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  • 1
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    Mass loading and velocity diffusion models for heavy pickup ions at comet Grigg-Skjellerup (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: We compare model predictions of cometary water group ion densities and the solar wind slow down with measurements made by the Giotto Johnstone plasma analyzer implanted ion sensor at the encounter with comet Grigg-Skjellerup (G-S) on July 10, 1992. The observed slope of the ion density profile on approach to the comet is unexpectedly steep. Possible explanations for this are discussed. We present also a preliminary investigation of the quasilinear velocity-space diffusion of the implanted heavy ion population at G-S using a transport equation including souce, convection, adiabatic compression, and velocity diffusion terms. Resulting distributions are anisotropic, in agreement with observations. We consider theoretically the waves that may be generated by the diffusion process for the observed solar wind conditions. At initial ion injections, waves are generated at omega approximately Omega(sub i) the ion gyrofrequency, and lower frequencies are predicted for diffusion toward a bispherical shell.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A12; p. 20,995-21,002
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  • 2
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    Mirror modes and fast magnetoacoustic waves near the magnetic pileup boundary of comet P/Halley (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Large-amplitude ultralow-frequency wave structure observed on both sides of the magnetic pileup boundary of comet P/Halley during the flyby of the Giotto spacecraft have been analyzed using suprathermal electron density and magnetic field observations. Upstream of the boundary, electron density and magnetic field magnitude variations are anticorrelated, while in the pileup region these quantities are clearly correlated. Both in front of and behind the pileup boundary the observed waves are quasi-perpendicular wave structures as a minimum variance analysis shows. A detailed comparison of our observations in the prepileup region with theoretical and numerical results shows that the mirror mode mode waves may have been generated by a mirror instability driven by the pressure anisotropy of the ring-type distributions of the heavy (water group) pickup cometary ions.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A12; p. 20,955-20,964
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  • 3
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    Velocity space diffusion and nongyrotropy of pickup water group ions at comet Grigg-Skjellerup (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The diffusion of water group cometary ions in velocity space at comet Grigg-Skjellerup was measured during the Giotto spacecraft encounter. The evolution of the collapsed pitch angle and energy distributions during the inbound and outbound passes shows that the timescale for energy diffusion may be similar to that for pitch angle diffusion. Fully isotropic pitch angle distributions were never seen. Also the bulk parameters of the three-dimensional distributions are examined. Transformation of these parameters into a field-aligned solar wind frame allows us to test the gyrotropy of the distributions. These observations imply that there were deviations from gyrotropy throughout the encounter becoming most important near to closest approach.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A12; p. 20,985-20,994
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  • 4
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    Low-frequency electromagnetic plasma waves at comet P/Grigg-Skjellerup: Analysis and interpretation (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The propagation and polarization characteristic of low-frequency electromagnetic wave fields near comet P/Grigg-Skjellerup (P/GS) are analyzed using magnetic field and plasma observations obtained by the Giotto magnetometer experiment and the Johnstone plasma analyzer during the encounter at the comet on July 10, 1992. The results have been physically interpreted.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A12; p. 20,937-20,953
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  • 5
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    Structured plasma sheet thinning observed by Galileo and 1984-129 (1993)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: On December 8, 1990, the Galileo spacecraft used the Earth for a gravity assist on its way to Jupiter. Its trajectory was such that it crossed geosynchronous orbit at approximately local midnight between 1900 and 2000 UT. At the same time, spacecraft 1984-129 was also located at geosynchronous orbit near local midnight. Several flux dropout events were observed when the two spacecraft were in the near-Earth plasma sheet in the same local time sector. Flux dropout events are associated with plasma sheet thinning in the near-profile of the near-Earth plasma sheet while 1984-129 provided an azimuthal profile. With measurements from these two spacecraft we can distinguish between spatial structures and temporal change. Our observations confirm that the geosynchronous flux dropout events are consistent with plasma sheet thinning which changes the spacecraft's magnetic connection from the trapping region to the more distant plasma sheet. However, for this period, thinning occurred on two spatial and temporal scales. The geosynchronous dropouts were highly localized phenomena of 30 min duration superimposed on a more global reconfiguration of the tail lasting approximately 4 hours.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A12; p. 21,323-21,333
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  • 6
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    Is there a classical Hyades lithium problem? (1990)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The distribution of surface lithium abundances in the Hyades cluster, as a function of effective temperature, has resisted a satisfactory theoretical explanation for more than two decades. Trends in the historically recomputed values of stellar opacity have stimulated a systematic investigation of the effects of larger opacities on lithium depletion during the premain-sequence evolution of low-mass stars. It is shown that the calculated depletion depends quite strongly on the opacities employed, and that both surface and interior opacities play an important role. By performing standard evolutionary calculations with parameterized increments in the opacities, the observed Hyades Li and T(eff) relation for G and K dwarfs have been determined in a manner consistent with the other observational constraints. The solution presented herein requires an increase in interior opacities at about 4 million K over older values by approximately less than 37 percent. Such a change is not incompatible with the historical trends in opacity computations and suggests that the resolution of this problem lies solely in the opacities, without recourse to other physical mechanisms such as turbulent diffusion or 'extra mixing'.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 348; L33-L36
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  • 7
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    A search for evidence of past and ongoing mass loss in the hot DO degenerates PG 1159 - 035, PG 1034 + 001, and KPD 0005 + 5106 (1990)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The results are reported of a search for evidence of mass loss from three very hot, helium-rich degenerate stars, PG 1034 + 001, PG 1159 - 035, and KPD 0005 + 5106, based upon a search for weak, velocity-displaced line features in high-resolution IUE spectra. The search was entirely negative for PG 1159 - 035 and resulted in the detection of Si IV in PG 1034 + 001 at the same redshifted velocity as the other detected far-ultraviolet resonance doublets. The presence of photospheric silicon in a DO star at 80,000 K is not predicted by current radiative acceleration theory. For KPD 0005 + 5106 there is no convincing far-ultraviolet spectroscopic detection of either past or ongoing mass loss. The high-ionization resonance doublets of C IV, Si IV, and N v share the same velocity broadening as the interstellar lines but are velocity displaced by + 10 km/s relative to the local interstellar medium. Their origin is ascribed to an expanding, photoionized H II region around KPD 0005 + 5106.
    Keywords: ASTROPHYSICS
    Type: Astronomical Journal (ISSN 0004-6256); 99; 908-916
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  • 8
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    Diffusive shock acceleration of energetic electrons and the method of generations (1990)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The diffusive shock acceleration of energetic electrons at a plane shock is studied by means of the method of generations developed by Petukhov et al. (1986), where the particles are binned according to the number of particle acceleration cycles. The method emphasizes the cyclic character of the shock acceleration process, and takes into account the particle energy increment per cycle, the energy loss per cycle, and the escape probability of particles downstream of the shock. In the limit of a small fractional momentum gain per cycle, the method reproduces the known steady-state semianalytic solution for constant diffusion coefficients on both sides of the shock. It is demonstrated that the method may also be applied to time-dependent shock-acceleration problems.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 362; 419-433
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  • 9
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    The hot DOA1 degenerate HZ 21 - A search for circumstellar/photospheric metals and peculiar absorption at He II (1990)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A high-resolution IUE spectrum of the hot DO1 degenerate HZ 21 was obtained by combining US1 + European 2 low-background observing shifts. The SWP image reveals a rich spectrum of interstellar absorption lines with an average velocity in the line of sight to HZ 21 of -30 km/s. However, there is no clear evidence of any highly or lowly ionized metal features which could be attributed to circumstellar, wind, or photospheric absorption. There is, however, a broad absorption trough at He II (1640) which was not unexpected, given the clear presence of He II (4686) absorption in this star's optical spectrum. The velocity width of He II (1640) appears consistent with photospheric absorption wings which appear to flank the geocoronal Ly-alpha emission feature. The He II (1640) feature reveals what appears to be a broad (310 km/s) emission reversal. Evidence is provided that the emission reversal is probably real.
    Keywords: ASTROPHYSICS
    Type: Astronomical Journal (ISSN 0004-6256); 99; 668-671
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