Engineering topics

Menietti, J. D.

Publications and source records attributed to Menietti, J. D..

At least 73 records · Page 4

NASA NTRS1987

The current-voltage relationship in auroral current sheets

The current-voltage relation within narrow auroral current sheets is examined through the use of high-resolution data from the high altitude Dynamics Explorer 1 satellite. The north-south perpendicular electric field and the east-west magnetic field are shown for three cases in which there are large amplitude, oppositely directed paired electric fields and narrow current sheets. These data are shown to indicate that there is a linear Ohm's law relationship between the current density and the parallel potential drop within the narrow current sheets. This linear relationship had previously been verified for large-scale auroral formations greater than 20 km wide at the ionosphere. The evidence shown here extends our knowledge down to the scale size of discrete auroral arcs.

Weimer, D. R.↗

NASA NTRS1986

Ray tracing survey of Z mode emissions from source regions in the high-altitude auroral zone

The ray tracing computer program described by Menietti and Lin (1985) was used to investigate the propagation of Z mode emissions from sources in the auroral zone. The calculation results indicate that the propagation is primarily perpendicular to the magnetic field line (horizontal propagation) for almost all sources considered, except when the wave source is assumed to be unrestricted by the cyclotron maser mechanism. While the propagation is initially downward, refraction rapidly increases the wave normal angle, and only propagation at near-constant altitude occurs. The frequency bandwidth estimated from the ray paths was found to be less than that observed by the DE 1 satellite. Large horizontal magnetic field gradients associated with field-aligned currents produce downward propagation but are still not sufficient to produce the observed bandwidths. The results suggest that wave growth may be due to mechanisms in addition to cyclotron maser resonance.

Menietti, J. D.↗

NASA NTRS1986

Modeling and investigative studies of Jovian low frequency emissions

The Voyager data in a newly produced noise-reduced and recalibrated format has been received. New color spectrograms were developed on high resolution color terminals which display this data. The production of these new spectrograms utilizing the new format data is quite important, because it eliminates a serious problem of noise contamination and miscalibration in the old data set. In addition, a new Jovian plasma model was introduced, which includes the Io torus and accounts for 7 ionic species. The new plasma model is important in the ray tracing of hectometric (HOM) and kilometric (KOM) radiation which may be influenced by the Io torus.

Menietti, J. D.↗

NASA NTRS1986

Modeling and investigative studies of Jovian low frequency emissions

Jovian decametric (DAM) and hectometric (HOM) emissions were first observed over the entire spectrum by the Voyager 1 and 2 flybys of the planet. They display unusual arc-like structures on frequency-versus-time spectrograms. Software for the modeling of the Jovian plasma and magnetic field environment was performed. In addition, an extensive library of programs was developed for the retrieval of Voyager Planetary Radio Astronomy (PRA) data in both the high and low frequency bands from new noise-free, recalibrated data tapes. This software allows the option of retrieving data sorted with respect to particular sub-Io longitudes. This has proven to be invaluable in the analyses of the data. Graphics routines were also developed to display the data on color spectrograms.

Menietti, J. D.↗

NASA NTRS1986

Evidence of high densities and ion outflows in the polar cap during the recovery phase

The composition and characteristics of the polar cap plasma for an Oct. 14, 1981 outflow of polar wind ions are examined using data from the DE 1 satellite. The on-board instruments included a plasma wave instrument, a retarding ion mass spectrometer (RIMS) and a high altitude plasma instrument (HAPI). The outflow took place at an altitude of about 19,000 km at a magnetic local time of about midnight. The total plasma density measured was about 50/cu cm, which was an order of magnitude higher than normally recorded at that location and altitude. The background hydrogen plasma was disturbed by highly collimated flows of hydrogen and oxygen ions. The H(+) ions had a mean energy of 0.15 eV and a density of 6-10/cu cm. The O(+) ions had an average density of 20/cu cm and a temperature of 0.26 eV. The total flux of outflowing H(+) and O(+) was about 10 million/sq cm per sec. The HAPI data indicated that the O(+) ions appeared in the dayside ionosphere and the H(+) ions detected by the RIMS originated in the nightside polar cap.

Gallagher, D. L.↗

NASA NTRS1986

Statistical study of enhanced ion fluxes in the outer plasmasphere

DE-1 measurements of ion outflows at E = 5 eV to 32 keV in the Northern Hemisphere outer plasmasphere are compiled in graphs and investigated statistically. The data comprise 40 dayside (6:00-12:00 magnetic local time) and 50 nightside (18:00-23:00) passages at magnetic activities Kp = 0-7 and include six magnetic storms and recoveries as well as quiet periods. Features noted include enhanced number fluxes during periods of increased magnetic activity, upward dayside and downward nightside flows, peak net H(+) fluxes greater than 10 to the 8th/sq cm sec, and greater field-aligned flows (but at lower ion temperatures) in the outer plasmasphere than in the plasma trough.

Menietti, J. D.↗

NASA NTRS1986

DE-1 observations of solar wind-magnetosphere coupling processes in the polar cusp

Measurements of hot plasma distribution functions in the midaltitude polar cusp by the Dynamics Explorer-1 spacecraft have provided new information on the processes of magnetosheath plasma injection and transport and the dependence of these phenomena on solar-wind parameters. Results on magnetosheath plasma injection have included: (1) localization of the region of plasma injection to the high-latitude cusp-magnetosheath interface, (2) movement of this region toward lower latitudes as B(z) becomes more southward, (3) maintenance of quasi-neutrality of suprathermal plasmas within the cusp, and (4) support for merging-type injection at the high-latitude cusp boundary for northward IMF. Plasma flow measurements from Dynamics Explorer-1 have led to a global IMF-dependent convection model which contains both merging and viscous driving forces.

Burch, J. L.↗

NASA NTRS1985

'Electron conic' signatures observed in the nightside auroral zone and over the polar cap

The high altitude plasma instrument, i.e., five electrostatic analyzers, on board the DE 1 satellite obtained data on electron conical distributions during two nightside auroral zone passes and one polar cap pass. The southern orbit apogee was 675 km, the northern, 24,000 km. Electrons confined to the outside of the loss cone were the highest energy particles detected. The upward integrated flux was greater than the downward flux, indicating the presence of a heating process perpendicular to the geomagnetic field.

Menietti, J. D.↗

NASA NTRS1985

Ray tracing of Z-mode emissions from source regions in the high-altitude auroral zone

Two-dimensional ray tracing of Z-mode radiation from sources in the auroral zone has been performed. The Z-mode waves are assumed to be excited by the cyclotron maser mechanism at frequencies near the electron gyrofrequency and at wave normal angles near 90 deg. The emission is found to propagate primarily perpendicular to the magnetic field line (horizontal propagation) for all sources considered, and thus not to substantially lower altitudes. The frequency bandwidth estimated from the ray paths is much less than that observed by DE-1. It is suggested that the wave source needs to be in the region of field-aligned current in order for the cyclotron maser mechanism to explain the broad frequency bandwidth of Z-mode emissions.

Menietti, J. D.↗

NASA NTRS1985

IMF By-dependent plasma flow and Birkeland currents in the dayside magnetosphere. I - Dynamics Explorer observations

Plasma, magnetic-field, and dc electric-field observations from Dynamics Explorers 1 and 2 are used to investigate the morphology of solar-wind ion injection, Birkeland currents, and plasma convection in the morning sector for both positive and negative interplanetary magnetic field (IMF) By components. The results of the study are used to construct a By-dependent global convection model for southward IMF. A significant element of the model is the coexistence of three types of convection cells ('merging cells', 'viscous cells', and 'lobe cells'). This model can account for observations of a nearly stationary (in local time) convection 'throat', a sunward-antisunward convection reversal zone at the polar-cap boundary in both the morning and afternoon quadrants, the morphology of solar-wind ion injection and transport in the mid-altitude polar cusp, and the By-dependent dawn-dusk asymmetry of polar-cap electron fluxes.

Burch, J. L.↗

NASA NTRS1985

Enhanced ion outflows measured by the DE 1 high altitude plasma instrument in the dayside plasmasphere during the recovery phase

Ion flow velocities both parallel and perpendicular to the magnetic field, and including the effects of spacecraft charging and spacecraft velocity, have been measured during the recovery phase of two large magnetic storms on October 14 and 21 of 1981. These measurements were made both inside and outside the plasmasphere and indicate unreported yet substantial outflows of ions within the dayside plasmasphere (October 14). Combined data from instruments on board the Dynamics Explorer satellite, including the high altitude plasma instrument, the energetic ion composition spectrometer, the retarding ion mass spectrometer, and the plasma wave instrument, indicate that these ions are most likely dominantly O(+) at energies at least as low as 5 eV. The nightside pass (October 21), which occurred during the recovery phase of a similar storm, showed no plasmaspheric outflows. The results indicate that a large contribution to the outflux into the dayside plasmasphere during the recovery period is due to E greater than 5 eV ions.

Menietti, J. D.↗

NASA NTRS1984

Correlated low-frequency electric and magnetic noise along the auroral field lines

Dynamics Explorer 1 measurements of intense low-frequency electric and magnetic noise observed at low altitudes over the auroral zone are described. The intensity of both the electric and magnetic fields decreases rapidly with increasing frequency. Most of the energy is at frequencies below the O(+) cyclotron frequency, and some evidence is found for a cutoff or change in spectral slope near that frequency. The magnetic to electric field ratio decreases rapidly with increasing radial distance and also decreases with increasing frequency. The polarization of the electric field in a plane perpendicular to the earth's magnetic field is essentially random. The transverse electric and magnetic fields are closely correlated, with the average Poynting flux directed toward the earth. The total electromagnetic power flow associated with the noise is substantial. Two general models are discussed to interpret these observations, one based on static electric and magnetic fields imbedded in the ionosphere and the other based on Alfven waves propagating along the auroral field lines.

Gurnett, D. A.↗

NASA NTRS1984

Jovian decametric arcs - An estimate of the required wave normal angles from three-dimensional ray tracing.

Three-dimensional ray tracing is applied to an analysis of variable radio wave normal angle effects on the associated decametric (DAM) arc structures in the Jovian magnetosphere. The analysis is bed on 1-40 MHz radio signature recorded during Voyage 1 and 2 passages. The frequencies considered are above the R-X cut-off, and several ratios of the emission frequency/source frequency. The ray tracing code is based on a cold plasma formula and integration of the Hasselgrove (1955) equations. It is assumed that the emission is in the R-X mode, the source lies at the foot of an Io flux tube, and the emission cone is hollow. Attention is focused on data for two intense, vertex-late, high curvature DAM arc. A possible source for the arcs is found to be doppler-shifted gyroemission from a beam of electrons with an energy of 10 keV. A value of 1.1 is set as the limit of the doppler shift of the DAM emissions.

Menietti, J. D.↗

NASA NTRS1984

Three dimensional ray tracing of the Jovian magnetosphere in the low frequency range

Ray tracing studies of Jovian low frequency emissions were studied. A comprehensive three-dimensional ray tracing computer code for examination of model Jovian decametric (DAM) emission was developed. The improvements to the computer code are outlined and described. The results of the ray tracings of Jovian emissions will be presented in summary form.

Menietti, J. D.↗

NASA NTRS1984

Three-dimensional ray tracing of the Jovian magnetosphere in the low-frequency range

Three-dimensional ray tracing of the Jovian DAM emission has been performed utilizing the O-4 magnetic field model (Acuna and Ness, 1979) and a realistic plasma model. Minimal assumptions about the emission mechanism have been made that include radiation in the right-hand extraordinary mode, propagating nearly perpendicular to the field line at source points located just above the RX cutoff frequency along Io flux tubes. Ray tracing has been performed in the frequency range from 2-35 MHz from successive Io flux tubes separated by ten degrees of central meridian longitude for a full circumference of northern hemisphere sources. The results show unusual complexity in model arc spectra that is displayed in a constant Io phase format with many similarities to the Voyager PRA data. The results suggest much of the variation in observed DAM spectral features is a result of propagation effects rather than emission process differences.

Menietti, J. D.↗

NASA NTRS1982

Three dimensional ray tracing Jovian magnetosphere in the low frequency range

Ray tracing of the Jovian magnetosphere in the low frequency range (1+40 MHz) has resulted in a new understanding of the source mechanism for Io dependent decametric radiation (DAM). Our three dimensional ray tracing computer code has provided model DAM arcs at 10 deg. intervals of Io longitude source positions for the full 360 deg of Jovian system III longitude. In addition, particularly interesting arcs were singled out for detailed study and modelling. Dependent decametric radiation arcs are categorized according to curvature--the higher curvature arcs are apparently due to wave stimulation at a nonconstant wave normal angle, psi. The psi(f) relationship has a signature that is common to most of the higher curvature arcs. The low curvature arcs, on the other hand, are adequately modelled with a constant wave normal angle of close to 90 deg. These results imply that for higher curvature arcs observed for from Jupiter (to diminish spacecraft motion effects) the electrons providing the gyroemission are relativistically beamed.

Menietti, J. D.↗

NASA NTRS1982

Plasma injection and transport in the mid-altitude polar cusp

DE-1 hot plasma observations in the mid-altitude polar cusp have shown evidence of a significant velocity filtering phenomenon which is consistent with a latitudinally narrow region of plasma injection located at a geocentric distance of about eight earth radii (in a dipole approximation). Plasma convection from the injection region into the polar cap results in a V-shaped log E vs. observed pitch angle relation at geocentric distances near 4 earth radii. This velocity filtering effect allows the measurement of much smaller flow velocities (about 10 km/sec) than have heretofore been possible with hot plasma measurements. The flows thus determined are consistent with ionospheric flows measured nearly simultaneously by the DE-2 spacecraft, although the magnitudes of the higher altitude flows are higher by a factor of 2 or more than an approximately r to the 3/2 power dipole-field mapping would predict.

Burch, J. L.↗

NASA NTRS1982

DE-1 observations of counterstreaming electrons at high altitudes

Observations of plasma at altitudes of 2-3 earth radii with the High Altitude Plasma Instrument (HAPI) on DE-1 indicate two distinct types of counterstreaming electron events. The type 1 event is characterized by two Maxwellian distribution functions, an isotropic high-temperature component and a field-aligned low temperature component. The type 2 event is distinguished by beams parallel and antiparallel to the magnetic field direction. The observations suggest two distinct mechanisms for accelerating counterstreaming electrons. Type 1 events appear to involve wave-particle interactions while type 2 events imply direct acceleration by oppositely-directed electric fields pointing toward the satellite along magnetic field lines.

Lin, C. S.↗