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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 289 records · Page 16
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NASA NTRS1985

Diurnal variation of galactic cosmic ray intensity on quiet days

A detailed study of the diurnal variation on long term basis was performed on geomagnetically quiet days using the experimental data of the cosmic ray intensity from the worldwide neutron monitoring stations. During the period when the polarity of the solar magnetic field in the Northern Hemisphere of the sun is negative the phase and amplitude of the diurnal anisotropy on quiet days was observed to remain almost constant. When the polarity of solar magnetic field in the Northern Hemisphere changes from negative to positive, a shift in the phase of the diurnal anisotropy on quiet days towards earlier hours is observed and the shift is found to be maximum during minimum solar activity periods 1953-54 and 1975-76. When the polarity of solar magnetic field changes from positive to negative in the Northern Hemisphere of the Sun the phase of the diurnal anisotropy on quiet days recovers to its usual direction of corotational anisotropy and is observed to remain almost constant till the polarity of the solar magnetic field does not change.

Kumar, S.↗
NASA NTRS1985

The large scale dynamics of the outer heliosphere and the long-term modulation of galactic cosmic rays

The network of cosmic ray observatories reaching across the heliosphere has given new insight into the process of solar modulation, establishing that the decreases occur principally in the outer heliosphere and are produced by interplanetary flow systems; that the hysteresis effects appear to be produced by changes in the rigidity dependence of the diffusion coefficient and that the predicted effects on the cosmic ray gradients associated with the reversal of the solar magnetic field polarity are not observed.

Mcdonald, F. B.↗
NASA NTRS1985

Large-scale solar modulation of greater than approximately 500 MeV/nucleon galactic cosmic rays seen from 1 to 30 AU

Using measurements of cosmic rays obtained by Cerenkov counters on Pioneer 10 and Pioneer 11 and neutron monitor data from earth, the spatial and temporal development of cosmic ray modulation during the last solar maximum were observed. The large-scale features of modulation and recovery are similar at these three sites and thus appear rotationally symmetric near the ecliptic plane. Outward propagating features characterize the radial dependence. The decline of the old cosmic ray cycle is marked by steplike decreases that propagate outward at nearly the solar wind velocity. During the start of the new cosmic ray cycle, recovery occurs first in the inner heliosphere and, after a lag comparable with that of the declining phase, appears later farther out. However, the direction of diffusive propagation is still inward, because the gradient remains positive. Forbush decreases are common at all three sites, and are evidently of great importance in understanding modulation. The largest decrease occurred during a short series of events in the summer of 1982 and had half the amplitude of the eleven year cycle.

Fillius, W.↗
NASA NTRS1983

On the localized nature of the Galactic cosmic rays

The escape of cosmic rays from the Galaxy at rigidities above and below 10 GV/c is investigated theoretically using a model with a diffusion coefficient of (1-10) x 10 to the 26th sq cm/sec (at 10 GV/c) and various values of the exponent delta. Some numerical results are presented in graphs, and it is found that at low energies the source nearest to the earth is at a distance of 100-300 pc, while the higher-energy diffusion length is 1-10 kpc (assuming in both cases that the diffusive medium is homogeneous and has a characteristic size greater than the respective lengths). For the case of a smaller storage volume, the lifetime to reach the boundary is inversely proportional to kappa.

Ormes, J. F.↗
NASA NTRS1983

The abundance of the radioactive isotope Al-26 in galactic cosmic rays

Satellite observations of the isotopic composition of aluminum in low energy cosmic rays (E/M = 200 MeV/amu) have been used to determine the abundance of the unstable isotope Al-26 (T1/2 = 0.87 Myr). The observed abundance ratio, Al-26/Al-27 = 0.036 (+0.037, -0.022), is in good agreement with previous balloon observations and yields a cosmic ray confinement time consistent with values based on the abundance of Be-10.

Wiedenbeck, M. E.↗
NASA NTRS1985

The galactic cosmic ray intensity minimum in the inner and outer heliosphere in solar cycle 21

The occurrence of the cosmic ray intensity minimum during solar cycle 21 at widely separated locations in the heliosphere is investigated. The data include in situ measurements by cosmic-ray detectors on the earth-orbiting satellite IMP 8, on the Voyagers 1 and 2 spacecraft, and on the Pioneers 10 and 11 spacecraft, located at distances from 1 to 27 AU. It is noted that Pioneer 10 and the other three spacecraft (Pioneer 11 and Voyagers 1 and 2) are on the opposite sides of the sun, with the latter moving toward the tail of the heliosphere. The data used in the study have been suitably corrected by removing solar flare particle contributions, Forbush decreases, and contamination by the radioisotope thermoelectric generator. It is evident that there exists a time delay in the occurrence of the cosmic ray intensity minimum registered by the IMP 8 and Pioneer 10 detectors, the latter being located at approximately 27 AU. It is shown that the data from the four spacecraft in deep space are on an average consistent with the IMP 8 data at 1 AU when suitably corrected, using 3 percent/AU for the radial gradient and 500 km/s for the average speed of propagation. An overview of the intensity profiles reveals reasonable agreement among them. However, specific intensity features appear at times to propagate with speeds in excess of the average value of about 500 km/s.

Venkatesan, D.↗
NASA NTRS1986

Measurements of hydrogen and helium isotopes in Galactic cosmic rays from 1978 through 1984

The propagation of light cosmic rays is examined using measurements of the relative abundances of the isotopes H-1, H-2 and He-3 and He-4 made with the ISEE 3 instrumentation. It is believed that cosmic ray particles experience spallation in traveling through the interstellar medium, thereby producing the isotopes examined in the present study. The isotopic ratios are therefore expected to yield data on outward migrating particles, which lose energy while moving toward extragalactic space, i.e., the 'leaky box approximation'. The energy ranges covered are 26-138 MeV/nucleon for H-1 and He-4, 24-89 MeV/nucleon for H-2 and 43-146 MeV/nucleon for He-3. Solar activity ranged from minimum to maximum over the observational period. Details of the experimental strategies, instrumentation features and calibration techniques employed with the particle detectors are provided. Histograms were geneated of the energies attributable to each particle track and mass ratios of the various isotopes were calculated over the measured energy ranges. Account was taken of errors introduced by solar modulation, and an escape path length of 5.6-7.8 g/sq cm was estimated for particle propagation through the Galaxy. The projected path length agrees with previous estimates based on data from heavier cosmic ray nuclei.

Kroeger, R.↗
NASA NTRS1987

Origin and propagation of galactic cosmic rays

The study of systematic trends in elemental abundances is important for unfolding the nuclear and/or atomic effects that should govern the shaping of source abundances and in constraining the parameters of cosmic ray acceleration models. In principle, much can be learned about the large-scale distributions of cosmic rays in the galaxy from all-sky gamma ray surveys such as COS-B and SAS-2. Because of the uncertainties in the matter distribution which come from the inability to measure the abundance of molecular hydrogen, the results are somewhat controversial. The leaky-box model accounts for a surprising amount of the data on heavy nuclei. However, a growing body of data indicates that the simple picture may have to be abandoned in favor of more complex models which contain additional parameters. Future experiments on the Spacelab and space station will hopefully be made of the spectra of individual nuclei at high energy. Antiprotons must be studied in the background free environment above the atmosphere with much higher reliability and presion to obtain spectral information.

Cesarsky, Catherine J.↗
NASA NTRS1986

The recovery phase of galactic cosmic ray modulation in the outer heliosphere

Energetic hydrogen and helium nuclei from the galaxy (cosmic rays) enter the solar system with varying degrees of difficulty during the 11-year solar magnetic cycle, resulting in spatial and temporal variations of particle intensities in the heliosophere. Observations of these variations from the ISEE-3, Voyager 1 and 2, and Pioneer 10 spacecraft during the period from early 1981 to early 1986 and for heliocentric distances ranging from 1 to 37 AU are reported. During this time the high energy intensities in the outer heliosphere have been increasing exponentially at a rate of approximately 20 percent/year with occasional interruptions due to enhanced solar activity. The radial intensity gradients measured in the ecliptic plane are observed to remain constant or to decrease at the time of the reversal of the solar magnetic poles in 1980.

Mcdonald, F. B.↗
NASA NTRS1986

He-3 in galactic cosmic rays

Cosmic-ray He-3/He-4 observations, including a new measurement around 65 MeV per nucleon from ISEE-3, are compared with interstellar propagation and solar modulation calculations in an effort to understand the origin of cosmic-ray He nuclei. A survey of spacecraft and balloon observations of the He-3/He-4 ratio shows improved consistency among measurements in the 50-300 MeV per nucleon energy range when a previously neglected contribution from atmospheric secondary He-3 is taken into account. These low-energy observations imply a mean escape length of 6-8 g/sq cm in the standard 'leaky box' model for cosmic-ray propagation in the Galaxy, a value consistent with that derived from studies of heavier nuclei. No evidence is found for an excess of low-energy He-3 such as that reported at high energies.

Mewaldt, R. A.↗
NASA NTRS1988

Galactic cosmic ray hydrogen and helium

This paper provides a summary of the spectra and propagation of the quartet of isotopes of hydrogen and helium which occur in arriving cosmic radiation. The unraveling of modulation effects is addressed, and the calculation of the local interstellar spectrum from a model of interstellar propagation is discussed. Two models for propagation are considered, one having an energy-independent mean pathlength and the other having an energy-dependent one.

Beatty, J. J.↗
NASA NTRS1988

Solar-cycle modulation of galactic cosmic rays

This paper describes a numerical solution of the spherically-symmetric, time-dependent, cosmic-ray modulation equation, compares it to data taken near the ecliptic, and probes the physical mechanisms responsible for the solar-cycle variations in the cosmic-ray intensity. With a physically defensible diffusion coefficient and other reasonable parameters, the spectra of protons and electrons can be simulated simultaneously over the cycle, the radial intensity gradients for particles greater than about 100 MeV, the overall intensity variation during the cycle, and the time lag in the recovery of low-energy particles behind high-energy ones, known commonly as the 'hysteresis.' The results suggest that cosmic-ray variations near the ecliptic are dominated by turbulent scattering regions in the heliosphere, overwhelming any effects of gradient and curvature drifts.

Perko, John S.↗
NASA NTRS1988

Galactic cosmic rays in three dimensions

A general conclusion is that the cosmic rays increase with increasing distance from the Sun at approximately 2 percent a.u. There is a strong correlation of the cosmic ray intensity with distance with the tilt of the heliospheric current sheet. Moreover, researchers find that the variation of the cosmic rays with time changes in alternate sun spot cycles. Finally, it seems that during alternate sun spot minima (1965 and 1985) the cosmic rays access to the inner solar system was along the equatorial current sheet, wheras in 1975 the cosmic rays came in over the poles. The recently discovered anomalous component of cosmic rays is very much related to this whole problem, and probably corresponds to particles being accelerated at the termination of the solar wind at some 50 to 100 astonomical units from the sun. In summary, many predictions of the models remain controversial in detail. Nonetheless, it appears now that we can expect more cosmic rays over the poles in the next sunspot cycle, and the intensity will continue to increase with heliocentric radius out to the interstellar medium.

Jokipii, J. R.↗
NASA NTRS1989

Elemental composition of low energy Galactic cosmic rays

This paper describes new measurements (at about 100 MeV/n) of elemental ratios in cosmic rays, made from Voyager 2 at about 22.5 AU. These data are characterized by a very low level of solar modulation, which results from the combination of the 1986-1987 solar minimum period and the large heliocentric distance. The data were obtained from one of the two HET telescopes on board Voyager 2, collected during 1986 and 1987; the charges were derived from a double dE/dx vs E analysis. The results on the secondary/primary elemental ratios are presented along with previous results obtained at 1 AU by IMP-8 and ISEE-3.

Ferrando, P.↗