Engineering topics

Ip, W.-H.

Publications and source records attributed to Ip, W.-H..

At least 37 records · Page 2

NASA NTRS1986

Plasma wave observations at comet Giacobini-Zinner

The plasma wave instrument on the International Cometary Explorer (ICE) detected strong ion acoustic waves together with electromagnetic whistlers and low-level electron plasma oscillations when the spacecraft was within two million km of the nucleus of comet Giacobini-Zinner. As ICE approached the anticipated bow-shock location, electromagnetic and electrostatic wave levels increased significantly, but even amidst this turbulence, the wave instrument detected structures with familiar bow shock characteristics that were correlated with observations of localized electron heating phenomena. Just beyond the visible coma, high-amplitude broadband waves were detected accounting for the significant electron heating observed in this region. Near closest approach, broadband electrostatic noise was detected together with a changing pattern of weak electron plasma oscillations that yielded a density profile for the outer layers of the cold plasma tail. Near the tail axis, the plasma wave instrument also detected a nonuniform flux of dust impacts, and a preliminary profile of the Giacobini-Zinner dust distribution for micrometer-sized particles is presented.

Scarf, F. L.↗

NASA NTRS1985

CCD observations of comet Tuttle 1980 XIII - The H2O(+) ionosphere

A CCD spectrum of comet Tuttle 1980h has been analyzed with emphasis on the emission of H2O(+) ions. The fine angular resolution (1.5 arcsec) and the capability of absolute brightness calibration of the CCD instrument made it possible to determine the spatial concentration of the H2O(+) ions of this faint comet and the total number of these ions confined within a spherical region. Solar photoionization of the H2O atmosphere can account for the production of the H2O(+) ions observed in the confined region. The dimension of the H2O(+) ionosphere at the time of observation was found to be comparatively small, implying the possible existence of an ionopause sharply delineating the extension of the cometary ionosphere.

Ip, W.-H.↗

NASA NTRS1984

A radio source occultation experiment with comet Austin 1982g, with unusual results

A radio source occultation by comet Austin 1982g was observed on September 15-16, 1982. A change in the apparent position of 1242 + 41 by 1.3 arcsec occurred when the source was 220,000 km away from the cometary ion tail. If this change was due to refraction by the cometary plasma, it indicates an electron density of the plasma of about 10,000/cu cm. When the radio source was on the other side of the plasma tail, at a distance of 230,000 km, the position angle of the electric vector of the radio source changed gradually over about 140 deg within two hours. This observation cannot be explained in terms of ionospheric Faraday rotation, and results from either an intrinsic change in the radio source or Faraday rotation in the cometary plasma due to a change in the direction and/or strength of the magnetic field. In the latter case, the cometary coma must have an electron density and a magnetic field strength orders of magnitude larger than current theories predict.

De Pater, I.↗

NASA NTRS1984

Limitation of electrostatic charging of dust particles in a plasma

It is shown that in a dusty plasma consisting of a plasma (density n and temperature T) and dust grains (density N and radius a) the charge on a grain is not given by its free-space value. Instead, the charge is reduced by a factor 1 + x. Except for the optically thin E and G rings, this factor is large. Usually electromagnetic forces on dust particles in Saturn's ring system are too small to produce observable effects. The current carried by dust particles moving relative to the plasma with a speed w is to a good approximation given by j = NQw. Thus, magnetic perturbations by the F ring should be much smaller than previously estimated.

Goertz, C. K.↗

NASA NTRS1984

A dawn-to-dusk electric field in the Jovian magnetosphere

It is shown that if Io-injected plasma is lost via a planetary wind a sun-fixed Birkeland current system may result. This is due to the fact that a current flows across a density gradient produced by the loss of plasma through the planetary wind in the tail. The divergent current is connected to field-aligned Birkeland currents which flow into the ionosphere at dawn and out of it at dusk. The closure currents in the ionosphere require a dawn-to-dusk electric field which at the orbit of Io is estimated to have a strength of a few mV/m. Independent estimates derived from the local time asymmetry of the torus u.v. emission indicate a field of 1.5 mV/m.

Ip, W.-H.↗

NASA NTRS1984

Electrodynamic processes in the ring system of Saturn

A number of recently observed Saturn ring phenomena are discussed in terms of their electrodynamic implications. Voyager 1 and 2 observations of the rotating near-radial spokes in the B Ring, waves and braids of the F Ring, and discrete episodic bursts of broadband radio emission, claimed by some to originate in a ring, are addressed. Several other phenomena are considered, including the origin and evolution of the diffuse E Ring and G Ring (which appear to be composed of fine dust), as well as the existence of a number of sharp discontinuities in the main ring system, within the context of gravitoelectrodynamics of charged dust in the magnetosphere.

Mendis, D. A.↗

NASA NTRS1983

Energetic particle microsignatures of Saturn's satellites

During Voyager's 1980 and 1981 encounters with Saturn, the low energy charged particle experiment observed satellite microsignatures in ions and electrons. Each of the five major satellites within Titan's orbit were associated with at least one absorption feature in the high time resolution data. Microsignatures are usually observed in the corotational wake region within about 25 deg in azimuth of a satellite, and do not generally occur at times predicted by a centered, aligned dipole. A better predictive model seems to be that of an aligned, axisymmetric field with significant quadrupole and octupole terms.

Carbary, J. F.↗

NASA NTRS1983

Magnetic drifts at Io - Depletion of 10-MeV electrons at Voyager 1 encounter due to a forbidden zone

A model for magnetic field lines is used to track the Voyager spacecraft trajectory past Io back to the equatorial plane in order to determine the cause of an observed depletion of 10 MeV electrons in Io's orbit. The field lines are in the vicinity of an Alfven wing, a noncompressive field perturbation. It is suggested that the Jovian magnetospheric drift features influence the behavior of particles around Io, which also has a zone of reduced convection velocity. An analytical model is developed that demonstrates that a forbidden zone may exist around Io due to the gradient drifts and reduced convection velocity, and projections of particles convecting to the Io surface agree well with the Voyager data for particle concentrations. Discrepancies in the measured 9 MeV fluxes with respect to Pioneer data indicate that a possible change in the Io conductance in the interval between the passages of the two spacecraft.

Goldstein, B. E.↗

NASA NTRS1983

Cosmic ray gradients in the outer heliosphere

Launched in 1972 and 1973 respectively, the Pioneer 10 and 11 spacecraft are now probing the outer heliosphere on their final escape from the sun. The data in this paper extend for almost an entire solar cycle from launch to early 1983, when Pioneer 10 was at a heliocentric distance of 29 AU and Pioneer 11, 13 AU. The UCSD instruments on board were used to study the gradient, and to look at the time and spatial variations of the cosmic ray intensities.

Fillius, W.↗

NASA NTRS1983

A 50 day periodicity in the cosmic ray anisotropy

A modulation of the anisotropy of relativistic interplanetary particles with a period of approximately 50 days is discussed. Even though it is not unusual for cosmic ray data to reflect the solar rotation frequency and its harmonics with periods of 25/n days (n being 1, 2, 3, . . .), it is not known how a periodicity of 50 days, which would be a subharmonic, is generated. Since the Cerenkov counter responds to all kinds of relativistic particles, it does not identify the particles that cause the 50 day east-west modulation. Jovian electrons are considered unlikely in that they are most abundant near their source, whereas the modulation has never been reported for ground-based neutron monitors. It is pointed out that since the Cerenkov threshold is somewhat lower than typical neutron monitor cutoffs, it is conceivable that the modulation affects nucleons just in this energy gap. It is also thought that there might be a uniformly distributed component of interplanetary electrons which undergo this peculiar modulation.

Fillius, W.↗

NASA NTRS1983

On the equatorial transport of Saturn's ionosphere as driven by a dust-ring current system

The diurnal modulation of the dust ring current of Saturn's D-ring causes field-aligned Birkeland currents to flow near the dawn and dusk terminators and close across the midlatitude ionosphere. One consequence of this current system is the establishment of a global convection pattern in the equatorial outer ionosphere. Outward motion of the dayside ionospheric plasma as well as the corresponding absorption effect of the inner ring system might be one physical cause of the depletion of the ionospheric content of Saturn.

Ip, W.-H.↗

NASA NTRS1983

On photochemical heating of cometary comae - The cases of H2O and CO-rich comets

It is found that the effect of energy addition due to photochemical processes in the cometary coma on the heating and accelerating of the expanding neutral gas is limited by the kinematic property of the energy transfer from the energetic fragments to the bulk flow. The effective scale length of the photochemical zone for an H2O comet is estimated to be only 10% of the dimension of the collision region due to the large mass ratio of H atom and H2O molecule. When this physical property is taken into consideration, the H2O gas is found to be expanding at a highly supersonic speed. In addition, it is shown that the addition of a mixture of CO and/or CO2 molecules in a medium-bright comet with total production rate of 10 to the 29th molecules/s would not change this conclusion since the photodissiation length scales of these molecules are a factor of 10-30 larger than that of the H2O molecules. Thus, most of the CO and CO2 molecules will be dissociated outside of the collision zone without depositing the excess kinetic energy to the neutral atmosphere. However, if the gas production rate is controlled by CO or CO2, the dimension of the collision zone of a CO comet at 1 AU will be increased by a factor of seven. The photochemical heating due to CO dissociation would thus be more effective, causing the atmospheric flow to be weakly supersonic. It is concluded that the neutral gas outflows from H2O comets and CO comets could be substantially different.

Ip, W.-H.↗

NASA NTRS1983

An outer ion coma model in two dimension

A simple cometary outer ion coma model is constructed by adopting published results from numerical MHD computations. The ion composition and density distributions so derived are used to highlight some of the basic features of cometary plasma flows and the need for cometary ion observations with absolute brightness calibration.

Ip, W.-H.↗

NASA NTRS1982

On charge exchange and knock-on processes in the exosphere of Io

One direct consequence of magnetospheric interaction of Io is the strong dynamical coupling of its neutral atmosphere with the corotating plasma. The absorption of the thermal ions and the associated neutral injection is an improtant issue not yet explored. As far as nonthermal escape of the neutral atmosphere is concerned, three processes stand out. That is, apart from sputtering, exospheric interactions like atom-ion knock-on collision and charge exchange recombination could be a significant source of the neutral clouds in the Jovian system. Using a current electrodynamic model of Io, both the absorption rate of the corotating thermal plasma and the production rates of new exospheric ions and the fast neutrals are considered. It is found that the source strength of the neutral atoms and molecules with speeds of about 100 km/sec could amount to 10 to the 26th/sec whereas exospheric neutrals emitted at lower speed (of about 10 km/sec) amounts to 4 x 10 to the 25th/sec. The generation of the new ions in connection with the streaming of the magnetospheric plasma around Io could also produce an asymmetric sputtering with a neutral flux of about 10 to the 27th/sec emitted from the region of Io which faces Jupiter. These results may be related to a number of sodium observations.

Ip, W.-H.↗

NASA NTRS1982

On the structure of the Io Torus

Since the plasma flow near Io is reduced, neutrals originating in charge exchange are not energetic enough to leave the Jovian system and are therefore distributed over an extensive region, as indicated by the sodium cloud. New ions subsequently created in the distributed neutral atomic cloud as a result of charge exchange or electron impact ionization are picked up by the corotating magnetic field, and the radial current driven by the pickup process cannot close in the Io torus, so that it must instead be connected to the planetary ionosphere by field-aligned currents. These field-aligned currents will flow away from the equator at the outer edge of the neutral cloud, and towards it at the inner edge. It is found that the Jovian ionospheric photoelectrons also cannot supply the current flowing away from the equator, so that torus ions accelerated by a parallel electric field could be involved. The parallel potential drop is large enough to push the torus into the Jovian atmosphere, explaining both the sharp, discontinuous change of flux tube content and ion temperature at L equals 5.6, as well as the generation of Auroral-type hiss at that point.

Goertz, C. K.↗

NASA NTRS1981

Atomic clouds as distributed sources for the Io plasma torus

Several recent developments have implications for the neutral particle environment of Jupiter. Very hot sulfur ions have been detected in the Io torus with gyrospeeds comparable to the corotation speed, a phenomenon that would result from a neutral sulfur cloud. Current evidence supports the hypothesis that extensive neutral clouds of oxygen and sulfur exist in the Jupiter magnetosphere and that they are important sources of ions and energy for the Io torus.

Brown, R. A.↗

NASA NTRS1981

Saturn's radio emissions - Rotational modulation

Observational data provided by Pioneer 11 and Voyager 1 have revealed that the Saturnian ring system has important effects on the structures of the planetary ionosphere as well as the magnetospheric environment, such as the unexpected rotational modulation of the Saturn kilometric radiation (SKR) and the Saturn electrostatic discharge (SED) as discovered by the Voyager planetary radio astronomy experiment. The challenge arises to devise a mechanism which can produce electrodynamic coupling between the ring system and the planetary ionosphere and/or magnetosphere in which the associated SKR and SED radio emissions are subject to longitudinal control. One candidate is the establishment of field-aligned current systems, connecting the rings and the ionosphere with longitudinal variation. Local acceleration processes in the vicinity of the rings are also needed. The interplay between the ring-current systems and the geometry of the dipole field and the rings is discussed, and a description is presented of some of the basic ingredients in the longitudinal modulations of SKR and SED.

Goertz, C. K.↗

NASA NTRS1980

Physical studies of the planetary rings

In this review paper, the physical properties of the Saturnian and Uranian rings as derived from ground-based observations are first discussed. Focus is then shifted to the study of the orbital dynamics of the ring particles. Numerical simulations of the evolutionary history of a system of colliding particles in differential rotation together with theoretical modeling of the inelastic collision processes are surveyed. In anticipation of the information returned from in situ measurements by space probes, interactions of the planetary rings with the interplanetary meteoroids and planetary magnetospheres are briefly considered. Finally, models of planetary ring origin are examined. In this connection, some recent work on the satellite resonant perturbation effects on the ring structure are also touched upon.

Ip, W.-H.↗