Engineering topics

Hall, D. T.

Publications and source records attributed to Hall, D. T..

14 records

NASA NTRS1995

Self-consistent solar wind-LISM interaction including neutrals: A 4-fluid model

We investigate the interaction between the heliosphere and local interstellar medium, including charge-exchange self-consistently between the neutral and ionized components. The system is modelled as consisting of neutral hydrogen, protons and electrons. The electrons are assumed to contribute to the dynamics via a pressure that is everywhere equal to the proton pressure. The fluid equations for the protons and hydrogen are solved self-consistently, assuming the two species interact only via charge exchange. The protons are treated as a single isotropic fluid. The hydrogen is decomposed into three isotropic fluids: hydrogen of interstellar origin with characteristic temperature of approximately 104 K, hydrogen produced via charge exchange in the post-shock solar wind, with characteristic temperature of approximately 105 K, and hydrogen produced via charge exchange in the supersonic solar wind, relatively cool but expanding radially outward with the solar wind speed. Two-Dimensional time-dependent numerical simulations of this system are performed to calculate the number density, temperature and bulk velocity of neutral hydrogen and plasma everywhere in the heliosphere. These quantities are compared to relevant solar wind observations. Implications for the pick-up process in the supersonic solar wind, as well as heating and deceleration of the wind, are addressed.

Williams, L. L.↗

NASA NTRS1995

Multi-dimensional modelling of the solar wind-LISM interaction including neutrals: A Boltzmann equation approach

The importance of interstellar neutrals in understanding and modelling the global interaction of the solar wind with the local interstellar medium is becoming increasingly apparent. Unfortunately the self-consistent inclusion of a neutral interstellar component into time-dependent, dynamical models is formidably difficult due to the extremely large mean free paths associated with the neutrals and the creation of essentially different neutral distributions from different interaction regions of the solar wind and LISM. In full generality, one has to address the problem by treating the neutrals kinetically with the appropriate extinction and creation source terms. In this paper, a limited set of simulations will be presented in which the solar wind and interstellar plasma is described as a 2D fully compressible time-dependent fluid while the interstellar neutral distribution is derived by solving the appropriate Boltzmann equation directly.

Zank, G. P.↗

NASA NTRS1995

New analysis of the Voyager UVS H Lyman-alpha emission of Saturn

The limb to limb Lyman-alpha reflectivities observed with the Voyager ultraviolet spectrometer (UVS) instruments during the fly-by of Saturn are reanalyzed using a revised H Lyman-alpha sensitivity for the Voyager 1 instrument. The new sensitivity reconciles the measured intensities to those of Voyager 2 and gives a coherent set of data. To fit the UV airglow observations, four sources are considered: (1) H resonance and H2 Rayleigh scattering of solar Lyman-alpha radiation, (2) the interplanetary Lyman-alpha radiation, (3) a possible internal source of unknown origin, (4) the possibility of atmospheric turbulence recently proposed to explain the Lyman-alpha bulge of Jupiter. The analysis supports neither a dominant collisional excitation source for the UV emissions nor the presence of strong atmospheric turbulence. The best fit, in terms of brightness but also in terms of shape of the limb to limb profile (that is to say independent on the absolute calibrations), is obtained for pure resonance and Rayleigh scattering of solar and interstellar wind line in an atmosphere enriched in atomic hydrogen up to three times the standard model. Influx of water from the rings of Saturn may provide a means for producing such enhanced H densities in the upper atmosphere.

Jaffel, L. Ben↗

NASA NTRS1994

Observations of interplanetary Lyman-alpha with the Galileo Ultraviolet Spectrometer: Multiple scattering effects at solar maximum

The Galileo Ultravilet Spectrometer Experiment (UVS) obtained a partial celestial sphere map of interplanetary Lyman-alpha (IP L alpha) on 13-14 December 1990 during the first Earth encounter. The Galileo spacecraft was near the downwind axis of the local interstellar medium flow. These UVS measurements sampled the downwind, anti-sunward hemisphere. The data were modeled using a hot model of the interplanetary hydrogen density distribution with the goal of studying multiple scattering effects in the inner solar system. The derived ratio in the downwind direction of the observed brightness and a single scattering model brightness, both normalized to unity in the upwind direction, is 1.82 +/- 0.2. This brightness ratio requires a multiple scattering correction which is 36% larger than can be accounted for by theoretical calculations. The hot model may require: (1) a temperature perturbation of the interstellar wind velocity distribution or (2) an additional downstream source of interplanetary hydrogen. However, a more likely exlanation which affects the hot model is the latitude dependence of the radiation pressure. This dependence, based on the known solar L alpha flux latitude variation at solar maximum, causes a downwind brightness enhancement by preferential focusing of H-atoms with trajectory planes containing the solar poles. This result implies that radiation pressure near the solar poles is nearly independent of solar cycle and is insufficient to lead to a net repulsion of hydrogen atoms by the sun, as can occur near the ecliptic plane during the solar maximum. In addition, the UVS performed 13 observations of IP L alpha while in cruise between Venus and the Earth in 3 directions fixed in ecliptic coordinates.

Ajello, J. M.↗

NASA NTRS1994

Hopkins Ultraviolet Telescope determination of the Io torus electron temperature

Sulfur ion emissions from the Io plasma torus observed by the Hopkins Ultraviolet Telescope (HUT) in 1990 December have been analyzed to determine the effective temperature of the exciting electrons. Spectra were obtained with a long slit that extended from 3.1 to 8.7 Jupiter radii R(sub J) on both dawn and dusk torus ansae. The average temperature of electrons exciting S(2+) emissions from the dawn ansa is (4800 +/- 2400) K lower than on the dusk ansa, a dawn-dusk asymmetry comparable in both sign and magnitude to that measured by the Voyager Ultraviolet Spectrograph (UVS) experiment. Emissions from S(2+) ions are generated in a source region with electron temperatures in the range 32,000-56,000 K; S(3+) ion emissions are excited by electrons that average 20,000-40,000 K hotter. This distinct difference suggests that the S(3+) emission source region is spatially separate from the S(2+) source region. Estimated relative aperture filling factors suggest that the S(3+) emissions originate from a region more extended out of the centrifugal plane than the S(2+) emissions.

Hall, D. T.↗

NASA NTRS1993

Heliospheric hydrogen beyond 15 AU - Evidence for a termination shock

The Voyager and Pioneer 10 spacecraft are moving upstream and downstream into the local interstellar flow, monitoring H Lyman alpha radiation resonantly scattered from heliospheric hydrogen. Voyager Cruise Maneuver observations obtained between 15 and 35 AU reveal that H Lyman alpha intensities in the upstream direction fall as r exp -0.75+/-0.05. Beyond 15 AU downstream, Pioneer 10 intensities fall as r exp -1.07+/-0.1. These trends cannot be simultaneously reproduced using a hot H distribution model that does not include termination shock structure. The Voyager H Lyman alpha intensities also show a distinctive trend to decrease less rapidly with increasing heliocentric distance. Between 15 and 20 AU, Voyager intensities fall as r exp -1, whereas between 30 and 35 AU they fall as r exp -0.35. This flattening trend implies that the upstream H density is increasing rapidly with heliocentric distance beyond about 25 AU. This steepening trend is significant because similar H density gradients are predicted in models which include the effects of the termination shock. Taken together, the Voyager and Pioneer 10 H Lyman alpha observations beyond 15 AU imply the existence of a solar wind termination shock, suggesting that it lies between 70 and 105 AU in the upstream direction.

Hall, D. T.↗

NASA NTRS1993

(abstract) Interplanetary Lyman-alpha Observations with the Galileo Ultraviolet Spectrometer: Solar Wind Latitude Variations and Multiple Scattering at Solar Maximum

The Galileo Ultraviolet Spectrometer obtained a Lyman-alpha celestrial sphere map on 13,14 December 1990 near the first Earth encounter, with spacecraft near the interstellar wind downwind axis. The data show solar flux longitudinal and latitudinal asymmetries which are modeled with He 10830 Ssolar images. The difference between the observed brightness and a single scattering model is attributed to multiple scattering effects, which we also calculate. The data constrain the solar wind flux latitude variation at solar maximum. Other 1990-1992 Galileo Lyman-alpha data will be discussed.

solar wind ultraviolet spectrometer solar flux cel↗

NASA NTRS1993

Detection of the hydroxyl radical in the Saturn magnetosphere

The detection of neutral OH molecules near the orbit of Tethys is reported. The findings suggest that neutral OH is one of the dominant species in Saturn's inner magnetosphere, implying a source rate for H2O 20 times greater than current theoretical estimates. One possible explanation is that the micrometeorite erosion rates of the inner satellites are significantly higher than expected.

Shemansky, D. E.↗

NASA NTRS1992

A reanalysis of Voyager UVS observations of Titan

Data from the Voyager 1 Ultraviolet Spectrometer (UVS) encounter at Titan were reanalyzed to reveal new spatial and spectral information. The observations, data reduction and spectrum analysis are discussed. The following conclusions were drawn: Titan's airglow is driven by a combination of mechanisms including both EUV deposition and magnetospheric particle precipitation; the emissions are distributed throughout a range of altitudes in the atmosphere; enhanced nightside emissions of the Helium 58.4 nm line may be the result of a thermospheric diurnal temperature variation; the CD2 spectrum, with enhanced Ly(alpha) and N-2 64.5 and 67.0 nm emission, implies that some of the high altitude excitation is due to impact dissociative excitation of N2 by electrons with E greater than 150 eV; the CD2 spectrum is consistent with the existence of a high altitude bright spot in the dayside disk with dimensions from 400 to 900 km.

Hall, D. T.↗

NASA NTRS1992

The distribution of atomic hydrogen in the magnetosphere of Saturn

Three sets of previously unpublished Voyager ultraviolet spectrometer observations of the Saturn system conducted by Voyager 1 and 2 are presented. Voyager 1 observations during the postencounter period provided a map of the distribution looking down on the equatorial plane. The reduced data show a nonuniform distribution in local time with a preponderance of emission on the duskside. The emission extends radially inward to the top of the Saturn atmosphere with stronger signals appearing close to the planet, strongly suggesting that the principal source is the sunlit Saturn atmosphere. In the subsolar sector of the magnetosphere no excess emission in the vicinity of Titan's orbit is detectable. Voyager 1 and 2 preencounter observations also show H Ly-alpha emissions increasing monotonically toward the planet but with a distinctive dawnside excess. It is concluded that the preliminary results reporting a toroidal hydrogen distribution with a cavity inside 8 RS are invalid because of limited spatial resolution and poor statistics.

Shemansky, D. E.↗

NASA NTRS1988

No cometesimals in the inner solar system

Ultraviolet measurements made by Voyager 2, apparently showing a rapid decrease in hydrogen Lyman-alpha emission with distance from the sun, were taken by Donahue et al. (1987) as evidence for a source of atomic hydrogen in the very local interstellar medium (VLISM). The suggested source of the hydrogen is a class of small comets at solar distances of about 1 AU. This claim has been adduced as evidence for a theory that the earth is subject to a large influx of cometary material significantly affecting atmospheric evolution. The Voyager 2 data have been reanalyzed, and it is shown that no source of hydrogen in the VLISM is required other than the inflow of neutral atoms.

Hall, D. T.↗

NASA NTRS1988

Large time scale variation in hydrogen emission from Jupiter and Saturn

The IUE and Voyager spacecraft observations of Jupiter and Saturn were combined to obtain a consistent measurement of temporal variation of the equatorial subsolar hydrogen emission. The outer planets appear to have rather independent behavior over time scales of the order of 10 yr, particularly in emission from the H Ly alpha line. The time interval from 1978 to the present shows variation of mean equatorial H Ly alpha brightness of 2 at Jupiter and 5 at Saturn. The relative magnitudes of the variations is sufficiently different to suggest that response to input from the Sun is at least nonlinear. The brightness of H2 band emission appears to be relatively more stable than H Ly alpha. There is evidence in IUE observations of a moderate increase in H2 band brightness with increasing time at Jupiter, in opposition to the variation in H Ly alpha.

Shemansky, D. E.↗

NASA NTRS1985

Electron impact excitation of H2 - Rydberg band systems and the benchmark dissociative cross section for H Lyman-alpha

The cross sections sigma R 1 (2p) for excitation of H Ly-alpha emission produced by electron impact on H2 is reexamined. A more accurate estimate for sigma R 1 (2p) is obtained based on Born approximation estimates of the H2 Rydberg system cross sections using measured relative excitation functions. The obtained value is (8.18 + or -1.2) x 10 to the -18th sq cm at 100 eV, a factor of 0.69 below the value universally applied to cross section measurements over the past decade. Cross sections for the H2 Rydberg systems fixed in magnitude by the Born approximation have also been obtained using experimentally determined excitation functions. Accurate analytic expressions for these cross sections allow the direct calculation of rate coefficients.

Shemansky, D. E.↗

NASA NTRS1985

Vacuum ultraviolet studies of electron impact of helium Excitation of He n1P0 Rydberg series and ionization-excitation of He(+) nl Rydberg series

The cross sections for the He I 1s2 1S-1snp 1P0 series have been measured using a relative flow method, with the absolute scale fixed by the H Ly-alpha dissociative excitation cross section standard. The results are compared with those obtained using a relative cross section data analysis by modified Born approximation, and good agreement is found. Cross sections for the ionization-excitation of the He II 121.51 nm and He II 164.04 nm transmissions have been measured, and the results strongly suggest that theoretical calculations of the reactions differ fundamentally from physical reality. The failure of the theory to describe experimental results stems from the neglect in the theory of electron correlation effects between the two orbital electrons.

Shemansky, D. E.↗