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West, H. I., Jr.

Publications and source records attributed to West, H. I., Jr..

16 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Chorus, energetic electrons and magnetospheric substorms

The paper investigates possible chorus generation mechanisms by means of simultaneous OGO-5 measurements of chorus, energetic (79 + or - 23 keV) electron fluxes and pitch angle distributions, and ambient magnetic fields. Further, the solar wind data during high-latitude dayside chorus events are examined. It is reported that the highly eccentric orbit had an apogee of 24 earth radii and an inclination of 30 deg to the geographic equator.

Tsurutani, B. T.↗
NASA NTRS1978

Multiple-satellite studies of magnetospheric substorms - Plasma sheet recovery and the poleward leap of auroral zone activity

Particle observations from pairs of satellites (OGO 5 and Vela 4A and 5A) during 28 plasma sheet thickening events indicate that thickening of the nighttime plasma sheet during substorms occurs in two main stages. The early stage involves single or multiple expansions of the near-earth plasma sheet at the onset of substorm expansions (Pi 2 bursts) on the ground, while the later stage of plasma sheet recovery starts near the time of maximum auroral zone bay activity. This stage is characterized by a large-scale thickening toward higher latitudes that occurs over a broad azimuthal scale and at heights that range from the ionosphere to beyond the Vela orbit. A detailed analysis of two-satellite observations during eight plasma sheet recoveries is presented, and events that occurred within 5 min in widely separated locations at small distances from the tail's midplane as well as events that occurred concurrently in the Vela orbit and at high latitudes in the near-earth region are revealed.

Pytte, T.↗
NASA NTRS1978

On the configuration of the magnetotail near midnight during quiet and weakly disturbed periods - State of the magnetosphere

Particle and field data are analyzed for a number of magnetically quiet days. It is shown that the magnetotail near midnight may exist for hours at a time in widely different quasi-stable configurations which, in their limit, can be characterized as being (1) fairly dipolelike to about 15 R(E), beyond which the field becomes taillike; and (2) dipolelike to only about 10 R(E), beyond which the field becomes taillike. These different states affect both the dynamics of the substorm and the evolution of the pitch angle distributions of energetic electrons in the near-earth plasma sheet past midnight. During quiet times, the plasma sheet in its azimuthal variation is at its thinnest in the region near midnight.

West, H. I., Jr.↗
NASA NTRS1978

Multiple-satellite studies of magnetospheric substorms - Distinction between polar magnetic substorms and convection-driven negative bays

Two types of prolonged auroral zone magnetic bay activity are studied. The first manifests frequent substorm expansions and the second (a convection bay) shows weak and infrequent substorm onset signatures during intense auroral zone bays. Data are obtained by multiple satellite recordings during a 5-hour interval of persistent auroral zone activity. The results are discussed in terms of: magnetic activity on the ground, solar wind and tail lobe conditions, onset and recovery of the 1122 UT substorm, plasma sheet variations during the convection bay, reappearance of substorm signatures in the tail of about 1613 UT, and magnetic field observations at synchronous orbit.

Pytte, T.↗
NASA NTRS1977

Multiple-satellite studies of magnetospheric substorms: Plasma sheet recovery and the poleward leap of auroral-zone activity

Particle observations from pairs of satellites (Ogo 5, Vela 4A and 5B, Imp 3) during the recovery of plasma sheet thickness late in substorms were examined. Six of the nine events occurred within about 5 min in locations near the estimated position of the neutral sheet, but over wide ranges of east-west and radial separations. The time of occurrence and spatial extent of the recovery were related to the onset (defined by ground Pi 2 pulsations) and approximate location (estimated from ground mid-latitude magnetic signatures) of substorm expansions. It was found that the plasma sheet recovery occurred 10 - 30 min after the last in a series of Pi bursts, which were interpreted to indicate that the recovery was not due directly to a late, high latitude substorm expansion. The recovery was also observed to occur after the substorm current wedge had moved into the evening sector and to extend far to the east of the center of the last preceding substorm expansion.

Pytte, T.↗
NASA NTRS1977

OGO 5 observations of Pc 5 waves - Particle flux modulations

An investigation is conducted concerning the modulations of particle fluxes associated with Pc 5 waves in the region beyond the plasmapause. A study of thermal flux modulations indicates that some of the density enhancements observed are not spatial structures but are spurious features caused by temporal flux variations associated with hydromagnetic waves. A resonance model of the energetic particle flux modulations is discussed. Energetic particle modulations are also considered. The reported observations reveal that modulations are dominant at energies of about 100 keV for electrons and at 100 keV to 1 MeV for protons. This may indicate that the bounce resonance interaction is not important for Pc 5 waves.

Kokubun, S.↗
NASA NTRS1976

Multiple-satellite studies of magnetospheric substorms - Radial dynamics of the plasma sheet

The radial dynamics of the nighttime plasma sheet during substorms is examined. The spatial dependence of plasma sheet variations at different radial distances is studied on the basis of simultaneous recordings from two closely spaced satellites. The simultaneous measurements of the plasma sheet behavior earthward and tailward of r = 15 earth radii confirm substorm models which predict a thinning of the near-earth plasma sheet before the formation of an X-type neutral line, followed by a thickening on the earthward side and a further thinning on the tailward side. Temporal correlations between the plasma sheet variations and substorm development on the ground are studied by obtaining accurate timing of individual substorm expansion onsets. In particular, during multiple onset storms, the near-earth plasma sheet is found to experience a series of multiple expansions and contractions, which usually occur in a one-to-one relationship with ground Pi 2 bursts and are well correlated with auroral zone and low-altitude magnetic disturbances.

Pytte, T.↗
NASA NTRS1975

Angular distributions of solar protons and electrons

High angular-resolution measurements of directional fluxes of solar particles in space have been obtained with detectors aboard OGO-5 during the cosmic ray event of Nov. 18, 1968. This is the only case on record for which sharply-defined directional observations of protons and electrons covering a wide rigidity range (0.3 MV to 1.5 GV) are available. The satellite experiment provided data for determining pitch-angle distributions with respect to the direction of the local interplanetary magnetic field lines during the lengthy highly anisotropic phase of the event. The results have been interpreted in the light of the temporal flux profiles and the state of the interplanetary medium.

Nielsen, E.↗
NASA NTRS1973

LLL electron and proton spectrometer on NASA's Orbiting Geophysical Observatory 5

The LLL energetic electron and proton spectrometer on NASA's Orbiting Geophysical Observatory 5 (OGO-5) operated successfully from launch - March 4, 1968 - until retirement in August 1971. Data recovery during this time was about 95% of the orbit except for the last few months. The electron spectrometer used a magnetic field for electron momentum selection which served also as an electron broom for a proton range - energy telescope. The energy range was approximately 60 to 2950 keV for electrons (seven channels) and 0.10 to approximately 94 MeV for protons (seven channels). The experiment was scanned relative to the stabilized OGO-5 for obtaining directional information. Excellent data were taken throughout the magnetosphere and in the interplanetary region. Studies were carried out in the areas of equatorial pitch-angle distributions, substorm dynamics and field topology, particle spectra (time history), particle spatial distributions, and solar particle events.

West, H. I., Jr.↗
NASA NTRS1973

Electron pitch angle distributions throughout the magnetosphere as observed on Ogo 5.

A survey of the equatorial pitch angle distributions of energetic electrons is provided for all local times out to radial distances of 20 earth radii on the night side of the earth and to the magnetopause on the day side of the earth. In much of the inner magnetosphere and in the outer magnetosphere on the day side of the earth, the normal loss cone distribution prevails. The effects of drift shell splitting - i.e., the appearance of pitch angle distributions with minimums at 90 deg, called butterfly distributions - become apparent in the early afternoon magnetosphere at extended distances, and the distribution is observed in to 5.5 earth radii in the nighttime magnetosphere. Inside about 9 earth radii the pitch angle effects are quite energy-dependent. Beyond about 9 earth radii in the premidnight magnetosphere during quiet times the butterfly distribution is often observed. It is shown that these electrons cannot survive a drift to dawn without being considerably modified. The role of substorm activity in modifying these distributions is identified.

West, H. I., Jr.↗