The inter-relation of phenomena on the sun and in the earth's ionosphere has been studied in order to establish a causal dependence which could result from travelling solar particles. In addition to examining a variety of data from a number of sources, an extensive R.F. noise recording program was undertaken to provide information on those regions of the solar and terrestrial atmospheres from which optical data were not available. The occasions on which there was an influx of such particles into the earth's ionosphere were established from magnetic data, from optical observations of the Polar Aurora, and from H.F. and V.H.F. radio observations close to the Auroral Zone. On the basis of these data the ionospheric disturbances for a 12 month period were related to prior distinctive solar events that were considered capable of producing the high energy particles. The selection was made from the optical, spectroscopic and radio data available. It was found that solar flares, eruptive prominences, and disappearing filaments were the most frequent sources of Earth-reaching particles, and that large sunspots contributed only occasionally to ionospheric disturbances. Moreover, the radio noise data permitted a fairly reliable estimate of the particle velocities in the solar corona, which could be used to predict the probability of subsequent terrestrial effects of the ejected particles. A correlation coefficient of +0.65 was found between probabilities predicted from the solar information and observed ionospheric disturbances which occurred two or three days later. The observational evidence on the solar noise bursts and on ionospheric storms could only be explained on the basis of a considerable distribution of velocities for the particles ejected from the sun. It was shown that a Maxwellian distribution of corpuscular velocities was a probable one. Moreover, the observations indicated that particles are frequently emitted from the sun with a distribution of velocities, but that only in the case of very large phenomena are there sufficient particles with the high energies necessary to overcome the sun's gravitational field and reach the earth. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

No description available.

A new "circuit" model is developed to study non-linear effects in holography. The model links classical signal detection theory to holography, for it clearly shows that the reconstructed images in a thin hologram can be computed from mathematical formulas obtained for the output of non-linear detectors. In preparation for hologram analysis, the results for the (time-) autocorrelation of electrical ʋth-law devices in response to signal plus noise are extended to memoryless non-linear detectors with arbitrary characteristics. Mathematical parallels are next established between holography and the non-linear detection of signals, and these are incorporated in the model. The ready-made formulas for electrical detectors apply directly to give formulas for the (space-)autocorrelation of holograms of diffuse objects. The autocorrelation function predicts distorted multi-pie images, their relative positions, orientations, widths, and strengths. The multiplicity of images is due to the generation of harmonics; the background halo-like noise components added to the faithful images are mainly due to the multiplicity of the first harmonic itself. The analysis is further expanded to include a still more general case in which the reflected light from the object is depolarized. A decrease in signal-to-noise ratio (decrease in fringe visibility); a loss of information, and an augmentation of non-linear distortion could be expected. A simple way of reducing these effects is suggested. Finally, holography with more than one reference beam is studied. An exact formulation is given for double-reference-beam holograms. For multiple-reference-beam holograms, only approximate closed forms are presented. It is shown that the addition of extra reference beams during the recording step of the hologram may amplify the faithful images without increasing their background noise significantly. Experiments confirm the theoretical expectations. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

NASA's Cassini mission to Saturn revolutionized modern understanding of the planet's vast and intricate ring system. We use stellar occultation data from Cassini's UVIS High Speed Photometer (HSP) to characterize the particle size distribution in the rings with two methods. First, we discern the sizes of the smallest particles at ring edges by forward-modeling observed diffraction signatures which appear as spikes in the signal, the shape and amplitude of which depends on the size and abundance of the smallest particles. We then probe the upper end of the size distribution using occultation statistics. Although the distribution of photon counts in the absence of ring particles follows Poisson statistics for which the variance is equal to the mean, random variations in the sizes and abundance of particles introduce excess variance. Previous studies have interpreted excess variance in stellar occultation data in terms of an effective particle size. The assumption of small particles is invalid in Saturn's A and B rings where ring particles cluster together into elongated structures called self-gravity wakes. We calculate the statistical moments within spiral density waves, undulating structures excited throughout Saturn's rings at locations of resonance with satellites. In our diffraction analysis, we find more detections of diffraction at edges near the outer A and B rings than at edges within the C ring and Cassini Division, consistent with the prediction that edges directly perturbed by satellites have a greater population of sub-cm particles than edges confined by other mechanisms. In our moments analysis, we find that the granola bar model for regularly spaced wakes cannot match the observed statistics of both density wave troughs and peaks with a single set of parameters S and W, which may indicate that wakes are more opaque in the wave crests due to compression than they are in the troughs.

Asteroids are the remnants of planet formation, and as such, they represent a record of the physical and chemical conditions in the early solar system and its evolution over the past 4.6 billion years. Asteroids are relatively accessible by spacecraft, and thus may be a source of the raw materials necessary for future human exploration and settlement of space. Those on Earth-crossing orbits pose impact hazards for which mitigation strategies must be developed. For these reasons, several missions to asteroids are in progress or planned with the support of the National Aeronautics and Space Administration (NASA) and other national space agencies. The study of asteroid composition and physical surface properties is vital to both our scientific understanding of the solar system's formation and evolution and to the development of asteroid missions and resource utilization schemes. This dissertation uses infrared spectroscopy to investigate the composition and physical properties of main-belt asteroid surfaces. Our efforts are focused on two populations that are especially relevant to constraining thermal and collisional processes in the asteroid belt: the "M-type" asteroids and primitive asteroid families. To investigate volatiles in the M-type asteroids, we obtained 2-4 micron spectra of six M-type asteroids using NASA's Infrared Telescope Facility. We find spectral signatures of hydrated minerals on all six asteroids, with evidence for rotational variability of hydration in one target. Diversity in the shape of the 3-micron feature in our sampled asteroids suggests there are different modes of hydration in the M-type population. Next, we carried out a thermal and compositional study of M-type asteroid (16) Psyche using 5-14 micron spectra from the Spitzer Space Telescope. Psyche is suspected to be a remnant iron core, and it is the target of an upcoming NASA mission. Using thermophysical modeling, we find that Psyche's surface is smooth and most likely has a thermal inertia of 5-25 J/m^2/K/s^(1/2), and a bolometric emissivity of 0.9, although a scenario with an emissivity of 0.7 and thermal inertia up to 95 J/m^2/K/s^(1/2) is possible if Psyche is somewhat larger than previously determined. From comparisons with laboratory spectra of silicate and meteorite powders, Psyche's emissivity spectrum is consistent with the presence of fine-grained ( < 75 micron) silicates. These silicates may include a magnesian pyroxene component. We conclude that Psyche is likely covered in a fine silicate regolith, which may also contain iron grains, overlying an iron-rich bedrock. Finally, we compared the mid-infrared properties of two primitive asteroids families, ancient Themis (~2.5 Gyr) and young Veritas (~8 Myr). Visible and near-infrared studies show spectral differences between the two families attributed to different degrees of space weathering. To test whether these differences are apparent in the mid-infrared, we analyzed the 5-14 micron Spitzer Space Telescope spectra of 11 Themis-family asteroids and 9 Veritas-family asteroids. We detect a broad 10-micron emission feature, attributed to fine-grained and/or porous silicate regolith, in all 11 Themis-family spectra and six of nine Veritas-family asteroids, with 10-micron spectral contrast ranging from 1% +/- 0.1% to 8.5% +/- 0.9%. Comparison with laboratory spectra of primitive meteorites suggests these asteroids are similar to meteorites with relatively low abundances of phyllosilicates. We used thermal modeling to derive diameters, beaming parameters and albedos for our sample. Asteroids in both families have beaming parameters near unity and geometric albedos in the range 0.031-0.14. Spectral contrast of the 10-micron silicate emission feature is not correlated with asteroid diameter; however, higher 10-micron contrast may be associated with flatter spectral slopes in the near-infrared. There is a slight trend of increasing 10-micron contrast with decreasing albedo in the Veritas asteroids, but not the Themis asteroids. Overall, our results indicate the Themis and Veritas family members show variation in regolith texture and/or structure within both families that is not directly related to family age.

Radiation from extraterrestrial sources is a concern for the safety of passengers and crew in high altitude aircraft. Cosmic radiation and solar particles constantly bombard the atmosphere with energy. Radiation levels from these sources can vary considerably depending on solar activity cycles and energetic particle events such as solar flares. In order to predict the effects of such events the nature of the radiation spectrum must be characterized, and the individual effects of each radiation type understood. The background radiation spectrum is known to good accuracy and prediction of radiation levels due to specific solar events is currently under investigation. This work begins the task of calculating the expected effects upon human tissue from these radiation sources with intervening air and aluminum. Proton and alpha radiation of 3000 MeV/nucleon and less is simulated using a software package called the High Energy Transport Code – Human Exploration and Development in Space (HETC-HEDS), and linear energy transfer to tissue surrogate is tabulated.

Thesis (Ph. D.)--Open University.

Published also as Thesis (Ph. D.)--Columbia University, 1917. / Bibliography: p. 130-144.

Published also as Thesis (Ph. D.)--Columbia University, 1917. / Bibliography: p. 130-144.

Thesis (Ph. D.)--Columbia University. / Vita. Published also without thesis note. Bibliography: p. 130-144.