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  • GEOPHYSICS  (106)
  • ASTRONOMY
  • Physics
  • 1990-1994  (151)
  • 1980-1984
  • 1991  (151)
  • 1
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    Asteroid radar astrometry (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Measurements of time delay and Doppler frequency are reported for asteroid-radar echoes obtained at Arecibo and Goldstone during 1980-1990. Radar astrometry is presented for 23 near-earth asteroids and three mainbelt asteroids. These measurements, which are orthogonal to optical, angular-position measurements, and typically have a fractional precision between 10 to the -5th and 10 to the -8th, permit significant improvement in estimates of orbits and hence in the accuracy of prediction ephemerides. Estimates are also reported of radar cross-section and circular polarization ratio for all asteroids observed astrometrically during 1980-1990.
    Keywords: ASTRONOMY
    Type: Astronomical Journal (ISSN 0004-6256); 102; 1490-150
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  • 2
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    Two encounters with the flank low-latitude boundary layer - Further evidence for closed field topology and investigation of the internal structure (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The structure of the flank low-latitude boundary layer (LLBL) is examined through differential energy spectra and particle angular anisotropies for traversals of the dawn flank (December 19, 1977) and dusk flank (July 7, 1978) during periods of predominantly northward magnetosheath field orientation. Spectra are presented that were obtained from combined ISEE 1 low-energy-proton and electron-differential-energy-analyzer and medium-energy-particle-instrument data extending over the 200-eV/q to 2-MeV energy range for the plasma sheet, stagnation region, outer LLBL, and magnetosheath regions. The stagnation region and the outer LLBL are each a mixture of plasma-sheet and magnetosheath populations, but the stagnation region contains a relatively higher fraction of plasma sheet particles, consistent with its placement earthward of the outer LLBL. Evidence for energization of thermal electrons appears during the dusk flank crossing. Bidirectional field-aligned ion distributions are observed with typically 5-to-1 enhancement of the flux along the magnetic field during certain portions of the dusk flank crossing.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 21
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  • 3
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    Modeling the growth phase of a substorm using the Tsyganenko model and multi-spacecraft observations - CDAW-9 (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The CDAW-9 Event C focused upon the early part of 3 May 1986 when a large substorm onset occurred at 0111 UT. By modifying the Tsyganenko 1989 magnetic field model, a model is constructed in which the near-earth current systems are enhanced with time to describe the observed development of the tail magnetic field during the growth phase. The cross-tail current intensity and the thickness of the current sheet are determined by comparison with three spacecraft in the near-earth tail. The location of the auroral bulge as recorded by the Viking imager is mapped to the equatorial current sheet. The degree of chaotization of the thermal electrons is estimated, and the consequences to the tail stability towards on tearing are discussed. It is concluded that the mapping of the brightening region in the auroral oval corresponds to the regions in the tail where the current sheet may be unstable towards ion tearing.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 18; 1963-196
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  • 4
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    Relativistic electron enhancements - Simultaneous measurements from synchronous and low altitude satellites (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: This paper presents, for the first time, simultaneous measurements of trapped relativistic electron enhancements at synchronous altitude and precipitating electrons in the bounce loss cone at low altitudes. The measurements show that the daily variations in the precipitation flux for L greater than 5 correlated well with the daily variations in the total flux at high altitude, both with respect to sudden enhancements as well as flux depletions. The daily averaged precipitating flux (E greater than 1 MeV) at L = 6.1 to 7.1 was about 0.3 percent of the daily averaged directional flux (E not less than 1.5 MeV) observed at synchronous altitude, whereas within narrow spikes the precipitating directional fluxes were often within a factor of 10 of the daily averaged trapped fluxes. Strong depletions in the synchronous altitude not less than 1.5 MeV electron intensities are shown to be associated with low-altitude measurements of the equatorward movement of precipitating spikes to lower L shells.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 18; 397-400
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  • 5
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    On open and closed field line regions in Tsyganenko's field model and their possible associations with horse collar auroras (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The boundary between open and closed field lines is investigated in the empirical Tsyganenko (1987) magnetic field model. All field lines extending to distances beyond -70 R(E), the tailward velocity limit of the Tsyganenko model are defined as open, while all other field lines, which cross the equatorial plane earthward of -70 R(E) and are connected with the earth at both ends, are assumed closed. It is found that this boundary at the surface of the earth, identified as the polar cap boundary, can exhibit the arrowhead shape, pointed toward the sun, which is found in horse collar auroras. For increasing activity levels, the polar cap increases in area and becomes rounder, so that the arrowhead shape is less pronounced. The presence of a net B(y) component can also lead to considerable rounding of the open flux region. The arrowhead shape is found to be closely associated with the increase of B(z) from the midnight region to the flanks of the tail, consistent with a similar increase of the plasma sheet thickness.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 3811-381
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  • 6
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    Ionospheric convection response to changing IMF direction (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: By combining ground-based and satellite-based measurements of ionospheric electric fields, conductivities and magnetic perturbations, it was possible to examine the characteristics of instantaneous, ionospheric convection patterns associated with changing directions of the interplanetary magnetic field (IMF). In response to a rapid southward-to-northward turning of the IMF on July 23, 1983, the ionospheric convection reconfigured over a period of 40 minutes. The configuration changed from a conventional two-cell pattern to a contracted four-cell pattern, with reversed convection cells in the high-latitude dayside, associated with a strong potential drop of about 75 kV. Later, in response to a gradual rotation of the IMF from the +Z through the -Y toward the -Z direction, the nightside cells disappeared and the dawn cell in the reversed pair wrapped around and displaced the dusk cell until a conventional two-cell pattern was reestablished, largely in accord with the qualitative model of Crooker (1988). The results suggest that multiple cells can arise as a result of strong southward to northward transitions in the IMF. They appear to persist for sometime thereafter.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 18; 721-724
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  • 7
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    The BLISS measurements of NO2 - Some new insights (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: The present study compares the diurnal variation of NO2 measured near 30 km by the BLISS in situ laser spectrometer with calculations from a photochemical model that includes a detailed description of multiple scattering. Even better agreement is found between the data and the model, both at sunset and during the day. The conclusions of an earlier study that the high-resolution in situ measurements of NO2 facilitated validation of the understanding of the diurnal chemistry of NO2 are confirmed.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 18; 2261-226
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  • 8
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    Intercomparisons of the solar irradiance measurements from the Nimbus-7 SBUV, the NOAA-9 and NOAA-11 SBUV/2, and the STS-34 SSBUV instruments - A preliminary study (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: Results are presented of solar irradiance measurements in the spectral range 160-400 nm at approximately 0.15-0.20-nm intervals and at 1-nm resolution performed continually since November 1978. Solar irradiance data from the Nimbus-7 SBUV satellite instrument, the SBUV/2 instruments on the NOAA-9 and NOAA-11 satellites, and the October 1989 flight of the Shuttle SBUV instrument are presented and compared. Uncertainties in the instruments' absolute and long-term radiometric calibrations, which vary among the four instruments, are discussed. Comparisons of the initial solar spectra from the four instruments show agreement to within approximately 10 percent, with spectral biases on the order of +/-4 percent. Irradiances measured by the two NOAA instruments and SSBUV agree to within about 5 percent overall from 270 to 360 nm, with spectral biases on the order of about +/-2 percent. The Nimbus-7 SBUV irradiances are an additional 5-10 percent lower in this region than those measured by the other three instruments.
    Keywords: GEOPHYSICS
    Type: Journal of Atmospheric and Terrestrial Physics (ISSN 0021-9169); 53; 993-997
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  • 9
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    Observations of ion-neutral collisional effects in the auroral E region (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Thermal ion energy distribution functions and local electric and magnetic fields were directly measured for the first time in the ionospheric E region. Measured ion distribution functions were fitted to shifted Maxwellian distributions, and their resulting ion drift velocities were compared with E x B/B-squared velocities from the double-probe electric field observations. The results show that the ion drift direction rotates with respect to the local electric field direction and that the ratio of the magnitudes of the ion velocity to the E x B/B-squared velocity decreases with decreasing altitudes. Using these observations, the quiet time ion-neutral collison frequencies and neutral wind velocities were estimated and found to be consistent with theoretical estimates. However, significant discrepancies between observations and theory are found in the disturbed E region near auroral particle precipitation regions. These data indicate that the auroral atmosphere is significantly perturbed due to Joule as well as particle heating effects.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 9761-977
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  • 10
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    Angular diameter measurements of stars (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Angular diameters determined with the Mark III Optical Interferometer are presented for 12 stars at wavelengths of 450 and 800 nm. The uniform disk diameters resulting from fits to the visibility observations have rms residuals of order 1 percent for the 800 nm measurements and less than 3 percent for the 450 nm measurements. The improvement over previous observations with this instrument is due to improved data analysis and the use of a wider range of baseline lengths. An analysis of the calibration systematics for the Mark III Optical Interferometer is included. There is good agreement between these measurements and previously published data. The changes in uniform disk diameter between wavelengths of 450 and 800 nm agree with models of stellar atmospheres.
    Keywords: ASTRONOMY
    Type: Astronomical Journal (ISSN 0004-6256); 101; 2207-221
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