Evidence for a helium flux in the lower thermosphere.
Helium flux in lower thermosphere, using diffusion theory for multicomponent gas mixture to determine equation for equilibrium distribution
Kasprzak, W. T.↗
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Helium diffusion in the sun
We calculate improved standard solar models using the new Livermore (OPAL) opacity tables, an accurate (exportable) nuclear energy generation routine which takes account of recent measurements and analyses, and the recent Anders-Grevesse determination of heavy element abundances. We also evaluate directly the effect of the diffusion of helium with respect to hydrogen on the calculated neutrino fluxes, on the primordial solar helium abundance, and on the depth of the convective zone. Helium diffusion increases the predicted event rates by about 0.8 SNU, or 11 percent of the total rate, in the chlorine solar neutrino experiment, by about 3.5 SNU, or 3 percent, in the gallium solar neutrino experiments, and by about 12 percent in the Kamiokande and SNO solar neutrino experiments. The best standard solar model including helium diffusion and the most accurate nuclear parameters, element abundances, and radiative opacity predicts a value of 8.0 SNU +/- 3.0 SNU for the C1-37 experiment and 132 +21/-17 SNU for the Ga - 71 experiment, where the uncertainties include 3 sigma errors for all measured input parameters.
Bahcall, J. N.↗
On the origin of the MeV energy nucleon flux associated with CIRs
Measurements of recurrent enhancements of interplanetary nucleon flux in the MeV energy range are presented and interpreted. The features recur at the solar rotation period in association with stream-stream plasma interaction regions corotating with the sun. At distances from the sun less than 1 AU, the maximum intensities of the hydrogen and helium components increase with increasing distance between 300%/AU and 600%/AU. A model is proposed which predicts the acceleration of nuclei from keV plasma energies to MeV energies by means of transit time damping of magnetosonic waves as solar wind plasma flows from the sun. Numerical solutions of the transport equation are derived to demonstrate that the model does not reproduce radial variations of hydrogen and helium fluxes, and estimates are made of the radial diffusive mean free path. Finally, the observations are found to be consistent with nucleon acceleration at the CIR shocks beyond 1 AU with subsequent diffusion toward the sun.
Christon, S. P.↗
Radiation considerations for interplanetary missions
Galactic cosmic radiation (GCR) poses a serious radiation hazard for long-duration missions. In designing a lunar habitat or Mars transfer vehicle, the worst-case radiation exposure determines shielding thickness and, hence, the weight of spacecraft. Using the spherically symmetric diffusion theory of the solar modulation of GCR, it was possible to use data on the differential energy spectra of hydrogen, helium, oxygen, and iron from 1954 to 1989 to show that the flux at 1 A.U. is determined by the diffusion parameter, K, which is a function of the time in the solar cycle. This analysis also showed that the solar minimum of 1976 to 1977 was the deepest minimum in the last 37 years. Using this theory, we have obtained the GCR spectra for all the nuclei and calculated the depth-dose as a function of aluminum shield thickness. Using the ICRP-26 definition of the quality factor, it is shown that the shielding required to stay below the LEO recommended annual limit of 50 cSv is 17.5 (+8, -3), g/sq cm of aluminum; if the limit is raised to 60 cSv, the required shielding is 9 (5, -1.5) g/sq cm. We also discuss the issues and shielding needs for protection against solar particle events.
Badwar, Gautam D.↗
Helioseismic Constraints on New Solar Models from the MoSEC Code
Evolutionary solar models are computed using a new stellar evolution code, MOSEC (Modular Stellar Evolution Code). This code has been designed with carefully controlled truncation errors in order to achieve a precision which reflects the increasingly accurate determination of solar interior structure by helioseismology. A series of models is constructed to investigate the effects of the choice of equation of state (OPAL or MHD-E, the latter being a version of the MHD equation of state recalculated by the author), the inclusion of helium and heavy-element settling and diffusion, and the inclusion of a simple model of mixing associated with the solar tachocline. The neutrino flux predictions are discussed, while the sound speed of the computed models is compared to that of the sun via the latest inversion of SOI-NMI p-mode frequency data. The comparison between models calculated with the OPAL and MHD-E equations of state is particularly interesting because the MHD-E equation of state includes relativistic effects for the electrons, whereas neither MHD nor OPAL do. This has a significant effect on the sound speed of the computed model, worsening the agreement with the solar sound speed. Using the OPAL equation of state and including the settling and diffusion of helium and heavy elements produces agreement in sound speed with the helioseismic results to within about +.-0.2%; the inclusion of mixing slightly improves the agreement.
Elliott, J. R.↗
Energization and transport of ions of ionospheric origin in the terrestrial magnetosphere
The work of this grant has been predominantly focused on ion outflows from two data sets: Prognoz 7 and Dynamics Explorer. The Prognoz analysis studied ion densities, temperatures, and flow velocities in the magnetotail. The work performed under this contract consisted of developing a program to load the raw data, computing the background subtraction of a strong sun pulse, and using the net count to calculate the low order moments of the distribution function. The study confirms the results of ISEE with regard to the supply of plasma from the cusp as a major source of plasmasheet plasma and goes beyond this to discuss the use of ion velocities as a way to examine the motions of the magnetotail. The abstract of the work to be reported is included as an appendix. The work on the DE/Retarding Ion Mass Spectrometer is separated into two categories: (1) classification of low-energy ion flows from high-latitudes, and (2) studies of the polar wind. Major publications resulting from this work are also included as an appendix to this report. The polar wind is in a category by itself as a result of the thermal escape of hydrogen and helium because of ambipolar diffusion through the heavier, oxygen-dominated topside ionosphere. The analysis of the polar wind reports the flux variability as a function of season, magnetic activity, etc. Much effort has been expended under this grant to complete a follow on study of the thermal structure of the polar wind. Extensive display tools and analysis software have been developed and used in an attempt to carry out this thermal analysis. The present work uses a constrained fit scheme that combines the ion densities and flow velocities derived from Chandler et al. and a spacecraft potential derived from an empirical relation to the total ion density to determine the remaining fit parameter, the ion temperature, via a least squares fit to the RIMS data.
Waite, J. Hunter JR.↗
The submillimeter spectrum of the cosmic background radiation
The diffuse brightness of the sky has been measured in six submillimeter passbands, using a rocket-borne, liquid helium-cooled, absolute radiometer. The flux measured at 1160 microns is in good agreement with the average of longer wavelength measurements of the temperature of the cosmic background radiation. The fluxes measured at 709 microns and 481 microns show a rapid decrease toward shorter wavelength, but correspond to significantly higher temperatures. No local source of this excess flux has been identified. The spectrum of the excess significantly constrains cosmological models. Data at 262, 137, and 102 microns are consistent with emission from interstellar dust.
Matsumoto, T.↗
Improved solar models constructed with a formulation of convection for stellar structure and evolution calculations without the mixing-length theory approximations
We have updated a previous attempt to incorporate within a solar model a treatment of convection based upon numerical simulations of convection rather than mixing-length theory (MLT). We have modified our formulation of convection for a better treatment of the kinetic energy flux. Our solar model has been updated to include a complete range of OPAL opacities, the Debye-Hueckel correction to the equation of state, helium diffusion due to gravitational settling, and atmospheres by Kurucz. We construct a series of models using both MLT and our revised formulation of convection and the compared results to measurements of the solar radius, the solar luminosity, and the depth of the solar convection zone as inferred from helioseismology. We find X(solar) = 0.702 +/- 0.005, Y(solar) = 0.278 +/- 0.005, and Z(solar) = 0.0193 +/- 0.0005.
Lydon, Thomas J.↗
Wind enhanced planetary escape: Collisional modifications
The problem of thermal escape is considered in which both the effects of thermospheric winds at the exobase and collisions below the exobase are included in a Monte Carlo calculation. The collisions are included by means of a collisional relaxation layer of a background gas which models the transition region between the exosphere and the thermosphere. The wind effects are considered in the limiting cases of vertical and horizontal flows. Two species are considered: terrestrial hydrogen and terrestrial helium. In the cases of terrestrial hydrogen the escape fluxes were found to be strongly filtered or throttled by collisions at high exospheric temperatures. The model is applied to molecular hydrogen diffusing through a methane relaxation layer under conditions possible on Titan. The results are similar to the case of terrestrial hydrogen with wind enhanced escape being strongly suppressed by collisions. It is concluded that wind enhanced escape is not an important process on Titan.
Curtis, S. A.↗
Non-thermal hydrogen in the Venus exosphere - The ionospheric source and the hydrogen budget
A nonthermal, or 'hot', Venus corona of H atoms has been observed by Mariners 5 and 10 and Venera 9. Of the sources investigated, reaction of H2 with ionospheric O(+) is still the strongest. It can explain the smaller densities but falls somewhat short of the largest (from Mariner 5). The subsequent recombination of OH(+), supplemented by solar-wind processes, may give an escape flux of 10 million atoms per sq cm/s. The low density of thermal H atoms on the day side has previously been attributed to either a large eddy diffusion coefficient or an escape flux tenfold greater than this. An alternative mechanism, suggested by Hartle and Mayr (1977) is supported: the hydrogen is swept to the night side by strong thermospheric winds. This process is analogous to the 'Johnson pump' for the terrestrial winter helium bulge. Large nightside bulges of H and H2 are predicted; the night/day density ratio is estimated to be as large as 100 for each.
Kumar, S.↗
A Computational Fluid Dynamic and Heat Transfer Model for Gaseous Core and Gas Cooled Space Power and Propulsion Reactors
A computational model based on the axisymmetric, thin-layer Navier-Stokes equations is developed to predict the convective, radiation and conductive heat transfer in high temperature space nuclear reactors. An implicit-explicit, finite volume, MacCormack method in conjunction with the Gauss-Seidel line iteration procedure is utilized to solve the thermal and fluid governing equations. Simulation of coolant and propellant flows in these reactors involves the subsonic and supersonic flows of hydrogen, helium and uranium tetrafluoride under variable boundary conditions. An enthalpy-rebalancing scheme is developed and implemented to enhance and accelerate the rate of convergence when a wall heat flux boundary condition is used. The model also incorporated the Baldwin and Lomax two-layer algebraic turbulence scheme for the calculation of the turbulent kinetic energy and eddy diffusivity of energy. The Rosseland diffusion approximation is used to simulate the radiative energy transfer in the optically thick environment of gas core reactors. The computational model is benchmarked with experimental data on flow separation angle and drag force acting on a suspended sphere in a cylindrical tube. The heat transfer is validated by comparing the computed results with the standard heat transfer correlations predictions. The model is used to simulate flow and heat transfer under a variety of design conditions. The effect of internal heat generation on the heat transfer in the gas core reactors is examined for a variety of power densities, 100 W/cc, 500 W/cc and 1000 W/cc. The maximum temperature, corresponding with the heat generation rates, are 2150 K, 2750 K and 3550 K, respectively. This analysis shows that the maximum temperature is strongly dependent on the value of heat generation rate. It also indicates that a heat generation rate higher than 1000 W/cc is necessary to maintain the gas temperature at about 3500 K, which is typical design temperature required to achieve high efficiency in the gas core reactors. The model is also used to predict the convective and radiation heat fluxes for the gas core reactors. The maximum value of heat flux occurs at the exit of the reactor core. Radiation heat flux increases with higher wall temperature. This behavior is due to the fact that the radiative heat flux is strongly dependent on wall temperature. This study also found that at temperature close to 3500 K the radiative heat flux is comparable with the convective heat flux in a uranium fluoride failed gas core reactor.
Anghaie, S.↗
The 1973-1984 Solar Modulation of Cosmic Ray Nuclei
As a continuation of the program of solar modulation studies, new measurements were carried out with the cosmic ray telescope on the Earth satellite IMP-8, of the intensity time variations and the energy spectra of galactic cosmic ray protons, helium, carbon and oxygen from 1980 through 1984 including the recent solar maximum. In order to test the applicability of a steady state model of solar modulation during a period which includes times of rapidly changing modulation, these fluxes were equated with the predictions of a conventional model of solar modulation which assumes equilibrium between modulation mechanisms. It is found that for a reasonable range of variations of the diffusion coefficient the model predictions can be made to agree with the measurements at essentially all times during the studied period. The model can account also for the observed hysteresis effects between cosmic rays of different rigidities.
Garcia-Munoz, M.↗
Energetic heavy ions with nuclear charge Z greater than or equal to 4 in the equatorial radiation belts of the earth - Magnetic storms
Direct in situ observations of trapped energetic heavy ions with nuclear charge Z greater than or equal to 4 at energies in the lower MeV range made with Explorer 45 during the period June-December 1972 are presented. It is noted that all measurements were carried out in the vicinity of the geomagnetic equatorial plane and that the data show the varying effects of four major magnetic storm periods. Orders of magnitude increases in the trapped heavy ion population are seen deep within the radiation belts following the August 1972 solar flare and magnetic storm events. Fluxes of the Z greater than or equal to 4 ions are found to decay faster than those of helium ions of comparable energies; typical decay times for these ions are found to be 24-40 days at L less than or equal to 4 and shorter at higher L shells. The observations are compared with the expected post-injection long-term behavior of atomic oxygen ions deduced from charge exhange, radial diffusive transport, and Coulomb collisions. Good agreement is found between theory and observations.
Spjeldvik, W. N.↗
Solar wind sputtering effects in the atmospheres of Mars and Venus
It is found through an investigation, combining Monte Carlo simulations and analytical techniques, of the direct collisional interaction of an energetic particle flux with the neutral components of a planetary atmosphere, that solar wind sputtering could provide an important exospheric mass sink on both Mars and Venus under appropriate conditions. The computed rates of helium loss in the Venusian atmosphere and of carbon, nitrogen and oxygen in the Martian atmosphere imply that sputtering would have a significant effect on the noble gas budget of Venus and dominate the chemical and photochemical loss processes of Mars. Because of diffusive separation of lighter elements and isotopes, and because the gravitational binding energy is proportional to the mass, the erosion process preferentially removes the lighter components of the atmosphere. Solar wind sputtering could therefore compete with other erosion mechanisms in generating substantial fractionation effects.
Watson, C. C.↗
Experimental investigation of contamination prevention techniques to cryogenic surfaces on board orbiting spacecraft
Within the simulation limitations of on-orbit conditions, it was demonstrated that a helium purge system could be an effective method for reducing the incoming flux of contaminant species. Although a generalized purge system was employed in conjunction with basic telescope components, the simulation provided data that could be used for further modeling and design of a specific helium injection system. Experimental telescope pressures required for 90% attenuation appeared to be slightly higher (factor of 2 to 5). Cooling the helium purge gas and telescope components from 300 to 140 K had no measurable effect on stopping efficiency of a given mass flow of helium from the diffuse injector.
Hetrick, M. A.↗
Experimental investigation of an axisymmetric free jet with an initially uniform velocity profile
An experimental investigation was conducted to determine the flow characteristics of a circular free helium jet having an initially uniform velocity profile. Complete velocity profiles are presented at Reynolds numbers of 1027 and 4571 at 0, 3, 6, 10, 15, and 20 nozzle diameters (where possible) from the nozzle exit. Centerline velocity decay and potential core length were obtained over a range of Reynolds numbers from 155 to 5349 at distances up to and including 25 nozzle diameters from the nozzle exit. The angles of spread associated with the diffusion of the jet downstream of the nozzle are also given. Axial jet momentum flux and entrained mass flux, at various distances downstream of the nozzle, are presented as a function of the jet Reynolds number.
Labus, T. L.↗
Kinetic and Transport Modeling for Entry Flows in Hydrogen-Helium Atmospheres
Several different thermochemical models for H2/He mixtures are used to simulate a 1-D shock in a viscous CFD code at conditions relevant for hypersonic entry into the ice and gas giants. These models range from a 6-species, 1-temperature (1-T) model, to more complex 17 and 25-species state-to-state (StS) models. Corresponding radiance profiles for each of these models are then computed using a radiation code, and results are compared to experimental data from the NASA Ames Electric Arc Shock Tube (EAST) facility. Overall, the 6-species model with a quasi-steady state (QSS) solver for atomic H and modified ionization rates, along with the 17 and 25-species StS models, are found to reproduce the magnitude of the radiance seen in the EAST experiments reasonably accurately. Two additional changes to the modeling of species diffusion fluxes are also investigated: the exclusion of a ∇W term, as well as the inclusion of Soret/ Dufour effects. It is determined that these changes to the transport modeling have a negligible impact on the flowfield and therefore the radiance predictions for the H2/He shock conditions considered in this study.
Outer planets↗
Our Sun IV: The Standard Model and Helioseismology: Consequences of Uncertainties in Input Physics and in Observed Solar Parameters
Helioseismic frequency observations provide an extremely accurate window into the solar interior; frequencies from the Michaelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) spacecraft, enable the adiabatic sound speed and adiabatic index to be inferred with an accuracy of a few parts in 10(exp 4) and the density with an accuracy of a few parts in 10(exp 3). This has become a Serious challenge to theoretical models of the Sun. Therefore, we have undertaken a self-consistent, systematic study of the sources of uncertainties in the standard solar models. We found that the largest effect on the interior structure arises from the observational uncertainties in the photospheric abundances of the elements, which affect the sound speed profile at the level of 3 parts in 10(exp 3). The estimated 4% uncertainty in the OPAL opacities could lead to effects of 1 part in 10(exp 3); the approximately 5%, uncertainty in the basic pp nuclear reaction rate would have a similar effect, as would uncertainties of approximately 15% in the diffusion constants for the gravitational settling of helium. The approximately 50% uncertainties in diffusion constants for the heavier elements would have nearly as large an effect. Different observational methods for determining the solar radius yield results differing by as much as 7 parts in 10(exp 4); we found that this leads to uncertainties of a few parts in 10(exp 3) in the sound speed int the solar convective envelope, but has negligible effect on the interior. Our reference standard solar model yielded a convective envelope position of 0.7135 solar radius, in excellent agreement with the observed value of 0.713 +/- 0.001 solar radius and was significantly affected only by Z/X, the pp rate, and the uncertainties in helium diffusion constants. Our reference model also yielded envelope helium abundance of 0.2424, in good agreement with the approximate range of 0.24 to 0.25 inferred from helioseismic observations; only extreme Z/X values yielded envelope helium abundance outside this range. We found that other current uncertainties, namely, in the solar age and luminosity, in nuclear rates other than the pp reaction, in the low-temperature molecular opacities, and in the low-density equation of state, have no significant effect on the quantities that can be inferred from helioseismic observations. The predicted pre-main-sequence lithium depletion is uncertain by a factor of 2. The predicted neutrino capture rate is uncertain by approximately 30% for the Cl-27 experiment and by approximately 3% for Ga-71 experiments, while the B-8 neutrino flux is uncertain by approximately 30%.
Boothroyd, Arnold I.↗