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Webber, W. R.

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

At least 55 records · Page 3
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NASA NTRS1987

A reexamination of the cosmic-ray helium spectrum and the He-3/He-4 ratio at high energies

Cosmic ray He spectral data collected by satellites in earth orbit were used to examine a recent measurement of He-3 of about 6 GeV/nucleon, an overabundance compared to predictions made with a leaky box approach. The spectral rigidity index at the time of the measurement was no more than 2.55, indicating a near-earth He-3/He-4 ratio of 0.17, although an index in the range 10-20 GV and a ratio of close to 0.24 is the usual value. New magnetic spectrometer data, however, show that a single spectral rigidity value for the near-earth He-3/He-4 ratio cannot be correct. The Fokker-Planck equation is used to demonstrate that solar modulation of the He flux yields an energy/abundance distribution close to observational data.

Webber, W. R.↗
NASA NTRS1986

Characteristics of large Forbush-type decreases in the cosmic radiation. II - Observations at different heliocentric radial distances

Cosmic ray data from IMP 8, Voyager 1 and 2, Pioneer 10 are used to investigate the heliocentric radial dependence of the characteristics of about 20 Forbush-type transient decreases which occurred from 1978 to 1984. These characteristics include the recovery time, the amplitude, and the time to decrease to minimum. It is found that the average recovery time is about 5 times longer at R = 30 AU than at 1 AU. The magnitudes of the transient decreases are observed to decrease about 1.5 percent/AU on average so that the magnitude of the decrease is half as great at R about 30 AU as at 1 AU. The time for the cosmic ray intensity to decrease to the minimum in the transient decrease is found to be greater at larger distances and is about 5 times longer at R = 30 AU than at 1 AU. The behavior of these effects as a function of radius is obviously related to the evolution of the disturbances causing the transient decreases as they propagate outward. A model of the Forbush-type decrease is proposed to explain the observed radial dependence of the recovery time and time to minimum of the decrease. The implications of these results for understanding the relationship between Forbush-type decreases and the 11-year variation are discussed.

Webber, W. R.↗
NASA NTRS1986

Characteristic recovery times of Forbush-type decreases in the cosmic radiation. I - Observations at earth at different energies

Data on 30 asymmetric Forbush decreases recorded by the IMP spacecraft at 1 AU and the Mt. Washington neutron monitor over the period 1972-84 are examined to characterize the recovery characteristics of cosmic rays after the events. The spacecraft data are concentrated at energies of 1.7 GV, while the terrestrial instruments recorded events at 5 GV. Attention is paid to the relative amplitudes of the recorded transient decreases, the characteristic recovery times, and the energy dependence of the amplitudes and recovery time. The recovery times were found to be equal at both energy levels, supporting a concept of energy independence for the recoveries. Also, no correlations were found between the recovery times and the occurrences of a solar magnetic field reversal or with phase in the solar modulation cycle. A time-dependent, two-dimensional model is defined, which expresses the cosmic ray particle distributions as a function of the decay of the disturbance, with a small dependence on the transport parameters of the cosmic rays.

Lockwood, J. A.↗
NASA NTRS1986

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

Spatial and temporal variations of the interplanetary cosmic ray oxygen spectrum over the 11 yr solar and cosmic ray modulation cycle are discussed. The variations were monitored with instrumentation on the Voyager 1 and 2 and the Pioneer 10 spacecraft over the period 1977-85. The analysis concentrates on 4-30 MeV nuclei, where the anomalous component of cosmic rays dominates. The number of particles of anomalous oxygen increased with increasing distance from the sun. Furthermore, the particles exhibited, on the average, higher energies after the middle of 1980, when a solar magnetic field reversal occurred. The Voyager 1 spacecraft, 24 deg out of the ecliptic plane, did not register as high a shift in energy during the same period. It is concluded that cosmic ray drift effects are closely associated with solar magnetic field polarity reversals, a situation which may be examined more fully during the solar minimum beginning in 1985.

Cummings, A. C.↗
NASA NTRS1985

Source spectral index of heavy cosmic ray nuclei

From the energy spectra of the heavy nuclei observed by the French-Danish experiment on HEAO-3, the source spectra of the mostly primary nuclei (C, O, Ne, Mg, Si, Ca and Fe) in the framework of an energy dependent leaky box model (Engelmann, et al., 1985) were derived. The energy dependence of the escape length was derived from the observed B/C and sub-iron/iron ratios and the presently available cross sections for C and Fe on H nuclei (Koch-Miramond, et al., 1983). A good fit to the source energy spectra of all these nuclei was obtained by a power law in momentum with an exponent gamma = -2.4+0.05 for the energy range 1 to 25GeV/n (Engelmann, et al., 1985). Comparison with data obtained at higher energy suggested a progressive flattening of these spectra. More accurate spectral indices are sought by using better values of the escape length based on the latest cross section measurements (Webber 1984, Soutoul, et al., this conference). The aim is also to extend the analysis to lower energies down to 0.4GeV/n (kinetic energy observed near Earth), using data obtained by other groups. The only nuclei for which a good data base is possessed in a broad range of energies are O and Fe, so the present study is restricted to these two elements.

Engelmann, J. J.↗
NASA NTRS1985

Charge and energy dependence of the residence time of cosmic ray nuclei below 15 GeV/nucleon

The relative abundance of nuclear species measured in cosmic rays at Earth has often been interpreted with the simple leaky box model. For this model to be consistent an essential requirement is that the escape length does not depend on the nuclear species. The discrepancy between escape length values derived from iron secondaries and from the B/C ratio was identified by Garcia-Munoz and his co-workers using a large amount of experimental data. Ormes and Protheroe found a similar trend in the HEAO data although they questioned its significance against uncertainties. They also showed that the change in the B/C ratio values implies a decrease of the residence time of cosmic rays at low energies in conflict with the diffusive convective picture. These conclusions crucially depend on the partial cross section values and their uncertainties. Recently new accurate cross sections of key importance for propagation calculations have been measured. Their statistical uncertainties are often better than 4% and their values significantly different from those previously accepted. Here, these new cross sections are used to compare the observed B/C+O and (Sc to Cr)/Fe ratio to those predicted with the simple leaky box model.

Soutoul, A.↗
NASA NTRS1985

Cosmic ray charge and energy spectrum measurements using a new large area Cerenkov x dE/dx telescope

In September, 1981, a new 0.5 square meter ster cosmic ray telescope was flown to study the charge composition and energy spectrum of cosmic ray nuclei between 0.3 and 4 GeV/nuc. A high resolution Cerenkov counter, and three dE/dx measuring scintillation counters, including two position scintillators were contained in the telescope used for the charge and energy spectrum measurements. The analysis procedures did not require any large charge or energy dependent corrections, and absolute fluxes could be obtained to an accuracy approximately 5%. The spectral measurements made in 1981, at a time of extreme solar modulation, could be compared with measurements with a similar telescope made by our group in 1977, at a time of minimum modulation and can be used to derive absolute intensity values for the HEAO measurements made in 1979 to 80. Using both data sets precise energy spectra and abundance ratios can be derived over the entire energy range from 0.3 to greater than 15 GeV/nuc.

Webber, W. R.↗
NASA NTRS1985

Results of a search for deuterium at 25-50 GC/c using a magnetic spectrometer

A method is presented for separately identifying isotopes using a Cerenkov detector and a magnet spectrometer. Simulations of the method are given for separating deuterium from protons. The simulations are compared with data gathered from the 1979 flight of the New Mexico State University balloonborne magnet spectrometer. The simulation and the data show the same general characteristics lending credence to the technique. The data show an apparent deuteron signal which is (11 + or - 3)% of the total sample in the rigidity region 38.5 to 50 GV/c. Until further background analysis and subtraction is performed this should be regarded as an upper limit to the deuteron/(deuteron+proton) ratio.

Golden, R. L.↗
NASA NTRS1985

Cosmic ray isotope measurements with a new Cerenkov X total energy telescope

Measurements of the isotopic composition of cosmic nuclei with Z = 7-20 are reported. These measurements were made with a new version of a Cerenkov x total E telescope. Path length and uniformity corrections are made to all counters to a RMS level 1%. Since the Cerenkov counter is crucial to mass measurements using the C x E technique - special care was taken to optimize the resolution of the 2.4 cm thick Pilot 425 Cerenkov counter. This counter exhibited a beta = 1 muon equivalent LED resolution of 24%, corresponding to a total of 90 p.e. collected at the 1st dynodes of the photomultiplier tubes.

Webber, W. R.↗
NASA NTRS1985

The Energy Spectrum of Jovian Electrons in Interplanetary Space

The energy spectrum of electrons with energies approx 10 to approx 180 MeV measured with the electron telescope on the Voyager 1 and 2 spacecraft in interplanetary space from 1978 to 1983 is studied. The kinetic energy of electrons is determined by double dE/dx measurements from the first two detectors (D sub 1, D sub 2) of a stack of eight solid state detectors and by the range of particle penetration into the remaining six detectors (D sub 3 to D sub 8) which are interleaved with tungsten absorbers. From 1978 to 1983 (radial range approximately 2 to a pproximately 12 AU) electrons of Jovian origin were clearly observable for electrons stopping in D(sub 3(E approximately greater than 4 MeV)) and in D(sub 4 (E approximately greater than 8 MeV)). For electrons stopping in D(sub 5(E approximately greather than 12 MeV)), the jovian flux dominated the galactic electron flux for a period of approximately one year near the encounter with Jupiter. Jovian electrons were also observed in D(sub 6(E approximately greater than 21 MeV)) but not in D(sub 7(E approximately greater than 28 MeV)). A detailed interpretation of the electron variations in all energy channels depends on an accurate subtraction of background induced by energetic protons of a few 100 MeV. This substraction is facilitated by laboratory calibration results at several energies. Further results on the differential energy spectrum of Jovian electrons and limits on the maximum detected energies will be reported.

Christon, S. P.↗
NASA NTRS1985

The intensity recovery of Forbush-type decreases as a function of heliocentric distance and its relationship to the 11-year variation

Recent data indicating that the solar modulation effects are propagated outward in the heliospheric cavity suggest that the 11-year cosmic ray modulation can best be described by a dynamic time dependent model. In this context an understanding of the recovery characteristics of large transient Forbush type decreases is important. This includes the typical recovery time at a fixed energy at 1 AU as well as at large heliocentric radial distances, the energy dependence of the recovery time at 1 Au, and the dependence of the time for the intensity to decrease to the minimum in the transient decreases as a function of distance. These transient decreases are characterized by their asymmetrical decrease and recovery times, generally 1 to 2 days and 3 to 10 days respectively at approx. 1 AU. Near earth these are referred to as Forbush decreases, associated witha shock or blast wave passage. At R equal to or greater than + or - 10 AU, these transient decreases may represent the combined effects of several shock waves that have merged together.

Lockwood, J. A.↗
NASA NTRS1985

The effects of newly measured cross sections in hydrogen on the production of secondary nuclei during the propagation of cosmic rays through interstellar H

The cross sections of six important cosmic ray source nuclei in hydrogen at several energies between 300 and 1800 MeV/nuc were measured. Significant differences, sometimes exceeding 50%, exist between these new measurements and the earlier semiempirical predictions, and a new set of semiempirical formulae are being determined that better describe this fragmentation. New cross sections were obtained so that the systematics of their effects on cosmic ray propagation through interstellar hydrogen can be examined.

Webber, W. R.↗
NASA NTRS1985

Measurements of the fragmentation of (40)Ar, (28)Si and (12)C in CH2C and H targets between 300 and 1500 MeV/nuc at the Bevalac

Studies of the fragmentation of various nuclei in CH2 and C targets were continued with the objective of obtaining cross sections in hydrogen for use in the cosmic ray propagation problem. New measurements include Fe-56. Measurements were made at 6 energies between 300 and 1700 MeV/nuc. C-12 measurements were made at six energies. Si-28 measurements were made at three energies and measurements were made at two energies. New data on C.-12 Si-28 and AR-44 nuclei are given. The data are compare it with the earlier semi-empirical predictions.

Webber, W. R.↗
NASA NTRS1985

Cherenkov-dE/dx-range measurements on cosmic ray iron group nuclei

A balloon experiment which combined a large area plastic detector unit with electronic dE/dx-C data is presented. The correlation of the electronic data with the range data of the plastic detector stack was achieved by rotating plastic detector disks which provided in this way the passive plastic detector with an incorporated time determination. The constant flux of cosmic ray particles with charge Z greater than two was used to gauge the time resolving system. Stopping cosmic ray iron group nuclei in the energy range 400 to 700 MeV/nuc are identified using their electronic scintillator and Cherenkov signals and their etch conelengths and range data. The precise knowledge of the particle's trajectory proposes refined pathlength corrections to the electronic data.

Sermund, G.↗
NASA NTRS1985

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

Data was used from the cosmic ray experiment on the Voyager spacecraft to measure the energy spectrum of anomalous O and He during the period 1977 to 1985. These spectra are found to change dramatically after the middle of 1980, with the peak of 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 of propagation of these particles.

Cummings, A. C.↗
NASA NTRS1985

Radial and latitudinal gradients of anomalous oxygen during 1977 - 1985

It is found that the radial gradient of anomalous O remains during 1977 to 1985 at approx. 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 is also found for a latitudinal gradient of approx. +3%/degree at low energies (7.1 to 10.6 MeV/nuc).

Webber, W. R.↗