Engineering topics

Hoffman, R. A.

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

At least 55 records · Page 3

NASA NTRS1981

The Low Altitude Plasma Instrument /LAPI/

The Low Altitude Plasma Instrument on the Dynamics Explorer-B spacecraft provides high resolution velocity space measurements of positive ions and electrons from 5 eV to 32 keV and a monitor of electrons with energies above 35 keV. It consists of an array of 15 parabolic electrostatic analyzers spanning 180 deg in angle and two Geiger-Mueller counters mounted on a one-degree of freedom scan platform. The platform is controlled by a magnetometer that allows placement of the array to selected angles with respect to the magnetic field. Each parabolic analyzer simultaneously measures electrons and positive ions. The temporal resolution and energy range of the measurements and the detector complement to be sampled are programmable by ground command

Winningham, J. D.↗

NASA NTRS1981

Dynamics Explorer Science Data Processing System

The Dynamics Explorer project has acquired the ground data processing system from the Atmosphere Explorer project to provide a central computer facility for the data processing, data management and data analysis activities of the investigators. Access to this system is via remote terminals at the investigators' facilities, which provide ready access to the data sets derived from groups of instruments on both spacecraft. The original system has been upgraded with both new hardware and enhanced software systems. These new systems include color and grey scale graphics terminals, an augmentation computer, micrographics facility, a versatile data base with a directory and data management system, and graphics display software packages.

Smith, P. H.↗

NASA NTRS1981

Magnetic field-aligned electron distributions in the dayside cusp

Observations of low-energy electron fluxes made over a 6 year period with the photoelectron spectrometer on the AE-C and AE-D satellite are used to investigate electron pitch angles in the low altitude cusp. A 16 point energy spectrum from 2 to 500 eV was obtained at every .25 s, and the location of the cusp was verified by the presence of protons detected by the low-energy electron instrument. Isotropic fluxes of precipitating electrons with Maxwellian energy spectra were observed in the low-altitude cusp, and the presence of low-energy electrons with pitch angles less than 15 deg was determined streaming into the cusp ionosphere. The energies of the streaming electrons sometimes appeared as a peak superimposed on the normal cusp Maxwellian background of isotropic electrons, and it is concluded that the energies and pitch angle distributions of the precipitating electrons may explain the enhancements of auroral emissions and discrete arcs.

Zanetti, L. J.↗

NASA NTRS1981

Nighttime observations of 0.2- to 26-keV electrons in the South Atlantic anomaly made by Atmosphere Explorer C

Atmosphere Explorer C satellite observations have determined that the flux of low energy ions in the South Atlantic Anomaly is much smaller than that of electrons, allowing the satellite's low-energy ion detector to monitor the high-energy background and correct the low-energy electron detector accordingly. It is shown that the electron spectra can be represented by a power law in the range 0.2-26.0 keV, with the spectral index close to -1.0, and that the mean energy flux carried by electrons in the middle of the anomaly is about 0.003 erg/sq cm/sec and may reach 5 times this value. Maps of the downward energy flux are included.

Gledhill, J. A.↗

NASA NTRS1981

Statistical study of precipitating electrons

Energy spectra of precipitating electrons are fitted to the sum of three distributions: a power law, a Maxwellian and a Gaussian. This fitting procedure determines seven parameters which characterize the essential features of each spectrum. These characteristic parameters are used to carry out various studies involving precipitating electrons. It is shown that the absence of the power-law population from a particular spectrum is related to the softness of the precipitating primary flux, that the Maxwellian temperature and the Gaussian peak energy have a positive correlation the strength of which varies with local time, that the upward moving Gaussian population has a loss cone distribution, and that the one dimensional velocity distribution parallel to the magnetic field occasionally displays a plateau or a hump on the tail.

Fontheim, E. G.↗

NASA NTRS1981

Observational evidence for a boundary layer source of dayside region 1 field-aligned currents

With the addition of magnetic field information to the Atmosphere Explorer C data base it is now possible to make simultaneous observations of charged particles, magnetic field perturbations, and ion drift velocity. The most recently compiled observations from this data set are presented, and attention is given to the observed relationships between the convection reversal boundary, the high-latitude limit of plasma sheet particles, and the high-latitude limit of the region 1 current sheets in the low-altitude ionosphere, taking into account the association with similar boundaries and regions in the distant magnetosphere. The observed relationships between these boundaries are further discussed in light of the current-generating mechanisms considered.

Bythrow, P. F.↗

NASA NTRS1981

Inference of the ring current ion composition by means of charge exchange decay

The analysis of data from the Explorer 45 (S3-A) electrostatic analyzer in the energy range 5-30 keV has provided some new results on the ring current ion composition. It has been well established that the storm time ring current has a decay time of several days, during which the particle fluxes decrease nearly monotonically. By analyzing the measured ion fluxes during the several day storm recovery period and assuming that beside hydrogen other ions were present and that the decays were exponential in nature, three separate lifetimes for the ions were established. These fitted decay lifetimes are in excellent agreement with the expected charge exchange decay lifetimes for H(+), O(+) and He(+) in the energy and L value range of the data.

Smith, P. H.↗

NASA NTRS1981

Study of inverted-V auroral precipitation events

The macroscopic properties of inverted V events measured by the Atmospheric Explorer-D satellite are described, along with the spatial distributions and the extents of individual events. The satellite carried 19 detectors, each comprising a cylindrical electrostatic analyzer followed by an electron multiplier. A total of 430 events was recorded, none of which occurred in cusp precipitation. The observed characteristics of the inverted V events are compared with particle distribution properties predicted by a double layer mechanism, an electrostatic shock model, extended field-aligned potential, and anomalous resistivity. Widespread, continuous electric fields were found parallel to the magnetic field lines, and anomalous resistivity was determined as the most favorable mechanism for producing the electron precipitation structure observed.

Hoffman, R. A.↗

NASA NTRS1980

Coincident particle observations from AE-C and ATS 6 during the October 28, 1977, geomagnetic storm

Simultaneous measurements of ion and electron energy spectra recorded during the October 28, 1977 geomagnetic storm by the AE-C and ATS 6 spacecraft when both were nearly on the same L shell are evaluated and compared. The orbit of the AE-C generates wide longitudinal coverage of the auroral oval, whereas the ATS 6, which is located at geosynchronous orbit, samples the equatorial particle population. It is shown that the coincident electron energy spectra obtained by the two satellites agree well above 5 keV and the spectra of electrons peak near 1 keV at ionospheric altitudes, while no peaks in energy appear at synchronous altitude.

Zanetti, L. J.↗

NASA NTRS1980

The spatial-temporal ambiguity in auroral modeling

The paper examines the time-dependent models of the aurora which show that various ionospheric parameters respond to the onset of auroral ionization with different time histories. A pass of the Atmosphere Explorer C satellite over Poker Flat, Alaska, and ground based photometric and photographic observations have been used to resolve the time-space ambiguity of a specific auroral event. The density of the O(+), NO(+), O2(+), and N2(+) ions, the electron density, and the electron temperature observed at 280 km altitude in a 50 km wide segment of an auroral arc are predicted by the model if particle precipitation into the region commenced about 11 min prior to the overpass.

Rees, M. H.↗

NASA NTRS1980

Energetic particle penetrations into the inner magnetosphere

The nose structures observed on 90-deg pitch angle ion spectrograms characteristic of energetic particle injection into the ring current region of the inner magnetosphere in the afternoon to midnight sector are examined in detail on the basis of Explorer 45 data. A statistical analysis of the time dependence of nose structures yields a highest probability of occurrence at around 2000 MLT, and most events are found to occur on successive passes. The appearance of nose events is also related to an enhancement or change in configuration of the geoelectric field which alters magnetospheric convection patterns to drive an ion front close to the earth, where it can be observed by Explorer 45. The observed characteristics of nose structures are interpreted in terms of adiabatic charged particle motions in the magnetosphere in a Volland-Stern convection electric field model, which is also applied to explain the energy spectra and dispersion in penetration distances of electrons and ions observed in the postmidnight to morning sectors.

Ejiri, M.↗

NASA NTRS1979

Fluctuations of inverted V electron fluxes

Fluctuations of low-energy electron fluxes during an inverted V event on Nov. 5, 1975 were detected by fixed-energy electrostatic analyzers used in the LEE (low-energy electron) experiment onboard the AE-D satellite. The flux fluctuations were observed at energies of 0.78, 1.38, and 2.62 keV, when these energies were below that of the monoenergetic beam. This suggests that the fluctuations might occur over a wide range of energies. The fluctuations were confined within a pitch angle of 15 degrees, the amplitude increasing with decreasing pitch angle. The oscillation frequencies centered around 1.75 Hz. Fluctuations at two energies were generally correlated. The heating of secondary electrons, deduced from an analysis of the distribution function, is attributed to anomalous heating by electron plasma wave turbulence, whose intensity was modulated by a low-frequency wave. The results are consistent with a model, according to which auroral precipitating electron beams enhance the anomalous resistivity at the topside ionosphere through parametric instabilities.

Lin, C. S.↗

NASA NTRS1979

A correlation between auroral kilometric radiation and inverted V electron precipitation

The paper discusses the direct relationship between the auroral kilometric radiation (AKR) power flux observed by the eccentric orbiting Hawkeye spacecraft and energetic electron precipitation in the nighttime auroral zone measured by the low-altitude polar orbiting AE-D spacecraft. The results of this study show that AKR is more closely associated with inverted V electron precipitation than with plasma sheet precipitation. It is found that as the observed AKR power flux increases, so does the simultaneously observed peak energy in inverted V electron precipitation increase. There is an evidence that suggests that the efficiency of generating AKR from inverted V particle participation ranges from 1/100,000% to a maximum efficiency of about 1%.

Green, J. L.↗

NASA NTRS1979

Characteristics of the inverted-V event

An attempt is made to determine the origin of the inverted-V precipitating electrons and mechanisms that could create the inverted-V structure. The energy and pitch angle structures are compared with predictions from several theories that have been proposed to explain the origin of inverted-V events. Data from the AE-D satellite indicate that the origin of the inverted-V precipitating electrons is in the magnetosphere, the most probable regions being the plasma sheet and the neutral sheet. The energy and pitch angle distributions show that electrons are trapped between the mirror points and the electric field potential. The observations suggest that field-aligned precipitating electrons have been heated, probably when they were accelerated by the parallel electric field. Qualitatively, the detailed structures of the inverted-V events favor the theory of anomalous resistivity.

Lin, C. S.↗

NASA NTRS1979

Motions of charged particles in the Magnetosphere under the influence of a time-varying large scale convection electric field

The motions of charged particles under the influence of the geomagnetic and electric fields were quite complex in the region of the inner magnetosphere. The Volland-Stern type large scale convection electric field was used successfully to predict both the plasmapause location and particle enhancements determined from Explorer 45 measurements. A time dependence in this electric field was introduced based on the variation in Kp for actual magnetic storm conditions. The particle trajectories were computed as they change in this time-varying electric field. Several storm fronts of particles of different magnetic moments were allowed to be injected into the inner magnetosphere from L = 10 in the equatorial plane. The motions of these fronts are presented in a movie format.

Smith, P. H.↗

NASA NTRS1979

Motions of charged particles in the magnetosphere under the influence of a time-varying large scale convection electric field

The motions of charged particles under the influence of the geomagnetic and electric fields are quite complex in the region of the inner magnetosphere. The Volland-Stern type large-scale convection electric field with gamma = 2 has been used successfully to predict both the plasmapause location and particle enhancements determined from Explorer 45 (S3-A) measurements. Recently introduced into the trajectory calculations of Ejiri et al. (1978) is a time dependence in this electric field based on the variation in Kp for actual magnetic storm conditions. The particle trajectories are computed as they change in this time-varying electric field. Several storm fronts of particles of different magnetic moments are allowed to be injected into the inner magnetosphere from L = 10 in the equatorial plane. The motions of these fronts are presented in a movie format. The local time of injection, the particle magnetic moments and the subsequent temporal history of the magnetospheric electric field play important roles in determining whether the injected particles are trapped within the ring current region or whether they are convected to regions outside the inner magnetosphere.

Smith, P. H.↗

NASA NTRS1978

The convection electric field model for the magnetosphere based on Explorer 45 observations

Large-scale electric fields in the magnetosphere, represented by the scalar potentials of corotation and convection fields, are examined in order to interpret both plasmapause positions at different local times and energetic particle penetrations inside the plasmapause observed by the Explorer 45 (S3-A) dc E field measurements and particle detectors. The Volland-Stern type convection electric field is assumed. A uniform dawn-dusk convection electric field which corresponds to an exponent (gamma) of unity is not consistent with the observational data, but the Volland-Stern model with gamma = 2 more likely represents the convection electric field during the hours of electric field increases during magnetic activity. In the time-independent case, the shape of the plasmapause does not depend on the intensity of the convection electric field, so that by using two plasmapause distances at different local times along the S3 outbound and inbound orbits the shape factor gamma of the convection field could be determined. The average value of gamma for 42 cases was found to be 2.4.

Ejiri, M.↗

NASA NTRS1978

Inference of the ring current ion composition by means of charge exchange decay

The analysis of the measured ion fluxes during the several day storm recovery period and the assumption that beside hydrogen other ions were present and that the decays were exponential in nature, it was possible to establish three separate lifetimes for the ions. These fitted decay lifetimes are in excellent agreement with the expected charge exchange decay lifetimes for H(+), O(+), and He(+) in the energy and L-value range of the data. This inference technique, thus, establishes the presence of measurable and appreciable quantities of oxygen and helium ions as well as protons in the storm-time ring current. Indications that He(+) may also be present under these same conditions were found.

Smith, P. H.↗