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At least 631 records · Page 35
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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

Radio evidence for shock acceleration of electrons in the solar corona

It is pointed out that the new class of kilometer-wavelength solar radio bursts observed with the ISEE-3 Radio Astronomy Experiment occurs at the reported times of type II events, which are indicative of a shock wave. An examination of records from the Culgoora Radio Observatory reveals that the associated type II bursts have fast drift elements emanating from them; that is, a herringbone structure is formed. It is proposed that this new class of bursts is a long-wavelength continuation of the herringbone structure, and it is thought probable that the electrons producing the radio emission are accelerated by shocks. These new events are referred to as shock-accelerated events, and their characteristics are discussed.

Cane, H. V.↗
NASA NTRS1981

Energetic electrons and plasma waves associated with a solar type III radio burst

Detailed in situ observations from the ISEE 3 spacecraft of energetic electrons, plasma waves, and radio emission for the type II solar radio burst of February 17, 1979, are presented. The reduced, one-dimensional electron distribution function is constructed as a function of time. Since the faster electrons arrive before the slower ones, a bump on tail distribution forms which is unstable to the growth of Langmuir waves. The plasma wave growth computed from the distribution function agrees well with the observed onset of the Langmuir waves, and there is qualitative agreement between variations in the plasma wave levels and in the development of regions of positive slope in the function. The evolution of the function, however, predicts far higher plasma wave levels than those observed. The maximum levels observed are approximately equal to the threshold for nonlinear wave processes, such as oscillation two-stream instability and soliton collapse.

Lin, R. P.↗
NASA NTRS1982

Nonlocal plasma turbulence associated with interplanetary shocks

Regions of plasma turbulence extending several tenths of an astronomical unit upstream or downstream of interplanetary shocks have been detected by the plasma wave instrument on ISEE 3. Highly impulsive electric field bursts at 1-10 kHz were found (hours upstream of quasi-parallel interplanetary shocks) whose average and peak amplitudes occasionally increased until the shock crossing, when they were suppressed. A 0.1-1 kHz electric field component was enhanced at nearly all shocks, and persisted downstream. A smooth, high-frequency continuum near and above the local electron plasma frequency was enhanced at, and persisted downstream of, every interplanetary shock studied. While no single interplanetary shock showed every effect, the ensemble of shocks contained at least one example of each type of plasma wave found upstream of the earth's bow shock.

Kennel, C. F.↗
NASA NTRS1982

A sub-Alfvenic solar wind - Interplanetary and magnetosheath observations

During much of an approximately 5-hour period on November 22, 1979, plasma and field instruments on ISEE 3 measured a solar wind flow that was simultaneously supersonic and sub-Alfvenic (about 320 km/s) due to an abnormally low ion density (about 0.07 per cu cm). The nature of the disturbed flow adjacent to the magnetosphere is examined. This examination suggests that the earth's bow wave retained its shock-like character when the solar wind flow was sub-Alfvenic.

Gosling, J. T.↗
NASA NTRS1982

Ducted auroral kilometric radiation

Certain discrete, intense wave signals attributed to auroral kilometric radiation (AKR) were observed with ISEE-l while it was within the plasmaspheric shadow zone for direct propagation. It is believed that wave ducting by thin depletions of the plasma density aligned with the magnetic field accounts for such signals, and that their discrete nature is caused by the satellite intercepting individual ducts. These ducts, which were also observed as coincident decreases of the upper hybrid resonance frequency, appeared to be twenty-percent depletions roughly one hundred kilometers across. The AKR, which is emitted approximately perpendicular to the magnetic field, apparently entered these ducts equatorward of the source after the waves had been refracted parallel to the duct axis. A diffuse background was also observed which is consistent with the leakage from similar ducts at lower L-values. These observations establish the existence of ducted AKR, its signature on the satellite wave spectrograms, and new evidence for depletion ducts within the plasmasphere.

Calvert, W.↗
NASA NTRS1982

Theory of the fine structure of auroral kilometric radiation

Recent data from ISEE 1 show auroral kilometric radiation (AKR) with finely separated bands in frequency. The observation that the AKR fine structure frequency separation is about equal to the ion cyclotron frequency at the AKR source is strong evidence for the interaction of AKR and electrostatic ion cyclotron (EIC) waves in the source, as proposed by Grabbe et al. (1980) to explain the origin of AKR. It is pointed out that no other wave of frequency close to the band separation is known to exist in the auroral source region. The fine structure observed in the source region AKR is the first evidence for EIC waves in the lower source region (3000 - 5000 km attitude), as required in the theory of Grabbe et al.

Grabbe, C. L.↗
NASA NTRS1982

Impulsive plasma transport through the magnetopause

The considered investigation is concerned with a study of the process of localized plasma entry on the basis of first principles. Since the magnetopause often moves at high velocities, an induction electric field must be involved. Attention is given to the case in which the adjacent magnetopause is at rest. Occasionally, a frame moving with the magnetopause in the interaction region is also considered. The employed model is based on the observations provided by ISEE. The considered plasma irregularity, which is denominated a 'cloud', may not have to be a large enhancement in number density or velocity. An examination of the involved physical relations shows that the plasma will create its own electric field, via a polarization current, to make possible a penetration through the magnetopause by the cloud. At the magnetopause the polarization current creates a charge separation electric field which promotes (rather than inhibits) continued flow through the moving magnetopause.

Heikkila, W. J.↗
NASA NTRS1981

Interplanetary particle observations associated with solar flare gamma-ray line emission

Observations of particle emissions during three solar flares which were observed to emit 2.22 MeV gamma rays as recorded by the Solar Maximum Mission are discussed. The 2.22 MeV line is produced by neutron capture by hydrogen, and additional attention is given to a 4.4 MeV emission line of June 7, 1980, with estimates made of the particle density 1 AU from the sun assuming a good magnetic connection between the earth and the sun. The measurements were made from the ISEE-3 and HELIOS-1 spacecraft. The connectedness of the earth and the sun in a magnetic field leads to conclusions that few particles actually escaped into interplanetary space.

Von Rosenvinge, T. T.↗
NASA NTRS1981

Unusual properties of particle events associated with solar flare gamma ray events

An attempt to establish a link between gamma rays observed at the earth with solar flare areas highly enriched with electrons is presented. Two hour averages of the counting rates of 5-100 MeV electrons and 25-145 MeV protons performed on the ISEE-3 spacecraft were correlated with solar flares with a maximum intensity of 6-11 MeV electrons and 25-44 MeV protons recorded by other observers. The coverage was part of the Solar Maximum Mission, which attained a 50 percent coverage. Gamma ray associated events showed high ratios of electrons to protons, about 0.2, and electrons with energies greater than 80 MeV were observed, which are values high enough to produce the highest gamma rays observed, 40 MeV. The available data for electron, proton, and rare isotope fluxes and the time histories of the fluxes are noted to provide a data base for further information on solar flares and particle acceleration mechanisms.

Evenson, P.↗
NASA NTRS1981

The relation of type II radio bursts to solar energetic particles observed at earth

Observations of particle events associated with type II radio bursts as recorded on the ISEE-3 spacecraft are reported. Data on burst events following flares of Sept. 23,1978 and Aug. 18, 1979 yield time histories of the particle fluxes, which exceeded 25 MeV energy. The profiles of the Category-1 type II bursts display a continuous release of particles, which was concluded by no decrease in the fluxes until after the passage of the shock. The continuous nature of the recorded events suggests that the secondary acceleration process associated with the type II bursts is maintained as the flare shock propagates through interplanetary space, and may be the source of the energetic particles which propagate with the shock.

Evenson, P.↗
NASA NTRS1981

Heavy-element abundances in He/3/-rich events

Abundances of the elements He through Fe observed on the ISEE-3 during He(3) rich events are reported. Ratios of He(3)-He(4) of not less than about 1, and Fe/O of greater than 1 are found, and rather large event-to-event variations in C, O, Ne, Mg, Si, and S are seen superposed upon the general trend of heavy element enhancement. It is concluded that the discontinuous variation of the neighboring elements in the He(3) rich events could result from differences in the electron temperature of the material prior to injection that give rise to differences in the dominant charge states and cyclotron frequencies of different elements.

Reames, D. V.↗
NASA NTRS1981

First-order Fermi acceleration of the diffuse ion population near the earth's bow shock

The flux of 30-65 keV particles observed by the ISEE-3 200 earth radii upstream is shown to be an upstream escape of the energetic ions in the earth's bow shock. A formal solution to the transport equation for the distribution function of energetic particles upstream from an isotropic monoenergetic source of particles/sq cm at a plane shock where the plasma changes speed is found, and escape conditions are defined. The efficiency of the acceleration is calculated to depend on the charge/particle, and fluxes near and far upstream of the shock are described analytically. Any model which takes into account shock acceleration by diffusive scattering with significant escape losses produces the observed spectrum close to the shock. The escape loss upstream is demonstrated to control the spectrum and the variation of flux and anisotropy with distance from the shock.

Forman, M. A.↗
NASA NTRS1982

Turbulent electric fields in the nightside magnetosphere

Electric field measurements from the long-wire double-probe instrument (baseline of 179 m) on ISEE 1 have shown the magnetospheric electric field on auroral L shells to be extremely turbulent during periods of magnetic activity. During intense activity these turbulent electric fields can penetrate to very low L values. The variational component of the electric field is typically larger than the DC value. Measurements are presented at frequencies up to 14 Hz. Magnitudes of over 40 m V/m (zero to peak) have been observed with spectral power levels in the 1-10 Hz range greater than m squareV/sq m Hz. The spectral shape of the most intense events was generally flatter than that predicted by two-dimensional hydromagnetic cascading of energy, which argues that the source of this turbulence must be driving the plasma near these frequencies. This in turn suggests that the instability is in the low-energy plasma.

Maynard, N. C.↗
NASA NTRS1982

Spatial structure of greater than 100 keV X-ray sources in solar flares

Recent ISEE 3 and Pioneer Venus Orbiter observations relevant to the altitude structure of the hard X-ray source down to 3000 km or less above the photosphere are reported. About 90 percent of the impulsive X-ray emission and about 70 percent of the gradual emission originates at altitudes 2500 km or less above the photosphere. In the 100-500 keV range, this altitude dependence is essentially independent of photon energy. Brightness of the impulsive X-ray source decreases rapidly with increase in altitude. As far as models of the impulsive phase are concerned, the thermal model with adiabatic compression and expansion of a magnetically confined plasma and the thin target model are not consistent with these observations. The thick target model and the dissipative thermal model are consistent with the observed low altitude of the hard X-ray source.

Kane, S. R.↗
NASA NTRS1982

The primary cosmic ray electron spectrum 1978-1980

In August of 1978 the ISEE-3 spacecraft carried the University of Chicago electron spectrometer into interplanetary space. Strong intensity variations of Jovian electrons have been observed in the energy range from 5 to 25 MeV in conjunction with 3 Jovian seasons. The spectral shape of the extra electrons has a shallow maximum around 15 MeV. No measurable long term variation in the average electron spectrum has been observed since 1968 up to energies of about 50 MeV. However, strong solar cycle modulation occurs at energies around a few hundred MeV.

Evenson, P.↗
NASA NTRS1982

The fine structure of the front side magnetopause during two successive crossings

The magnetopause crossings of November 10, 1977, which are considered to be representative of a class of structures, are investigated, and a procedure is developed to derive approximate surface currents near the magnetopause from ISEE 1 magnetic field data by assuming a constant speed of the magnetopause along its normal for each crossing of the magnetopause layer. Changes in the magnetic field indicate that current sheets exist on each side of the magnetopause layer, which is characterized by an irregular magnetic field and a magnetosheath-like energy distribution. No boundary layer plasma is observed on the magnetospheric field lines inside the magnetopause, and electromagnetic energy is found to be dissipated at the two edges of the magnetopause layer where the current layers are observed.

Formisano, V.↗
NASA NTRS1982

The magnetopause as sensed by energetic particles, magnetic field, and plasma measurements on November 20, 1977

The position of the trapping boundary for ions not less than 24 keV in the vicinity of the magnetopause was determined using energetic particle three-dimensional distributions observed by ISEE 1 and 2. For a short period of time on November 20, 1977, definite periods were observed when the subsolar magnetopause current and magnetosheath plasma penetration layers were not overlapping the trapping region for energetic particles whose outer edge is the trapping boundary. The trapping boundary was found to be in rapid continual motion, representing a sharp, well-defined boundary, and the penetration of the magnetosheath plasma inside this boundary can be limited to a distance comparable to the plasma ion gyroradius of not greater than about 100 km, although plasma penetration was seen up to 200 km earthward of the trapping boundary.

Fritz, T. A.↗