Engineering topics

Northrop, T. G.

Publications and source records attributed to Northrop, T. G..

At least 19 records

NASA NTRS1992

A self-consistent model for the particles and fields upstream of an outgassing comet. II - A time-dependent description

In a previous paper, the global variation of the magnetic field and solar wind flow parameters in the unshocked region upstream of an outgassing comet was determined using a kinetic treatment for the cometary ions. Two different assumptions were made concerning the cometary ion distribution function: the pickup cometary ions formed either a velocity space gyrotropic ring distribution or a velocity space isotropic shell distribution in the solar wind frame of reference. In the present paper the general case is considered wherein the newly picked up ions are elastically pitch angle scattered from the initial ring distribution to a shell with some characteristic time scale. Using theoretically determined parameters which reflect the conditions expected at Comet Kopff (a typical short-period comet and a possible target for the CRAF/Cassini mission) for various heliocentric distances, how the pitch angle scattering rate affects the global morphology is determined.

Flammer, K.↗

NASA NTRS1992

Dusty plasmas

Dust grains immersed in plasma become charged. The charge is determined by the plasma characteristics, by secondary and photoemission from the grain, by grain velocity, and at any given instant by the past time history of the charging currents. This charge affects the Coulomb drag on a grain moving through the plasma. It affects the motion of the grain in an electromagnetic field of a planetary magnetosphere, and it is involved in the formation of the spokes in Saturn's rings and in the erosion of the rings by micrometeorites. And finally, it affects the coagulation rate of dust into larger bodies.

Northrop, T. G.↗

NASA NTRS1991

A self-consistent model for the particles and fields upstream of an outgassing comet. I - Gyrotropic and isotropic ion distributions

The magnetic field and solar wind flow parameters in the vicinity of a weakly outgassing comet are determined using a self-consistent model which treats the cometary ions kinetically. Two different assumptions are made concerning the cometary ion distribution function; the pickup cometary ions from either a velocity space gyrotropic ring distribution or a velocity space isotropic shell distribution in the solar wind frame of reference. The individual currents due to the solar wind and cometary ions, which determine the magnetic field, are calculated in each case. Using theoretically determined parameters which reflect the conditions expected to comet Kopff (a typical short-period comet and one possible target for the future CRAF/Cassini mission) for various heliocentric distances, the way in which global field and flow characteristics are influenced by the nature of the cometary ion distribution function is determined.

Flammer, K. R.↗

NASA NTRS1990

Plasma drag on a dust grain due to Coulomb collisions

Expressions are given for the drag due to Coulomb collisions on a charged dust grain moving through a plasma. The commonly used Chandrasekhar (1943) result does not include large scattering angles or plasma collective effects. An equation given by Morfill et al. (1980) which does include large scattering angles, is limited to one-dimensional plasma particle motion and can give at least an order of magnitude too much drag. This paper also makes use of an analogy between the drag problem and problems in electrostatics. This analogy permits generalization to any isotropic plasma of an observation made by Chandrasekhar for a Maxwellian, namely, that the drag is independent of the presence or absence of plasma particles moving faster than the grain. Finally, the contribution of plasma collective effects to the drag is studied with the inclusion of large scattering angles.

Northrop, T. G.↗

NASA NTRS1989

Gyrophase drifts and the orbital evolution of dust at Jupiter's Gossamer Ring

The 'gyrophase drift' phenomenon in Jupiter's fine-dust 'gossamer ring' is presently shown to exceed the plasma-drag drift, and may be able to move small, charged grains either toward or away from synchronous radius. The grain gyrophase drifts toward the higher temperature in the presence of a radial gradient in plasma temperature; gyrophase drift will also occur in conjunction with a radial gradient in the relative concentrations of different plasma ion species, or even due to plasma-grain velocity variation associated with the grain's cycloidal motion through the plasma. The Poynting-Robertson drift is noted to be diminutive by comparison with either the plasma-drag or gyrophase drifts.

Northrop, T. G.↗

NASA NTRS1987

A micrometeorite erosion model and the age of Saturn's rings

An erosional model of Saturn's rings is proposed based on theoretical studies of the high charge-to-mass ratio particles in Saturn's ring plane, and assuming that the B and C rings were initially formed as one ring with the optical thickness of the present B ring. The erosion rate is calculated using data from observed micrometeorite fluxes, and a ring age of 4.4-76 Myr is determined which is inconsistent with the 4.5-Gyr ring lifetime required by the cosmogonic ring hypothesis. The sharpness of the transition between the B and C rings suggests that the principal mass loss is through particles moving at a few m/sec with respect to the parent bodies from which they were eroded.

Northrop, T. G.↗

NASA NTRS1986

Adiabatic theory in regions of strong field gradients

The theory for the generalized first invariant for adiabatic motion of charged particles in regions where there are large gradients in magnetic or electric fields is developed. The general condition for an invariant to exist in such regions is that the potential well in which the particle oscillates change its shape slowly as the particle drifts. It is shown how the Kruskal (1962) procedure can be applied to obtain expressions for the invariant and for drift velocities that are asymptotic in a smallness parameter epsilon. The procedure is illustrated by obtaining the invariant and drift velocities for particles traversing a perpendicular shock, and the generalized invariant is compared with the magnetic moment, and the drift orbits with the actual orbits, for a particular case. In contrast to the magnetic moment, the generalized first invariant is better for large gyroradii (large kinetic energies) than for small gyroradii. Expressions for the invariant when an electrostatic potential jump is imposed across the perpendicular shock, and when the particle traverses a rotational shear layer with a small normal component of the magnetic field are given.

Whipple, E. C.↗

NASA NTRS1986

The electrostatics of a dusty plasma

The potential distribution in a plasma containing dust grains were derived where the Debye length can be larger or smaller than the average intergrain spacing. Three models were treated for the grain-plasma system, with the assumption that the system of dust and plasma is charge-neutral: a permeable grain model, an impermeable grain model, and a capacitor model that does not require the nearest neighbor approximation of the other two models. A gauge-invariant form of Poisson's equation was used which is linearized about the average potential in the system. The charging currents to a grain are functions of the difference between the grain potential and this average potential. Expressions were obtained for the equilibrium potential of the grain and for the gauge-invariant capacitance between the grain and the plasma. The charge on a grain is determined by the product of this capacitance and the grain-plasma potential difference.

Whipple, E. C.↗

NASA NTRS1985

Relationships among the harmonic coefficients of scan plane anisotropies

Frequently, a detector on a spinning spacecraft measures the flux of charged particles whose velocities lie in a plane containing the magnetic field. This flux may be Fourier analyzed as a function of the spacecraft roll angle. Relationships among the Fourier coefficients are derived using the adiabatic solution of the Vlasov equation. These relationships depend only on the fact that the lowest order (in gyroradius) of the distribution function is a function of the magnetic moment and that the first-order term is a function of the lowest order. These relationships may be used to separate the proton from the electron counts registered in Saturn's inner magnetosphere by the University of California's Cerenkov counter on Pioneer 11.

Northrop, T. G.↗

NASA NTRS1985

Electrons and cosmic ray produced protons in Saturn's inner magnetosphere

The Cerenkov detector on Pioneer 11 previously observed Crand protons above 600 MeV in Saturn's inner magnetosphere, mixed with a poorly understood background of energetic electrons. The electron count is separated from the proton counts and the first-order angular distributions are established for each species. To do this the theoretical relationships among the harmonic coefficients of the count rate is used as a function of spacecraft roll angle. The majority of the counts were electrons with energy above several MeV; i.e., with drift periods shorter than the satellite orbital resonance. The electrons have isotropic pitch angle distributions, and the protons pancake over most of the region between Nimas and the rings, although there is a small region of dumbbell proton distributions in the vicinity of Janus and Epimetheus.

Northrop, T. G.↗

NASA NTRS1985

A Note on Jupiter's Gossamer Ring

The newly discovered gossamer ring of Jupiter, composed largely of micron-sized grains, exhibits a significant peak very near the synchronous radius. It is believed that this peak is associated with an unobserved source consisting of large bodies that straddle the synchronous orbit. An alternative evolutionary mechanism is offered, which is the gyrophase drift towards the synchronous orbit, of fine grains, which necessarily are electrostatically charged within the Jovian magnetosphere.

Mendis, D. A.↗

NASA NTRS1985

Electrons and Cosmic Ray Produced Protons in Saturn's Inner Magnetosphere

Northrop, T. G.↗

NASA NTRS1985

The electrostatics of a dusty plasma

Voyager 1 and 2 observations of the Saturnian ring system have led to the discovery of several interesting phenomena associated with its fine dust component. The dust grains are immersed in a plasma. The present paper is concerned with the electrostatics of a dusty plasma, giving particular attention to the capacitance and charge of an individual grain in the presence of neighboring grains and the surrounding plasma. The Poisson equation and gauge considerations are discussed along with a solution of the Poisson equation for several models, taking into account impermeable grains, permeable grains, and a spherical capacitor model. A comparison of potential shapes for three models is conducted, and the effect of neighbors on charging currents is explored.

Whipple, E. C.↗

NASA NTRS1985

Electrons and cosmic ray produced protons in Saturn's inner magnetosphere

The Cerenkov detector on Pioneer 11 previously observed Crand protons above 600 MeV in Saturn's inner magnetosphere, mixed with a poorly understood background of energetic electrons. The electron count is separated from the proton counts and the first-order angular distributions are established for each species. To do this the theoretical relationships among the harmonic coefficients of the count rate is used as a function of spacecraft roll angle. The majority of the counts were electrons with energy above several MeV; i.e., with drift periods shorter than the satellite orbital resonance. The electrons have isotropic pitch angle distributions, and the protons pancake over most of the region between Mimas and the rings, although there is a small region of dumbbell proton distributions in the vicinity of Janus and epimetheus.

Northrop, T. G.↗

NASA NTRS1984

Dust in magnetised plasmas - Basic theory and some applications

In this paper the theory of charged test particle motion in magnetic fields is reviewed. This theory is then extended to charged dust particles, for which gravity and charge fluctuations play an important role. It is shown that systematic drifts perpendicular to the magnetic field and stochastic transport effects may then have to be considered none of which occur in the case of atomic particles (with the exception of charge exchange reactions). Some applications of charged dust particle transport theory to planetary rings are then briefly discussed.

Northrop, T. G.↗

NASA NTRS1983

The inner edge of Saturn's B ring

The sharp, 90-km wide transition from an optical depth of 0.2 in the C ring to 1 in the B ring begins at 91,970 km from Saturn's center. This radius is found to be almost exactly at the inward stability limit of charged particles launched in the ring plane at the local Kepler velocity, provided these particles have large charge to mass ratio. The zonal harmonic models of Saturn's magnetic field from the Voyager data and the gravitational field model from Pioneer data are essential to get the very close agreement between theory and observation. The theoretical stability limits are 91,973 + or - 145 km from Voyager 1 magnetic field data and 91,991 + or 145 km from Voyager 2 magnetic data. The zonal harmonic magnetic field lines are not perpendicular to the ring plane. Therefore, in addition to the magnetic mirror, gravitational, and centrifugal forces, an unknown force must be postulated to produce equilibrium in the ring plane and make the stability calculation meaningful.

Northrop, T. G.↗

NASA NTRS1983

The adiabatic motion of charged dust grains in rotating magnetospheres

Adiabatic equations of motion are derived for the micrometer-sized dust grains detected in the Jovian and Saturn magnetospheres by the Pioneer 10 and 11 spacecraft. The adiabatic theory of charged particle motion is extended to the case of variable grain charge. Attention is focused on the innermost and outermost limits to the grain orbit evolution, with all orbits tending to become circular with time. The parameters such as the center equation of motion, the drift velocity, and the parallel equation of motion are obtained for grains in a rotating magnetosphere. Consideration is given to the effects of periodic grain charge-discharge, which are affected by the ambient plasma properties and the grain plasma velocity. The charge-discharge process at the gyrofrequency is determined to eliminate the invariance of the magnetic moment and cause the grain to exhibit radial movement. The magnetic moment increases or decreases as a function of the gyrophase of the charge variation.

Northrop, T. G.↗

NASA NTRS1982

Stability of negatively charged dust grains in Saturn's ring plane

The boundary between the inner and outer parts of Saturn's B ring is located at the theoretical limit of stability of dust grains with large negative charge to mass ratio. A grain inside of this stability limit will move along magnetic field lines and strike Saturn if given a slight velocity component normal to the ring plane. Outside of this marginal stability radius, a perturbed grain merely oscillates back and forth through the ring plane. The theoretical location of the marginal stability radius is at 1.625 Saturn radius. Observations by Pioneer 11 and Voyager 2 in the infared see the boundary as a prominent change in ring brightness at this radius. The occultation of delta-Scorpii by the rings in the ultraviolet seen by Voyager 2 shows about a factor of two change in optical depth beginning very close to this radius.

Northrop, T. G.↗