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Mewaldt, R. A.

Publications and source records attributed to Mewaldt, R. A..

At least 127 records · Page 7
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NASA NTRS1979

Satellite measurements of the isotopic composition of galactic cosmic rays

The individual isotopes of galactic cosmic ray Ne, Mg, and Si at 100 MeV/nucleon were clearly resolved with an rms mass resolution of 0.20 amu. The results suggest the cosmic ray source is enriched in Ne-22, Mg-25, and Mg-26 when compared to the solar system. The ratio of (Mg-25)+(Mg-26) to Mg-24, which is approximately 0.49 compared to the solar system value of 0.27, suggest that the cosmic ray source and solar system material were synthesized under different conditions.

Mewaldt, R. A.↗
NASA NTRS1978

The radial diffusion coefficient of 1.3 - 2.3 MeV protons in recurrent proton streams

The paper presents anisotropy measurements of 1.3-2.3 MeV protons in the interplanetary space during recurrent events for which the radial intensity variation is measured within the radial range from about 0.3 to about 4 AU for the period from June, 1973 through April, 1976. The simultaneous measurements of the diffusive anisotropy and the radial gradient are used to make a direct estimate of the interplanetary radial diffusion coefficient, which is independent of any particular solution of the propagation equation. IMP 7 and 8 is required to be sunward of the earth and hence outside of the magnetosphere; if both spacecraft are sunward, measurements from the one farthest from the magnetosphere are used. The resulting values for the radial diffusivity coefficient near 1 AU lie in the range (3 to 9) x 10 to the 20th sq cm/sec, corresponding to scattering mean free paths between 0.03 and 0.1 AU with a mean of 0.06 AU.

Mewaldt, R. A.↗
NASA NTRS1978

A cosmic ray isotope spectrometer

The paper describes a new instrument to be flown on ISEE-C which is optimized to measure the isotopic composition of solar and galactic cosmic rays in the 5-250 MeV/nucleon range. A mass resolution of not greater than 0.3 AMU should be achieved for all elements with charge Z not exceeding 30.

Althouse, W. E.↗
NASA NTRS1977

Neutral particle background in cosmic ray telescopes composed of silicon solid state detectors

The energy loss-spectrum of secondary charged particles produced by the interaction of gamma-rays and energetic neutrons in silicon solid state detectors has been measured with a satellite-borne cosmic ray telescope. In the satellite measurements presented here two distinct neutral background effects are identified: secondary protons and alpha particles with energies of about 2 to 100 MeV produced by neutron interactions, and secondary electrons with energies of about 0.2 to 10 MeV produced by X-ray interactions. The implications of this neutral background for satellite measurements of low energy cosmic rays are discussed, and suggestions are given for applying these results to other detector systems in order to estimate background contamination and optimize detector system design.

Mewaldt, R. A.↗
NASA NTRS1976

The isotopic composition of hydrogen and helium in low-energy cosmic rays

The isotopes H-2 and He-3 have been identified in low-energy cosmic rays during solar-quiet periods from 1973 January to 1974 October. These observations, made with the electron/isotope spectrometer on IMP-7, cover the energy intervals 5-29 MeV per nucleon for H-2 and 7-50 MeV per nucleon for He-3. The energy spectra of H-1, H-2, and He-3 fall rapidly with decreasing energy, giving H-2/H-1 and He-3/H-1 ratios which are essentially independent of energy as expected from current theories of the solar modulation of galactic cosmic rays. The measured He-4 spectrum, however, is essentially flat below 40 MeV per nucleon, suggesting that there may be contributions from a local, nonsolar source of He-4. Comparisons of the H-1, H-2, and He-3 observations with calculated spectra at 1 AU imply a mean interstellar path length of 7 plus or minus 2 g/sq cm. However, present low-energy measurements of H and He isotopes at 1 AU do not discriminate between possible cosmic-ray source spectra.

Mewaldt, R. A.↗
NASA NTRS1976

Observations of Jovian electrons at 1 AU

It has recently been suggested that electrons of Jovian origin are responsible for the quiet-time increases in the electron intensity greater than 3 MeV observed at 1 AU. Using data from electron/isotope spectrometers on Imp 7 and 8, the temporal behavior of quiet-time electrons at 1 AU is studied over the period October 1972 through May 1975. The 1- to 6-MeV electron intensity is found to vary by a factor of about 10 from one quiet-time period to another, including numerous short-term enhancements characteristic of the quiet-time increases reported during 1964-1972. The magnitude and frequency of the increases grow abruptly and remain high for an approximate 4-month period beginning with the time at which the interplanetary field line first connects earth and Jupiter. It is suggested that the interconnection of the interplanetary field with an extended Jovian magnetotail of about 2 AU in length could result in the observed longitudinal distribution of Jovian electrons of 1 AU.

Mewaldt, R. A.↗
NASA NTRS1975

Enrichment of heavy nuclei in He-3-rich flares

IMP-7 observations of five solar-flare particle events characterized by He-3 enrichment are reported which show that such events are also enriched in nuclei with charges (Z) of at least 6. The ratio of these nuclei to H-1 at approximately 3 MeV/nucleon was found to be enriched by about 10 to 100 times, while the ratio with respect to He-4 was enriched by about 3 to 30 times. It is suggested that the simultaneous enhancement of He-3 and the heavier nuclei as well as the absence of H-2 and H-3 during the observed events may be partly due to a preferential acceleration process which depends on the ratio of the square of the charge to the atomic weight of the nuclei.

Hurford, G. J.↗
NASA NTRS1975

The isotopic composition of hydrogen and helium in low energy cosmic rays

The Caltech Electron/Isotope Spectrometer on IMP-7 has been used to identify the isotopes H-2 and He-3 in low energy cosmic rays during solar quiet periods from October 1972 to October 1974. These observations cover the energy intervals 5 to 29 MeV/nuc for H-2 and 7 to 50 MeV/nuc for He-3. The energy spectra of H-1, H-2, and He-3 all fall rapidly with decreasing energy, giving H-2/H-1 and He-3/H-1 ratios essentially independent of energy as expected from adiabatic acceleration. The measured He-4 spectrum, however, was essentially flat over this energy interval, and therefore the H-2/He-4 ratio observed at 1 AU is not simply related to the interstellar abundances of these nuclei. However, comparisons of the H-2/H-1 and He-3/H-1 ratios with calculated spectra are possible.

Mewaldt, R. A.↗
NASA NTRS1975

Isotopic composition of the anomalous low energy cosmic ray nitrogen and oxygen

The isotopic composition of the enhanced fluxes of cosmic ray nitrogen and oxygen observed below 30 MeV/nuc is of interest, whether the nuclei are a sample from some nearby galactic source region that is underabundant in carbon, or a sample of the neutral interstellar medium. Enhanced fluxes in the 6 to 12 MeV/nuc energy interval were measured over a two year period with the Caltech Electron/Isotope Spectrometer on IMP-7. The observed low-energy nitrogen and oxygen nuclei are predominantly N-14 and O-16, with upper limits (84% confidence level) of N-15/N below 0.26, O-17/0 below 0.13, and O-18/0 below 0.12 for other isotopes in the 6-12 MeV/nuc energy interval. The implications of these results for the origin of the enhanced nitrogen and oxygen fluxes are discussed.

Mewaldt, R. A.↗
NASA NTRS1975

Observations of low energy interplanetary electrons

Observations of the temporal behavior of low energy (0.16-6 MeV) at 1 AU are reported. The electron intensity is found to vary by a factor of more than five from one quiet time to another, including short-term enhancements of the type reported at higher electron energies. Over a period of about four months, beginning with the time at which the interplanetary field line first connects earth and Jupiter, the magnitude and frequency of the increases grow abruptly and remain high. The observed longitudinal distribution of Jovian electrons could be the result of the interconnection of the interplanetary field with an extended Jovian magnetotail.

Mewaldt, R. A.↗
NASA NTRS1975

The quiet time spectra of low energy hydrogen and helium nuclei

Measurements of the 1972-1973 quiet time hydrogen and helium spectra from 1.3-40 MeV/nuc are discussed. For both spectra the relative-intensity minimum occurs at lower energies than those reported for earlier years. There is no evidence of a low energy turnup in the He spectrum down to 2.4 MeV/nuc. The spectra indicate that the galactic component dominates down to about 10 MeV; a stable, non-solar He-4 component extends from higher energies down to about 2.4 MeV/nuc. At lower energies the periods of minimum H and He intensity do not coincide, and the relative abundance of H and He at 1.3-2.3 MeV/nuc is variable, with H/He ratios ranging from about 3 to about 10. The observations suggest that the 1.3-2.3 MeV/nuc protons and alphas are of solar origin.

Mewaldt, R. A.↗
NASA NTRS1975

The elemental composition of 4-30 MeV/nuc cosmic ray nuclei with Z between 1 and 8

The fluxes of elements from lithium through oxygen at energies down to about 4 MeV/nucleon were measured during solar quiet periods from October 1972 to September 1974 by spectrometers aboard IMPs 7 and 8. The energy spectra of Li, Be, B, and C are consistent with those expected from the adiabatic deceleration of higher energy galactic cosmic rays. The He, N, and O fluxes are greatly enhanced, with relative abundances of O/He of about 0.25, O/B of about 100, and O/C of about 30 at energies of 5 to 12.5 MeV/nucleon. There is qualitative agreement between the observed He, N, O, and Ne fluxes and the source abundances estimated from the model proposed by Fisk et al. (1974); however, the observed hydrogen flux is about 1000 times lower than the source hydrogen.

Mewaldt, R. A.↗
NASA NTRS1975

Implications of time variations for the origin of low energy cosmic ray nitrogen and oxygen nuclei

Time variations have been observed in the quiet time intensities of 5-27 MeV/nuc nitrogen and oxygen and 13-25 MeV/nuc helium nuclei measured by spectrometers aboard IMPs 7 and 8. There is no significant correlation of the O variations with variations in the low intensity fluxes of 1-2 MeV solar protons, but the O intensity is well correlated with the modulation of galactic cosmic rays. These observations indicate that the enhanced low energy N and O fluxes are not of solar origin, and suggest that the acceleration mechanism required by the model of Fisk et al. (1974) must be located far enough from the sun that the singly charged ions, once accelerated, undergo considerable modulation in penetrating to 1 AU. Comparison of the time variations of He and O fluxes may yield information on the charge state of these nuclei, since these variations are strongly correlated and exhibit similar hysteresis effects when compared with the neutron monitor.

Mewaldt, R. A.↗
NASA NTRS1975

A search for solar flare positrons

The detection of solar gamma-ray line emission and observations of the isotopes H2, H-3, and He-3 in solar cosmic rays provide direct evidence for the occurrence of high energy nuclear reactions in solar flare events. Appreciable numbers of other reaction products, including positrons with energies near about 1 MeV, should also be produced in such events. We have searched for positrons in the 0.16-1.6 MeV energy interval during 5 H-3 rich solar particle events. Based on calculations of positron and He-3 production at the sun, and using a simplified model of interplanetary propagation, we might expect comparable fluences of positrons and He-3 to be observed. Summing over these 5 events, we find the 0.16 to 1.6 MeV positron fluence to be a maximum of about 10% of the He-3 fluence with more tnan 1 MeV/nuc. This suggests that other processes, such as preferential trapping by the solar magnetic field, may be important.

Mewaldt, R. A.↗
NASA NTRS1974

The energy spectrum of 0.16 to 2 MeV electrons during solar quiet times

New observations of the quiet-time energy spectrum of 0.16- to 2-MeV electrons have been made with the Caltech Electron/Isotope Spectrometer which was launched on IMP-7 in September 1972. Earlier measurements of quiet-time electrons in this energy range by other groups have resulted in spectra differing by more than an order of magnitude in intensity. A minimum quiet-time flux level somewhat lower than the lowest previously reported spectra and consistent with an extrapolation of the spectrum measured at higher energies is found. A galactic secondary source of knock-on electrons is consistent with these results and with independent studies of the interstellar spectra of cosmic-ray nuclei, provided that solar modulation does not suppress the 0.16- to 2-MeV electron flux by more than a factor of about 3. Although not required, other recently suggested sources may also contribute to the observed fluxes.

Hurford, G. J.↗
NASA NTRS1974

The energy spectrum of 0.16 to 2 MeV electrons during solar quiet times

New observations of the quiet-time energy spectrum of 0.16 to 2 MeV electrons were made with the Caltech Electron/Isotope Spectrometer which was launched on IMP-7 in September 1972. Earlier measurements of quiet-time electrons in this energy range by other groups have resulted in spectra differing by more than an order of magnitude in intensity. A minimum quiet-time flux somewhat lower than the lowest previously reported spectra and consistent with an extrapolation of the spectrum measured at higher energies was found. A galactic secondary source of knock-on electrons is consistent with the results and with independent studies of the interstellar spectra of cosmic ray nuclei provided that solar modulation does not suppress the 0.162 MeV electron flux by more than a factor of approximately 3. Although not required, other recently suggested sources may also contribute to the observed fluxes.

Hurford, G. J.↗
NASA NTRS1974

Observations of low energy hydrogen and helium isotopes during solar quiet times

Results of a new quiet-time measurement of the relative abundance of cosmic-ray H-2 and He-4. The observations were made in selected time intervals between September 1972 and February 1973 with the Caltech Electron/Isotope Spectrometer on IMP-7. In the energy interval from 13 to 29 MeV/nucleon, an upper limit to the H-2 to He-4 ratio of less than 0.06 is found. This new upper limit is significantly lower than finite H-2/He-4 ratios measured in earlier years by other workers. Possible implications of this new result are discussed.

Hurford, G. J.↗