Engineering topics

Hood, L. L.

Publications and source records attributed to Hood, L. L..

At least 37 records · Page 2

NASA NTRS1991

Mesospheric effects of solar ultraviolet variations - Further analysis of SME IR ozone and Nimbus 7 SAMS temperature data

In order to improve the constraints on models of the mesospheric response to solar UV variations, an analysis is conducted of the Solar Mesosphere Explorer (SME) IR ozone data and Nimbus 7 stratosphere and mesosphere sounder (SAMS) temperature data. Maximum low-altitude ozone and temperature-response amplitudes occur at about the same altitude, where a strong coupling between photochemical and thermal components of the mesospheric response is suggested by the simultaneous positive temperature and negative ozone response maxima. Increased Lyman-alpha dissociation of water vapor and temperature feedback are theorized to account for the negative ozone response. HO(x) chemical heating can increase as ozone destruction increases, and can therefore account for the positive temperature response.

Hood, L. L.↗

NASA NTRS1991

Magnetic effects of large-scale impacts on airless planetary bodies

The analysis of lunar orbital and sample data combined with laboratory measurements of impact-produced plasmas suggest that large-scale impacts on planetary surfaces may have had significant magnetic effects. These effects may potentially explain part of all lunar crustal magnetization and, by extension, may be responsible for producing paleomagnetism on other airless silicate bodies in the solar system. Theoretical studies are presented of the magnetic field and remanent magnetization effects of basin-scale impacts on the Moon. The specific case of a Moon exposed to the solar wind plasma flow and its embedded magnetic field is investigated. It is shown that maximum compressed field amplitudes occur antipodal to the impact point in agreement with the observed tendency for orbital magnetic anomalies to be concentrated antipodal to young large lunar basins. Generalization of these results to include magnetic effects of impacts on other airless or nearly airless bodies in the solar system is presented.

Hood, L. L.↗

NASA NTRS1991

Stratospheric dynamical effects of solar ultraviolet variations - Evidence from zonal mean ozone and temperature data

Cross-spectral analysis of low-latitude average Nimbus 7 stratospheric and mesospheric sounder (SAMS) temperature deviations versus Nimbus 7 solar backscattered ultraviolet (SBUV) solar 205 nm flux measurements in the 1-3 mbar pressure range yields significant coherence at periods near 27 and 13 days. This supports earlier correlative evidence that the ozone response to solar ultraviolet variations is supplemented by a coupled temperature response in the upper stratosphere. Comparisons of improved one-dimensional radiative photochemical model calculations with ozone and temperature response measurements at low latitudes yields agreement only in the case of ozone at levels below 3 mbar. An additional, presumably dynamical, component of the upper stratospheric response is suggested. Cross-spectral analysis of low-latitude average SAMS temperature deviations versus higher-latitude temperature fluctuations of opposite sign in the winter hemisphere also yields significant coherency at periods near 27 and 13 days. Latitudinal temperature oscillations of this type result from interference between stationary and traveling wave components at stratospheric heights. It is therefore hypothesized that solar ultraviolet variations may force or modulate traveling waves with periods near 27 and 13 days in the upper stratosphere. The existence of such waves with higher amplitude near solar maximum than near solar minimum may help to explain observed solar cycle variations of zonally averaged winds and temperature in the upper stratosphere.

Hood, L. L.↗

NASA NTRS1991

The relict magnetism of meteorites

Meteorite paleomagnetic studies are reviewed and evaluated. Possible mechanisms for producing early solar system magnetic fields that could have been responsible for the magnetization of some or all meteorite classes are explored. A detailed review of paleointensity results derived from the various classes of meteorites is given, and the likelihood that the results might related to solar or nebulawide magnetic fields is considered. The implications of meteorite magnetism for early solar system evolution are discussed.

Cisowski, S. M.↗

NASA NTRS1990

A Probable Approx. 2400 Year Solar Quasi-cycle in Atmospheric Delta C-14

A 2200 to 2600 year quasi-periodicity is present in atmospheric delta C-14 records after removal of long-term trends due to the geomagnetic dipole amplitude variation. This periodicity consists of both a long-term variation of the mean and a superposed, approximately recurring pattern of century-scale variations. The strongest of these latter variations occur near maxima of the approx. 2400 year delta C-14 cycles. The residual record can be modeled to first order as an amplitude modulation of a century-scale periodic forcing function by a approx. 2400 year periodic forcing function. During the last millennium, the largest century-scale variations (occurring near the most recent 2400 year delta C-14 maximum) are known to be mainly a consequence of the pronounced Maunder, Sporer, and Wolf solar activity minima, as verified by independent proxy solar activity records. Therefore, during this period, amplitude modulation has been occurring primarily in the sun and not in the terrestrial radiocarbon system. It is therefore inferred that the approx. 2400 year forcing function is mainly solar although some secondary terrestrial feedback into the delta C-14 record is likely. This conclusion has implications for the predictability of future pronounced solar activity minima and for the interpretation of certain minor Holocene climatic variations.

Hood, L. L.↗

NASA NTRS1990

A Mechanism Involving Solar Ultraviolet Variations for Modulating the Interannual Climatology of the Middle Atmosphere

In years of low solar activity, free traveling wave modes in the upper stratosphere are dominated by atmospheric normal modes such as the 16-day wave. However, within a 4-year interval centered on the 1980 to 1981 solar maximum, cross-spectral analyses of zonal mean satellite temperature data versus the solar UV flux demonstrate significant power near 27 and 13 days, providing indirect evidence that short-term UV variations were capable of exciting traveling planetary-scale waves in the upper stratosphere. Previous theoretical and observational work has indicated that interference between traveling waves and stationary waves forced from below (and the resulting oscillating latitudinal heat transports) plays a likely role in the initiation of stratospheric warmings. Researchers therefore hypothesize that the initiation of a major stratospheric warming in the upper stratosphere and lower mesosphere may depend to some extent on the amplitude of longer-period 27-day traveling waves in the upper stratosphere. This would represent a new mechanism for solar UV effects on stratospheric climatology that may be relevant to the interpretation of some recent long-term correlative results.

Hood, L. L.↗

NASA NTRS1989

Radial diffusion in the Uranian radiatian belts - Inferences from satellite absorption loss models

Low-energy charged particle (LECP) phase space density profiles available from the Voyager/1986 Uranus encounter are analyzed, using solutions of the time-averaged radial diffusion equation for charged particle transport in a dipolar planetary magnetic field. Profiles for lower-energy protons and electrons are first analyzed to infer radial diffusion rate as a function of L, assuming that satellite absorption is the dominant loss process and local sources for these particles are negligible. Satellite macrosignatures present in the experimentally derived profiles are approximately reproduced in several cases, lending credence to the loss model and indicating that magnetospheric distributed losses are not as rapid as satellite absorption near the minimum satellite L shells for the particles. Diffusion rates and L dependences are found to be similar to those previously inferred in the inner Jovian magnetosphere (Thomsen et al., 1977) and for the inner Saturnian magnetosphere (Hood, 1985). Profiles for higher energy electrons and protons are also analyzed using solutions that allow for the existence of significant particle sources as well as sinks. Possible implications for radial diffusion mechanisms in the Uranian radiation belts are discussed.

Hood, L. L.↗

NASA NTRS1989

Stratospheric effects of solar ultraviolet variations on the solar rotation time scale

A summary is presented of some current work on measurement and interpretation of stratospheric ozone and temperature responses to observed short term solar ultraviolet variations. Although some studies have yielded provisional evidence for a nearly in-phase ozone-solar cycle relationship, they extend at most over only one or two 11 year cycles so the statistical significance of the correlations is not large. Similarly, the relatively short lengths of individual satellite data sets combined with the problem of estimating the effect of changes in instrument sensitivity (drift) during the observing period have complicated attempts to infer long term or solar cycle ozone trends. The solar rotation and active region development time scale provides an alternate time scale for which detailed studies of middle atmospheric ozone and temperature responses to solar ultraviolet variability are currently possible using available satellite data sets. At tropical latitudes where planetary wave amplitudes are relatively small, clear correlative evidence for the existence of middle atmospheric ozone and temperature responses to short term solar ultraviolet variations has been obtained in recent years. These measurements will ultimately allow improved empirical and theoretical calculations of longer term solar induced ozone and temperature variations at low and middle latitudes.

Hood, L. L.↗

NASA NTRS1989

Radial diffusion and losses of energetic protons in the 5 to 12 R(S) region of Saturn's magnetosphere

Phase space density profiles for nearly equatorial mirroring energetic protons, derived from Pioneer 11 measurements during the 1979 Saturn encounter, were analyzed using solutions of the time-averaged radial diffusion equation in a dipolar planetary magnetic field. Then, several loss models were considered, ranging from a model that takes into account satellite absorption only to the one that accounts for satellite plus ring-E absorption plus added distributed losses, and the corresponding form of the time-averaged radial diffusion coefficient was determined for each case by a minimum-variance fit to the data-derived phase space densiy profiles. The model comparisons clearly indicated the need for distributed losses, in addition to satellite absorption, in order to explain the smooth decline of the experimental energetic proton phase space density profiles in the 5-12-Saturn radius region of Saturn's magnetosphere.

Hood, L. L.↗

NASA NTRS1989

The lunar swirls - Distribution and possible origins

Correlative data analysis and theoretical modeling methods are used to evaluate models involving both meteoroid and cometary impact processes which can explain the correlation in location of the strongest lunar magnetic anomalies with swirl-like high-albedo and low-albedo markings of the Reiner Gamma class. The results indicate that the relatively strong magnetization of basin antipode zones may have favored the formation of swirls in the same regions. The solar wind deflection model is tested by calculating the trajectories of simulated solar wind ions deflected by the Lorentz force in the presence of model crustal magnetic fields.

Hood, L. L.↗

NASA NTRS1989

Investigation of the Saturn dust environment from the analysis of energetic charged particle measurements

In order to assist the Cassini project in evaluating risks of collisions with particulate matter in the rotational equatorial plane, available Pioneer 11 and Voyager energetic charged particle data were reinvestigated to constrain the column mass density of absorbing material within several radial ranges. Within the orbit of Mimas, CRAND proton phase space densities maximize near 2.67 R sub s and exhibit secondary maximum at 2.43 R sub s. From the condition that sources must exceed losses near these maxima and using available theoretical models for CRAND proton production rates, upper bounds on the column mass density at these two radial locations are calculated. Detailed fits of radial diffusion models to the observed flux maxima yield somewhat more restrictive upper limits. The upper limits compare to a lower limit on the column mass density, estimated from previous model calculations by Van Allen. Aside from continuous rings, longitudinally limited, low optical depth clouds of particulates may exist in orbit with several of the inner satellites including Mimas and Enceladus. A brief review of Voyager energetic particle microsignatures that suggest the presence of material co-orbiting with these two satellites is presented. Finally, Pioneer 11 and Voyager measurements of low energy electron fluxes exhibit minima near the location of the tenuous E Ring centered on approx. 4 R sub s. Pioneer 11 pitch angle distributions appear to support the possibility that direct absorption by Ring E particulates produced the observed flux decreases.

Hood, L. L.↗

NASA NTRS1988

Stratospheric ozone and temperature responses to short-term solar ultraviolet variations - Reproducibility of low-latitude response measurements

Two independent 22-month time intervals of NIMBUS 7 solar backscattered UV (SBUV) ozone and stratospheric and mesospheric sounder (SAMS) temperature measurements for the upper stratosphere at low latitudes are analyzed to calculate mean responses to observed changes in solar ultraviolet spectral irradiance occurring on the time scale of the solar rotation period. Average cross-correlation functions of both SBUV ozone and SAMS temperature versus the solar 205 nm flux are in substantial agreement for these two intervals. Linear regression methods are applied to estimate response amplitudes or sensitivities. The derived sensitivities and phase lags relative to the 205 nm flux are also in approximate agreement for the two separate intervals although the temperature response measurements exhibit larger deviations. These results support the validity of previously reported measurements on the 27-day time scale, and impose firmer constraints on proposed theoretical models for the response of the stratosphere to solar UV forcing on both short and long time scales.

Hood, L. L.↗

NASA NTRS1988

Lunar surface magnetic field concentrations antipodal to young large impact basins

Planetary electron reflection magnetometry data of the Apollo 15 and 16 subsatellites indicate the presence of major regions of strong surface magnetic fields near the antipodes of four large, young, ringed impact basins. While mechanisms yielding surface magnetic field concentrations antipodal to impact basins are not yet clear, the present results' indicated period of strong lunar magnetic fields, at between about 3.85 and 3.6 aeons, is consistent with lunar paleomagnetic sample data. The origin of the strong surface magnetic fields is discussed.

Lin, R. P.↗

NASA NTRS1988

Stratospheric responses to solar ultraviolet variations - Comparisons with photochemical models

At latitudes and altitudes where transport effects on ozone mixing ratio can be neglected, ozone and temperature responses to short-term changes in solar ultraviolet flux can be used to calculate implied bounds on two basic photochemical parameters: the odd oxygen photochemical relaxation time and the chemical sensitivity of odd oxygen to local temperature changes. Seasonally averaged values of these parameters are derived from response measurements and are compared with model calculations between 0.7 and 4 mbar at tropical latitudes. Results are in agreement within measurement and model uncertainties at all pressure levels. Model values of mean ozone and of the response of ozone to perturbations are functions of the same photochemical data. Therefore this agreement provides a constraint which should be considered when changes are proposed in model input data for the purpose of improving agreement between measured and calculated high-altitude ozone.

Hood, L. L.↗

NASA NTRS1987

Magnetic field and remanent magnetization effects of basin-forming impacts on the moon

Maps of the distribution of lunar surface magnetic fields produced by the electron reflection method have shown that the largest observed concentrations of lunar crustal magnetization occur antipodal (diametrically opposite) to four relatively young large impact basins: Imbrium, Orientale, Serenitatis, and Crisium. A model is proposed here for the formation of these magnetization concentrations (or 'magcons') in which the partially ionized vapor cloud produced in a hypervelocity (greater than 10 km/s) basin-forming impact expands around the moon forcing a preexisting ambient magnetic field to be concentrated for a brief (less than 1 day) time period in the antipodal zone. Acquisition of magnetic remanence during the period of compressed field amplification may occur by one of several mechanisms, such as shock remanence by impact of solid secondaries ejected from the basin-forming event. The model implies that basin-forming impacts have played a major role in determining the large-scale distribution of crustal magnetization detectable from lunar orbit.

Hood, L. L.↗

NASA NTRS1987

Magnetospheric environments of outer planet rings - Influence of Saturn's axially symmetric magnetic field

Saturn's main rings exist within a zone of negligible magnetospheric losses and surface alteration effects, substantially due to the solid-body absorption of inwardly diffusing magnetospheric particles. This process is presently shown to be especially efficient in the inner magnetosphere of Saturn, due to the near-axial symmetry of the planetary magnetic field relative to the equatorial rotation plane; under the assumption of comparable diffusion rates, the inward magnetospheric particle transport is far more inhibited in the inner Saturnian magnetosphere than in the same regions of Jupiter and Uranus, even when only rings of comparable widths and depths are considered. In light of this, ring particle surface exposure to the ion fluxes of the radiation belt remains a prepossessing rationale for low Uranian ring albedos.

Hood, L. L.↗

NASA NTRS1987

Lunar magnetization concentrations (MAGCONS) antipodal to young large impact basins

Electron reflection measurements from Apollo 15 and 16 subsatellites show that patches of strong surface magnetic fields ranging in size from less than about 7 km to greater than 500 km are distributed over the surface of the Moon. With the exception of a few regions, no obvious association to surface geology has been found. Researchers examined the antipodes of 23 winged impact basins for which electron reflection measurements are available. It was concluded that the apparent temporal variations for the basin antipodes may reflect real variations in the lunar magnetic field.

Lin, R. P.↗

NASA NTRS1987

Geophysical constraints on lunar bulk composition and structure - A reassessment

Theoretical lunar mantle seismic velocity and density profiles were calculated for a series of specific bulk composition models, thermal models, and evolutionary models. It was found that most of the calculated seismic profiles are in qualitative agreement with the Nakamura (1983) observational mantle seismic velocity model. With respect to the density modeling results, the most interesting finding is that none of the considered models yield mantle density increases that are sufficient to match the adapted upper bound on the lunar moment-of-inertia factor (0.3928). The results suggest that the moon possesses a dense metallic core with a mass in the range of 1 to 4 percent of the lunar mass and that the alumina-rich models of Morgan et al. (1978) and Taylor (1982) are best able to match the middle mantle velocity increase that characterizes the Nakamura model.

Hood, L. L.↗