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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

The solar elongation distribution of low-frequency radio bursts

Over 500 days of low-frequency (less than 1 MHz) radio observations from the IMP-6 spacecraft have been accumulated to produce a two-dimensional map (frequency vs elongation) of solar type III burst occurrences. This map indicates that most solar bursts in this frequency range are observed at the second harmonic of the plasma frequency, rather than the fundamental. The map also shows that the solar wind electron density varies as an inverse power of heliocentric distance, with the exponent somewhat less than 2 to perhaps 3 or higher.

Kaiser, M. L.↗
NASA NTRS1976

The interplanetary acceleration of energetic nucleons

Corotating proton and electron streams are the dominant type of low-energy (i.e., 0.1-10 MeV per nucleon) particle event observed at 1 AU. The radial dependence of these events has been studied between 1 and 4 AU using essentially identical low-energy detector systems on IMP-7, Pioneer-10, and Pioneer-11. It had been expected that at a given energy, the intensity of these streams would decrease rapidly with heliocentric distance due to the effects of interplanetary adiabatic deceleration. Instead, it is observed that from event to event, the intensity either remains roughly constant or increases significantly (more than an order of magnitude) between 1 and 4 AU. It appears that interplanetary acceleration processes are the most plausible explanation. Several possible acceleration models are discussed.

Mcdonald, F. B.↗
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

The low energy cosmic ray H-2 and He-3 spectra and the anomalous He-4 component

Measurements of the 1974 low-energy spectra of H-2, He-3, and He-4 with the IMP-8 satellite reveal the continued presence of the anomalous flat He-4 spectrum first reported in 1972 for roughly below 50 MeV/nucleon. Measurements of the isotope quartet H-1, H-2, He-3, He-4 are reported and compared, for 1974 quiet time, and are compared to 1965 observations at near solar minimum. Comparison of H-3 and He-3 data with accepted models of interstellar propagation and solar modulation supports a galactic origin for these isotopes down to about 10 MeV/nucleon. No evidence is found for a low-energy breakdown or other anomalous behavior of galactic propagation or solar modulation models capable of accounting for the anomalous He-4 component.

Garcia-Munoz, M.↗
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

Anomalous composition and energy spectra of cosmic rays below 20 MeV/nucleon

The relative abundances and energy spectra of C, N, O, Ne, and Fe are investigated as a function of the interplanetary low energy proton intensity using observations obtained over a 17-month period by IMP-8. The oxygen intensity was enhanced when proton activity was at its lowest level, while all other heavy elements almost disappeared. Nuclei of less than 3 MeV/nucleon showed a strong intensity fluctuation strongly correlated with the interplanetary proton flux and displayed a velocity anisotropy along the garden-hose angle, indicating that they are of solar origin. The anomalous oxygen hump between 3 and 20 MeV is apparently not of solar origin. The observations are compatible with the model proposed by Fisk et al. (1974), in which these oxygen nuclei are considered to be interstellar neutral oxygen atoms which are ionized and accelerated to the observed energies by magnetic field irregularities in the outer solar system.

Klecker, B.↗
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

Anisotropy measurements of nearly 50 KeV solar protons

The Energetic Particles Experiment on IMP-7 measures the angular distribution of 50-200 keV solar protons in 16 sectors. The velocity of 50 keV protons may be less than 5 times that of the solar wind. A generalized nonlinear Compton-Getting point transformation into the co-moving frame that contains no assumptions as to the angular distribution of either the spectrum or intensity is presented. Nearly 50 keV proton data in the spacecraft frame exhibit an anisotropy ratio that is large (not less than 5) and radial throughout the October 29, 1972 event lasting more than 9 days at this energy. This anisotropy argues against impulsive injection and diffusive decay in the inner solar system. Application of the transformation to the data reveals a long lasting residual anisotropy in the co-moving frame with protons streaming from the sun. Differences between the co-moving frame and solar wind frame velocities suggest residual electric fields upstream from the bow shock.

Gold, R. E.↗
NASA NTRS1976

Anomaly in the quiet-time helium spectrum at 1 MeV per nucleon

The IMP-8 satellite observed (in 1974-1975) an unexpected hump in the solar helium spectrum between 0.6 and about 3 MeV per nucleon and an enhanced helium abundance of up to 70 MeV per nucleon. The energy spectrum of helium below 0.6 MeV per nucleon was consistent with the proton spectrum below 1.5 MeV, while the proton-to-alpha ratio was about 30. The origin of this anomalous low-energy, quiet-time helium is as yet undecided. The alpha particles could be of solar origin, could have been accelerated in the heliosphere or could be of galactic origin.

Gloeckler, G.↗
NASA NTRS1976

Correlated observations of several auroral substorms on February 17, 1971

The purpose of this study is to correlate in detail auroral activity with the corresponding disturbances in the magnetotail. The auroral data were recorded by optical instruments aboard an airplane flying over the Arctic Ocean along the Alaska meridian and by the Alaska meridian chain of all-sky cameras. The corresponding magnetotail observations were made by various instruments on Vela 6A and Imp 5; the interplanetary magnetic field was monitored concurrently by Explorer 35. Three successive substorms were observed on February 17, 1971. Each substorm was readily identified by the classical auroral and magnetic signatures which accompanied its onset. The observed variations of plasma and magnetic field in the magnetotail were consistent with the idea that a neutral line formed in the range X sub SM between -12 and -18 R sub E at the onset of each substorm expansive phase and then moved tailward past X sub SM = -18 R sub E some tens of minutes afterward. The Z component of the tail magnetic field decreased rather steadily for a period of 1-2 1/2 hours after each substorm and until the onset of the next expansive phase, reaching a minimum value just before each onset.

Hones, E. W., Jr.↗
NASA NTRS1975

Electron plasma oscillations associated with type III radio emissions and solar electrons

Results of an extensive search for electron plasma oscillations associated with type III radio noise bursts are presented which were obtained by analyzing 87 type III bursts detected in plasma-wave and charged-particle measurements carried out by IMP 6, 7, and 8. Only one case is found for which plasma oscillations were associated with electrons of solar origin; at least eight events are identified in which no plasma oscillations were detected even though electrons from solar flares were clearly evident. The type III emissions are compared with similar radiation coming from upstream of earth's bow shock at the harmonic of the local electron plasma frequency, and quantitative calculations of the rate of conversion from plasma oscillatory energy to electromagnetic radiation are performed. The results show that electron plasma oscillations are seldom observed in association with solar electron events and type III radio bursts at 1.0 AU and that neither the type III emissions nor the radiation from upstream of the bow shock can be adequately explained by a current model for the coupling of electron plasma oscillations to electromagnetic radiation. Several possible explanations are considered for this discrepancy between theory and observations.

Gurnett, D. A.↗
NASA NTRS1976

Fluxes of protons above 50 keV and electrons above 30 keV at approximately 35 earth radii. I - Velocity anisotropies and plasma flow in the magnetotail. II Morphology and flow patterns in the magnetotail

Imp-7 data were used to study the morphology and flow patterns of energetic proton and electron events in the magnetotail and magnetosheath at 35 earth radii. The patterns are deduced from the probability of occurrence and the spatial distribution of the events. The distribution of 50 to 200 keV proton and 30 to 90 keV electron fluxes shows that in the magnetotail (i.e., inside the magnetopause) the proton and electron events occur primarily in the plasma sheet region within plus or minus 10 earth radii of the neutral sheet; fluxes of 50- to 200-keV protons are also found in the magnetosheath, along with occasional fluxes of 30- to 90-keV electrons; the plasma sheet is well defined (at 35 earth radii) by both the proton and electron probabilities; the frequency of the electron events in the magnetotail shows a positive correlation with the geomagnetic Kp index in the dawn and dusk sectors.

Roelof, E. C.↗
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 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

Influence of the interplanetary magnetic field on the occurrence and thickness of the plasma mantle

The response of the plasma mantle to the orientation of the interplanetary magnetic field (IMF) has been studied by correlating Heos 2 plasma and Imp 6 magnetic field data. The mantle is nearly always present when the IMF has a southward component and often also when the field has a weak northward component. In addition, the mantle appears increasingly thicker with greater southward components. On the other hand, the mantle is thin or missing (from the region where it is normally found) when the average IMF has a strong northward component. This result supports the idea that polar cap convection plays a dominant role in the formation of the plasma mantle: mantle plasma originates in the magnetosheath, enters the magnetosphere through the day side polar cusps, and is transported across the cusp to the night side by means of a convection electric field whose magnitude is controlled by the orientation of the IMF.

Sckopke, N.↗
NASA NTRS1976

Plasma wave interactions with energetic ions near the magnetic equator

An intense band of electromagnetic noise is frequently observed near the magnetic equatorial plane at radial distance from about 2 to 9 earth radii. Recent wide band wave form measurements with the Imp 6 and Hawkeye 1 satellites have shown that the equatorial noise consists of a complex superposition of many harmonically spaced lines. Several distinctly different frequency spacings are often evident in the same spectrum. The frequency spacing typically ranges from a few hertz to a few tens of hertz. The purpose of this paper is to suggest that these waves are interacting with energetic protons, alpha particles, and other heavy ions trapped near the magnetic equator. The possible role that these waves play in controlling the distribution of the energetic ions is considered.

Gurnett, D. A.↗
NASA NTRS1976

Correlation of bow shock plasma wave turbulence with solar wind parameters

The rms field strengths of electrostatic and electromagnetic turbulence in the earth's bow shock, measured in the frequency range 20 Hz to 200 kHz with the Imp 6 satellite, are examined. The largest strengths of electrostatic turbulence occur when the upstream electron-to-proton temperature ratio is large and the proton temperature is small. No substantial correlation is found among the rms field strengths of electrostatic turbulence, the Alfven Mach number, the ratio of particle to magnetic-field pressure, and the shock normal angle. These results indicate that the strength of electrostatic turbulence in the bow shock is determined by the kinetic properties of the solar-wind plasma. The largest strengths for electromagnetic turbulence occur when the upstream particle density is large and when the shock normal angle is closer to 90 deg, supporting a previous conclusion that whistler waves comprise the electromagnetic turbulence in the bow shock.

Rodriguez, P.↗