SEARCH · Engineering Papers
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Harmonic analysis of pressures on ring baffles, force measurements for solid and perforated ring baffles, and inertia damping coefficient for model of liquid sloshing - fuel tank
Large anticyclonic rings are shed from the retroflecting North Brazil Current (NBC) near 8 deg N in the tropical western Atlantic. New subsurface velocity and temperature measurements within three such rings are presented here and are found to be consistent with previous in situ and remotely sensed NBC ring measurements. A high-resolution numerical model of the Atlantic Ocean forced by monthly wind stress and an imposed meridional overturning cell is found to shed NBC rings that approximate those observed. The model rings are more surface-intensified than those observed and somewhat smaller in diameter. Both observed and modeled NBC rings move northwestward along the coast of South America with a speed of 8-16 cm/s, considerably slower than predicted by analytical theories describing westward ring propagation. At least 2-3 rings per year separate from the NBC retroflection. Annually, 1-3 rings translate intact from their formation region near 50 deg W to the islands of the southeastern Caribbean, where they disintegrate after a lifetime of about 100 days. This study is the first to make use of subsurface temperature and velocity data to compute the volume of the anomalous ring core. NBC rings may be responsible for 3-4 Sv of direct mass transport across the equatorial-tropical gyre boundary or 20-25% of the total upper ocean cross-gyre transport required by the Atlantic meridional overturning cell. Translating NBC rings may contribute 20% of the total meridional heat transport by the ocean at this latitude.
During a geomagnetic storm on 24 May 2000, the IMAGE Extreme Ultraviolet (EUV) camera observed a plasmaspheric density trough in the evening sector at L-values inside the plasmapause. Forward modeling of this feature has indicated that plasmaspheric densities beyond the outer wall of the trough are well below model expectations. This diminished plasma condition suggests the presence of an erosion process due to the interaction of the plasmasphere with ring current plasmas. We present an overview of EUV, energetic neutral atom (ENA), and Far Ultraviolet (FUV) camera observations associated with the plasmaspheric density trough of 24 May 2000, as well as forward modeling evidence of the lie existence of a plasmaspheric erosion process during this period. FUV proton aurora image analysis, convolution of ENA observations, and ring current modeling are then presented in an effort to associate the observed erosion with coupling between the plasmasphere and ring-current plasmas.
The most prominent oscillatory feature observed in the Voyager 1 radio occultation of Saturn's rings is identified as a one-armed spiral bending wave excited by Titan's-1:0 nodal inner vertical resonance. Ring particles in a bending wave move in coherently inclined orbits, warping the local mean plane of the rings. The Titan-1:0 wave is the only known bending wave that propagates outward, away from Saturn, and the only spiral wave yet observed in which the wave pattern rotates opposite to the orbital direction of the ring particles. It is also the first bending wave identified in ring C. Modeling the observed feature with existing bending wave theory gives a surface mass density of about 0.4 g/sq cm outside the wave region and a local ring thickness of less than about 5 meters, and suggests that surface mass density is not constant in the wave region.
Observations of radar returns from Saturn's rings, together with radio interferometry of their absorption of radiation from the disk, combine to require an effective radius of ring particles of about 6 cm or larger. It is suggested that the ring particles may also include, in addition to the known ice constituent, a mixture of the clathrated hydrate of methane and ammonia hydrate. A two-density model for ring particles is possible in which a matrix of low density contains many nodules of higher-density ice particles; in this case, radii nearly as large as the observed ring thickness would be possible. Improved resolution in radio observations at 21 cm or, if necessary, at longer wavelengths for narrow ring openings is perhaps the most useful method for determining upper limits on the particle size.
The self-consistent treatment of the RC ion dynamics and EMIC waves, which are thought to exert important influences on the ion dynamical evolution, is an important missing element in our understanding of the storm-and recovery-time ring current evolution. For example, the EMlC waves cause the RC decay on a time scale of about one hour or less during the main phase of storms. The oblique EMIC waves damp due to Landau resonance with the thermal plasmaspheric electrons, and subsequent transport of the dissipating wave energy into the ionosphere below causes an ionosphere temperature enhancement. Under certain conditions, relativistic electrons, with energies 21 MeV, can be removed from the outer radiation belt by EMIC wave scattering during a magnetic storm. That is why the modeling of EMIC waves is critical and timely issue in magnetospheric physics. This study will generalize the self-consistent theoretical description of RC ions and EMIC waves that has been developed by Khazanov et al. [2002, 2003] and include the heavy ions and propagation effects of EMIC waves in the global dynamic of self-consistent RC - EMIC waves coupling. The results of our newly developed model that will be presented at the meeting, focusing mainly on the dynamic of EMIC waves and comparison of these results with the previous global RC modeling studies devoted to EMIC waves formation. We also discuss RC ion precipitations and wave induced thermal electron fluxes into the ionosphere.
An experimental investigation of the vibration characteristics of a 60 deg conical shell model of a planetary entry vehicle is described and the results presented. Model configurations include the shell with or without one or two Z-ring stiffeners and with or without a simulated payload. Tests were conducted with the model clamped at the small diameter and with the model suspended at the simulated payload. Additionally, calculated results obtained from application of several analytical procedures reported in the literature are presented together with comparisons between experimental and calculated frequencies and meridional mode shapes. Generally, very good frequency agreement between experimental and calculated results was obtained for all model configurations. For small values of circumferential mode number, however, the frequency agreement decreased as the number of ring stiffeners increased. Overall agreement between experimental and calculated mode shapes was generally good. The calculated modes usually showed much larger curvatures in the vicinity of the rings than were observed in the experimentally measured mode shapes. Dual resonances associated with modal preference were noted for the shell without Z-ring stiffeners, whereas the addition of stiffeners produced resonances for which the model responded in two or more modes over different sections of the shell length.
We present simulation results from a one-way coupled global MHD model (Block-Adaptive-Tree Solar-Wind Roe-Type Upwind Scheme, BATS-R-US) and kinetic ring current models (Comprehensive Ring Current Model, CRCM, and Fok Ring Current, FokRC). The BATS-R-US provides the CRCM/FokRC with magnetic field information and plasma density/temperature at the polar CRCM/FokRC boundary. The CRCM uses an electric potential from the BATS-R-US ionospheric solver at the polar CRCM boundary in order to calculate the electric field pattern consistent with the CRCM pressure distribution. The FokRC electric field potential is taken from BATS-R-US ionospheric solver everywhere in the modeled region, and the effect of Region II currents is neglected. We show that for an idealized case with southward-northward-southward Bz IMF turning, CRCM-BATS-R-US reproduces well known features of inner magnetosphere electrodynamics: strong/weak convection under the southward/northward Bz; electric field shielding/overshielding/penetration effects; an injection during the substorm development; Subauroral Ion Drift or Polarization Jet (SAID/PJ) signature in the dusk sector. Furthermore, we find for the idealized case that SAID/PJ forms during the substorm growth phase, and that substorm injection has its own structure of field-aligned currents which resembles a substorm current wedge. For an actual event (12 August 2000 storm), we calculate ENA emissions and compare with Imager for Magnetopause-to-Aurora Global Exploration/High Energy Neutral Atom data. The CRCM-BATS-R-US reproduces both the global morphology of ring current and the fine structure of ring current injection. The FokRC-BATS-R-US shows the effect of a realistic description of Region II currents in ring current-MHD coupled models.
The space environment is a complex system defined by regions of differing length scales, characteristic energies, and physical processes. It is often difficult, or impossible, to treat all aspects of the space environment relative to a particular problem with a single model. In our studies, we utilize several models working in tandem to examine this highly interconnected system. The methodology and results will be presented for three focused topics: 1) Rapid radiation belt electron enhancements, 2) Ring current study of Energetic Neutral Atoms (ENAs), Dst, and plasma composition, and 3) Examination of the outflow of ionospheric ions. In the first study, we use a coupled MHD magnetosphere - kinetic radiation belt model to explain recent Akebono/RDM observations of greater than 2.5 MeV radiation belt electron enhancements occurring on timescales of less than a few hours. In the second study, we present initial results of a ring current study using a newly coupled kinetic ring current model with an MHD magnetosphere model. Results of a dst study for four geomagnetic events are shown. Moreover, direct comparison with TWINS ENA images are used to infer the role that composition plays in the ring current. In the final study, we directly model the transport of plasma from the ionosphere to the magnetosphere. We especially focus on the role of photoelectrons and and wave-particle interactions. The modeling methodology for each of these studies will be detailed along with the results.
The proposed work addresses two main objectives: (1) to pursue the development of the random diffraction screen model for analytical/computational characterization of the extinction and near-forward scattering by ring models that include particle crowding, uniform clustering, and clustering along preferred orientations (anisotropy). The characterization is crucial for proper interpretation of past (Voyager) and future (Cassini) ring, occultation observations in terms of physical ring properties, and is needed to address outstanding puzzles in the interpretation of the Voyager radio occultation data sets; (2) to continue the development of spectral analysis techniques to identify and characterize the power scattered by all features of Saturn's rings that can be resolved in the Voyager radio occultation observations, and to use the results to constrain the maximum particle size and its abundance. Characterization of the variability of surface mass density among the main ring, features and within individual features is important for constraining the ring mass and is relevant to investigations of ring dynamics and origin. We completed the developed of the stochastic geometry (random screen) model for the interaction of electromagnetic waves with of planetary ring models; used the model to relate the oblique optical depth and the angular spectrum of the near forward scattered signal to statistical averages of the stochastic geometry of the randomly blocked area. WE developed analytical results based on the assumption of Poisson statistics for particle positions, and investigated the dependence of the oblique optical depth and angular spectrum on the fractional area blocked, vertical ring profile, and incidence angle when the volume fraction is small. Demonstrated agreement with the classical radiative transfer predictions for oblique incidence. Also developed simulation procedures to generate statistical realizations of random screens corresponding to uniformly packed ring models, and used the results to characterize dependence of the extinction and near-forward scattering on ring thickness, packing fraction, and the ring opening angle.
Diffraction model fits to the light curves obtained during April 18, 1984 and August 20, 1985 stellar occultations that revealed incomplete rings, or 'arcs' around Neptune, are found to indicate arc radial widths of 8-26 km, and normal optical depths of 0.07-0.14. On the basis of the Neptune pole direction ascertained from Voyager navigation analyses, it is confirmed that the arc observed on August 20, 1985, together with the Hubbard et al. (1986) and Covault et al. (1986) arcs, seem to match the radius of the outermost ring found in Voyager images analyzed by Smith et al. (1989).
Global simulations of Earth's magnetosphere in the solar wind compute the pathways of plasma circulation through the plasma sheet. We address the pathways that supply and drain the plasma sheet, by coupling single fluid simulations with Global Ion Kinetic simulations of the outer magnetosphere and the Comprehensive Ring Current Model of the inner magnetosphere, including plasmaspheric plasmas. We find that the plasma sheet is supplied with solar wind plasmas via the magnetospheric flanks, and that this supply is most effective for northward IMF. For southward IMF, the innermost plasma sheet and ring current region are directly supplied from the flanks, with an asymmetry of single particle entry favoring the dawn flank. The central plasma sheet (near midnight) is supplied, as expected, from the lobes and polar cusps, but the near-Earth supply consists mainly of slowly moving ionospheric outflows for typical conditions. Work with the recently developed multi-fluid LFM simulation shows transport via plasma "fingers" extending Earthward from the flanks, suggestive of an interchange instability. We investigate this with solar wind ion trajectories, seeking to understand the fingering mechanisms and effects on transport rates.
A new representation is developed for that portion of the magnetospheric magnetic field which is produced by currents on the magnetopause or outside it. For a magnetopause which is a paraboloid of rotation and which confines all field lines of magnetospheric sources, the fields due to magnetopause currents are derived for two sources, a dipole and a two-parameter model of the ring current, each arbitrarily inclined to the earth-sun line. Methods are also developed for accomodating changes in the size of the magnetospheric cavity and for using nonparaboloidal boundaries, open at the nightside. Neutral points and polar cusps are derived, and in agreement with observations it is found that a growth of the ring current shifts them equatorward. By extrapolation from these results, it is argued that the observation of such shifts during magnetic storms may help resolve the relative importance of two processes proposed for magnetic storms.
Assuming that the micron-sized particles making up the bright Jovian ring are fragments of erosive collisions between micrometeoroid projectiles and large parent bodies, a physical model of the ring is calculated. This leads to a well-defined size distribution for the ejecta, whose optical properties can be compared with observation. The (most likely silicate) ejecta material maximum diameter is estimated to be 0.1 micron, and most likely the result of Io volcanic activity. The impact model's determination of ejecta size distribution in turn determines the structure of the ring, with the largest ejecta forming the bright ring, medium-sized ejecta forming a disk that extends to the Jovian atmosphere, and small ejecta forming a faint halo whose structure is dominated by electromagnetic forces.
TWINS is the first mission to perform stereo imaging of the Earth's ring current. The magnetic storm on 22 July 2009 is the largest storm observed since TWINS began routine stereo imaging in June 2008. On 22 July 2009, the Dst dropped to nearly -80nT at 7:00 and 10:00 UT. During the main phase and at the peak of the storm, TWINS 1 and 2 were near apogee and moving from pre-dawn to post-dawn local time. The energetic neutral atom (ENA) imagers on the 2 spacecraft captured the storm intensification and the formation of the partial ring current. The peak of the ENA emissions was seen in the midnight-to-dawn local-time sector. The development of this storm has been simulated using the Comprehensive Ring Current Model (CRCM) to understand and interpret the observed signatures. We perform CRCM runs with constant and time-varying magnetic field. The model calculations are validated by comparing the simulated ENA and ion flux intensities with TWINS ENA images and in-situ ion data from THEMIS satellites. Simulation with static magnetic field produces a strong shielding electric field that skews the ion drift trajectories toward dawn. The model's corresponding peak ENA emissions are always eastward than those in the observed TWINS images. On the other hand, simulation with a dynamic magnetic field gives better spatial agreements with both ENA and insitu particle data, suggesting that temporal variations of the geomagnetic field exert a significant influence upon global ring current ion dynamics.
During intense geomagnetic storms, the magnetopause can move in as far as geosynchronous orbit, leaving the satellites in that orbit out in the magnetosheath. Spacecraft operators turn to numerical models to predict the response of the magnetopause to solar wind conditions, but the predictions of the models are not always accurate. This study investigates four storms with a magnetopause crossing by at least one GOES satellite, using four magnetohydrodynamic models at NASA's Community Coordinated Modeling Center to simulate the events, and analyzes the results to investigate the reasons for errors in the predictions. Two main reasons can explain most of the erroneous predictions. First, the solar wind input to the simulations often contains features measured near the L1 point that did not eventually arrive at Earth; incorrect predictions during such periods are due to the solar wind input rather than to the models themselves. Second, while the models do well when the primary driver of magnetopause motion is a variation in the solar wind density, they tend to overpredict or underpredict the integrated Birkeland currents and their effects during times of strong negative interplanetary magnetic field (IMF) Bz, leading to poorer prediction capability. Coupling the MHD codes to a ring current model, when such a coupling is available, generally will improve the predictions but will not always entirely correct them. More work is needed to fully characterize the response of each code under strong southward IMF conditions as it relates to prediction of magnetopause location.
The main subject of the discussed investigation is the infrared brightness variation of the A, B, and C rings, as a function of solar elevation. Existing models for the B ring are not entirely satisfactory. A description is presented of a model applicable to all three rings. This model and other existing models are compared with the available data. Particular attention is given to the 20-micrometer observations of Saturn's rings. It is found that the brightness variation of the A and B rings with solar elevation angle can be well modeled by an optically thick monolayer of particles, or equivalently, a flat sheet, emitting on one side only. This points to a temperature contrast between the two sides of the ring plane. The infrared brightness of the C ring increases as the solar elevation angle with respect to the ring plane decreases.
TWINS is the first mission to perform stereo imaging of the Earth's ring current. The magnetic storm on 22 July 2009 was at the time the largest storm observed since TWINS began routine stereo imaging in June 2008. On 22 July 2009, the Dst dropped to nearly .80 nT at 0700 and 1000 UT. During the main phase, and at the peak of the storm, TWINS 1 and 2 were near apogee and moving between predawn and postdawn local time. The energetic neutral atom (ENA) imagers on the two spacecraft captured the storm intensification and the formation of the partial ring current. The peak of the high-altitude ENA emissions was seen in the midnight-to-dawn local time sector. The development of this storm has been simulated using the comprehensive ring current model (CRCM) to understand and interpret the observed signatures. We perform CRCM runs with constant and time-varying magnetic field. The model calculations are validated by comparing the simulated ENA and ion flux intensities with TWINS ENA images and in situ ion data from a THEMIS satellite. Simulation with a static magnetic field produces a strong shielding electric field that skews the ion drift trajectories toward dawn. The model's corresponding peak ENA emissions are always more eastward than those in the observed TWINS images. On the other hand, the simulation with a dynamic magnetic field gives better spatial agreement with both ENA and in situ particle data, suggesting that temporal variations of the geomagnetic field exert a significant influence upon global ring current ion dynamics.