Engineering topics

Webber, W. R.

Publications and source records attributed to Webber, W. R..

At least 73 records · Page 4

NASA NTRS1985

The cosmic ray interplanetary radial gradient from 1972 - 1985

It is now established that the solar modulation of cosmic rays is produced by turbulent magnetic fields propagated outward by the solar wind. Changes in cosmic ray intensity are not simultaneous throughout the modulation region, thus requiring time dependent theories for the cosmic ray modulation. Fundamental to an overall understanding of this observed time dependent cosmic ray modulation is the behavior of the radial intensity gradient with time and heliocentric distance over the course of a solar modulation cycle. The period from 1977 to 1985 when data are available from the cosmic ray telescopes on Pioneer (P) 10, Voyager (V) 1 and 2, and IMP 8 spacecraft is studied. Additional data from P10 and other IMP satellites for 1972 to 1977 can be used to determine the gradient at the minimum in the solar modulation cycle and as a function of heliocentric distance. All of these telescopes have thresholds for protons and helium nuclei of E 60 MeV/nucleon.

Webber, W. R.↗

NASA NTRS1985

Solar modulation and interplanetary gradients of the galactic electrons flux, 1977 - 1984

The flux of electrons with energy from approx. 10 to 180 MeV measured with the electron telescope on the Voyager 1 and 2 spacecraft in the heliocentric radial range 1 - 22 AU between 1977 and 1984 is reported. Jovian electrons were clearly observable between 1978 and 1983 (radial range 2 - 12 AU) at energies below approx. 50 MeV. Above approx. 50 MeV the electron intensity exhibited temporal variations generally related to the 11 year modulation of protons 75 MeV. The overall magnitude of the electron intensity changes between the maximum intensity observed in 1977 and the minimum intensity in 1981 was a factor approx. 2, also comparable to that observed for 75 MeV protons. By early 1985 the electron intensity had apparently recovered to the level observed in 1977 whereas the proton intensity was still about 20% lower. A detailed interpretation of these electron variations in all energy channels depends on an accurate subtraction of background induced by energetic protons of a few 100 MeV. This subtraction is facilitated by calibration results at several energies.

Christon, S. P.↗

NASA NTRS1985

The energy spectrum of Jovian electrons in interplanetary space

The energy spectrum of electrons with energies approximately 10 to approximately 180 MeV measured with the electron telescope on the Voyager 1 and 2 spacecraft in interplanetary space from 1978 to 1983 is reported. The kinetic energy of electrons is determined by double dE/dx measurements from the first two detectors (D1,D2) of a stack of eight solid state detectors and by the range of particle penetration into the remaining six detectors (D3 to D8) which are interleaved with tungsten absorbers.

Christon, S. P.↗

NASA NTRS1985

Changes in the energy spectrum of anomalous oxygen and helium during 1977 - 1985

Data from the cosmic-ray experiment on the Voyager spacecraft was used to assure the energy spectrum of anomalous O and He during the period 1977 to 1985. These spectra change dramatically after the middle of 1980, with the peak or plateau region of the differential spectrum shifting to a higher energy. This change appears to be related to the reversal of the solar magnetic field and could imply that particle drifts are important to the acceleration or propagation of these particles.

Cummings, A. C.↗

NASA NTRS1985

Radial and latitudinal gradients of anomalous oxygen during 1977 - 1985

The radial gradient of anomalous O remained constant during 1977 to 85 at approximately 10 to 15%/AU although the intensity changes by more than a factor of 100. These results can be used to deduce that most of the modulation of the intensities of these particles is occurring beyond 27 AU. Evidence for a latitudinal gradient of approximately +3%/degree at low energies (7.1 to 10.6 MeV/nucl) was also found.

Webber, W. R.↗

NASA NTRS1985

Solar modulation and interplanetary gradients of the galactic electron flux, 1977 - 1984

The flux of electrons with energy from approximately 10 to 180 MeV measured ith the electron telescope on the Voyager 1 and 2 spacecraft in the heliocentric radial range 1 to 22 AU between 1977 and 1984 is reported. Jovian electrons were clearly observable between 1978 and 1983 (radial range 2 to 12 AU) at energies below approximately 50 MeV. Above approximately 50 MeV the electron intensity exhibited temporal variations generally related to the 11 year modulation of protons 75 MeV. The overall magnitude of the electron intensity changes between the maximum intensity observed in 1977 and the minimum intensity in 1981 was a factor approximately 2, also comparable to that observed for 75 MeV protons. By early 1985 the electron intensity has apparently recovered to the level observed in 1977 whereas the proton intensity was still about 20% lower. A detailed interpretation of these electron variations in all energy channels depends on an accurate subtraction of background induced by energetic protons of a few 100 Mev. This subtraction is facilitated by calibration results at several energies. Further results on these temporal variations and limits on possible interplanetary gradients will be reported.

Christon, S. P.↗

NASA NTRS1985

Cross section measurements at the Bevelac and their implications for cosmic ray studies

Measurements of cross sections at the Bevelac using C-12 and Fe-56 ions incident on H targets at several energies between 300 and 1700 MeV/nuc are reported. Of particular interest are the Be and B cross sections from C-12 which are about 30 percent less than the semiempirical predictions below about 1 GeV/nuc. These cross sections are presently being incorporated into an updated galactic-propagation program. Results are also presented on the charge and isotopic resolution of the cosmic-ray telescope used in these studies.

Webber, W. R.↗

NASA NTRS1984

Evidence that the anomalous cosmic-ray component is singly ionized

Data from the Cosmic Ray Subsystem on the Voyager spacecraft were used to measure the spectra of He, C, N, O, and Ne with 4-124 MeV per nucleon (for O) during solar minimum conditions near the end of 1977. By subtracting both a low-energy interplanetary component and the high-energy galactic component, the energy spectra of the 'anomalous' cosmic-ray species He, N, O, and Ne are determined. The energy dependences of such spectra are found to depend upon the charge state of the particles, their source spectra, and the rigidity dependence of the diffusion coefficient. If the source spectra at the boundary of the modulation region have similar power-law energy dependences, then the observed energy spectra indicate that the anomalous particles are singly ionized.

Cummings, A. C.↗

NASA NTRS1984

Integral radial cosmic ray gradients in the solar system from 1972 to 1982

The cosmic-ray telescope data on IMP 8, Voyagers 1 and 2, and Pioneer 10 have been used to determine the integral radial cosmic-ray intensity gradients for energies greater than 60 MeV per nucleon. When the 26 day average counting rates are time-shifted for the effects of the outward propagation speed of the solar modulation, the average integral radial gradients (Gr) are 3.0 + or - 0.35 (percent/AU) from 1975 to 1982. Gr is independent to the first order of the cosmic-ray intensity and radial heliocentric distance out to about 30 AU. After the large transient variations in the cosmic-ray intensity in mid-1982, the integral gradients returned to their original values.

Lockwood, J. A.↗

NASA NTRS1984

Observations of the dynamics of the cosmic ray modulation

New measurements of the outward propagation of the cosmic ray modulation and large transient decreases at a radial distance of 10-30 AU from the earth are reported. Also, new observations of the hysteresis effect in the 11-year modulation are presented for two solar cycles, and an attempt is made to reproduce the long-term modulation at earth from the observed transient or Forbush decreases. The results indicate that steady-state, spherically symmetric models of the heliosphere are not capable of explaining the 11-year variation. A time-dependent solution to the cosmic ray transport equation is required to describe the solar cosmic ray modulation.

Webber, W. R.↗

NASA NTRS1983

A measurement of the energy spectra and relative abundance of the cosmic-ray H and He isotopes over a broad energy range

The measurements reported of these isotopes were made using two sets of detectors during the same minimum modulation period in 1977. One measurement was made with a balloon-borne telescope, the other with telescopes on the Voyager spacecraft. It is noted that together they provide the widest energy range yet available for studying these isotopes: 14-150 MeV per nucleon for H2 and 10-290 MeV per nucleon for He-3. The simultaneous helium isotope observations are used to give a mutually consistent picture of galactic propagation and solar modulation. The data define the form of the interstellar H-1 and He-4 spectra, an interstellar matter path length for both H-1 and He-4, and a total residual modulation for He-4. The H-2 observations suggest a picture that is very similar for the galactic propagation of H-1 and He-4.

Webber, W. R.↗

NASA NTRS1983

Temporal variations of the anomalous oxygen component

Data from the cosmic ray experiment on Voyagers 1 and 2 was used to examine anomalous oxygen in the time period from launch in 1977 to the end of 1981. Several time periods were found where large periodic (typically 26 day) temporal variations of the oxygen intensity between approximately 5 - 15 MeV/nuc are present. Variations in intensity by up to a factor of 10 are observed during these periods. Several characteristics of these variations indicate that they are not higher energy extensions of the low energy particle (approximately 1 MeV/nuc) increases found in many corotating interaction regions (CIR's). Many of these periodic temporal variations are correlated with similar, but much smaller, recurrent variations in the 75 MeV proton rate. Voyager 1 and Voyager 2 counting rates were compared to estimate the local radial gradient for both the protons and the oxygen. The proton gradients during periods of both maximum and minumum fluxes are consistent with the overall positive radial gradients reported by others from Pioneer and near-Earth observations, supporting the view that these variations are due to local modulation of a source outside the radial range of project measurements. In contrast, the oxygen gradients during periods of maximum proton flux differ in sign from those during minimum proton fluxes, suggesting that the origin of the oxygen variations is different from that of the protons.

Cummings, A. C.↗

NASA NTRS1983

Voyager measurements of the energy spectrum, charge composition, and long term temporal variations of the anomalous components in 1977-1982

The large collecting area and wide energy range of the cosmic ray experiment on Voyager 1 and 2 was used to examine the energy spectra, charge composition, and long term temporal variations of the anomalous components in 1977-1982. Individual energy spectra are obtained for 17 separate quiet time periods during the time interval. The composite spectra of anomalous He, N, O, and Ne are obtained to a new level of precision. This includes the spectral shape and the relative abundance. Essentially, the spectral shape of N, O, and Ne appear to be similar. The ratios of anomalous N and Ne to O are found to be different from both the solar cosmic ray and galactic cosmic ray source composition. Some evidence is found for the enhancement of Ar as well. In the case of elements such as C, Mg, S, and Fe it is difficult to separate a possible lower intensity anomalous component from a quasi-steady interplanetary component that appears to be present at the lowest energies. The long term temporal variations of the anomalous He and O components were studied from 1977-82, a period from minimum to maximum in the modulation cycle. The tracking between these anomalous component intensities and the integral intensity of 75 MeV protons is striking; however, the intensity decrease of the anomalous components is much greater.

Webber, W. R.↗

NASA NTRS1983

Fragmentation of 710 and 1050 MeV/nuc Fe nuclei in CH2 and C targets - Isotopic cross sections for H targets

Fe nuclei of incident energies of 710 and 1050 MeV/nuc in CH2 and C targets are fragmented in order to measure isotopic cross sections in hydrogen targets. The telescope used to measure the charge and mass of the fragments is a Cerenkov x total energy telescope. The data are reported for z = 20-25 fragments. The number of individual events corresponding to each charged fragment is obtained and analyzed. Ratios of partial cross sections on C and CH2 targets indicate that simple scaling exists for isotope fractions as well as for charge fractions in different targets. The cross sections in hydrogen are obtained directly by a simple subtraction. Excitation functions for Mn and Cr nuclei are presented.

Webber, W. R.↗

NASA NTRS1983

Fragmentation of approximately 500 MeV/nuc Ne and O nuclei in CH2, C and H targets - Charge and isotopic cross sections

Fragmentation of Ne-20 and O-16 nuclei in CH2 and C targets at several energies between 400 and 1000 MeV/nuc is carried out in order to obtain cross sections in hydrogen for use in the galactic cosmic ray propagation problem. Charge changing and isotopic cross sections for the fragments with Z greater than or equal to 4 are obtained. The hydrogen cross sections are obtained by a subtraction procedure. The predictions of the semi-empirical model of Tsao and Silberberg (1979) are given and in general there is good agreement between the measurements and the predictions.

Webber, W. R.↗

NASA NTRS1983

Asymmetries in the modulation of protons and helium nuclei over two solar cycles

Measurements of proton and helium spectra over two cycles of solar modulation show large asymmetries in modulation. These effects are apparent at sunspot minimum conditions in 1965 and 1977, where a much larger intensity is observed in 1977, and also at sunspot maximum conditions where a much lower intensity is observed in 1981 relative to 1970. The details of this asymmetry are discussed and possible causes are considered.

Webber, W. R.↗

NASA NTRS1983

Cerenkov detectors for cosmic ray telescopes employing the Cerenkov x total energy technique of mass identification

Considerable progress has been made regarding the evolution of the 'Cerenkov x total energy technique' for mass identification of cosmic ray nuclei since the introduction of telescopes employing this technique by Webber et al. (1973). Thus, significant improvements in mass resolution have been made. These improvements are mainly related to the resolution of the Cerenkov counter. The present investigation is, therefore, concerned with the properties of various types of Cerenkov detectors. In addition, a description is provided of the characteristics of a large area (approximately 0.5 sq m-ster) cosmic ray isotope telescope, which is being developed for use on balloons or spacecraft.

Webber, W. R.↗

NASA NTRS1983

Studies of low energy cosmic rays - The anomalous component

Data from the cosmic ray subsystem on the Voyager spacecraft is used to measure the spectra of He, C, N, O, and Ne with about 4-124 MeV/nuc (for O) near 1 AU. By subtracting both a low-energy solar/interplanetary component and the high-energy galactic component the energy spectra of the anomalous cosmic-ray species He, N, O, and Ne have been determined. It is suggested that the shapes of these spectra carry information about the charge state of the particles and the rigidity dependence of the diffusion coefficient. For similar power-law source spectra at the boundary of the modulation region, the location of features in the energy spectra indicates that the anomalous particles are singly ionized.

Webber, W. R.↗