Engineering topics

Imhof, W. L.

Publications and source records attributed to Imhof, W. L..

At least 19 records

NASA NTRS1996

Magnetospheric Atmospheric X-ray Imaging Experiment (MAXIE)

This report summarizes the activities sponsored by the Office of Naval Research for the Magnetospheric Atmospheric X-ray Imaging Experiment (MAXIE). The MAXIE instrument was developed as a joint activity of Lockheed, The Aerospace Corporation, and the University of Bergen, Norway. Lockheed was responsible for the overall management of the program, interfacing with the appropriate government agencies, the overall electrical and mechanical design, flight software, environmental testing, spacecraft integration activities, on orbit checkout, and data processing activities. The Magnetospheric Atmospheric X-ray Imaging Experiment (MAXIE), the ONR 401 experiment, is the first in a new class of satellite-borne remote sensing instruments. The primary innovation is the ability to obtain rapid, sequential, images with high sensitivity of the earth's X ray aurora from a low altitude polar orbiting satellite. These images can be used to identify dynamic temporal variations in the three-dimensional (energy and position) distribution of electron precipitation into the atmosphere. MAXIE was launched on the TIROS NOAA-13 satellite on 9 August 1993. The experiment performed well during its turn-on sequence; however, the spacecraft bus failed on 21 August 1993. New spacebased technologies successfully used in MAXIE were mixed-mode ASIC microcircuits, a zero torque scanning system with associated viscoelastic damping, a paraffin stow release mechanism, a parallel integrating PHA processor, a low noise Si(Li) sensor telescope, and an advanced thermal cooling system. MAXIE's on orbit operation, control of penetrating particle backgrounds, and scientific data indicated good overall performance.

Imhof, W. L.↗

NASA NTRS1995

Relativistic electron fluxes in May 1992 and their effect on the middle atmosphere

Enhancements in the fluxes of relativistic electrons trapped within the Earth's magnetosphere have been measured by the high-energy particle spectrometer, part of the particle environment monitor on the upper atmosphere research satellite (UARS). The largest increase in the electron fluxes with energies greater than 1 MeV observed on UARS from October 1991 through July 1994 was in early May 1992. The fluxes of trapped electrons in the drift loss cone and locally precipitating electrons showed differing buildup and decay rates as a function of invariant latitude. Increases of more than 2 orders of magnitude were observed in drift loss cone fluxes at magnetic latitudes of 40 deg-66 deg and in precipitating fluxes from 48 deg to 66 deg. The energy flux contained in the most intense local precipitation observed was approximately 0.1 erg/sq cm/s, entering the atmosphere and creating up to 1000 ion pairs/cu cm/s at 55-km altitude. The daily averaged energy flux from directly precipitating electrons with energies greater than 1 MeV deposited greater than 10(exp 20) erg/d worldwide into the atmosphere for the period May 12-21, 1992, producing greater than 10(exp 31) odd nitrogen molecules below 60-km altitude.

Gaines, E. E.↗

NASA NTRS1994

Relativistic electron flux comparisons at low and high altitudes with fast time resolution and broad spatial coverage

Analyses are presented for the first high-time resolution multisatellite study of the spatial and temporal characteristics of a relativistic electron enhancement event with a rapid onset. Measurements of MeV electrons were made from two low-altitude polar orbiting satellites and three spacecraft at synchronous altitude. The electron fluxes observed by the low-altitude satellites include precipitating electrons in both the bounce and drift loss cones as well as electrons that are stably trapped, whereas the observations at geosynchronous altitude are dominated by the trapped population. The fluxes of greater than 1 MeV electrons at low-satellite altitude over a wide range of L shells tracked very well the fluxes greater than 0.93 MeV at synchronous altitude.

Imhof, W. L.↗

NASA NTRS1994

Simultaneous measurements of waves and precipitating electrons near the equator in the outer radiation belt

An investigation of wave-particle interactions is made using several simultaneous electron and wave measurements performed at near-equatorial positions from the Combined Release and Radiation Effects Satellite (CRRES) satellite. Bursts of electron precipitation were observed, most frequently at local times near dawn. Examples of bursts are presented in which the fluxes of the precipitating electrons and the wave intensities are correlated with coefficients as high as 0.7. During bursts the frequencies of the enhanced waves spanned a wide range from 311 Hz to 3.11 kHz, and the energies of the enhanced electrons were in the range 1.7 keV to 288 keV. The changes of the precipitating fluxes were generally less pronounced at the lowest energies. On the basis of electron-cyclotron resonant calculations using the cold plasma densities and ambient magnetic fields taken from the CRRES measurements it was found that the wave frequencies and precipitating electron energies were generally consistent with those expected from electron resonance with parallel propagating whistler waves. The electron data of principal concern here were acquired in and about the loss cone with narrow angular resolution spectrometers covering the energy range 340 eV to 5 MeV. The wave data included electric field measurements spanning frequencies from 5 Hz to 400 kHz and magnetic field measurements from 5 Hz to 10 kHz.

Imhof, W. L.↗

NASA NTRS1993

Observations of the UARS Particle Environment Monitor and computation of ionization rates in the middle and upper atmosphere during a geomagnetic storm

In this paper we present observations made by the Particle Environment Monitor (PEM) instruments during the geomagnetic storm of 8-9 November, 1991. Ionization and energy deposition rates as functions of altitude in the middle and upper atmosphere by incident electrons and positive ions in the storm interval are computed. The suite of PEM instruments provides a systematic measurement of energetic particles and their associated X-rays over an energy range not fully covered by previous satellite missions.

Sharber, J. R.↗

NASA NTRS1992

Simultaneous equatorial measurements of waves and precipitating electrons in the outer radiation belt

Simultaneous wave and precipitating electron measurements near the equator in the outer radiation belt have been made from the CRRES satellite. The electron data of principal concern here were acquired in and about the loss cone with narrow angular resolution spectrometers covering the energy range 340 eV to 5 MeV. The wave data included electric field measurements spanning frequencies from 5 Hz to 400 kHz and magnetic field measurements from 5 Hz to 10 kHz. This paper presents examples in which the variations in electron fluxes in the loss cone and the wave intensities were correlated. These variations in electron flux were confined to pitch angles less than about 30 deg. The association between the flux enhancements and the waves is consistent with wave-induced pitch angle diffusion processes.

Imhof, W. L.↗

NASA NTRS1992

Global spectroscopy and imaging of atmospheric X-ray bremsstrahlung - Instrumentation and initial results from the PEM/AXIS instrument aboard the Upper Atmosphere Research Satellite

The Atmospheric X-ray Imaging Spectrometer (PEM/AXIS) aboard NASA's Upper Atmosphere Research Satellite provides continuous horizon to horizon images, both day and night, of the 3- to 100-keV X-ray flux emitted from the top of the atmosphere. AXIS achieves a spatial resolution to better than 100 km using a one-dimensional array of 16 passively cooled silicon detectors. The primary purpose of this instrument is to provide a global monitor of electron energy input to the upper atmosphere. We describe the design, development, and calibration of AXIS and provide an assessment of its excellent on-orbit performance. The unique capabilities of X-ray imaging spectrometers are demonstrated through an analysis of specific examples from October and November 1991. Important new developments for follow-on instruments also will be described.

Chenette, D. L.↗

NASA NTRS1992

Auroral X-ray imaging from high- and low-earth orbit

Observations of bremsstrahlung X-rays emitted by energetic electrons impacting the earth's atmosphere can be used for remotely sensing the morphology, intensity, and energy spectra of electron precipitation from the magnetosphere. Two auroral X-ray imagers, developed for future spaceflight, will be discussed. Polar Ionospheric X-ray Imaging Experiment is scheduled for launch on the NASA International Solar-Terrestrial Physics/Global Geospace Science program POLAR satellite in May, 1994. The POLAR orbit, with an apogee and perigee of 9 and 1.8 R(E) (earth radii), respectively, affords the opportunity to image the aurora from high altitude above the north pole continuously for several hours. The Magnetospheric Atmospheric X-ray Imaging Experiment is scheduled for launch aboard the NOAA-I satellite in late 1992. The 800-km polar orbit passes over both the northern and southern auroral zones every 101 minutes. The presentation emphasizes the experimental approaches used to exploit these very different orbits for remote sensing of the earth's auroral zones.

Mckenzie, D. L.↗

NASA NTRS1991

Relativistic electron enhancements - Simultaneous measurements from synchronous and low altitude satellites

This paper presents, for the first time, simultaneous measurements of trapped relativistic electron enhancements at synchronous altitude and precipitating electrons in the bounce loss cone at low altitudes. The measurements show that the daily variations in the precipitation flux for L greater than 5 correlated well with the daily variations in the total flux at high altitude, both with respect to sudden enhancements as well as flux depletions. The daily averaged precipitating flux (E greater than 1 MeV) at L = 6.1 to 7.1 was about 0.3 percent of the daily averaged directional flux (E not less than 1.5 MeV) observed at synchronous altitude, whereas within narrow spikes the precipitating directional fluxes were often within a factor of 10 of the daily averaged trapped fluxes. Strong depletions in the synchronous altitude not less than 1.5 MeV electron intensities are shown to be associated with low-altitude measurements of the equatorward movement of precipitating spikes to lower L shells.

Imhof, W. L.↗

NASA NTRS1988

Observations of intense trapped electron fluxes at synchronous altitudes

The concept of flux limiting in the outer radiation belt proposed by Kennel and Petschek (1966) has been tested in a dynamic situation by using data acquired with instruments aboard the SCATHA satellite. A case-by-case analysis of 12 events for evidence of flux limiting under various magnetospheric conditions is made. The reuslts indicate qualitative agreement with the flux limiting theory for all the events studied. Even the quiescent events and hard-spectrum events are consistent with flux limiting. The limiting flux level at any instant appears to depend strongly on the recent history of the trapped electrons and plasma in the outer magnetosphere.

Davidson, G. T.↗

NASA NTRS1985

Morphological study of energetic electron precipitation events using the satellite bremsstrahlung X ray technique

The precipitation of energetic electrons into the atmosphere is investigated with simultaneous measurements of bremsstrahlung X-rays, emitted from different local time sectors. The measurements were performed from the low-altitude, polar-orbiting satellite P78-1 with an array of X-ray spectrometers (21-139 keV). Magnetic local-time (MLT) profiles of the intensities and energy spectra were obtained for a broad span of L values. From approximately 0930 to 1400 MLT the average X-ray intensity was found to decrease with increasing magnetic local time, whereas from 2200 to 0200 MLT no clear trend with local time was observed. The data presented are noted to correspond closely with the relative electron precipitation (REP)-type events, though no evidence of a distinct group of REP was found.

Imhof, W. L.↗

NASA NTRS1985

Precipitation of radiation belt electrons by man-made waves A comparison between theory and measurement

The temporal and spectral shape and the absolute flux level of particle pulses precipitated by a VLF transmitter are examined from a theoretical point of view. A test-particle model of the gyroresonant wave-particle interaction is applied to the parameters of the observed cases for calculating the precipitation characteristics. The temporal shapes of the precipitation pulses are found to be controlled (1) by the pitch angle dependence of the particle distribution near the edge of the loss cone and (2) by the multiple interaction of the particles with the waves due to significant atmospheric backscatter.

Inan, U. S.↗

NASA NTRS1983

Narrow spectral peaks in electrons precipitating from the slot region

Narrow L-dependent peaks commonly occur in the energy spectra of electrons precipitating from the inner radiation belt at L approximately equal to 1.5-1.85, and the cause of the peaks has been attributed to cyclotron resonance interactions with waves generated by VLF transmitters. In the slot region, L-dependent peaks have also been reported at L approximately equal to 2-3.5, but these have been predominately wider and consistent with their origin being cyclotron resonance interactions involving naturally occurring hiss. The infrequent occurrence of narrow peaks in electrons precipitating at L greater than or approximately equal to 2 is investigated. From coordinated wave and plasma density measurements it is found that if the narrow peaks are formed by first-order cyclotron resonant interactions occurring close to the magnetic equator between narrow band waves and the trapped electron population, then the equatorial plasma density gradients are unusually steep. This finding is consistent with the evidence previously obtained by other techniques for structured plasma density profiles in that L shell region of space.

Imhof, W. L.↗

NASA NTRS1983

X-ray sensor development for magnetospheric research

The ARPA-301 data on the P78-1 satellite provide information on the long-term X-ray environment; specifically, the distributions in total X-ray intensities and energy spectral parameters and the spatial extents of the X-ray sources. The average X-ray intensity near noontime was found to decrease with increasing local time whereas the opposite trend occurs around midnight. At both of these local times, the average flux increases with increasing level of geomagnetic activity. The distributions in flux are also considered for given local time intervals and these span a large dynamic range. The temporal correlation between the X-ray flux in different local time sectors is found to decrease with increasing difference in local time. The average spectral shapes do not depend significantly upon local time or geomagnetic activity.

Imhof, W. L.↗

NASA NTRS1983

Computer simulation study of multiple germanium gamma-ray sensor arrays

Design studies of large high resolution gamma-ray spectrometer systems employing multiple sensors are greatly facilitated by the use of computer simulation techniques. A study has been made by interfacing a detector geometry code with the EGS-3 version of the Stanford Linear Accelerator Center Monte-Carlo Code which has been modified to run on our VAX computer. The detector responses have been simulated for input photons with energies between 0.1 and 10 MeV. Examples are given to illustrate the performance of complex sensor arrays with different configurations of the anticoincidence system. Of particular interest is a design composed of an array of 7 detector elements each consisting of a thin planar Ge (1 cm thick) detector mounted in front of and in tandem with a large coaxial Ge detector.

Nakano, G. H.↗

NASA NTRS1982

Coordinated measurements of slot region electron precipitation by plasmaspheric wave bands

Simultaneous measurements have been made of slot region precipitating electron spectra, wave frequency distributions and plasma density profiles using the P78-1 low altitude satellite and ISEE 1 spacecraft. Broad bands are often observed in the electron energy spectra with a well-defined low energy cutoff which decreases in energy with increasing L value. This is consistent with calculations for first-order cyclotron resonance with waves at the high frequency cutoff of the band. On the basis of electron spectra and plasma densities observed from upper hybrid resonance frequencies, hiss cutoff frequencies are calculated for an assumed first-order cyclotron resonance interaction and compared with wave spectral density profiles. Plasma wave and electron measurements are generally consistent, supporting the precipitation mechanism hypothesis.

Imhof, W. L.↗

NASA NTRS1981

The significance of VLF transmitters in the precipitation of inner belt electrons

The launch of the P78-1 low-altitude satellite containing a high-resolution electron spectrometer and the ISEE spacecraft with a plasma wave experiment have made it possible to perform simultaneous measurements of the waves, the plasma densities, and the precipitating electrons. It was found that nearly monochromatic waves in the range from 10 kHz to 25 kHz were frequently present in the near equatorial regions, their frequencies, narrow bandwidth, and geographic locations being consistent with various ground-based VLF transmitters as the sources. In the energy spectra of the precipitating electrons observed at low altitudes, narrow and at times multiple peaks often appear with central energies that decrease with increasing L shell. The narrow widths suggest that the interactions take place within a restricted range of latitudes which, based on other considerations, are likely equatorial.

Imhof, W. L.↗

NASA NTRS1981

Role of energetic particles in charging/discharging of spacecraft dielectrics

The role that energetic particles in the substorm plasma have on the charging and discharging of typical dielectric layers used on spacecraft was investigated using spectra and pitch angle distributions measured in situ on the SCATHA spacecraft prior to and during a few kilovolt differential charging event in eclipse conditions. The particle spectra was input to deposition codes that determine the dose rate as a function of depth in Kapton and Teflon layers used in an experiment on SCATHA. The calculated ambient dose rates of a few rads/sec throughout the bulk of the samples are sufficiently high that radiation damage levels are reached on the time scale of 1 year. Surface dose is a factor of 100 higher. Bulk conductivity profiles were obtained from the dose rates using empirical relationships. The radiation induced bulk conductivities calculated at the peak charging time are found to be smaller than the intrinsic dark conductivity range of solar conditioned Kapton but higher than the corresponding value for Teflon.

Reagan, J. B.↗