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Roeder, J. L.

Publications and source records attributed to Roeder, J. L..

14 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

Towards Inner Magnetosphere Particle and Field Models

We report on the initial stages of an effort to construct comprehensive empirical models of the plasma and fields in the inner magnetosphere. The models are based not only on archival data (e.g., SCATHA) but also on current spacecraft mission data (e.g., POLAR). In this component of the effort, we incorporate the recently archived data provided by the SCATRA (Spacecraft Charging AT High Altitude) satellite. The SCAT14A satellite was in a near-geostationary orbit and was in operation for more than a decade. In this paper, we focus on the SCATRA plasma and magnetic field data from approximately the first two years of operation. The time-series data are binned according to spatial location and geomagnetic activity. Examples of statistical, empirical models from this initial effort are presented; even the simply-constructed preliminary models reveal such subtle features as the previously reported warm (greater than 100 eV) plasma density enhancement near the magnetic equator.

Spence, H. E.↗
NASA NTRS1994

Low-latitude boundary layer near noon: An open field line model

We propose that many features of the cusp and low-latitude boundary layer (LLBL) observed near noon MLT can be explained by interpreting the LLBL as being on open lines with an inner boundary at the separatrix between open and closed magnetic field lines. This interpretation places the poleward boundary of the LLBL and equatorward boundary of the cusp along the field line that bifurcates at the cusp neutral point. The interpretation accounts for the abrupt boundary of magnetosheath particles at the inner edge of the LLBL, a feature that is inconsistent with LLBL formation by diffusion onto closed field lines, and for the distribution of magnetosheath particles appearing more as one continuous region than as two distinct regions across the noon cusp/LLBL boundary. Furthermore, we can explain the existence of energetic radiation belt electrons and protons with differing pitch angle distributions within the LLBL and their abrupt cutoff at the poleward boundary of the LLBL. By modeling the LLBL and cusp region quantitatively, we can account for a hemispherical difference in the location of the equatorial boundary of the cusp that is observed to be dependent on the dipole tilt angle but not on the interplanetary magnetic field (IMF) x component. We also find important variations and hemispherical differences in that the size of the LLBL that should depend strongly upon the x component of the IMF. This prediction is observationally testable. Finally, we find that when the IMF is strongly northward, the LLBL may include a narrow region adjacent to the magnetopause where field lines are detached (i.e., have both ends connected to the IMF).

Lyons, L. R.↗
NASA NTRS1992

Energetic and magnetosheath energy particle signatures of the low-latitude boundary layer at low altitudes near noon

The low-latitude boundary layer (LBL) and its separation from the cusp have previously been identified using observations of particle precipitation at magnetosheath energies. Using S3-3 satellite observations, we have determined that these identifications can also be made from energetic particle observations on polar-orbiting satellites. It is found that the equatorward boundary of the LBL is identifiable as an approximately discontinuous decrease in 33-keV electron fluxes from low to high latitudes. Both the energetic ion and electron fluxes decrease discontinuously at the boundary between the LBL and the cusp or polar cap. A distinct LBL is nearly always identifiable in energetic particle measurements in the 10-14 MLT region when counting rates are statistically significant. The identifications obtained using the energetic particle measurements have been compared to those obtained using criteria developed by Newell and Meng (1988, 1989) for magnetosheath energy particle precipitation. In this way, we have evaluated the accuracy of both techniques and used the energetic particle measurements to supplement the identifications obtained using the Newell and Meng criteria. We propose that the Newell and Meng threshold on ion energy flux can be reduced by a factor of 6. This modification provides identification of the LBL for lower ion intensity levels than has previously been thought possible. Source, acceleration, and scattering processes have also been studied within and in the vicinity of the LBL. Observed trapped pitch angle distributions of energetic electrons imply that the LBL is at least partially on closed field lines. Strong scattering of energetic protons is found within and equatorward of the LBL and thus must occur at least partially along closed field lines. Field-aligned electron acceleration by parallel electric fields can be discerned within and poleward of the LBL, but a more detailed analysis is necessary for a statistical study. Conical ion acceleration was seen relatively frequently within the LBL and about half as often poleward of the LBL. Neither acceleration process could be identified anywhere equatorward of the LBL.

Roeder, J. L.↗
NASA NTRS1992

Energetic and magnetosheath energy particle signatures of the low-latitude boundary layer at low altitudes near noon

The energetic and magnetosheath energy particle signature of the low-latitude boundary layer (LBL) and its separation from the cusp are investigated using particle data from the polar-orbiting S3-3 satellite. The LBL and its boundaries, as determined from the energetic particles are compared with the LBL and boundaries determined from a modified set of low-energy plasma criteria based on Newell and Meng (NM, 1988, 1989). Excellent agreement was found in 68 percent of the 19 orbits where the LBL was identified by the energetic electron data. In 10 percent of the remaining orbits, the modified NM criteria did not provide numerical LBL identification, though a clear boundary region was visually identifiable in the data. For the remaining orbits, the modified NM criteria identified the LBL, but not its full latitudinal extent.

Roeder, J. L.↗
NASA NTRS1989

Plasma wave observations during electron beam experiments at high altitudes

Electron beam experiments on the nearly geosynchronous P78-2 satellite conducted in 1979 resulted in observations of intense radiation near the local electron gyrofrequency. These signals resembled naturally occurring f(ce) waves during the same period. The amplitude of the simulated waves depended upon beam parameters. During 50-eV beam operations, current levels of 10 microamps produced strong emissions. Current levels of 1 microamp and 100 microamps did not. These emissions correspond in time to previously reported observations of electron distributions, which suggest heating of the local thermal plasma. Sufficient power is in the observed emissions to explain some aspects of the heated electron distributions observed during the experiments.

Olsen, R. C.↗
NASA NTRS1988

Longitudinal differences in electron precipitation near L = 4

The origin of the significant differences revealed in data on electron precipitation characteristics obtained above Siple Station, Antarctica, and Kerguelen Islands was investigated. The two stations are both in the Southern Hemisphere at nearly the same magnetic latitude (L=4) and at longitudes that place them roughly at equal distances east and west of the center of the South Atlantic magnetic anomaly. The primary data used in the study were counting rates from rocket-borne parachute-deployed scintillation counters and VLF data from ground-based and rocket-borne receivers. The two locations were found to differ in two major respects: (1) the precipitation background at Kerguelen Islands is very low, with high levels of wave activity being required to produce any detectable precipitation, and (2) X-ray microbursts, very common at Siple, were found to be essentially absent at Kerguelen. This observation supports models of the microburst generation process which predict maximum pitch angle scatterings of only a few tenths of a degree.

Bering, E. A., III↗
NASA NTRS1987

Electron cyclotron harmonic waves observed by the AMPTE-IRM plasma wave experiment following a lithium release in the solar wind

An unexpected occurrence following the second lithium release by the AMPTE-IRM spacecraft in the solar wind on September 20, 1984, was the appearance of electron cyclotron harmonic emissions. These emissions began about 50 s after the release and continued for several minutes. Narrow-band emissions polarized perpendicular to the magnetic field with amplitudes of approximately 0.00001 V/m were observed in each of the first five harmonic bands. The diffuse emissions extended from below the lowest measured frequency channel to above the highest narrow-band emission with a maximum below the electron cyclotron frequency. It will be shown that these observations are inconsistent with their generation by several ion beam instabilities.

Roeder, J. L.↗
NASA NTRS1987

Observation of nonlinear wave decay processes in the solar wind by the AMPTE IRM plasma wave experiment

Nonlinear wave decay processes have been detected in the solar wind by the plasma wave experiment aboard the Active Magnetospheric Particle Tracer Explorers (AMPTE) IRM spacecraft. The main process is the generation of ultralow-frequency ion acoustic waves from the decay of Langmuir waves near the electron plasma frequency. Frequently, this is accompanied by an enhancement of emissions near twice the plasma frequency. This enhancement is most likely due to the generation of electromagnetic waves from the coalescence of two Langmuir waves. These processes occur within the electron foreshock in front of the earth's bow shock.

Koons, H. C.↗
NASA NTRS1987

Electron precipitation near L = 4 - Longitudinal variation

Observations of penetrating electron precipitation at two L of about 4 stations in the southern hemisphere, Kerguelen and Siple Station, Antarctica, are summarized. The nearly complete absence of precipitation at Kerguelen and its frequent occurrence at Siple Station is explained in terms of their location with respect to the center of the South Atlantic geomagnetic anomaly at L of about 4. Conclusions which result from this explanation are discussed along with questions which remain concerning penetrating electron precipitation.

Sheldon, W. R.↗
NASA NTRS1985

X ray microbursts and VLF chorus

On January 4, 1978, at 1140 UT, a SuperArcas sounding rocket was launched from Siple Station, Antarctica (L = 4.2, 76 deg S, 84 deg W), during a geomagnetically disturbed period (Kp = 6-) with intense X-ray and VLF chorus activity. The parachuted payload observed an intense microburst precipitation event of 10-minute duration. These data have been correlated with measurements of FLF chorus by receivers on the ground at both Siple and its magnetic conjugate point, Roberval, Quebec. Detailed one-to-one correspondence between the microbursts and the chorus was not a consistent feature of the data. Time series analysis fo the data did indicate a significant correlation between the Siple X-ray precipitation and the Roberval VLF waves with an arrival time delay of 0.1 + or 0.3.

Roeder, J. L.↗
NASA NTRS1983

Quiet-time electron precipitation at L = 4 in the South Atlantic anomaly

A Superarcas sounding rocket was launched from Siple Station, Antarctica in January 1978, during a prolonged geomagnetically quiet period with very low VLF activity. The observed electron flux is the first direct measurement in the high latitude mesosphere of the quiet-time precipitating electrons in the South Atlantic anomaly. The integrated flux agrees resonably well with satellite measurements and predictions. The calculated energy spectrum of the precipitating electrons is much softer than typical measured quiet-time trapped spectra reported from satellite measurements after extended periods of low magnetic activity.

Benbrook, J. R.↗
NASA NTRS1980

The results from the X ray bremsstrahlung experiment of Project Trigger

Analysis of the bremsstrahlung X-ray data produced the following conclusions. The counting rate observed before the explosion was an expected normal quiet background. An unexpected increase in the counting rate below 20 keV was observed after the explosion, which may have been an effect of the explosion. A significant 0.13-H2 peak in the power spectral density of the X-ray flux was observed, which is interpreted as evidence for a particle-wave-particle interaction involving a bounce resonance stimulated by the explosion. Dispersed continuation of this interaction may have been responsible for delayed long-term precipitation.

Bering, E. A.↗
NASA NTRS1980

X-ray measurements during the Araks experiments

Measurements of the X-ray bremsstrahlung precipitation were made during the Araks experiments. The following conclusions were deduced from the data: with several exceptions the ambient X-ray flux observed was the normal quiet background expected for the location and existing conditions. The X-ray flux at energies below 20 keV was in excess of that predicted by theory. No obvious effects of the Araks experiments were seen in the data. However, a superposed epoch analysis of the X-ray data revealed a very small amount of precipitation associated with the electron gun.

Roeder, J. L.↗