Engineering topics

Sentman, D. D.

Publications and source records attributed to Sentman, D. D..

30 records · Page 2

NASA NTRS1979

Magnetospheric multiharmonic instabilities

The paper discusses linear convective growth rates of instabilities of electrostatic multiple electron cyclotron harmonic waves in a plasma consisting of a hot electron component with a loss-cone type of free energy source and a cold electron component of presumably ionospheric origin. When the ratio of cold to hot electron temperature is small, the cold upper hybrid frequency controls the harmonic bands that can be nonconvectively stable. When this ratio increases above a few times 0.01, nonconvective instability disappears simultaneously for each harmonic band, when the density ratio is less than unity. A consistent interpretation of the spatial localization and harmonic frequency bandwidths of the observed waves can be made assuming linear convective saturation, provided that the cold electrons have temperatures considerably in excess of those in the ionosphere.

Ashour-Abdalla, M.↗

NASA NTRS1978

Correction to 'Recirculation of energetic particles in Jupiter's magnetosphere'

An error in Pioneer 11 data reduction software has, when present, caused a phase shift of 180 deg in the assignment of spacecraft roll angles. The corrected analysis of the pitch angle distributions of energetic particles in Jupiter's magnetosphere reveals significant proton anisotropies directed toward the planet in the southern hemisphere, contrary to the authors' (1975) original report. In the northern hemisphere, both proton and electron anisotropies are directed away from the planet, as reported previously. The revised data show that the claim of direct evidence for the hypothesis of recirculation of energetic particles in the Jovian magnetosphere is invalid. It is suggested that indirect evidence still supports the hypothesis, although the recirculation process must be weaker than originally envisioned and obscured by other processes.

Sentman, D. D.↗

NASA NTRS1978

Whistler mode noise in Jupiter's inner magnetosphere

A study is made of the amplitude and spectral extent of whistler mode noise in the inner magnetosphere of Jupiter. It is found that the 'hat-shaped' pitch angle distributions of energetic electrons (21 and 31 MeV at L=3) are consistent with those predicted in the presence of a band-limited spectrum of whistler mode noise. The equatorial maximum linear growth rate of parallel propagating whistlers are consistent with those necessary to limit the energetic electron intensities by the whistler mode instability. It is noted that the wave phase speeds before wave reflection can occur at high latitudes and that wave growth is limited to a disk-like region centered around the magnetic equator. The frequency extent of the whistler mode noise spectrum may be estimated by the range of frequencies maximally unstable to equatorial linear growth. A value is found for the spectral density of the broadband whistler mode noise necessary to balance radial diffusion of energetic electrons above the critical range, and an expression is derived for the energetic electron system response to fluctuations about the limiting flux value.

Sentman, D. D.↗

NASA NTRS1976

Angular distributions of electrons of energy greater than 0.06 MeV in the Jovian magnetosphere

Measurements of the angular distribution of electron intensity in the Jupiter magnetosphere were made by the University of Iowa experiment on Pioneer 10. The data were from three directional particle detectors with effective integral threshold of 0.06, 0.55 and 0.5 MeV respectively. It was discovered that the central core of the magnetosphere for radial distances less than 12 Jovian radii is dominated by pitch angle distributions strongly peaked at alpha = 90 degrees, while the region of radial distances 12-25 Jovian radii shows bidirectional and approximately equal maxima at alpha = 0 and 180 degrees. Substantial field-aligned streaming was detected on two occasions, each of about 1 hour duration. No distinctive effects on angular distributions were observed near the L shells of satellites.

Sentman, D. D.↗

NASA NTRS1975

Recirculation of energetic particles in Jupiter's magnetosphere

A significant new finding from analysis of Pioneer 11 observations in the magnetosphere of Jupiter is that there is net streaming of both electrons E above 40 keV and E above 560 keV and protons in the range from .61 to 3.41 MeV away from the planet along high-latitude field lines. This result is compatible with the recent suggestion of Nishida that energetic particles undergo trans-L shell diffusion at low altitudes without significant change of energy. This provides a plausible explanation for the remarkable pitch angle distributions near the equator in the range of L values from 12 to 25; the presence of particles of about 1 MeV energy at the outer edge of the magnetosphere; and hence, via conventional inward diffusion processes, the presence of those having magnetic moments of several hundred MeV per gauss in the inner magnetosphere. The recirculation of energetic particles emerges as an important dynamical feature of the Jovian magnetosphere.-

Sentman, D. D.↗

NASA NTRS1975

Pioneer 11 observations of energetic particles in the Jovian magnetosphere

A preliminary report is presented of energetic electrons and protons observed with the University of Iowa instrument on Pioneer 11. A graph shows absolute, spin-averaged unidirectional intensities of electrons and protons as a function of time during traversal of the central magnetosphere. Another graph shows the effects of the Jovian satellites Io and Amalthea on particle intensities. It is pointed out that a full analysis of satellite effects is the most promising technique for understanding the physical dynamics of the magnetosphere of Jupiter.

Van Allen, J. A.↗

NASA NTRS1975

Angular distributions of electrons of energy E sub e greater than 0.06 MeV in the Jovian magnetosphere

The results of an angular distribution analysis of the electron intensity data recorded near Jupiter for the period from 26 November to 14 December 1973 are presented. The data were from three directional particle detectors with effective integral electron energy thresholds of 0.06, 0.55, and 5.0 Mev, respectively. It was found that the central core of the magnetosphere, within 12 Jupiter radii, is dominated by pitch angle distributions strongly peaked at alpha = 90 deg, while the region from 12 to 25 Jupiter radii shows bidirectional and approximately equal maxima at alpha = 0 and 180 deg. Bidirectional angular distributions in the magnetodisc out to the radius of the magnetopause strongly suggest quasi-trapping on closed field lines as the predominant situation. Substantial field aligned, unidirectional streaming was detected on only two occasions. No distinctive effects on angular distributions were discerned near the L-shells of satellites.

Sentman, D. D.↗

NASA NTRS1974

The magnetosphere of Jupiter as observed with Pioneer 10. I - Instrument and principal findings

Description of the first in situ observations of energetic electrons of energy greater than 0.06 MeV in the magnetosphere of Jupiter during November-December 1973. The outer magnetosphere has the form of a thin disklike quasi-trapping region extending from about 20 to over 100 Jovian radii. This magnetodisk is confined near the magnetic equatorial plane and has approximate axial symmetry about the magnetic axis of the planet. The observations inside a radial distance of 12 Jovian radii are well organized by a centered dipolar model of the planet's magnetic field with a tilt of 9.5 plus or minus 0.5 deg to the rotational axis and with pole at a system III longitude of 230 plus or minus 3 deg. Absolute omnidirectional intensities of electrons within the stable trapping region inside 20 Jovian radii are given for five energy ranges greater than 0.06, 0.55, 5.0, 21, and 31 MeV.

Van Allen, J. A.↗

NASA NTRS1974

A search for 5 min periodic structure in solar 2 cm emission

Two hundred and eighty-five hours of solar data obtained from a 2 cm radiometer during 1968-1969 were analyzed for evidence of periodic structure related to the 5 min periodic chromospheric oscillations detected in optical line emissions. A power spectral analysis of the data failed to show any statistically significant periodic activity in the frequency range 1-15 mHz for data organized according to solar activity in H-alpha, soft solar X-rays (2-12 A), and several microwave frequencies (3-15 GHz). A small shift in power from low to higher frequencies in the power spectrum of the 2 cm data was found to be correlated with H-alpha and X-ray activity. This power shift is attributed to a relative increase in chromospheric turbulence at altitudes common to H-alpha, X-ray, and 2 cm emission.

Sentman, D. D.↗

NASA NTRS1974

Energetic electrons in the magnetosphere of Jupiter

Observations of energetic electrons (above 7 MeV) show that the outer magnetosphere of Jupiter consists of a thin disklike, quasi-trapping region extending from about 20 to 100 planetary radii. This magnetodisk is confined to the vicinity of the magnetic equatorial plane and appears to be an approximate figure of revolution about the magnetic axis of the planet. Hard trapping is observed within a radial distance of about 20 Jovian radii. The omnidirectional intensity of electrons with energy above 21 MeV between 3 and 20 Jovian radii is described by a provisional expression in terms of radial distance and magnetic latitude. There is tentative evidence for mild effects of the Galilean satellite Europa and possibly Io and Ganymede but not Callisto.

Van Allen, J. A.↗

NASA NTRS1974

Basic elements of power spectral analysis

The basic elements of power spectral analysis with emphasis on the Blackman-Tukey method are presented. Short discussions are included on the topics of pre-whitening, frequency and spectral windows, and statistical reliability. Examples are included whenever possible, and a FORTRAN subroutine for calculating a power spectrum is presented.

Sentman, D. D.↗

NASA NTRS1973

A search for periodic structure in solar 2 cm microwave radiation

A power spectral analysis of 285 hr of 2 cm microwave intensity data showed no statistically significant ( 96% confidence) periodicities in the frequency range 1 to 15 mHz. No correlation was found between 2 cm periodicities and solar activity in H alpha, X-ray, and several microwave frequencies. A small shift of power toward higher frequencies in the power spectrum of the 2 cm data was found to be correlated with solar H alpha and X-ray activity. Using the statistical properties of power spectra, an expression for the ratio of the minimum detectable peak-to-peak to ambient temperature at chromospheric heights may be derived. Applied to a model for oscillation bursts in quiescent supergranules, and using the most significant results of experiments to detect the microwave periodicities, this expression yields an upper limit of approximately .0015.

Sentman, D. D.↗