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Jokipii, J. R.

Publications and source records attributed to Jokipii, J. R..

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Acceleration and transport of galactic cosmic rays

A recently proposed cosmic-ray acceleration mechanism is combined with a model of transport and loss from the Galaxy to construct a model for galactic cosmic rays. Various aspects of the cosmic-ray spectrum, including the secondary-to-primary ratio as a function of energy, are found to fit naturally into the overall picture. A solution to the dynamical-halo model with energy-dependent diffusion coefficient (kappa) is presented. This model is consistent with the observed spectra if kappa is proportional to the square root of the kinetic energy. The observed flattening of the secondary-to-primary ratio below about 1-2 GeV per nucleon is shown to be a natural consequence of the model. Similarly, the primary electron spectrum is predicted to flatten below about 1-2 GeV, which is consistent with deconvolution of the local nonthermal radio spectrum, although the flattening is not quite as much as indicated by the observations.

Jokipii, J. R.↗
NASA NTRS1979

A preview of astrophysical particle acceleration

Basic mechanisms of energy particle acceleration in astrophysics are reviewed including deterministic (shock waves, parallel electric fields) and stochastic (turbulence, Fermi acceleration, and its variations) mechanisms. Particular consideration is given to more general mechanisms devised for open geometries.

Jokipii, J. R.↗
NASA NTRS1978

Al-26 and the partial ionization of the solar nebula

It is shown that an even distribution of the amount of Al-26 equal to 10 to the minus tenth power of the amount of H (the amount of Al-26 suggested by scaling from cosmic abundances) will lead to a small but significant partial ionization of the nebular gas. Calculation of the ionization state and consequent magnetic Reynolds number for the solar nebula shows that the presence of Al-26 will result in strong coupling of the gas to magnetic fields. In the absence of Al-26, K-40 will still result in substantial ionization, but the degree of magnetic coupling is much more model dependent.

Consolmagno, G. J.↗
NASA NTRS1978

Cosmic-ray streaming perpendicular to the mean magnetic field. II - The gyrophase distribution function

The distribution function of cosmic rays streaming perpendicular to the mean magnetic field in a turbulent medium is reexamined. Urch's (1977) discovery that in quasi-linear theory, the flux is due to particles at 90 deg pitch angle is discussed and shown to be consistent with previous formulations of the theory. It is pointed out that this flux of particles at 90 deg cannot be arbitrarily set equal to zero, and hence the alternative theory which proceeds from this premise is dismissed. A further, basic inconsistency in Urch's transport equation is demonstrated, and the connection between quasi-linear theory and compound diffusion is discussed.

Forman, M. A.↗
NASA NTRS1978

Effects of particle drift on cosmic ray transport. II - Analytical solution to the modulation problem with no latitudinal diffusion

An analytical solution to a model of the modulation of galactic cosmic rays in the presence of particle drifts is presented and discussed. The solution assumes an energy-independent radial diffusion coefficient proportional to distance and no latitudinal diffusion, and includes energy-independent particle drift velocities similar to those expected in a Parker spiral magnetic field with an equatorial current sheet. The solutions clearly demonstrate the large effects of drifts on the modulated cosmic-ray intensity. For values of the radial diffusion coefficient and particle drift velocity which are plausible for 1-GV-rigidity protons, the logarithmic radial gradient in the inner solar system is reduced by more than a factor of 5 over the value calculated in the absence of drifts. It is found that even for much smaller values of particle drift velocity and radial diffusion coefficient, such as might be expected for protons with energies of the order of 10 MeV, the effects of the drifts can be substantial.

Isenberg, P. A.↗
NASA NTRS1978

Fermi acceleration and the structure of interstellar turbulence

Recent observations at length scales of at least about 10 to the 18th cm and about 10 to the 11th cm suggest the possibility of a generally turbulent interstellar gas with a Kolmogorov spectrum. The total r.m.s. fluctuation in fluid properties is of the same order as the mean. The consequences of such a turbulence spectrum for cosmic rays are discussed. If the turbulence is composed of Alfven waves propagating in both directions along the average magnetic field, the rate of Fermi acceleration is found to be significant. Consequences of the turbulence for spatial transport and the general dynamics of the interstellar gas, as well as possible energy sources for the turbulence, are discussed. It is concluded that Fermi acceleration by interstellar turbulence is a possible acceleration mechanism for galactic cosmic rays.

Jokipii, J. R.↗
NASA NTRS1977

Cosmic ray investigation for the Voyager missions: Energetic particle studies in the outer heliosphere - and beyond

The Voyager mission cosmic ray detector system, consisting entirely of solid-state charged-particle detectors, is designed to measure the energy spectrum of electrons in the range 3 to 110 MeV and the energy spectra and elemental composition of cosmic ray nuclei from hydrogen through iron in the range 1 to 500 MeV/nuc; for isotopes of hydrogen through sulfur the range is approximately 2 to 75 MeV/nuc. The cosmic ray investigation deals with the energy content, origin and acceleration process, and dynamics of cosmic rays in the galaxy, with particular attention given to low-energy phenomena in interstellar space and the outer solar system. The precise measurement of three-dimensional stream patterns of nuclei from H to Fe, as well as electrons over a wide energy range, is the data acquisition program emphasized.

Stone, E. C.↗
NASA NTRS1977

Cosmic rays in a dynamical halo. I - Age and matter traversal distributions and anisotropy for nuclei. II - Electrons

Recent measurements of the mean lifetime of galactic cosmic rays (at least 10 million yr) have suggested a dynamical halo model of cosmic-ray confinement to the Galaxy. A one-dimensional version of the model is analyzed to determine the ranges of parameters required to give self-consistent values of the mean lifetime, matter traversed, anisotropy, and interstellar gamma-ray flux. It is found that the ranges of the parameters are: disk scale height of 4000-10,000 pc, average convection velocity of no more than 60 km/s, and diffusion coefficient of (3-10) by 10 to the 28th power sq cm/sec for particles with charges of the order of 10 GV. Some consequences for this model of a much shorter lifetime are also discussed. The energy spectrum and spatial distribution of electrons in the dynamical halo model are considered. A gradual steepening of the electron energy spectrum beginning at several GeV energy is found, as indicated by observations. The decrease of electron density with height in the halo depends on the particle energy and may be much steeper than that for nuclei. The models give a halo with a scale height of the order of 3 kpc for several-GeV electrons, which is shown to be consistent with radio observations.

Owens, A. J.↗
NASA NTRS1977

Turbulent scattering in an exponential atmosphere - A wave-optical solution

The thin-screen model for scattering of radiation by a turbulent atmosphere is generalized to the case where the mean phase change decreases exponentially as a function of distance across the screen. This is of interest because it models the effect of turbulent scattering in an isothermal average atmosphere. We present both the change in average intensity and average bending angle in terms of the r-m-s scattering angle introduced by the turbulence. We find, for example, that the effect of turbulent scattering on the mean scattering angle (and hence Doppler residuals) is approximately the same as that obtained from more heuristic calculations. The effect depends on wavelength only to the extent that the refractivity is a function of wavelength.

Hubbard, W. B.↗
NASA NTRS1977

The occultation of Epsilon Geminorum by Mars - Analysis of McDonald data

An analysis of observations of the occultation of Epsilon Gem by Mars on April 8, 1976, is presented. The data were obtained by three neighboring telescopes at McDonald Observatory. Intensity fluctuations on time scales of the order of 100 ms were observed simultaneously at the three telescopes. As the observations compare well with predictions of turbulent scintillation theory, it is concluded that such fluctuations were probably largely the effect of stellar scintillations in the Martian atmosphere. The stellar diameter is included as a parameter in the theory but in a way which differs from previously published interpretations of occultations of extended sources by planetary atmospheres. Scintillations govern the experimental uncertainty in the deduction of the scale height of the high Martian atmosphere. A density scale height of 9.9 + or - 2.5 km is obtained at an altitude of 74 + or - 8 km above the mean surface. For CO 2 gas, this result corresponds to a temperature of 190 + or - 50 K.

Africano, J.↗
NASA NTRS1977

Effects of particle drift on cosmic-ray transport. I - General properties, application to solar modulation

Although gradient and curvature drifts are explicitly contained in the general equations of cosmic-ray transport, they have been almost universally neglected in applications of these equations. The drifts are evaluated explicitly for the Parker (1965) spiral magnetic field, and it is shown that for particles with rigidities greater than about 0.3 GV in the solar wind, they are larger than the solar-wind velocity over much of the heliosphere. Hence most current models of solar modulation and solar-flare particle events neglect terms which in many cases are as important as those retained. Calculations are presented which demonstrate the importance of the effects for simple modulation models. It is concluded that comparisons of presently available model calculations with observations do not provide a fair test of transport theory since they neglect drifts, which may substantially reduce the net modulation.

Jokipii, J. R.↗
NASA NTRS1977

Effects of particle drifts on the solar modulation of galactic cosmic rays

Gradient and curvature drifts in an Archimedean-spiral magnetic field are shown to produce a significant effect on the modulation of galactic cosmic rays by the solar wind. The net modulation, heliocentric radial gradient, and average energy change of particles which reach the inner solar system are significantly reduced. The effects of drifts are due to the fact that cosmic rays for which the drift velocity is comparable to the wind velocity or larger, have more rapid access to the inner solar system than in the absence of drifts.

Jokipii, J. R.↗
NASA NTRS1977

Effects of stream-associated fluctuations upon the radial variation of average solar wind parameters

A numerical MHD model in both spherically symmetric time-dependent and corotating equatorial flow approximations is used to compute the effects of nonlinear fluctuations due to solar wind streams upon radial gradients of average solar wind parameters. Significant effects of correlations between fluctuations on the gradients of azimuthal magnetic field, radial velocity, density, and azimuthal velocity are found. It is found that nonlinear fluctuations are a significant effect in determining the radial gradients of the solar wind at distances as small as 0.2 AU; at distances greater than 1 AU, nonlinear fluctuations dominate the behavior of the radial gradients.

Goldstein, B. E.↗
NASA NTRS1977

Stellar occultations by turbulent planetary atmospheres. I - A heuristic scattering model. II - The Beta Scorpii events

Effects of atmospheric turbulence on stellar-occultation inversion procedures are investigated using a heuristic scattering model that is believed to reproduce the essential features of turbulence. A quantitative estimate is made of the size of the error in deducing the mean refractivity profile of a planetary atmosphere, taking into account constant as well as exponential scattering. It is shown that ordinary turbulence has no important effect on the average intensity profile in a stellar occultation but could have an important instantaneous effect. A critical examination of possible manifestations of turbulent scattering during occultations of Beta Sco by Jupiter indicates that all observed phenomena during these events can be understood in terms of scintillations produced by turbulence.

Hubbard, W. B.↗
NASA NTRS1976

Consequences of a lifetime greater than 10 to the 7th power years for galactic cosmic rays

The implications of recent determinations of the cosmic-ray lifetime are discussed. It is concluded that the observations are consistent with a 'dynamical halo' model in which cosmic rays are confined in an outward-moving galactic halo by self-generated hydromagnetic waves. Alternative models which do not incorporate a halo, but which have the cosmic rays propagate in regions of reduced density in the galactic disk, are also briefly discussed.

Jokipii, J. R.↗
NASA NTRS1976

The spiral structure of the interplanetary magnetic field

The Pioneer 10 data concerning the heliocentric radial variation of the interplanetary magnetic field are examined. Incorporating the systematic increase in solar wind velocity observed during the mission leads to much improved agreement between the usual Parker Archimedean spiral and the data. The effects of correlations between short-period fluctuations of speed and azimuthal field are discussed. It is concluded that during the period under study they did not substantially affect the average magnetic field.

Parker, G. D.↗