn decay with height above the troposphere to the nonzero mean of local tropospheric winds. There is no evidence of significant atmospheric absorption at the observed wavelengths, which correspond to spectral regions of weak methane absorption.
On the black hole lens and its foci
1989, Advances in Space ResearchMethods developed for radio occultation studies of planetary atmospheres are used to predict the electro-magnetic focusing properties of a refracting lens representation of a Schwarzschild black hole. The inifinity of foci are of three distinct types: a principal forward axial focus which is by far the strongest; higher-order forward axial foci; and backward conical foci with axial nulls.
Dual-wavelength radio observations during the nearly diametric occultation of Voyager 1 by Titan provided 3.6- and 13-cm data on the equatorial neutral atmosphere. Intensity and frequency fluctuations occurred on time scales from about 0.1 to 1.0 sec at both wavelenghts whenever the radio path passed within 90 km of the surface, indicating the presence of variations in refractivity on length scales from a few hundred meters to a few kilometers. Strong intensity scintillations were observed for ray periapsis between about 5 and 25 km altitude, while weak scintillations occurred with ray periapsis between the surface and 5 km, and between 25 and 90 km. As the intensity fluctuations showed similar variation with altitude during ingress and egress, the small-scale structure is not considered to be a localized phenomenon. Comparisons of measured intensity spectra with theory show best agreement when horizontal variations in refractivity are included in the theoretical models; stable layers alone are insufficient to explain this result. Above 25 km, the altitude profile of intensity scintillations closely agrees with the predictions of a simple theory based on the characteristics of internal gravity waves propagating with little or no attenuation through the vertical stratification in Titan's atmosphere. These observations support a hypothesis of stratospheric gravity waves, possibly driven by a cloud-free convective region in the lowest few kilometers of the atmosphere. The net upward flux of energy and an upper bound on the net upward flux of horizontal momentum due to gravity waves are approximately 2 erg cm−2 sec−1 and 3 · 10−2 dyne cm−2, respectively. This energy flux is not significant in determining the thermal structure of the atmosphere below 150 km, but the vertical transport of momentum by gravity waves could be important in maintaining the zonal stratospheric circulation. The enhanced scintillations between 5 and 25 km may be associated with stratified layers.
Theory of radio occultation by Saturn's rings
1982, IcarusThe radio occultation technique is developed here as a new method for the study of the physical properties of planetary ring systems. Particular reference is made to geometrical and system characteristics of the Voyager dual-wavelength (13 and 3.6 cm) experiment at Saturn. The rings are studied based on the perturbations they introduce in the spectrum of coherent sinusoidal radio signals transmitted through the rings from a spacecraft in the vicinity of the planet to Earth. Two separate signal components are identified in a perturbed spectrum: a sinusoidal component that remains coherent with the incident signal but is reduced in intensity and possibly changed in phase, and a Doppler-broadened incoherent component whose spectral shape and strength are determined by the occultation geometry and the radial variation of the near-forward radar cross section of illuminated ringlets. Both components are derived in terms of the physical ring properties starting from a conventional radar formulation of the problem of single scattering on ensembles of discrete scatterers, which is then generalized to include near-forward multiple scattering. The latter is accomplished through special solutions of the equation of transfer for particles that are larger than the wavelength. When the occultation geometry is optimized, contributions of an individual ringlet to a perturbed spectrum can be identified with radial resolution on the order of a few kilometers for the coherent component and a few hundred kilometers for the incoherent one, thus permitting high-resolution reconstruction of the radial profile of the optical depth, as well as reconstruction of the radar cross section of resolved ringlets. Simultaneous estimates of the optical depth and radar cross section of a ringlet at 3.6 cm-gl allow separation of its aerial density and particle size, if the particles are of known material and form a narrow size distibution with radii greater than several tens of centimeters. This separation is also achieved for radii ⪅10 cm from differential effects on the coherent signal parameters at 3.6- and 13-cm wavelengths. For the more general case of a broad size distribution modeled by a power law, the absence of differential effects on the coherent signal binds the minimum size to be ⪆10 cm. In this case, the radius inferred from an estimate of the radar cross section represents an equivalent radius, which is strongly controlled by the maximum size of the distribution provided that the power index is in the range 3 to 4. On the other hand, detection of differential coherent signal extinction determines an upper bound on the maximum size and a lower bound on the power index, assuming water-ice particles. These bounds, together with an inferred equivalent size, constrain the size distribution at both its small and large ends.