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Caltech Authors (103,751 recursos)
Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 4,561

1. Neoproterozoic glacial record in the Death Valley region, California and Nevada - Abolins, Mark; Oskin, Rebecca; Prave, Tony; Summa, Catherine; Corsetti, Frank
The Neoproterozoic succession in the Death Valley region contains a physical- and chemostratigraphic record of glaciation. Direct evidence for glaciation includes dropstones, glacially influenced diamictites, and cap carbonates in the Neoproterozoic Kingston Peak Formation. Within this formation, glacially influenced deposits and cap carbonates occur at two distinct horizons, suggesting at least two glacial episodes. Cap-like carbonates and sequence boundaries elsewhere in the succession may indicate additional glacial intervals. The basal Beck Spring Dolomite has fades and isotopic characteristics commonly associated with cap carbonates, and the rest of the succession is punctuated by numerous sequence boundaries including a prominent incised horizon in the uppermost Johnnie Formation. This horizon...

2. Some transition and cavitation inception observations on a 1.5 cal ogive - Arakeri, V. H.; Acosta, A. J.
Transition observation on a 1. 5 cal ogive were carried out by Schlieren technique of flow visualization up to Re_D of 1.26 x 10^6. Good agreement is found between computed position of transition by Smith method and those observed by Schlieren technique for tunnel velocities greater than 50 fps (Re_D> 7.85 x 10^5). Cavitation under desinent conditions at tunnel velocities of 30 fps and 40 fps was found to occur within the transition region of the boundary layer. At 50 fps good agreement is found between the present value of inception cavitation index, the value of desinent cavitation index measured...

3. The Palomar Transient Factory - Nugent, Peter; Cao, Yi; Kasliwal, Mansi
Astrophysics is transforming from a data-starved to a data-swamped discipline, fundamentally changing the nature of scientific inquiry and discovery. New technologies are enabling the detection, transmission, and storage of data of hitherto unimaginable quantity and quality across the electromagnetic, gravity and particle spectra. The observational data obtained during this decade alone will supersede everything accumulated over the preceding four thousand years of astronomy. Currently there are 4 large-scale photometric and spectroscopic surveys underway, each generating and/or utilizing hundreds of terabytes of data per year. Some will focus on the static universe while others will greatly expand our knowledge of transient...

4. Activated-complex theory: current status, extensions, and applications - Marcus, R. A.
The activated-complex theory of chemical reactions has proved to be very useful in interpreting rate data on a wide variety of chemical reactions. The topics so treated include preexponential factors of bimolecular gas-phase reactions, preexponential factors at high pressures of unimolecular reactions, salt and polar solvent effects on bimolecular-reaction rates, and recombination rates of radicals under certain conditions. In conjunction with additional concepts, the theory has also found extensive use in such subjects as pressure effects on unimolecular reactions (so-called RRKM theory}, chemical activation, and electron-transfer reactions both in solution and at electrodes. In the present chapter the assumptions and several derivations of the theory are considered. Some of...

5. Chemical Engineering - Roth, Dana L.

6. Ionic Charge States of High Energy Solar Energetic Particles in Large Events - Sollitt, L. S.; Stone, E. C.; Mewaldt, R. A.; Cohen, C. M. S.; Leske, R. A.; Wiedenbeck, M. E.; von Rosenvinge, T. T.
We present a novel technique to infer average ionic charge states of high energy (≥ 10 MeV/nuc) solar energetic particles (SEPs) in large solar events. In some large SEP events, it is observed that higher energy SEPs decay in intensity more rapidly than at lower energies. Furthermore, this energy dependence varies with particle species, as would be expected if the decay timescale depended on a rigidity-dependent diffusive mean free path. By comparing the decay timescales of nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron to a reference element, such as carbon, charge states are inferred for these elements in several SEP...

7. Heavy Ion and Electron Release Times in Solar Particle Events - Mewaldt, R. A.; Cohen, C. M. S.; Haggerty, D. K.; Gold, R. E.; Krimigis, S. M.; Leske, R. A.; Ogliore, R. C.; Roelof, E. C.; Stone, E. C.; von Rosenvinge, T. T.; Wiedenbeck, M. E.
Using data from the SIS and EPAM instruments on ACE we have measured the onset times of 6 - 88 MeV/nuc ions and 38 - 315 keV electrons in 11 solar energetic particle (SEP) events from 1997 through 2002. We find that heavy ions are generally released later than electrons, by as much as ∼50 minutes. There is an apparent correlation between the release times (and the inferred release distances) and the 3He/4He ratio.

8. Onsets and Release Times in Solar Particle Events - Tylka, A. J.; Cohen, C. M. S.; Dietrich, W. F.; Krucker, S.; McGuire, R. E.; Mewaldt, R. A.; Ng, C. K.; Reames, D. V.; Share, G. H.
The time at which solar energetic particles (SEPs) are first released into interplanetary space, and its relation to CMEs and various photon emissions, are important clues to the site and nature of the SEP acceleration mechanism [1- 5,7,8,10,11]. We examine velocity dispersion among onsets in electrons and ions from Wind, ACE, and IMPB, as well as available neutron monitors, to determine the solar release time. We present results for two large impulsive events (1 May 2000 and 14 April 2001) and three western ground level events (GLEs; 6 November 1997, 6 May 1998, and 15 April 2001). In the impulsive events,...

9. Impulsive Flare Material: A Seed Population for Large Solar Particle Events? - Mewaldt, R. A.; Cohen, C. M. S.; Mason, G. M.; Desai, S.; Leske, R. A.; Mazur, J. E.; Stone, E. C.; von Rosenvinge, T. T.; Wiedenbeck, M. E.
It has been suggested that gradual solar energetic particle (SEP) events enriched in ^3He and Fe result from the shock acceleration of remnant interplanetary material from previous impulsive flares. We test this hypothesis by comparing the number densities of Fe ions during quiet periods, ^3He-rich periods, and Ferich SEP events. We find that there is not a sufficient density of suprathermal Fe during typical quiet periods (or ^3He-rich periods) to account for the overall enrichment of Fe in and suggest other possibilities for interpreting these events.

10. Possible Detection of Large Solar Particle Event at Balloon Altitudes during the 2001-2002 TIGER Flight - Geier, S.; Barbier, L. M.; Binns, W. R.; Christian, E. R.; Cummings, J. R.; de Nolfo, G. A.; Israel, M. H.; Link, J. T.; Mewaldt, R. A.; Mitchell, J. W.; Schindler, S. M.; Scott, L. M.; Stone, E. C.; Streitmatter, R. E.; Waddington, C. J.
The Trans-Iron Galactic Element Recorder (TIGER) was launched on December 21, 2001 and flew for about 32 days on a long-duration balloon mission from McMurdo Base in Antarctica. On December 26, 2001 at about 5:30 UT, a ground-level solar particle event (M7.6 flare) was observed by a number of neutron monitors. The SIS instrument aboard the ACE spacecraft measured the elemental composition and particle energy spectra up to ∼150MeV/nuc. While not designed to operate under such conditions, TIGER data for the same period show interesting variations in the count rate and composition of the measured particles that may be related to...

11. Measurements of the Ultra-Heavy Galactic Cosmic-Ray Abundances between Z=30 and Z=40 with the TIGER Instrument - Link, J. T.; Barbier, L. M.; Binns, W. R.; Christian, E. R.; Cummings, J. R.; de Nolfo, G. A.; Geier, S.; Israel, M. H.; Mewaldt, R. A.; Mitchell, J. W.; Schindler, S. M.; Scott, L. M.; Stone, E. C.; Streitmatter, R. E.; Waddington, C. J.
The Trans-Iron Galactic Element.Recorder (TIGER) instrument was launched in December 2001 from McMurdo, Antarctica. TIGER is a cosmic-ray telescope that uses four scintillation counters, two Cherenkov detectors and a scintillating fiber hodoscope to determine the charge (Z) and energy of a particle. During the 31.8 day flight it measured ~100 ultra-heavy galactic cosmic-ray (GCR) events with Z > 30 and demonstrated charge resolution sufficient to resolve the individual elemental abundances in this region. The abundances of the Ultra-Heavy GCRs in this range can be used to distinguish between GCR source models. We present our measurements and discuss the implications for the...

12. High Energy Ionic Charge State Composition In Recent Large Solar Energetic Particle Events - Labrador, A. W.; Leske, R. A.; Mewaldt, R. A.; Stone, E. C.; von Rosenvinge, T. T.
The ionic charge states of solar energetic particles (SEPs) provide information on the temperature of source materials and on conditions during acceleration and transport. SAMPEX/MAST measures mean ionic charge states at > 15 MeV/nuc using the geomagnetic rigidity filter technique. Charge state measurements by MAST for gradual SEP events suggest a continuum of charge states correlated with abundance ratios for a variety of elements, similar to what is observed at lower energies. In cases where lower energy measurements are also available, the combined measurements indicate energy dependent charge states. We have completed ionic charge state measurements for 17 SEP events from solar...

13. Voyager Observations of Anomalous Cosmic Ray Gradients and the Role of Diffusion and Drifts in the Outer Heliosphere - Cummings, A. C.; Stone, E. C.
We find that the diffusion mean free path at 1.5 GV inferred from the gradient of anomalous cosmic rays (ACRs) observed by Voyager 1 and 2 (V1 and V2) has an approximately linear radial distance dependence at solar maximum. At solar minimum the mean free path is a factor of 10 or more larger than at solar maximum. We find that the expected dependence on current sheet tilt of the ACR O V1/V2 gradient has not become evident as of early 2003, likely because the tilt has yet to decrease below ˜40° . It appears that from mid-2000 up to...

14. Chemical Engineering - Roth, Dana L.

15. Optimal navigation functions for nonlinear stochastic systems - Horowitz, Matanya B.; Burdick, Joel W.
This paper presents a new methodology to craft navigation functions for nonlinear systems with stochastic uncertainty. The method relies on the transformation of the Hamilton-Jacobi-Bellman (HJB) equation into a linear partial differential equation. This approach allows for optimality criteria to be incorporated into the navigation function, and generalizes several existing results in navigation functions. It is shown that the HJB and that existing navigation functions in the literature sit on ends of a spectrum of optimization problems, upon which tradeoffs may be made in problem complexity. In particular, it is shown that under certain criteria the optimal navigation function is...

16. Reactive switching protocols for multi-robot high-level tasks - Raman, Vasumathi
This paper considers a team of cooperative, homogeneous robots operating in a nondeterministic environment, performing complex high-level tasks. We concurrently solve the problems of dynamic task assignment and reactive planning in a centralized fashion, allowing goal reassignment as needed in response to changes in the environment. To this end, we model the team of robots as a switched system whose various modes must be activated to satisfy high-level specifications, and describe a formal framework for synthesizing switching protocols for this system. The robots' continuous dynamics under a specific task allocation, as well as their operating environment, are encoded as formulas...

17. A Compositional Approach to Stochastic Optimal Control with Co-safe Temporal Logic Specifications - Horowitz, Matanya B.; Wolff, Eric M.; Murray, Richard M.
We introduce an algorithm for the optimal control of stochastic nonlinear systems subject to temporal logic constraints on their behavior. We compute directly on the state space of the system, avoiding the expensive pre-computation of a discrete abstraction. An automaton that corresponds to the temporal logic specification guides the computation of a control policy that maximizes the probability that the system satisfies the specification. This reduces controller synthesis to solving a sequence of stochastic constrained reachability problems. Each individual reachability problem is solved via the Hamilton-Jacobi-Bellman (HJB) partial differential equation of stochastic optimal control theory. To increase the efficiency of...

18. Finding quantum algorithms via convex optimization - Childs, Andrew M.; Landahl, Andrew J.; Parrilo, Pablo A.
In this paper we describe how to use convex optimization to design quantum algorithms for certain computational tasks. In particular, we consider the ordered search problem, where it is desired to find a specific item in an ordered list of N items. While the best classical algorithm for this problem uses log_2 N queries to the list, a quantum computer can solve this problem much faster. By characterizing a class of quantum query algorithms for ordered search in terms of a semidefinite program, we find quantum algorithms using 4 log_(605) N ≈ 0.433 log_2 N queries, which improves upon the previously best known...

19. Control Over a Network: Using Actuation Buffers to Reduce Transmission Frequency - Epstein, Michael; Shi, Ling; Di Cairano, Stefano; Murray, Richard M.
We consider a discrete time linear feedback control system with additive noise where the control signals are sent across a network from the controller to the actuators. Due to network considerations it is desired to reduce the transmission frequency of the control signals. We show that by including a finite sequence of predicted control signals in each communication packet the frequency of transmission can be reduced by transmitting only when the previously sent sequence has run out, although as a consequence the closed loop error will increase. We introduce a communication protocol, which we call Input Difference Transmission Scheme (IDTS),...

20. Presolar (?) Corundum in the Orgueil Meteorite - Huss, G. R.; Hutcheon, I. D.; Wasserburg, G. J.; Stone, J.
The discovery in chondritic meteorites of diamond, SiC, and poorly crystallized graphite that formed around other stars demonstrated conclusively that presolar dust survived the formation of the solar system to be incorporated into meteorites. The presolar nature of these grains is shown by the highly unusual isotopic compositions of their constituent elements. To date, all recognized types of presolar grains have been carbon rich and apparently formed around carbon stars, those with C/O greater than 1. The discovery of the first oxygen-rich grain with isotopic characteristics consistent with a presolar origin is reported. Oxygen-rich grains presumably form only around stars...

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