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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.

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At least 37 records · Page 2
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NASA NTRS2001

Transport of Photoelectrons in the Nightside Magnetosphere

Kinetic modeling results are analyzed to examine the transport of photoelectrons through the nightside inner magnetosphere. Two sources are considered, those on the dayside from direct solar illumination, and those at all local times from light scattered by the upper atmosphere and geocorona. For the dayside photoelectrons, a natural filter exists oil the nightside for these particles, with Coulomb collisions eroding the distribution at low energies and low L shells, and magnetospheric convection compressing the flux tubes as they corotate towards dawn. It is shown that, for low activity levels, a band of photoelectrons forms between L=4 and 6 that extends throughout the nightside local times and into the morning sector. For the scattered light photoelectrons, the trapped zone throughout the nightside is filled for energies less than 30 eV. Convective compression on the nightside and expansion on the dayside yields an accelerated population up to 50 eV at high L shells around dawn. Modeled energy and pitch angle distributions are presented to show the development and features of these populations. Such distinctive features should be observable in satellite spectrometer measurements.

Khazanov, G. V.↗
NASA NTRS2013

Correlations Between Variations in Solar EUV and Soft X-Ray Irradiance and Photoelectron Energy Spectra Observed on Mars and Earth

Solar extreme ultraviolet (EUV; 10-120 nm) and soft X-ray (XUV; 0-10 nm) radiation are major heat sources for the Mars thermosphere as well as the primary source of ionization that creates the ionosphere. In investigations of Mars thermospheric chemistry and dynamics, solar irradiance models are used to account for variations in this radiation. Because of limited proxies, irradiance models do a poor job of tracking the significant variations in irradiance intensity in the EUV and XUV ranges over solar rotation time scales when the Mars-Sun-Earth angle is large. Recent results from Earth observations show that variations in photoelectron energy spectra are useful monitors of EUV and XUV irradiance variability. Here we investigate photoelectron energy spectra observed by the Mars Global Surveyor (MGS) Electron Reflectometer (ER) and the FAST satellite during the interval in 2005 when Earth, Mars, and the Sun were aligned. The Earth photoelectron data in selected bands correlate well with calculations based on 1 nm resolution observations above 27 nm supplemented by broadband observations and a solar model in the 0-27 nm range. At Mars, we find that instrumental and orbital limitations to the identifications of photoelectron energy spectra in MGS/ER data preclude their use as a monitor of solar EUV and XUV variability. However, observations with higher temporal and energy resolution obtained at lower altitudes on Mars might allow the separation of the solar wind and ionospheric components of electron energy spectra so that they could be used as reliable monitors of variations in solar EUV and XUV irradiance than the time shifted, Earth-based, F(10.7) index currently used.

EUV↗
NASA NTRS1972

Rocket measurements of conjugate photoelectrons during the total solar eclipse of 7 March 1970 over Wallops Island.

Results of measurements made with a retarding potential analyzer on a Nike-Tomahawk rocket during the totality of the solar eclipse, showing definite evidence for the existence of photoelectrons from the conjugate hemisphere. Photoelectrons are observed in the altitude range from 120 to 260 km. The observed flux in the energy range from 2 to 30 eV is relatively constant above about 200 km, but decreased below that altitude. The flux of 5-eV energy electrons above 200 km altitude is about 10 to the 7th power electrons/cm/sec/eV. Higher-energy electrons were also observed, and it is possible that the energy content of these observed fluxes of conjugate-point photoelectrons is sufficient to maintain the observed electron densities and temperatures during the total eclipse.

Maier, E. J.↗
NASA NTRS1972

The 6300 A predawn enhancement - Excitation by photoelectrons from the magnetic conjugate point.

Incoherent scatter and tilting filter photometer data from Arecibo Observatory were combined to search for a contribution to the 6300 A predawn enhancement due to impact excitation by photoelectrons from the magnetic conjugate point. Prior to about 105 deg conjugate solar zenith angle, calculations of the 6300 A intensities due to dissociative recombination gave good agreement with the observed intensities. Then, with the arrival of conjugate photoelectrons (confirmed by plasma line enhancements), the observed intensities rose above the calculated, the difference increasing until a plateau was achieved at a conjugate solar zenith angle of about 95 deg. The difference or residual intensity is attributed to direct photoelectron impact excitation. Significant day-to-day variations in the onset angle and intensity have a strong negative correlation with ionospheric electron density.

Wickwar, V. B.↗
NASA NTRS1972

Direct observation of the lunar photoelectron layer.

The Charged-Particle Lunar Environment Experiment (CPLEE), a part of the Apollo 14 ALSEP, is an ion-electron spectrometer capable of measuring ions and electrons with energies between 40 eV and 50 keV. Accordingly, the instrument, with apertures 26 cm above the surface, has detected a layer of photoelectrons, or photoelectron gas above the sunlit lunar surface with energies ranging up to 200 eV. The experimental data for periods when the moon was in the earth's magnetotail for electron energies between 40 and 200 eV follows a defined power-law spectrum. The implications of this measurement are two-fold in that the lunar surface potential can be immediately determined to be at least 200 V, and a value of the photoelectron yield of the lunar surface material for photon energies above 40 eV may be computed.

Reasoner, D. L.↗
NASA NTRS1973

Photoelectrons and solar wind/lunar limb interaction.

It is suggested that boundary conditions for solar wind/lunar limb interactions are active. The 'whole-moon' limb does not evoke a shock cone, because warm (approximately equal 13 eV/electron) solar wind electrons are replaced by cool (less than or equal to 2 eV/electron) photoelectrons that are ejected from the generally smooth areas of the lunar terminator illuminated at glazing angles by the sun. A localized volume of low thermal pressure is created in the solar wind by these cool photoelectrons. The solar wind expands into this turbulence-suppressive volume without shock production. Conversely, directly illuminated highland areas exchange hot photoelectrons (greater than 20 eV/electron) for warm solar wind electrons. The hot electrons generate a localized pressure increase in the adjacent solar wind flow which evokes a shock streamer in the solar wind. Shock streamers are identifiable by a coincident increase in the magnitude of the solar wind magnetic field immediately external to the lunar wake. Shock occurrence is controlled by lunar topography, solar activity in the hard ultraviolet (greater than 20 eV), solar wind electron density and thermal velocity, and the intensity of the solar wind magnetic field.

Criswell, D. R.↗
NASA NTRS1974

Photoelectron angular distributions as a probe of anisotropic electron-ion interactions

Expressions are given for atomic photoelectron angular distributions in LS coupling in which the role of anisotropic final state electron-ion interactions emerges explicitly. Calculations of photoelectron angular distributions for atomic sulfur are presented in which these anisotropic interactions produce pronounced deviations from the predictions of the Cooper-Zare model. Such effects are expected to be a general feature of photoelectron angular distributions for most open shell atoms.

Dill, D.↗
NASA NTRS1973

The effect of conjugate photoelectron impact ionization on the pre-dawn ionosphere

The measured photoelectron fluxes are used to show that ionization by photoelectrons is not sufficiently large to affect significantly the nighttime ionosphere. The measured downward photoelectron flux used in the calculations is shown in a table. This flux was measured on December 9, 1971 at an altitude of 1400 km, when the local zenith angle was 159 deg and the conjugate zenith angle was 85.2 deg.

Nagy, A. F.↗
NASA NTRS1973

Photoelectrons and lunar limb shocks

It is suggested that boundary conditions for solar wind/lunar limb interactions are active. The 'whole-moon' limb does not evoke a shock cone because warm solar wind electrons are replaced by cool photoelectrons that are ejected from the generally smooth areas of the lunar terminator illuminated at glancing angles by the sun. A localized volume of low thermal pressure is created in the solar wind by these cool photoelectrons. The solar wind expands into this volume without shock production. Directly illuminated highland areas exchange hot photoelectrons for warm solar wind electrons. The hot electrons generate a localized pressure increase in the adjacent solar wind flow which evokes a shock streamer in the solar wind. Shock streamers are identifiable by a coincident increase in the magnitude of the solar wind magnetic field. Shock occurrence is controlled by lunar topography, solar activity in the hard ultraviolet, solar wind electron density and thermal velocity, and the density of the solar wind magnetic field.

Criswell, D. R.↗
NASA NTRS1974

Photoelectron angular distributions as a probe of anisotropic electron-ion interactions

Expressions are given for atomic photoelectron angular distributions in LS coupling in which the role of anisotropic final state electron-ion interactions emerges explicitly. Calculations of photoelectron angular distributions for atomic sulfur are presented in which these anisotropic interactions produce clear deviations from the predictions of the Cooper-Zare model. Such effects are expected to be a general feature of photoelectron angular distributions for most open-shell atoms.

Dill, D.↗
NASA NTRS1975

Calculated photoelectron pitch angle and energy spectra

Calculations of the steady-state photoelectron energy and angular distribution in the altitude region between 120 and 1000 km are presented. The distribution is found to be isotropic at all altitudes below 250 km, while above this altitude anisotropies in both pitch angle and energy are found. The isotropy found in the angular distribution below 250 km implies that photoelectron transport below 250 km is insignificant, while the angular anisotropy found above this altitude implies a net photoelectron current in the upward direction.

Mantas, G. P.↗
NASA NTRS1975

Observations of OI 7774 emission excited by conjugate photoelectrons

Observations and computer calculations of O I 7774 A airglow emissions excited by conjugate photoelectrons have been carried out. The observations were made at McDonald Observatory, Texas using a 2 m grille spectrometer from December 1972 to June 1973. The zenithal emission intensity during conjugate photoelectron precipitation was fairly constant at 2-4 R until conjugate sunset, after which it diminished steadily and ceased near a conjugate solar zenith angle of 105 (plus or minus 3) deg. A predawn enhancement in both O I 7774 A and forbidden O I 6300 A was observed to commence near 102 deg. The computations utilize the two-stream technique of Nagy and Banks (1970) to obtain the escaping photoelectron flux and the local excitation rates of the oxygen emissions. Good agreement with the observations is obtained for the dependence of the emission rate on conjugate solar zenith angle.

Christensen, A. B.↗
NASA NTRS1976

On the energy deposition of photoelectrons in the atmosphere of Venus

Vertical components of photoelectron fluxes in the atmosphere of Venus are computed by solving an appropriate form of the Boltzmann equation in the cases where there is no flux of either photoelectrons or solar-wind particles across the ionopause and where photoelectrons are free to escape from the atmosphere. It is assumed that Venus has no magnetic field and that the atmosphere is composed of carbon dioxide, atomic oxygen, and helium. The results are plotted as a function of altitude for several energies in the range from 100 eV to a cutoff of the order of 1 eV. Heating rates for the two upper boundary conditions and the case of no spatial transport are determined which show that transport effects dominate at altitudes greater than about 200 km. Electron temperatures are calculated for the adopted model atmosphere and ionosphere by solving the pertinent conservation equation, and excitation rates are computed for the CO Cameron band as well as the CO2(+) A and B bands.

Mccormick, P. T.↗
NASA NTRS1977

Comparison between calculated and measured photoelectron fluxes from Atmosphere Explorer C and E

The two-stream method has been used to calculate photoelectron fluxes. These results have been compared to those obtained by the photoelectron spectrometer on Atmosphere Explorers C and E. At both low altitudes, and higher altitudes (where transport effects are significant) the calculated and measured photoelectron fluxes yield good agreement. It is suggested that (1) better photoionization and scattering cross-section data will yield improved flux calculations, and (2) measurements of the pitch angle distribution are necessary for better high-altitude comparisons.

Nagy, A. F.↗
NASA NTRS1978

Photoelectrons in the upper atmosphere - A formulation incorporating effects of transport

A technique for solving the problem of photoelectron spatial transport and spectral degradation is presented. The developed model, which allows for a relatively complete description of the relevant transport and collisional physics, can be applied to study processes at either low or high altitude, and may be used to investigate interhemispheric transport. The technique involves dividing the spatial dimension into a set of discrete ordinates and calculating the photoelectron intensity for a specific energy at each point in space and at a set of angle points. The selected range of the energy spectrum was covered completely by a set of energy bins, and the photoelectron intensity in the highest energy bin was calculated first by assuming that the only source term was derived from photoionization. The method is similar to a method used in radiative transfer theory. Spatial and angular redistribution of electrons play a dominant role at heights above 400 km.

Prather, M. J.↗
NASA NTRS1978

Measurement of the daytime photoelectron energy distribution from AE-E with improved energy resolution

A substantial improvement in energy resolution for photoelectron energy spectra in the 150-320 km altitude range is reported. The spectra were obtained with the AE-E satellite. The improvement is based on a data analysis technique which makes use of spectra obtained from the spinning spacecraft when the potential of the sensor varies slightly with respect to the ionospheric plasma as a consequence of the v x B induced voltage in the spacecraft body. The results permit determination of the line shape and area of the O and N2 304A solar radiation-produced photoelectron lines as a function of altitude. The photoelectron lines are found to be essentially unbroadened by coulomb scattering below 200 km, but substantial broadening occurs by 250 km. At 300 km, the characteristic line shape due to coulomb scattering with a degradation to low energy and a sharp high energy side can be observed readily.

Lee, J. S.↗
NASA NTRS1981

A model of solar flux attenuation during eclipse passage and its effects on photoelectron emission from satellite surfaces

The basic theory of solar flux attenuation by the earth's atmosphere is reviewed and a model of the time-varying flux observed by a satellite during eclipse passage developed. The general model is applied to the specific problem of variations in photoelectron flux during penumbral passage and the effects of wavelength, solar activity, and atmospheric constituents on photoelectron emission investigated. Predictions of the photoelectron current expected from tungsten and aluminum surfaces are then successfully compared with actual observations from the ATS-5 and Injun 5 satellites confirming the validity of the model.

Garrett, H. B.↗
NASA NTRS1983

A simple theoretical model for calculating and parameterizing the ionospheric photoelectron flux

A method for calculating the ionospheric photoelectron flux is developed which uses the concept of average electron energy loss to simplify the calculation of the degraded electron spectrum. This method requires only a knowledge of the total inelastic electron impact cross sections and can be used for the calculation of all secondary ion and excited state production rates. The simple calculation reduces the computing time by a factor of 10 and considerably reduces storage requirements. It is found that the ionospheric photoelectron flux in the local equilibrium region is directly proportional to the attenuated solar EUV flux, which is a function only of the total neutral column density, and is independent of the neutral density composition. Thus, it is shown that electron impact cross section can be chosen so that this method may also be used to parameterize the measured ionospheric photoelectron fluxes.

Richards, P. G.↗