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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 199 records · Page 11
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NASA NTRS1990

Energetic charged particle angular distributions near (r less than or equal to 2 Neptune radii) and over the pole of Neptune

Energetic ion (greater than 28 keV) and electron (greater than 22 keV) pitch angle distributions very close to (r less than or equal to 2 Neptune radii) and over the north planetary pole of Neptune are presented using data from the Low Energy Charged Particle and magnetometer Experiments on the Voyager 2 spacecraft. The particle data are temporally structured and spectrally soft; a similarity with earth-like auroral signatures has previously been noted. However, the pitch angle distributions (showing trapped distributions at high magnetic latitudes) do not support an earth-like auroral interpretation, and alternative explanations for the temporal dynamics must be sought. Between r of about 1.6 and 2.0 Neptune radii and in the vicinity of the magnetic equator, the higher energy ion and electron pitch angle distributions (E greater than or equal to 80 keV) display dramatic 'bite-outs' at 90 deg. This bite-out feature could be caused by interactions with the newly discovered ring 1989N3R.

Mauk, B. H.↗
NASA NTRS1990

Electrons in the ionopause boundary layer of Venus

Measurements made by the retarding potential analyzer on board the Pioneer Venus Orbiter (PVO) have indicated the presence of both magnetosheath electrons and atmospheric photoelectrons in the plasma mantle region of Venus, which lies just outside the ionopause. A theoretical model of the spatial and energy distribution of electrons in the plasma mantle has been constructed using the two-stream electron transport method and the time-dependent electron energy equation. Magnetic field lines parallel to the ionopause are assumed. Transport and collisional processes, including electron scattering by neutrals and ions, are taken into consideration. The calculated electron spectra in the mantle show signatures of both solar wind electrons and photoelectrons. Cold ionospheric electrons are present only for altitudes below the ionopause. The calculated electron temperatures agree very well with the PVO data both in the mantle region and in the magnetized ionosphere.

Gan, LU↗
NASA NTRS1990

Possible second harmonic gyroemission at Uranus

During the inbound trajectory toward Uranus, the Planetary Radio Astronomy Instrument on board the Voyager 2 spacecraft observed narrow-band smooth (n-smooth) emission at frequencies centered near 60 kHz. By assuming models of the plasma density for the dayside magnetosphere of Uranus and by using cold plasma theory together with stringent observational constraints, ray-tracing calculations were performed to determine the source location and mode of the n-smooth emission. Ray-tracing calculations suggest that the n-smooth emission with sources near the magnetic equator may be fundamental X mode for certain conditions or second harmonic gyroemission. If the emission is second harmonic gyroemission, the fundamental emission at 30 kHz is expected but apparently not observed. These findings are discussed in the context of the most recent developments in the theory of the cyclotron maser instability.

Menietti, J. D.↗
NASA NTRS1991

Particle flows to shape and voltage surface discontinuities in the electron sheath surrounding a high voltage solar array in LEO

This paper discusses the numerical modeling of electron flows from the sheath surrounding high positively biased objects in LEO (Low Earth Orbit) to regions of voltage or shape discontinuity on the biased surfaces. The sheath equations are derived from the Two-fluid, Warm Plasma Model. An equipotential corner and a plane containing strips of alternating voltage bias are treated in two dimensions. A self-consistent field solution of the sheath equations is outlined and is pursued through one cycle. The electron density field is determined by numerical solution of Poisson's equation for the electrostatic potential in the sheath using the NASCAP-LEO relation between electrostatic potential and charge density. Electron flows are calculated numerically from the electron continuity equation. Magnetic field effects are not treated.

Metz, Roger N.↗
NASA NTRS1991

Polarization model applied to Uranian radio emission

The total power and the degree of circular polarization as measured by the Planetary Radio Astronomy experiments on the Voyager spacecraft are modeled. For a source near the electron cyclotron frequency, the degree of circular polarization is determined by the angle between the wave vector and the field. It is shown that the observed strong circular polarization of Uranian smooth low-frequency (SLF) can be modeled as emission that is beamed along the direction of the magnetic field in a filled cone. The main observational constraints of SLF emission from Uranus are met by conjugate sources at about 21 deg from the magnetic equator.

Sawyer, C. B.↗
NASA NTRS1992

A thermosphere/ionosphere general circulation model with coupled electrodynamics

A new simulation model of upper atmospheric dynamics is presented that includes self-consistent electrodynamic interactions between the thermosphere and ionosphere. This model calculates the dynamo effects of thermospheric winds, and uses the resultant electric fields and currents in calculating the neutral and plasma dynamics. A realistic geomagnetic field geometry is used. Sample simulations for solar maximum equinox conditions illustrate two previously predicted effects of the feedback. Near the magnetic equator, the afternoon uplift of the ionosphere by an eastward electric field reduces ion drag on the neutral wind, so that relatively strong eastward winds can occur in the evening. In addition, a vertical electric field is generated by the low-latitude wind, which produces east-west plasma drifts in the same direction as the wind, further reducing the ion drag and resulting in stronger zonal winds.

Richmond, A. D.↗
NASA NTRS1992

Equatorial bubbles updrafting at supersonic speeds

Plasma and electric field observations from two satellite encounters with equatorial plasma bubbles updrafting at velocities of about 2 km/s are presented. These large, upward velocities are consistent with an adaptation of Chandrasekhar's model for the motion of plasma blobs supported against gravity by a magnetic field. Vector magnetic field measurements, available during one of the bubble encounters show a perturbation of about 150 nT, directed radially outward from the earth, near the western wall of deepest plasma depletion. This magnetic variation is too large to be caused by simple shunting of the g x B current along the bubble's edge. Rather, it is Alfvenic in nature, radiating from a generator located near the magnetic equator, in the plasma outside the bubble's leading edge. A heuristic model of a depleted flux tube with constant circular cross section moving upward through a background plasma predicts most of the measurements' qualitative features.

Aggson, Thomas L.↗
NASA NTRS1992

Plasma wave observations at Neptune

Phenomena detected by the plasma wave instrument during the Voyager 2 flyby of Neptune are reviewed. Particular attention given to radio emissions, electron plasma oscillations in the solar wind upstream of the bow shock, electrostatic electron cyclotron waves and upper hybrid resonance (UHR) waves, whistler mode noise, and dust impacts. The radio emissions which occur in a broad range of about 5 to 50 kHz are considered to be generated by mode conversion from UHR waves at the magnetic equator. The inner magnetosphere has relatively low plasma wave intensities (less than 100 microV/m). Many small micron-sized dust particles which were detected striking the spacecraft had the maximum impact rate of about 280 impacts per sec at the bound ring plane crossing, and about 110 impacts per sec at the outbound ring plane crossing. Most of the particles were concentrated in a dense disk, about one thousand km thick, near the equatorial plane.

Gurnett, D. A.↗
NASA NTRS1992

On the standing wave mode of giant pulsations

In order to determine the standing wave mode of giant pulsations, a systematic survey of magnetic field data from the AMPTE CCE spacecraft and from ground stations located near the geomagnetic foot point of CCE was made. One giant pulsation was associated with a compressional wave, while no giant pulsation was observed in association with transverse wave events. The CCE magnetic field record for the giant pulsation exhibited a remarkable similarity to a giant pulsation observed from the ATS 6 geostationary satellite near the magnetic equator. It is concluded that the compressional nature of the giant pulsation is due to an odd-mode standing wave structure, which places a strong constraint on the generation mechanism of giant pulsations.

Takahashi, K.↗
NASA NTRS1994

SEPAC co-investigator support

Mission preparation and support, data reduction and analysis, and modelling of beam phenomenon activities are described with respect to the Space Experiments with Particle Accelerators (SEPAC) project. The SEPAC Electron Beam Accelerator was used in observing the life time and drift velocity of ionization streaks in the upper atmosphere by ground based radars. A second active source, the plasma contactor (PC) electrically neutralized the shuttle orbiter during beam injections. Several observations of plasma density enhancements were correlated with orbiter crossings of the magnetic meridian of the Jicamarca, Peru radar located near the magnetic equator. One of several successful experiments was the artificial aurora experiment in which downward directed electron beam pulses were observed by the onboard AEPI camera. Theoretical and modelling efforts focused on phenomena related to electron beam and plasma cloud injection from spacecraft.

Source record↗
NASA NTRS1995

Elliptically polarized bursty radio emissions from Jupiter

We report a new component of Jovian radio emission observed by the Ulysses spacecraft when Ulysses was at high Jovigraphic latitudes (greater than or approximately = 30 deg north or south of the Jovian magnetic equator). This bursty high-latitude emission is elliptically polarized in the right-hand sense when observed from northern latitudes and in the left-hand sense when observed from southern latitudes, consistent with extraordinary mode. The orientation of the polarization ellipse is observed to systematically vary with time relative to the observer. It is argued that the elliptically-polarized nature of the emission is intrinsic to the source region.

Reiner, M. J.↗
NASA NTRS1994

Unusual satellite-electron signature within the Uranian magnetosphere and its implications regarding whistler electron loss processes

It has been reported that during the outbound (nightside) portion of the Voyager 2 encounter with the Uranian magnetosphere, intense whistler mode emissions were observed near the magnetic equator (lambda(sub m) approx. 16 deg) and at L shell values between approx. 5.5 and approx. 9 R(sub U). Comprehensive calculations of whistler-driven pitch angle diffusion, in previous work, have yielded strong diffusion electron lifetimes of approx. 1 hour for 20 to 40 keV electrons. In this paper we report on an unusual and sharply defined charged particle feature that: (1) involved electrons between 22 and 35 keV, (2) was observed during the time period of the intense whistler mode observations, (3) was aligned very accurately and sharply with the minimum L shell position (L approx. 7.5) of the satellite Ariel, and (4) has an appearance that suggests that electrons were removed only at and beyond Ariel's minimum-L. On the basis of our conclusion that the signature was caused by electron interactions with either Ariel or materials distributed along Ariel's orbit, the signature could not have been generated for at least 10 hours prior to its observation. Thus the calculated whistler loss times are in apparent conflict with the signature observation. A scenario of events is proposed to explain the data that involves substormlike electron acceleration on the Uranian nightside and a subsequent sculpting of the electron spatial distributions via interactions with Ariel or materials distributed along Ariel's orbit. The possibility exists that the accurate alignment of the sharp electron feature with Ariel's minimum-L, and the absorptionlike character of the feature, are accidental, and that the feature is caused by dynamical processes (e.g., substorms). In this case the dynamical processes must be quite dissimilar to those occurring in the Earth's magnetosphere.

Mauk, B. H.↗
NASA NTRS1995

An interpretation of a mysterious 3.0- to 4.6-kHz emission band observed on Voyager 2 near Neptune

A whistler mode interpretation is provided for the narrowband signal (f approx. 3 - 4.6 kHz, Delta f approx. 200 - 800 Hz) detected by the plasma wave instrument on Voyager 2 during its encounter with Neptune. Our analysis indicates that this signal may have been generated in a limited spatial region and that it propagated to other regions of the Neptunian magnetosphere in the nonducted whistler mode with wave normal vectors lying close to the whistler mode resonance cone. The observed frequency variation of the emission along the Voyager 2 trajectory is consistent with this interpretation. The source location is estimated to be near the magnetic equator at L approx. 4 and dipole longitude of 111 deg W (260 deg W longitude in Neptune coordinate system). The source frequency and bandwidth are estimated to be 3.6 kHz and 300 Hz, respectively. The waves most likely would have been generated by energetic electrons with 2- to 20-keV parallel energy via a gyroresonance mechanism. Our interpretation of the narrowband emissions places the following limits on the Neptunian thermal plasma density and temperature: (1) N(sub e, min) greater than 0.16 el/cu cm for 1.2 R(sub N) less than R less than 5 R(sub N), (2) N(sub e, max) = 597.5/cu cm at R - 1.3 R(sub N), (3) T(sub e, max) less than 500-1000 K at R approx. 5 R(sub N). It is also possible that the weak UV aurora observed near Neptune could have been caused by the precipitation of energetic particles by the narrowband emission as a result of wave particle interactions.

Sonwalkar, Vikas S.↗
NASA NTRS1994

VHF radar and rocket observations of equatorial spread F on Kwajalein

VHF radar data from the Summer 1990 Equatorial Spread F campaign on Kwajalein are presented. The Cornell 50 MHz portable radar interferometer (CUPRI) operated concurrently with the Altair UHF incoherent scatter radar throughout July and August and supported two sounding rocket flights on July 30 and August 2. This experiment provided the first opportunity to simultaneously diagnose equatorial spread F using the three prime experimental techniques: VHF/UHF coherent scatter, incoherent scatter, and in situ probe measurements of electric field and density fluctuations. The intensity of the coherent echoes observed was consistent with typical Jicamarca spread F observations, but chains of periodic, large-scale plasma upwellings were observed more often and for much longer durations on Kwajalein than have been seen over Peru. CUPRI also measured Doppler frequencies in one upwelling corresponding to 1200-m/s plasma drift velocities. This measurement agrees with recent observations of supersonic drift rates at the magnetic equator by spacecraft. Near the most active localized plasma upwellings, interferometer data reveal that the zonal drift rate of plasma irregularities can vary sharply in space, as one would expect for two-dimensional incompressible flow. We introduce a semiempirical model of the three-dimensional spectrum of F region irregularities that is consistent with the one-dimensional spectra of density fluctuations observed by sounding rockets and with the axial ratio of irregularities determined recently. Normalized to data from one of the rocket flights on Kwajalein, the model predicts the 3-m scattering cross-section measured by CUPRI to within a few decibels.

Hysell, D. L.↗
NASA NTRS1994

Observations of Pc5 micropulsation-related electric field oscillations in equatorial ionosphere

A 54.95-MHz coherent backscatter radar, an ionosonde and the magnetometer located at Trivandrum in India (8.5 deg N, 77 deg E, 0.5 deg N dip angle) recorded large-amplitude ionospheric fluctuations and magnetic field fluctuations associated with a Pc5 micropulsation event, which occurred during an intense magnetic storm on 24 March 1991 (A(sub p) = 161). Simultaneous 100-n T-level fluctuations are also observed in the H-component at Brorfelde, Denmark (55.6 deg N gm) and at Narsarsuaq, Greenland (70.6 deg N gm). Our study of the above observations shows that the E-W electric field fluctuations in the E- and F-regions and the magnetic field fluctuations at Thumba are dominated by a near-sinusoidal oscillation of 10 min during 1730-1900 IST (1200-1330 UT), the amplitude of the electric field oscillation in the equatorial electrojet (EEJ) is 0.1-0.25 mV/m and it increases with height, while it is about 1.0 mV/m in the F-region, the ground-level H-component oscillation can be accounted for by the ionospheric current oscillation generated by the observed electric field oscillation in the EEJ and the H-component oscillations at Trivandrum and Brofelde are in phase with each other. The observations are interpreted in terms of a compressional cavity mode resonance in the inner magnetosphere and the assoicated ionospheric electric field penetrating from high latitudes to the magnetic equator.

Reddy, C. A.↗
NASA NTRS1994

The streaming-trapped ion interface in the equatorial inner magnetosphere

Spacecraft measurements of core ions on L=4-7 field-lines typically show trapped ion distributions near the magnetic equator, and frequently indicate field-aligned ion streams at higher latitudes. The nature of the transition between them may indicate both the microphysics of hot-cold plasma interactions and overall consequences for core plasma evolution. We have undertaken a statistical analysis and characterization of this interface and its relation to the equatorial region of the inner magnetosphere. In this analysis, we have characterized such features as the equatorial ion flux anisotropy, the penetration of field-aligned ionospheric streams into the equatorial region, the scale of the transition into trapped ion populations, and the transition latitude. We found that most transition latitudes occur within 13 deg of the equator. The typical values of equatorial ion anisotropies are consistent with bi-Maxwellian temperature ratios of T(sub perpendicular)/T(sub parallel) in the range of 3-5. The latitudinal scales for the edges of the trapped ion populations display a rather strong peak in the 2-3 deg range. We also found that there is a trend for the penetration ratio, the anisotropy half width, and the transition scale length to decrease with a higher equatorial ion anisotropy. We may interpret these features in terms of Liouville mapping of equatorially trapped ions and the reflection of the incoming ionospheric ion streams from the equatorial potential peaks associated with such trapped ions.

Lin, J.↗
NASA NTRS1995

Bounce-Averaged Hamiltonian for Charged Particles in an Axisymmetric but Nondipolar Model Magnetosphere

In order to facilitate bounce-averaged guiding center simulations of geomagnetically trapped particles, we express the kinetic energy of a particle with magnetic coordinates (L,phi) as an analytic function of the first two adiabatic invariants (M, J) and the L value of the field line. The magnetic field model is axisymmetric, consisting of a dipolar B field plus a uniform southward magnetic field parallel to the dipole moment mu(sub E). This model magnetosphere is surrounded by a circular equatorial neutral line whose radius b is an adjustable parameter. The L value of a field line is (by definition) inversely proportional to the flux enclosed by the corresponding magnetic shell of equatorial radius r(sub 0), and the L value at the neutral line (r(sub 0) = b) is denoted L*. The azimuthal coordinate phi measures magnetic local time. The best functional representation found for the normalized difference (L(exp 3)a(exp 3)/mu(sub E))(B(sub m) - B(sub 0)) between mirror-point field B(sub m) and equatorial field B(sub 0) along any field line is a 5-term expansion in powers (2/3 through 6/3) of the quantity X equivalent to (La/mu(sub E))(exp 1/2)K, where K equivalent to (J(exp 2)/8m(sub 0)M)(exp 1/2) is an adiabatically conserved quantity independent of particle energy, m(sub 0) is the rest mass of the particle, and a is the radius of the Earth. This functional form is motivated by results for limiting cases in which particles mirror very near and very far from the magnetic equator. Expansion coefficients corresponding to various powers of X are obtained from least squares fits to numerically computed results for X as a function of L and B(sub m). These are accurately expressible as fourth-order polynomials in (r(sub 0)/b)(exp 3), hence indirectly as functions of L/L* = 3La/2b. This representation, which leads (except for a manageably small region of parameter space) to better than 1% accuracy in the specification of B(sub m) as a function of K and L, allows bounce-averaged guiding center simulations to be performed without actually tracing the bounce motions of individual particles. Bounce-averaged drifts L' (meridional) and phi' (azimuthal) are proportional to derivatives of the Hamiltonian H (sum of kinetic and potential energies) with respect to phi and L, respectively. Our formulation thus provides a computationally efficient method for tracing the bounce-averaged adiabatic motion (conserving all three invariants) and nonadiabatic transport (violating the third invariant while conserving the first two invariants) of geomagnetically trapped particles in the model magnetosphere.

Schulz, Michael↗
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.↗