Browsing by Author "Michel, F. Curtis"

Now showing 1 - 7 of 7
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    Acceleration of ambient ions in the Lunar atmosphere
    (1981) Bulgher, Debra Lynn; Freeman, John W.; Dufour, Reginald J.; Michel, F. Curtis
    The Apollo Suprathermal Ion Detector Experiments (SIDEs) deployed on the lunar surface observe sporadic bursts of positive ions throughout the local lunar night. Evidence exists that the source of these events is most likely the dayside lunar atmosphere. Ionization of the neutral lunar atmosphere by photoionization or chargeexchange produces a population of ions over the sunlit portion of the moon. These ions are accelerated by the interplanetary electric field, executing cycloidal trajectories that may bring them onto the nightside hemisphere of the moon. A nightside surface electric field directed radially inward can then further accelerate these ions onto the lunar surface, altering their energies and trajectories such that they are detected by the SIDE. It is possible to obtain ion trajectories that intersect various positions on the nightside of the moon in this manner. For several nighttime ion events, synthetic particle data have been produced with this model which are in agreement with the observed data. Parameters of the nightside surface potential distribution are determined for the selected events. A profile of the expected surface potential over the nightside hemisphere of the moon indicates a variable surface potential. The height distribution over which the potential is effective becomes increasingly larger, while the magnitude of the surface potential becomes increasingly negative, when approaching local lunar midnight. Assuming an exponential neutral nimber density profile for all atmospheric constituents, indirect observations of the neutral lunar atmosphere can be made by the SIDE. Combined with the effect of the surface potential on the height distribution of these ions, number densities for various species in the lunar atmosphere determined in this manner are in agreement with reported values from various other experiments also deployed on the lunar surface.
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    Aggregation of grains in a turbulent presolar disk
    (1983) Wieneke, Bernhard; Clayton, Donald W.; Michel, F. Curtis; Reiff, Patricia H.
    The growth and evolution of grains in the protostellar nebula is investigated within the context of turbulent-low mass disk models developed by previous investigators. Because of grain collisions promoted by the turbulent velocities, particles aggregate to millimeter size in times of order 1^3 yrs. During the growth the particles acquire a large inward radial velocity due to gas drag (Weidenschilling, 1977) and spiral into the sun. The calculations indicate that the final size of the particles does not exceed a few centimeters. This result is not very sensitive to the specific nebula parameters. For all conditions investigated it seems impossible to grow meter- or kilometer-sized bodies which could decouple from the gas motion. An additional argument is given which shows that only particles smaller than centimeter size can survive drift into the growing sun by being transported radially outwards by turbulent mixing. This agrees well with the maximum size of inclusions and chondrules. Since sedimentation of grains and subsequent dust disk instability is effectively inhibited by turbulent stirring, the formation of planetesimals and planets can not be explained in the above scenario without further assumptions.
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    Magnetic Compton scattering in pulsar magnetospheres
    (1993) Sturner, Steven John; Michel, F. Curtis
    In the large magnetic fields associated with highly magnetized neutron stars, the Compton cross section exhibits a resonance at the local cyclotron energy. In this work I describe applications of magnetic Compton scattering to models of both rotation powered and accretion powered pulsars. Radio pulsars are generally considered to be rapidly rotating, highly magnetized neutron stars. The rapid rotation coupled with the large magnetic field induces large electric fields that can accelerate electrons in the neutron star magnetosphere to large energies. I have produced a Monte Carlo code to model $\gamma$-ray emission from rotation powered pulsars utilizing pair cascades initiated by Comptonized photons. Previous polar cap $\gamma$-ray emission models have relied on pair cascades initiated by curvature radiation photons. This Monte Carlo model can reproduce the double peak pulse profiles often observed from rotation powered pulsars and explain the trend for harder spectra from slower pulsars. X-ray Pulsars are thought to be highly magnetized neutron stars accreting matter from an ordinary stellar companion. The accreting matter is channeled onto the neutron star polar cap by the magnetic field. This material produces a hot spot on the neutron star surface that emits X-rays. I have investigated the effects of radiation pressure due to these X-rays on the accreting material. The radiation pressure is greatly enhanced by the resonance in the magnetic Compton cross section. Because the electron cyclotron energy varies with distance from the neutron star, the energy dependent X-ray spectrum maps to a spatially varying radiation force. This force can exceed the force of gravity over a limited region of the X-ray pulsar magnetosphere. I postulate that when this occurs matter can be elevated above the neutron star surface outside the accretion column. This material will act as an energy dependent "lamp shade" that will produce pulse profiles that vary with photon energy. This model is capable of reproducing the energy dependent pulse profiles observed from the X-ray pulsars 4U 1626-67, 4U 1538-52, 4U 1907+09, and Vela X-1.
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    Pulsar slowdown rates
    (1975) Ellison, Donald C.; Michel, F. Curtis
    A model for the age distribution of pulsars is developed and compared with the observed distribution. This model gives a value of n~4 in the phenomenological slowdown relation, [equation]. A value for the galactic birthrate of visible pulsars is also found and is approximately 6 x 1 to the -16 pc to the -3 yr to the -1. The magnetic field decay time is found to be no smaller than 15 million years and the model is consistent with a constant magnetic field. In addition, observational selection effects are considered in fitting measured values of n and Q to the above slowdown relation. We find n = 4. ± .6 and the distribution of log 1 k appears to be gaussian with a mean of -17. and a width of 1..
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    Rotating magnetospheres: a three dimensional approach
    (1973) Morris, Clifford Dean; Michel, F. Curtis
    Several authors have calculated the torque due to a stellar wind on a rotating body with an axisymmetric magnetic field. These discussions are restricted to the equatorial plane because the magnetic field structure is unknown elsewhere. Their treatment is here extended to the non-equatorial case. The axisymmetric magnetic field is described by a scalar function f. The various conservation equations allow the plasma flow to be described in terms of f. Although f has not been determined in the general case, it can be obtained in two special cases when the particle inertia is neglected. In the general case the Alfven critical points are obtained in terms of f, and with an assumption of a "minimum torque" solution, conditions on the angular momentum flow are found.
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    The beam-plasma discharge revisited (a new theoretical model)
    (1983) Llobet Ribeiro, Xavier (b. 1963); Bernstein, William; Haymes, Robert C.; Michel, F. Curtis
    Many features related to the ignition of the Beam-Plasma Discharge can be explained with the requirement of a critical density, which in case of a self-made plasma translates into a critical current. However, other characteristics indicate that this requirement which arises from the geometry, is a necessary but not sufficient condition. In this thesis it is shown that a second requirement must exist, also taking the form of a minimum current. With these two constraints most of the ignition-related features can be explained. A preliminary analysis of the discharge showing how the second requirement arises is presented.
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    The quiescent rotator
    (1982) Polstorff, Juergen; Michel, F. Curtis; Wolf, Richard A.; Dufour, Reginald J.
    The problem of a physically self-consistent pulsar magnetosphere has existed since the introduction of the Goldreich and Julian model. This work presents a method for creating a self consistent charge-separated finite magnetosphere which would surround a pulsar with magnetic dipole and spin axis aligned. The finite magnetosphere configuration is allowed due to the existence of vacuum gaps and acts to make the pulsar surface emission free. This produces a quiescent rotator that is electrically static. Two examples using discrete charges to model the magnetosphere are presented and the magnetospheric configuration associated with them is examined.