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Cummings, A. C.

Publications and source records attributed to Cummings, A. C..

At least 55 records · Page 3
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NASA NTRS1995

Anomalous cosmic ray oxygen gradients throughout the heliosphere

We have used data from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), Ulysses, Voyager 1, Voyager 2, and Pioneer 10 spacecraft to determine the radial and latitudinal gradients of anomalous cosmic ray oxygen at 10 MeV/nuc during the last half of 1993. These five spacecraft cover radial distances from 1 AU (SAMPEX) to 58 AU (P10) and latitudes to 41 deg S (Ulysses) and 32 deg N (V1). We find that the radial gradient is a decreasing function of radial distance, approximately r(exp -n), with n = 1.7 +/- 0.7. The large-scale radial gradient between the inner and outer heliosphere is much smaller than it was during the last solar minimum period in approximately 1987. The latitudinal gradient is small and positive, 1.3 +/- 0.4 %/deg, as opposed to the large and negative latitudinal gradients found during 1987, but similar to the small positive latitudinal gradient measured during 1976 for anomalous cosmic ray helium. These observations confirm that effects of curvature and gradient drift in the large scale magnetic field of the Sun are important for establishing the three-dimensional intensity distributions of these particles in the heliosphere during periods of solar minimum conditions.

Cummings, A. C.↗
NASA NTRS1993

The return of the anomalous cosmic rays to 1 Au in 1992

New observations of low energy (approximately 1 to 200 MeV/nuc) cosmic rays measured by three newly launched experiments on Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) during 1992 and 1993 show the strong presence of anomalous cosmic ray (ACR) nitrogen and oxygen, well before the approaching solar minimum. When compared with ACR temporal variations over the past two solar cycles we find that the 1992-1993 fluxes are approximately 5 to 10 times their level at corresponding neutron monitor counting rates in 1969-1970 and 1985.

Mewaldt, R. A.↗
NASA NTRS1993

New evidence for geomagnetically trapped anomalous cosmic rays

We report new observations of 15 MeV/nuc or greater trapped heavy ions with Z equal to or greater than 2, made on the polar-orbiting SAMPEX spacecraft in late 1992 and early 1993. A trapped population that includes He, N, O, and Ne is found to be located at L = 2. We conclude that the observed N, O, and Ne ions are 'anomalous' cosmic rays, trapped by the mechanism proposed by Blake and Friesen (1977). While it is not expected that this mechanism would also trap anomalous He, the characteristics of the trapped He population are generally consistent with those of N, O, and Ne.

Cummings, J. R.↗
NASA NTRS1993

Estimate of the distance to the solar wind termination shock from gradients of anomalous cosmic ray oxygen

The radial gradient of anomalous cosmic ray oxygen measured instantaneously between Voyager 2 and Pioneer 10 during 1985-1988 is correlated with the inferred tilt of the heliospheric neutral sheet. This is consistent with a simple model in which the radial gradient is related to the length of the neutral sheet between the two spacecraft. With this model we show that the radial gradient and the tilt of the neutral sheet near the solar wind termination shock can be inferred from the Voyager and Pioneer observations. By comparing the time history of the inferred tilt with that derived from solar observations, we estimate that the termination shock was at 62 +5/-7 AU at solar minimum in 1987. At solar maximum the shock should be located at about 90 AU due to the increased pressure of the solar wind.

Cummings, A. C.↗
NASA NTRS1990

The Advanced Composition Explorer

The Advanced Composition Explorer (ACE) was recently selected as one of two new Explorer-class missions to be developed for launch during the mid-1990s. ACE will observe particles of solar, interplanetary, interstellar, and Galactic origins, spanning the energy range from that of the solar wind to Galactic cosmic ray energies. Definitive studies will be made of the abundance of nearly all isotopes from H to Zn, with exploratory isotope studies extending to Zr. The scientific objectives, instrumentation, spacecraft, and mission approach that were defined for ACE during the Phase-A study period are summarized.

Stone, E. C.↗
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

The Advanced Composition Explorer

The Advanced Composition Explorer (ACE) was recently selected as one of two new Explorer-class missions to be developed for launch during the mid-1990's. ACE will observe particles of solar, interplanetary, interstellar, and galactic origins, spanning the energy range from that of the solar wind (approx. 1 keV/nucleon) to galactic cosmic ray energies (several hundred MeV/nucleon). Definitive studies will be made of the abundance of nearly all isotopes from H to Zn (1 less than or = Z less than or = 30), with exploratory isotope studies extending to Zr (Z = 40). To accomplish this, the ACE payload includes six high-resolution spectrometers, each designed to provide the optimum charge, mass, or charge-state resolution in its particular energy range, and each having a geometry factor optimized for the expected flux levels, so as to provide a collecting power a factor of 10 to 1000 times greater than previous or planned experiments. The payload also includes several instruments of standard design that will monitor solar wind and magnetic field conditions and energetic H, He, and electron fluxes. The scientific objectives, instrumentation, spacecraft, and mission approach that were defined for ACE during the Phase-A study period are summarized.

Stone, E. C.↗
NASA NTRS1988

Evidence for anomalous cosmic-ray hydrogen

Evidence is presented for the emergence of a measurable anomalous cosmic-ray hydrogen component in 1987 which may account for 20-40 percent of the total hydrogen flux at 60 MeV. Comparing this flux with that of anomalous cosmic-ray helium, the H I/He I ratio in the very local interstellar medium is estimated at 4, consistent with determinations from solar ultraviolet backscatter measurements.

Christian, E. R.↗
NASA NTRS1988

Toward a descriptive model of galactic cosmic rays in the heliosphere

Researchers review the elements that enter into phenomenological models of the composition, energy spectra, and the spatial and temporal variations of galactic cosmic rays, including the so-called anomalous cosmic ray component. Starting from an existing model, designed to describe the behavior of cosmic rays in the near-Earth environment, researchers suggest possible updates and improvements to this model, and then propose a quantitative approach for extending such a model into other regions of the heliosphere.

Mewaldt, R. A.↗
NASA NTRS1988

The anomalous cosmic-ray component

This brief report is intended to update the anomalous component section of the summary report of the galactic cosmic-ray working group (Mewaldt et al., 1987), which was drafted at the March 1987 Workshop on the Interplanetary Charged Particle Environment at the Jet Propulsion Laboratory. The description of the spectrum of the anomalous cosmic-ray component is contained in section 3.3 of that report. That description is based on data analyzed through day 310 of 1986, and in it we proposed that the energy spectrum of the various species of the anomalous component could be derived by scaling from two generic spectra. Two generic spectra were required because the energy spectrum of the anomalous component changed shape near the time of the solar magnetic field reversal in 1980. These two generic spectra are shown in Figure 2 of the summary report.

Cummings, A. C.↗
NASA NTRS1987

Latitudinal and radial gradients of anomalous and galactic cosmic rays in the outer heliosphere

Measurements from instruments on Voyagers 1 and 2 and Pioneer 10 are used to derive simultaneous radial and latitudinal gradients of anomalous cosmic-ray oxygen during the latter part of 1985. It is found that in the energy interval from 7.1-30.6 MeV/nucleon the latitudinal gradient is -3 to -4 percent/deg. The sign of the latitudinal gradient is opposite to that reported for the last solar cycle when the solar magnetic field polarity was reversed, as predicted by propagation models in which curvature and gradient drifts are significant. The ratios of the radial and latitudinal gradients of anomalous oxygen, anomalous helium, and galactic cosmic rays are similar, also in agreement with drift theory predictions. Based on drift theory, these observed ratios, which are near unity, lead to estimates of the perpendicular diffusion coefficient/particle velocity ratio in the range 3-8 x 10 to the 22nd sq cm/sec at 24.9 AU for particles with rigidities of about 2-3 GV.

Cummings, A. C.↗
NASA NTRS1986

Solar cycle variations of anomalous He-4 as deduced by studies of cosmic ray He-3

Voyager 1 and 2 observations have been used to derive the energy spectrum of 'anomalous' cosmic-ray (ACR) He using a technique based on the separation of the isotopes of He. Previously reported observations show that the ACR oxygen spectrum underwent a change in shape following the solar magnetic field reversal in 1980; in this paper evidence for a corresponding change in the ACR He spectrum is presented. It is found that the energy spectra of ACR He and oxygen and Galactic-cosmic-ray He-4 and He3 can be reproduced both before and after the field reversal using a simple solar modulation model in which the effects of curvature and gradient drifts are arbitrarily neglected.

Cummings, A. C.↗
NASA NTRS1986

Energetic charged particles in the Uranian magnetosphere

The cosmic ray instrument on Voyager 2 obtained detailed charged particle data on the path the spacecraft followed through the newly-discovered Uranian magnetosphere. Notably, the electron counts did not rise above background until the spacecraft was deep in the magnetosphere. The data further indicated that the satellites Miranda, Ubriel and Ariel impose a sweep effect on MeV electrons, which move radially inward from some source in the outer magnetosphere or the magnetotail. The energetic particle data also supported a firm pitch angle dependence for proton energy spectra in the 1-8 MeV range. Finally, the particle density data were commensurate with a dipole magnetic field tilted 60.1 deg in relation to the planet and rotating every 17.4 hr, close to estimates from other instruments.

Stone, E. C.↗
NASA NTRS1986

Differential measurement and model calculations of cosmic ray latitudinal gradient with respect to the heliospheric current sheet

Simultaneous magnetic field and charged particle measurements from the Voyager spacecraft with heliographic latitude separations of more than 10 deg are used to investigate the distribution of about 1-GeV galactic cosmic ray protons with respect to the heliospheric current sheet in the outer solar system. By comparing the ratio of cosmic ray flux at Voyager 1 to that at Voyager 2 during periods of relatively quiet interplanetary conditions when the spacecraft are either both north or both south of the heliospheric current sheet, an average latitude component of the gradient of the cosmic ray flux on opposite sides of the current sheet is derived under restricted interplanetary conditions of -0.22 + or - 0.03 pct/deg, equivalent to a decrease of about 1 percent/AU away from the current sheet at about 12 AU. The results for these limited periods are in qualitative agreement with propagation models incorporating particle drifts.

Christon, S. P.↗
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

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

Differential measurement of cosmic-ray gradient with respect to interplanetary current sheet

Simultaneous magnetic field and charged particle measurements from the Voyager spacecraft at heliographic latitude separations from 10 deg. to 21 deg. are used to determine the latitude gradient of the galactic cosmic ray flux with respect to the interplanetary current sheet. By comparing the ratio of cosmic ray flux at Voyager 1 to that a Voyager 2 during periods when both spacecraft are first nort and then south of the interplanetary current sheet, we find an estimate of the latitudinal gradient with respect to the current sheet of approximately -0.15 + or 0.05% deg under restricted interplanetary conditions.

Christon, S. P.↗