Engineering topics

Mcdonald, F. B.

Publications and source records attributed to Mcdonald, F. B..

At least 19 records

NASA NTRS2012

Sudden Intensity Increases and Radial Gradient Changes of Cosmic Ray Mev Electrons and Protons Observed at Voyager 1 Beyond 111 AU in the Heliosheath

Voyager 1 has entered regions of different propagation conditions for energetic cosmic rays in the outer heliosheathat a distance of about 111 AU from the Sun. The low energy 614 MeV galactic electron intensity increased by 20over a time period 10 days and the electron radial intensity gradient abruptly decreased from 19AU to 8AU at2009.7 at a radial distance of 111.2 AU. At about 2011.2 at a distance of 116.6 AU a second abrupt intensity increase of25 was observed for electrons. After the second sudden electron increase the radial intensity gradient increased to18AU. This large positive gradient and the 13 day periodic variations of 200 MeV particles observed near theend of 2011 indicate that V1 is still within the overall heliospheric modulating region. The implications of these resultsregarding the proximity of the heliopause are discussed.

protons↗

NASA NTRS1994

The cosmic-ray oxygen and helium spectra measured at Pioneer 10 over the time of the 1987 modulation minimum, and implications for the He/O source ratio

At the time of the recent sunspot minimum in 1987 when the cosmic-ray intensity within the heliosphere was a maximum, Pioneer 10(P10) was at a radial distance of approximately 42 AU. The solar modulation parameter Phi is estimated to have reached a minimum value of approximately 150 MV at P10 at that time, as compared with a minimum value of approximately 500 MV at the Earth. Thus P10 is in a sense effectively approximately 0.7 of the way to the heliospheric modulation boundary whatever its physical distance in astronomical units. Accordingly, the spectra of the various cosmic-ray species are much nearer to the interstellar spectra than any spectra ever before measured. We have therefore examined the penetrating high energy telescope (HET) data from P10 for a 2.5 yr period around 1987 to obtain a statistically accurate spectrum of oxygen nuclei over the energy range from 90 to 450 MeV/nucleon for comparison with a 'reference' interstellar spectrum. A similar spectrum is derived for helium nuclei. These two spectra and their ratios constitute important 'reference' spectra for all of the heavier cosmic-ray nuclei. The cosmic-ray source ratio required to explain the observed He/O ratio, when a full Galactic propagation calculation is carried out including secondary He-4 and He-3 production, is 15.9. The implications of this very low ratio and a comparison with the He/O ratio measured at high energies are discussed.

Webber, W. R.↗

NASA NTRS1994

Study of cosmic-ray H and He isotopes at 23 AU

We have measured the spectra of H and He isotopes during the 1987 solar minimum with the cosmic-ray detector system (CRS) on the Voyager 2 spacecraft. By carrying out the measurement near solar minimum and at large heliospheric distances, the effects of solar modulations were reduced. In particular, the adiabatic energy losses were smaller, and these results from 23 AU over the solar minimum period of cycle 21 represent observations at energies not accessible from previous measurements near 1 AU. The modulated spectra with the diffusion coefficient constant k(sub 0) = 3.15 x 10(exp 22) sq cm/s (which corresponds to a solar modulation parameter of 360 MV at 23 AU and 500 MV at 1 AU) agree well with both our data at 23 AU and the previous solar minimum measurements at 1 AU. The measured H-1 and H-2 spectra are both consistent with the calculated spectra, using standard Galactic and heliospheric propagation models without invoking an anomalous hydrogen component. With the fixed modulation parameter of 360 MV, the mean pathlengths, source spectra, and cross sections were varied to study the effects of different input parameters on the spectra and relative abundances. At this stage of our work, we have not found any strong evidence from the low-energy H-2 and He-3 data that H-1 and He-4 should have a different propagation history, or different types of source spectra from the heavier cosmic-ray nuclei.

Seo, E. S.↗

NASA NTRS1994

Effects of the intense solar activity of March/June 1991 observed in the outer heliosphere

The properties of the large-scale global merged interaction region (GMIR) generated by the intense solar events of March and June 1991 are studied using the available solar wind, interplanetary magnetic field, and energetic particle data from the observing network of Pioneer 10 and Voyagers 1 and 2 in the outer heliosphere. At heliocentric distances extending to 55 AU the delayed effects of this enhanced solar activity are observed in the form of large inceases in the solar wind velocity and interplanetary magnetic field and significant decreases in the galactic cosmic ray intensity. For low-energy ions (5-MeV protons) there was a single long-lived event extending over a period of some 6 months. Near the strongest interplanetary disturbances the H and He spectra are best represented by similar exponentials in momentum/nucleon (i.e., particle velocity at these at these energies). Over the rest of the event the characteristic momentum for He, (P(sub 0))(sub He) is generally approximately 0.66 for hydrogen. These spectra and the consistently low H/He ratio (25.3) at 2 MeV/nucleon closely resemble that observed in corrotating interaction regions events. Despite the strong north/south asymmetry in the solar activity, the interplanetary disturbances produced the same net decrease in the galactic cosmic ray intensity of ions greater than 70 MeV at the three widely separated spacecraft when the effects of the long-term recovery are taken into account. A comparison of the relative intensity of MeV ions at these three spacecraft suggest that the most intense solar events occurred on the back side of the Sun in time periods adjacent to the March and June episodes of solar activity. It is argued that this GMIR as a system is responsible for the low-frequency radio emission observed by the Voyager Plasma Wave experiment some 1.46 years after the onset of the March 1991 activity.

Mcdonald, F. B.↗

NASA NTRS1994

Voyager measurements of the isotopic composition of cosmic-ray aluminum and implications for the propagation of cosmic rays

We report a new measurement of the cosmic-ray isotopic composition of aluminum in the low-energy range form 75 to 206 MeV per nucleon.This measurement was made using the high-energy telescope of the CRS experiment on the Voyager 1 and 2 spacecraft during the time period from 1977 to 1993 with an average solar modulation level about 497 MV, roughly the same as at Earth near sunspot minimum. We obtain approximately 430 Al events of which approximately 35 are Al-26 and 395 are Al-27. The Al isotopes were separated with an average mass resolution sigma of 0.35 amu. Our interpretation of the isotopic composition of cosmic-ray aluminum is based on a standard Leaky-Box model for the interstellar propagation of cosmic-ray nuclei using the latest cross sections of the New Mexico-Saclay collaboration as well as a disk-halo diffusion model. From our observed ratio Al-26/Al-27 of 8.3 +/- 2.4 % we deduce an average interstellar density of about 0.52 (+0.26, -0.2) atoms per cu cm. This density is larger than the value of 0.28 (+0.14, -0.11) atoms per cu cm we found from an analysis of the observed abundance of the longer lived Be-10 made using data from the Voyager detectors over almost the same time interval and using essentially the same propagation program.

Lukasiak, A.↗

NASA NTRS1994

Cosmic-ray isotopic composition of C, N, O, Ne, Mg, Si nuclei in the energy range 50-200 MeV per nucleon measured by the Voyager spacecraft during the solar minimum period

The isotopic composition of C, N, O, Ne, Mg, Si cosmic ray nuclei has been measured in the energy range 50-200 MeV per nucleon using data collected by the High-Energy Telescope of the cosmic-ray subsystem experiment on the Voyager 1 and 2 spacecraft. These data were collected during the period of minimum solar activity in 1986-1988 at an average distance of 27 AU with an effective solar modulation that was much less than at the Earth. The isotope analysis, based on the energy loss - total energy method, has a mass resolution of 0.2 amu for carbon and 0.4 amu at silicon. We find a (C-13)/(C-12) ratio slightly lower and a (O-18)/(O-16) ratio slightly enhanced over their solar system value. We also observe the previously reported enhancement of the (Ne-22)/(Ne-20) ratio relative to solar at the cosmic-ray source but only a weak, if any, enhancement of the (Mg-25)/(Mg-24), (Mg-26)/(Mg 24), and (Si-30)/(Si-28) ratios.

Lukasiak, A.↗

NASA NTRS1994

The isotopic composition of cosmic-ray beryllium and its implication for the cosmic ray's age

We report a new measurement of the cosmic-ray isotopic composition of beryllium in the low-energy range from 35 to 113 MeV per nucleon. This measurement was made using the High Energy Telescope of the CRS experiment on the Voyager 1 and 2 spacecraft during the time period from 1977 to 1991. In this overall time period of 14 years the average solar modulation level was about 500 MV. The cosmic-ray beryllium isotopes were completely separated with an average mass resolution sigma of 0.185 amu. The isotope fractions of Be-7, Be-9, and Be-10 obtained are 52.4 +/- 2.9%, 43.3 +/- 3.7%, and 4.3 +/- 1.5%, respectively. The measured cosmic-ray abundances of Be-7 and Be-9 are found to be in agreement with calculations based on standard Leaky-Box model for the interstellar propagation of cosmic-ray nuclei using the recent cross sections of the New Mexico-Saclay collaboration. From our observed ratio Be-10/Be = 4.3 +/- 1.5% we deduce an average interstellar density of about 0.28 (+0.14, -0.11) atoms/cu cm, and acosmic-ray lifetime for escape of 27 (+19, -9) x 10(exp 6) years. The surviving fraction of Be-10 is found to be 0.19 +/- 0.07. Modifications to the conclusions of the Leaky-Box model when a diffusion + convection halo model for propagation is used are also considered.

Lukasiak, A.↗

NASA NTRS1993

The role of merged interaction regions and drafts in the heliospheric modulation of cosmic rays beyond 20 AU - A computer simulation

Voyager 2 magnetic field measurements are used to simulate merged interaction and rarefaction regions (MIRs and RRs) for 1985-1989 via numerical solutions of the time-dependent, axially symmetric transport equation of cosmic rays in the heliosphere, together with the concurrent use of the wavy neutral sheet as a time-dependent drift parameter. This drift approach was found to be more successful, because it was able to reproduce the intensity levels, the factor modulation, and latitudinal gradients for 1 GeV protons at 23 AU.

Potgieter, M. S.↗

NASA NTRS1993

Cosmic ray modulation and the distant heliospheric magnetic field - Voyager 1 and 2 observations from 1986 to 1989

The relation between the cosmic ray (CR) intensity and the magnetic field strength observed by Voyager 1 and 2 during the period from 1986 through most of 1989 beyond 19 AU is examined. It is shown that these observations generally confirm the relation between changes in the CR intensity and the magnetic field intensity observed beyond 10 AU from 1981 to 1985. The CR intensity decreases when merged interaction regions (MIRs) are strong and dominating, and it fluctuates about a plateau when the effects of corotating MIRs are balanced by rarefaction regions. The CR intensity increases when the MIRs are weak and the magnetic field strength is relatively low.

Burlaga, L. F.↗

NASA NTRS1992

Concerning solar sources for Cycle 22 solar wind activity in the heliosphere

Beginning in 1989, the active phase of the present solar cycle became manifest in the outer heliosphere as large disturbances in solar wind velocity as observed by the Ames plasma analyzers aboard Pioneer 10 (46-50 AU heliocentric distance) and Pioneer 11 (about 28 AU). Inner heliospheric baseline plasma observations from the Pioneer Venus Orbiter (0.7 AU) and IMP 8 (1 AU) are useful for attempts to correlate solar events with the outer heliospheric disturbances. With regard to the onset of activity at Pioneer 11, Pioneer Venus observations are pertinent, and some of these in turn correspond with CMEs (coronal mass ejections) observed in SMM coronagraph data. In particular, enhanced solar wind speeds observed at Pioneer Venus during December 1988 to February 1989 are associated with seven large solar wind shocks (or shock candidates); corresponding CMEs may be identified. Two of these seven shocks were identified as candidates for a precursor to the onset of the disturbances at Pioneer 11. At Pioneer 10 the disturbed period includes two large disturbances, associated with the passage of shocks. There are several candidate CMEs in the SMM observations, one of which may be associated with the second Pioneer 10 shock.

Mihalov, J. D.↗

NASA NTRS1991

Studies of low-energy Galactic cosmic-ray composition at 22 AU. I - Secondary/primary ratios

Data from the High Energy Telescope of the CRS experiment on Voyager 2 have been used to measure the intensity, spectra, and elemental abundances of Galactic cosmic rays from Be to Ni at about 100 MeV/n. The charge resolution of this telescope varies from sigma = 0.034 for oxygen to sigma = 0.11 for iron. The solar modulation deceleration parameter Phi relevant for these data is estimated to be around 300 MV (Phi = 150 MeV/n for particles with A/Z = 2), an unprecedently low level for such measurements. This low modulation parameter is a result of the measurements being made in the outer heliosphere at a heliocentric distance of 22 AU, and centered on the solar minimum period of cycle 21. The results on secondary/primary ratios are used to test the Leaky-Box model of cosmic ray propagation, using the most recent cross sections data in hydrogen and helium, and taking into account the effects of the ionized fraction of the interstellar medium. It is found that all the secondary/primary ratios up to P/S are completely consistent with an exponential pathlength distribution (PLD). This PLD shape also accounts for the Sc-V/Fe ratio.

Ferrando, P.↗

NASA NTRS1991

Cosmic ray modulation - Voyager 2 observations, 1987-1988

The intensity profile of cosmic rays above 70 MeV observed by Voyager 2 and its relation to the interplanetary magnetic field and plasma at the beginning of the new modulation cycle from day 190, 1987 to day 345, 1988 in the region from 23.3 AU to 27.8 AU is analyzed. The cosmic ray intensity profile was approximately a series of four plateaus separated by three steps in which the intensity dropped abruptly. Each step was associated with a region in which the magnetic field, density and temperature were higher than average. The plateaus were associated with regions in which the magnetic field was alternately strong and weak. The solar wind within 200 AU during this interval can be roughly pictured as consisting of three shells between which the flow was quasiperiodic with a 26 day periodicity. The latitudinal extent of the shells in the northern hemisphere was probably less than 33 deg, since no steps were observed by Voyager 1. Drift motions might play a role during the recovery phase, just prior to the onset of the new modulation cycle, in the plateau regions between the shells, within the shells where drifts in various directions might mimic diffusion, and close to 1 AU, where large regions of intense magnetic fields have not yet formed. However the principal decreases in the cosmic ray intensity in the outer heliosphere during 1987 and 1988 were associated with the passage of broad regions of intense magnetic fields, consistent with the diffusion/convection model.

Burlaga, L. F.↗

NASA NTRS1989

Energetic charged particles in the magnetosphere of Neptune

The Voyager 2 cosmic ray system (CRS) measured significant fluxes of energetic greater than or equal to about 1 megaelectron volt (MeV) trapped electrons and protons in the magnetosphere of Neptune. The intensities are maximum near a magnetic L shell of 7, decreasing closer to the planet because of absorption by satellites and rings. In the region of the inner satellites of Neptune, the radiation belts have a complicated structure, which provides some constraints on the magnetic field geometry of the inner magnetosphere. Electron phase-space densities have a positive radial gradient, indicating that they diffuse inward from a source in the outer magnetosphere. Electron spectra from 1 to 5 MeV are generally well represented by power laws with indices near 6, which harden in the region of peak flux to power law indices of 4 to 5. Protons have significantly lower fluxes than electrons throughout the magnetosphere, with large anisotropies due to radial intensity gradients. The radiation belts resemble those of Uranus to the extent allowed by the different locations of the satellites, which limit the flux at each planet.

Stone, E. C.↗

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.↗

NASA NTRS1988

The charge state of the anomalous cosmic-ray component

H, He, and O cosmic-ray energy spectra obtained in the outer heliosphere by the Voyager 2 and Pioneer 10 spacecraft during the most recent periods of declining solar activity are presented graphically and analyzed. The recovery of high-rigidity particles is shown to be significantly delayed relative to that of the low-rigidity particles. This finding and the demodulated source spectra are attributed to the presence of singly ionized anomalous He and O, as predicted by Fisk et al. (1974).

Mcdonald, F. B.↗

NASA NTRS1987

Large-scale fluctuations between 13 AU and 25 AU and their effects on cosmic rays

The reasons for the temporal and latitudinal large-scale fluctuations in the magnetic-field strength observed between 13 and 25 AU by Voyagers 1 and 2 in 1984-1985, and their effect on the cosmic ray intensity profile are investigated. The study of low-frequency and intermediate-frequency fluctuations determined from magnetic-field-intensity profile suggests the presence of discontinuities near the equator during both years. The intensity of cosmic ray nuclei of greater than 75 MeV during 1984 was found to fluctuate about a constant value, because the decreases of intensity caused by the passage of large merged interaction regions balanced the increases of intensity associated with rarefied regions. The intensity of cosmic ray nuclei above 75 MeV increased during 1985 because there were few large interactions then. The cosmic ray intensity increased more rapidly at low than at high latitudes. The model of Burlaga et al. (1985) was found to provide good first-order fits to the cosmic ray intensity profiles.

Burlaga, L. F.↗

NASA NTRS1986

Formation of a compound stream between 0.85 AU and 6.2 AU and its effects on solar energetic particles and galactic cosmic rays

This paper describes the formation of a compound stream as a result of the interaction and coalescence of a series of five streams (a slow transient stream, two corotating streams, and two exceptionally fast transient streams) observed by the Helios B (HB) satellite near 0.85 AU. The compound stream and two merged interaction regions were also observed by the Voyager 1 satellite near 6.2 AU when it was nearly radially aligned with the HB. Closely associated with this compound stream was one of the largest solar energetic particle events observed beyond 5 AU. The relationship of this compound stream and its magnetic fields to the intensity profiles of solar energetic particles and galactic cosmic rays is discussed.

Burlaga, L. F.↗

NASA NTRS1986

Variations of galactic cosmic rays with heliolatitude in the outer heliosphere

On 1 January 1986, Voyager 1 was at a heliolatitude of 30 deg above the ecliptic plane at a radial distance of 25.4 AU. The galactic cosmic ray latitudinal gradients in the outer heliosphere measured at that time were found to be negative, about 1-2 percent/AU, show no strong energy dependence and are approximately half the radial gradient.

Mcdonald, F. B.↗