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Armstrong, T. P.

Publications and source records attributed to Armstrong, T. P..

At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Ions of Jovian origin observed by Voyager 1 and 2 in interplanetary space

Burst-like and long-lived ion fluxes (E greater than 30 keV) of Jovian origin have been observed in interplanetary space by the LECP instrument on the Voyager 1 and 2 spacecraft. Burst (few minute duration) events are observed at distances greater than 0.6 AU from Jupiter. These events are highly anisotropic and possess steep energy spectra, while long-lived (greater than 8 hour duration) events have relatively steady fluxes at low energies, strong anisotropies that decay with time, and a variable high energy component. Both types of events usually display simultaneous onsets and sharp cutoffs for all energies, an excess of atomic number Z not less than 6 particles compared to solar and interplanetary events, and particle flow directions pointed away from Jupiter along the local interplanetary magnetic field. The origin for the long-lived events appears to be inside the bow shock of the planet.

Zwickl, R. D.↗
NASA NTRS1980

Energetic /approximately 100-keV/ tailward-directed ion beam outside the Jovian plasma boundary

The hot plasma instrument on the Voyager-2 spacecraft measured a nearly monoenergetic (100 keV) ion beam several hours after crossing the Jovian plasma boundary on the nightside of the planet. The beam, deduced to consist primarily of heavy ions, persisted for about four hours and originated from the general direction of Jupiter. The energy density of the beam was about several times the energy density of the magnetic field (beta greater than 1). This beam, a product of an as yet not understood Jovian plasma acceleration mechanism, provides a dramatic example of the energetic dynamics of Jupiter's magnetosphere.

Krimigis, S. M.↗
NASA NTRS1980

Recurrent energetic particle events associated with forward/reverse shock pairs near 4 AU in 1978

The elemental composition at energies near 1 MeV/nucleon has been studied for three recurrent energetic particle events during the period August-October, 1978, with the low-energy-charged-particle instruments on Voyager 1/2 near 4 AU. These events are associated with forward/reverse shock pairs in the solar wind as identified by the MIT plasma experiments on Voyager. In all three events, the flux increases associated with the reverse shocks showed enhancements in the He/H and CNO/H ratios compared to typical solar flare composition. The average C/O ratio for the three events was 0.68 which is larger than for normal solar flares but somewhat smaller than the average value found for corotating events at 1 AU during the previous solar minimum (1974-1976).

Hamilton, D. C.↗
NASA NTRS1979

A study of solar flare electron events from October 1972 through December 1974 from Imp 7 and 8

Data from the Johns Hopkins University Applied Physics Laboratory Charged Particle Measurement Experiment aboard Imp H and J were searched for solar flare produced intensity increases in greater than 0.2-MeV electrons during the 26-month period from October 1972 through December 1974. Of the 44 solar electron events found during this period, 31 were isolated for a detailed statistical study. Systematics among the characteristics of the electron profiles (e.g., peak intensity times and count rates) and those of the associated flares (e.g., H-alpha onset times, H-alpha importance class, heliocentric coordinates, etc.) were examined, and the significant results are presented in several scatter plots. The results reveal that the time delay between the flare onset and the arrival of the peak electron intensity at 1 AU (time to maximum) is a function of the flare's deviation in heliolongitude from the solar region which was well connected to the earth via a magnetic flux tube; the well-connected flares produced electron intensity maxima in the least time.

Decker, R. B.↗
NASA NTRS1979

Hot plasma environment at Jupiter - Voyager 2 results

Preliminary results are reported from measurements made with the low-energy charged particle (LECP) instrument on Voyager 2 as it approached and traversed the Jovian magnetosphere. The primary objectives of the LECP instrument were to make measurements of the hot plasma (no less than about 20 keV and no less than about 28 keV for electrons and ions, respectively), to characterize the composition of the hot plasma and energetic-particle population, and to determine the particle flows and spatial distributions. In addition, the effects associated with the possible wake of Ganymede are discussed. Attention is given to inbound and outbound passes, along with Jovian plasma characteristics. The results suggest that the Jovian magnetosphere is confined by a plasma boundary rather than a conventional magnetopause. Inside the plasma boundary there exists a discontinuity at about 50-60 Jupiter radii, and the region inside this discontinuity is termed the 'inner plasmasphere'.

Krimigis, S. M.↗
NASA NTRS1979

Low-energy charged particle environment at Jupiter - A first look

Preliminary results of measurements obtained by the low energy charged particle instrument on board the Voyager 1 spacecraft during its traversal of the Jovian magnetosphere are reported. The instrument consists of the low energy particle telescope and the low energy magnetospheric particle analyzer, designed to perform measurements in the inner and outer magnetosphere respectively. Ions and electrons comprising the Jovian magnetosphere were first detected at a distance of about 600 Jupiter radii from the planet, with the first bow shock crossing at 85.6 Jupiter radii. Upon crossing the magnetopause at about 67 Jupiter radii, the flows of electrons and ions were observed to change direction from away from the planet to the corotational direction. The hot plasma near the magnetosphere boundary is comprised predominantly of protons, sulfur and oxygen. Selective particle absorption near the Io flux tube indicates some form of particle deflection by Io. Fluxes in the outbound region were found to be enhanced from 90 to 160 deg longitude, and 5- and 10-hour low energy particle flux periodicities were observed.

Krimigis, S. M.↗
NASA NTRS1979

Hydrogen over helium enhancement in successive solar flare particle events from the same active region

An analysis of all of the identified solar-flare-associated energetic particle events in the 1972-1975 period observed with instruments aboard the IMP 7 and IMP 8 satellites has revealed at least eight occasions when more than one particle-producing flare occurred within the same McMath active plage region during its transit of the visible solar disk. A strong tendency for second flares to produce hydrogen over helium (p/alpha) enhanced energetic particle fluxes when compared with the first flare in the 1.8-10.0 MeV per nucleon range emerged in these multiflare regions. The p/alpha enhancement is apparently transient, and for flares separated by at least about 100 hours the p/alpha ratio tends toward its preflare value. It is suggested that the substrate plasma in an active region may be enriched prior to a flare in elements heavier than hydrogen and the composition may not be significantly altered during subsequent acceleration, escape, and propagation. Thus, the preflare history of the active region must be added to the list of factors influencing observed solar-particle-event composition.

Briggs, P. R.↗
NASA NTRS1979

Particle acceleration by propagating interplanetary shocks

The process of charged particle acceleration in interplanetary shocks has been simulated for typical parameters. Since space probe observations of charged particle fluxes are the principle means of inferring the action of an acceleration mechanism, the simulation was designed to follow particles backwards in time from a given observing point, usually 1 AU. This allowed a full diagnosis of which observed particles had interacted with an oncoming shock, where the interacting occurred, and by how much the energy had been changed by the shock interaction. Simple assumptions about the preshock energy spectrum allow the construction of a full temporal profile of the expected intensity, anisotropy, and energy spectrum. The simulations are apparently capable of reproducing the main features of the less than 10 MeV/nuc ion flux enhancements observed at interplanetary shocks using only a laminar interplanetary magnetic field, Archimidean spiral, and a spherical oblique shock with typical speed, strength, and shock normal-magnetic field angle.

Armstrong, T. P.↗
NASA NTRS1978

Magnetosheath bursts of predominantly medium nuclei observed with Imp 8 on February 16, 1974

This paper presents observations made with detectors aboard Imp 8 of a unique series of charged particle bursts that occurred on February 16, 1974, while the spacecraft was traversing the dawn magnetosheath region. This event was unlike any other observation of Imp 7 and 8 in 5.5 years of operation in orbit. The measurements are most directly interpreted as an intense (6000/(sq cm s sr MeV/nucleon)), highly collimated beam flowing in the antisolar direction composed of medium (carbon, nitrogen, or oxygen) nuclei. Both the degree of collimation in arrival direction and the composition are unique to this event. The event is possibly an important signature of terrestrial O(+) ions escaping from the magnetosphere.

Armstrong, T. P.↗
NASA NTRS1978

Z-rich solar particle event characteristics 1972-1976

It is found in the reported investigation that Z-rich solar particle events usually have large and prolonged anisotropies in addition to an extremely variable charge composition that varies not only from event to event but also throughout the event. These observations suggest that one can no longer regard the event-averaged composition of solar particle events at low energies as providing an unbiased global sample of the solar atmospheric composition. The variability from event to event and among classes of events is just too great. However, the tendency for the Z-rich events to be associated with both the low-speed solar wind at or just before the onset of solar wind streams and with active regions located in the western hemisphere, indicates that charge composition studies of solar particle events can yield a better knowledge of the flare acceleration process as well as the inhomogeneous nature of magnetic field structure and particle composition in the solar atmosphere.

Zwickl, R. D.↗
NASA NTRS1978

Simultaneous multispacecraft observations of energetic proton bursts inside and outside the magnetosphere

Energetic proton bursts were observed simultaneously on Imp 6, 7, and 8 both inside and outside the earth's magnetosphere during the period October 1972 through August 1974. It is found that proton bursts are present nearly simultaneously in the vicinity of the outer belt trapping boundary, in the low-latitude magnetotail, in the magnetosheath, and upstream from the bow shock. Large intensity differences, time delays, and differences in energy spectra recorded by the three spacecraft are described. It is suggested that energetic protons and electrons are probably accelerated inside the plasma sheet and propagate to various regions both inside and outside the magnetosphere.

Sarris, E. T.↗
NASA NTRS1978

The quiet-time low energy nucleon spectrum during 1975

The paper reports measurements during quiet time periods in 1975 over the range of Ep from about 0.3 to 5 MeV for protons and E-alpha from about 0.7 to 11 MeV/nuc for helium nuclei. The data are obtained from the JHU/APL experiments on earth-orbiting IMP-7 and IMP-8 spacecraft. Both spacecraft are spinning, and data from the experiment are acquired in both spin-averaged and sectored form. It is shown that the earth's atmosphere continues to be a most important contributor to the proton population in the Ep range of 0.29-0.5 MeV, and that the 1975 intensities are virtually identical to those measured in 1973, suggesting the absence of solar modulation at these low (no more than about 5 MeV/nuc) energies. Lack of solar modulation argues against a galactic origin.

Krimigis, S. M.↗
NASA NTRS1977

The Low Energy Charged Particle /LECP/ experiment on the Voyager spacecraft

The Low Energy Magnetosphere Particle Analyzer (LEMPA) and the Low Energy Particle Telescope (LEPT) on board the Voyager spacecraft are described. The LEMPA has an energy range of about 10 keV to greater than 11 MeV for electrons and about 15 keV to greater than 150 MeV for protons and heavier ions; the LEPT covers a range of about 0.05 to 40 MeV/nucleon with good energy and species resolution. The LEPT and LEMPA subsystems are mounted on a stepping platform which rotates at variable rate through eight angular sectors. Comprehensive measurements of particles in the Jovian, Saturnian, Uranian, and interplanetary environments are expected from the experiment.

Krimigis, S. M.↗
NASA NTRS1977

Acceleration and modulation of electrons and ions by propagating interplanetary shocks

The theory and observation of shock spikes are reviewed. Models of the orbits of charged particles and their energization during interaction with discontinuities under various geometries are studied. In situ observations of shock spikes are discussed. These have shown that sharp spikes are found sometimes ahead of and sometimes behind the shock, with the highest energies being ahead. One generally observes larger, field-aligned ion anisotropies ahead of shocks rather than behind. Intensity modulation of protons, alpha particles, medium nuclei, and iron have been observed.

Armstrong, T. P.↗
NASA NTRS1976

Location of the source of magnetospheric energetic particle bursts by multispacecraft observations

During a magnetic substorm on Oct. 16, 1973 a number of magnetospheric bursts of energetic particles were observed simultaneously by IMP-6 and IMP-7 in the magnetotail. Detailed anisotropy measurements of 210 and 290 keV protons provide for the first time an indication of the location of the source of the energetic magnetospheric particles as well as evidence for its movement with speeds from 30 to more than 80 km/sec, in association with the intensification of the westward auroral electrojet during a magnetic bay at the station closest to the local time of the spacecraft (also local midnight). The observations indicate that energetic particles are accelerated to greater than 1.85 MeV in a moving and localized region in the geomagnetotail.

Sarris, E. T.↗