Engineering topics

Michel, F. C.

Publications and source records attributed to Michel, F. C..

At least 19 records

NASA NTRS1990

Gamma-ray bursts from extinct neutron stars

The paper concentrates on disks around old extinct pulsars, that can produce gamma-ray bursts owing to viscous evolution of the disk bringing it into the near vicinity of the neutron star, with runaway ionization of the disk and simultaneous precipitation of this plasma onto the neutron star. An old extinct pulsar is modeled as a magnetized neutron star circled by a ring of cold dense matter with an orbital period approximately equal to the rotational period of the neutron star. The numerical estimates produced are found to be consistent with the observed properties of gamma-ray bursters.

Michel, F. C.↗

NASA NTRS1987

A magnetospheric simulation at the Space Station

It is proposed that a strong magnet (terrella) be flown at or near the Space Station to create an artificial magnetosphere in a laboratory setting. The relative flow of the ionosphere past the terrella will constitute a plasma wind that will interact with the magnetic field of the terrella to produce a localized magnetosphere. This object could then be extensively studied using diagnostic probes attached to the Space Station or with free flyers. The space and storage requirements would be minimal, since the experiment would be conducted outside the Space Station. The total equipment would consist of several terrella (with varying surface conductivities), approximately 3 small magnetometer/plasma diagnostic packages, and several gas canisters for upstream seeding. Power requirements would be approximately 60 watts. Several track mounted tethers, each approximately or 200 m long in length, with track parallel to the orbital motion and 100 m long, are also needed. Astronaut time needed would be minimal in the tethered configuration (approximately 4 man hours/week). A free flying configuration, while not needing the tether track, would require much more human interaction.

Lopez, R. E.↗

NASA NTRS1986

A magnetospheric simulation at the space station

It is proposed that a strong magnet (terrella) be flown at or near the Space Station to create an artificial magnetosphere in a laboratory setting. The relative flow of the ionosphere past the terrella will constitute a plasma wind that will interact with the magnetic field of the terrella to produce a localized magnetosphere. This object could then be extensively studied using diagnostic probes attached to the Space Station, or with free flyers. The space and storage requirements would be minimal, since the experiment would be conducted outside the space station. The total equipment would consist of several terrella (with varying surface conductivities), approximately 3 small magnetometer/plasma diagnostic packages, and several gas canisters for upstream seeding. Power requirements would be approximately 60 watts. Several track mounted tethers, each approximately or 200 m long in length, with track parallel to the orbital motion and 100 m long, are also needed. Astronaut time needed would be minimal in the tethered configuration (approximately 4 man hours/week). A free flying configuration, while not needing the tether track, would require much more human interaction.

Lopez, R. E.↗

NASA NTRS1985

Durability of the accretion disk of millisecond pulsars

Pulsars with pulsation periods in the millisecond range are thought to be neutron stars that have acquired an extraordinarily short spin period through the accretion of stellar material spiraling down onto the neutron star from a nearby companion. Nearly all the angular momentum and most of the mass of the companion star is transferred to the neutron star. During this process, wherein the neutron star consumes its companion, it is required that a disk of stellar material be formed around the neutron star. In conventional models it is supposed that the disk is somehow lost when the accretion phase is finished, so that only the rapidly spinning neutron star remains. However, it is possible that, after the accretion phase, a residual disk remains in stable orbit around the neutron star. The end result of such an accretion process is an object that looks much like a miniature (about 100 kilometers), heavy version of Saturn: a central object (the neutron star) surrounded by a durable disk.

Michel, F. C.↗

NASA NTRS1985

Electrosphere of an aligned magnetized neutron star

A fundamentally new self-consistent solution for the electrosphere of an aligned magnetized neutron star is presented. Unlike previous models the electrospheres are finite in extent. This avoids the light cylinder problem. The results may provide a basis for pulsar models.

Krause-Polstorff, J.↗

NASA NTRS1985

Gamma-ray bursts from remnant neutron star disks

The consequences of a disk of matter orbiting an old neutron star are examined. When the inner edge of the disk approaches close to the star, due to internal viscous drag, runaway ionization of the disk occurs and the resulting plasma is precipitated to the surface of the neutron star, thereby producing a gamma-ray burst. Rough numerical estimates of the occurrence rate are given and found to be consistent with gamma-ray burst observations. The estimates indicate that energies of 10 to the 39th ergs or more could be released with rise times as fast as 0.3 ms. Consideration is given to explanations of the March 5, 1979 event (Cline et al., 1980). Some possible observational searches for optical or IR emission from such disks at the locations of known burst sources and pulsars are discussed.

Michel, F. C.↗

NASA NTRS1985

Pulsar space charging

It is found that stable self-consistent static solutions for a pulsar magnetosphere can be constructed with the magnetosphere having vacuum gaps separating the positive and negative regions of space charge. The magnetosphere is confined well within the light cylinder thus avoiding the problems of the Goldreich and Julian model. The total system charge is a free parameter in the family of models.

Krause-Polstorff, J.↗

NASA NTRS1985

Relativistic wind termination - Jets and synchronotron nebulae

An idealized model describing the termination of a relativistic wind owing to its interaction with surrounding nonrelativistic matter, such as the interaction of a pulsar wind with the supernova remnant shell is examined. It is assumed that the large-scale electric and magnetic fields out to the termination distance are controlled by the wind source, in contrast to previous work treating the wind as an isotropic MHD flow. Two effects are found: forced synchrotron radiation from the bulk of the injected particles, and formation of oppositely directed jets of ultrarelativistic particles.

Michel, F. C.↗

NASA NTRS1984

Relativistic charge-separated winds

An investigation is made into the effects of including charge separation in three-dimensional MHD wind models. Attention is focused on the energy gain experienced by particles in the wind zone of a magnetized rotator, e.g., a pulsar. An expression is defined for the energy gained by a particle emitted from the polar cap of a rotating neutron star while crossing the polar axis from the wind zone to the equatorial plane. The stellar wind is treated in terms of global electrodynamics and solutions are obtained for a neutral sheet wind, a monopole wind, and a simple wind model. The particle acceleration Lorentz factors are found to be a quadratic of those formerly predicted by MHD theory and in agreement with observations of energies around the Crab pulsar.

Michel, F. C.↗

NASA NTRS1984

Hydraulic jumps in 'viscous' accretion disks

It is proposed that the dissipative process necessary for rapid accretion disk evolution is driven by hydraulic jump waves on the surface of the disk. These waves are excited by the asymmetric nature of the central rotator (e.g., neutron star magnetosphere) and spiral out into the disk to form a pattern corotating with the central object. Disk matter in turn is slowed slightly at each encounter with the jump and spirals inward. In this process, the disk is heated by true turbulence produced in the jumps. Additional effects, such as a systematic misalignment of the magnetic moment of the neutron star until it is nearly orthogonal, and systematic distortion of the magnetosphere in such a way as to form an even more asymmetric central 'paddle wheel', may enhance the interaction with inflowing matter. The application to X-ray sources corresponds to the 'slow' solutions of Ghosh and Lamb, and therefore to rms magnetic fields of about 4 x 10 to the 10th gauss. Analogous phenomena have been proposed to act in the formation of galactic spiral structure.

Michel, F. C.↗

NASA NTRS1983

Fast pulsars with disks

The observed properties of the pulsar PSR1937+214 are compared with predictions of the disk model. It is assumed that an isolated magnetized rotating neutron star is ringed by a fluid disk with a 0.00001 solar mass, and relative rotations of the star and the disk produce potential differences across the disk. A Faraday disk dynamo is also formed between the disk and the star, and allows the polar cap current to return from the disk to the star through auroral arcing. Preferential regions of the star are recipients of a return current controlled by the surface magnetic field structure, which configures the pulsing emissions. The disk model predicts the average luminosity to be 10 to the 31st erg/sec, and an emission of 3 x 10 to the 30th erg/sec was detected. Only one-millionth of the output of the emissions is in the radio region, and the X and gamma ray emissions are in the normal range for pulsars. It is concluded that PSR1937+214 behaves within the predictions of the disk model and is not a new kind of object.

Michel, F. C.↗

NASA NTRS1983

Radio pulsar disk electrodynamics

Macroscopic physics are discussed for the case of a disk close to an isolated, magnetized, rotating neutron star that acts as a Faraday disk dynamo, while the disk acts as both a load and a neutral sheet. This sheet allows the polar cap current to return to the neutron star, splitting a dipolar field into two monopolar halves. The dominant energy loss is from the stellar wind torque, and the next contribution is dissipation in the auroral zones, where the current returns to the star in a 5 cm-thick sheet. The disk itself may be a source of visible radiation comparable to that in pulsed radio frequency emission. As the pulsar ages, the disk expands and narrows into a ring which, it is suggested, may lead to a cessation of pulsed emission at periods of a few sec.

Michel, F. C.↗

NASA NTRS1981

Pulsar disk systems

It is proposed that radio pulsars have the same basic physical features as X-ray pulsars. Specifically, it is suggested that active radio pulsars are rotating neutron stars surrounded by fossil disks left over from the collapse event and that energy is extracted from the rotation of the neutron star by interaction with the disk to produce pulsar luminosity. Attention is given to a model in which the neutron star acts as a unipolar generator (or Faraday disk dynamo) and the disk acts as a load. The self-excitation of the disk/pulsar system is considered along with aspects of disk persistence, pulsed emission, and magnetically field aligned currents to the disk. An investigation is conducted regarding the possibility of a deposition of material in the form of a disk about a pulsar, taking into account questions concerning disk survival. It is found that there are some promising features regarding a disk system.

Michel, F. C.↗

NASA NTRS1980

Permanent magnetic trapping

It is shown that the global nature of trapped particle motion in phase space makes it entirely plausible that particles with pitch angles near 90 deg are trapped for extremely long times, probably indefinitely. The long observed lifetimes of trapped particles can immediately be accounted for by this former population. For particles with low pitch angles an 'overstability' develops which serves to produce rapid variations in pitch angle and, presumably, random walk into the loss cone.

Michel, F. C.↗

NASA NTRS1979

Ionosphere of Venus - First observations of the dayside ion composition near dawn and dusk

Independent Bennett radio-frequency ion mass spectrometers on the Pioneer Venus bus and orbiter spacecraft obtained in situ measurements of the composition of the ionosphere of Venus. The spectrometer on the bus explored the dawn region while the spectrometer on the orbiter explored the duskside region. Information on the ion composition in the topside, the lower ionosphere, and the upper ionosphere is presented. Below the O(+) peak near 200 km, the ions are found to exhibit scale heights consistent with a neutral gas temperature of about 180 K near the terminator. In the upper ionosphere, scale heights of all species reflect the effects of plasma transport.

Taylor, H. A., Jr.↗

NASA NTRS1979

Ionosphere of Venus - First observations of the effects of dynamics on the dayside ion composition

Data obtained by Bennett radio-frequency ion mass spectrometers indicate that the ionosphere envelope, dominated above 200 km by O(+), responds dramatically to variations in the solar wind pressure. The pressure compresses the thermal ion distributions from heights as great as 1800 km inward to 280 km. At the thermal ion boundary, or ionopause, the ambient ions are swept away by the solar wind, while at higher altitudes energetic ion currents are detected. Within the ionosphere, ion convection stimulated by the solar wind interaction causes pass-to-pass differences in the ion scale heights.

Taylor, H. A., Jr.↗

NASA NTRS1979

Shape integral method for magnetospheric shapes

A method is developed for calculating the shape of any magnetopause to arbitrarily high precision. The method uses an integral equation which is evaluated for a trial shape. The resulting values of the integral equation as a function of auxiliary variables indicate how close one is to the desired solution. A variational method can then be used to improve the trial shape. Some potential applications are briefly mentioned.

Michel, F. C.↗

NASA NTRS1978

A phenomenological pulsar model

Particle injection energies and rates previously calculated for the stellar wind generation by rotating magnetized neutron stars are adopted. It is assumed that the ambient space-charge density being emitted to form this wind is bunched. These considerations immediately place the coherent radio frequency luminosity from such bunches near 10 to the 28th erg/s for typical pulsar parameters. A comparable amount of incoherent radiation is emitted for typical (1 second) pulsars. For very rapid pulsars, however, the latter component grows more rapidly than the available energy sources. The comparatively low radio luminosity of the Crab and Vela pulsars is attributed to both components being limited in the same ratio. The incoherent radiation essentially has a synchotron spectrum and extends to gamma-ray energies; consequently the small part of the total luminosity that is at optical wavelengths is unobservable. Assuming full coherence at all wavelengths short of a critical length gives a spectral index for the flux density of -8/3 at higher frequencies. The finite energy available from the injected particles would force the spectrum to roll over below about 100 MHz, although intrinsic morphological factors probably enter for any specific pulsar as well.

Michel, F. C.↗