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  • GEOPHYSICS  (182)
  • 1990-1994  (182)
  • 1985-1989
  • 1991  (182)
  • 1
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    Geomagnetic activity associated with earth passage of interplanetary shock disturbances and coronal mass ejections (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Coronal mass ejection events (CMEs) are important occasional sources of plasma and magnetic field in the solar wind at 1 AU, accounting for approximately 10 percent of all solar wind measurements in the ecliptic plane during the last solar activity maximum. Using a recently appreciated capability for distinguishing CMEs in solar wind data in the form of counterstreaming solar wind electron events, this paper explores the overall effectiveness of shock wave disturbances and CMFs in general in stimulating geomagnetic activity. The study is confined to the interval from mid-August 1978 through mid-October 1982, spanning the last solar activity maximum, when ISEE 3 was in orbit about the L1 Lagrange point 220 Re upstream from earth. It is found that all but one of the 37 largest geomagnetic storms in that era were associated with earth passage of CMEs and/or shock disturbances, with the large majority of these storms (27 out of 37) being associated with interplanetary events where earth encountered both a shock and the CME driving the shock (shock/CME events). Although CMEs and/or shock disturbances were increasingly the cause of geomagnetic activity as the level of geomagnetic activity increased, many smaller geomagnetic disturbances were unrelated to these events.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 7831-783
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  • 2
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    Sources of field-aligned currents in the auroral plasma (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Data from the Dynamics Explorer 1 High Altitude Plasma Instrument (HAPI) and magnetometer are used to investigate the sources of field-aligned currents in the nightside auroral zone. It is found that the formula developed by S. Knight predicts the field-aligned current density fairly accurately in regions where a significant potential drop can be inferred from the HAPI data; there are, however, regions in which the proportionality between potential drop and field-aligned current does not hold. In particular, occurrences of strong upward field-aligned current associated not with inverted-V events but instead with suprathermal bursts are noted. In addition, upward field-aligned currents are often observed to peak near the edges of inverted-V events, rather than in the center as would be predicted by Knight.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 18; 45-48
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  • 3
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    The role of the ionosphere in coupling upstream ULF wave power into the dayside magnetosphere (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A series of recent studies of Pc 3 magnetic pulsations in the dayside outer magnetosphere has given new insights into the possible mechanisms of entry of ULF wave power into the magnetosphere from a bow shock-related upstream source. A comparison is made of data from two 10-hour intervals on successive days in April 1986 and then a possible model for transmission of pulsation signals from the magnetosheath into the dayside magnetosphere is presented. Clear interplanetary magnetic field magnitude control of dayside resonant harmonic pulsations and band-limited very high latitude pulsations, as well as pulsation-modulated precipitation of what appear to be magnetosheath/boundary layer electrons are shown. It is believed that this modulated precipitation may be responsible for the propagation of upstream wave power in the Pc 3 frequency band into the high-latitude ionosphere, from whence it may be transported throughout the dayside outer magnetosphere by means of an 'ionospheric transistor'. In this model, modulations in ionospheric conductivity caused by cusp/cleft precipitation cause varying ionospheric currents with frequency spectra determined by the upstream waves; these modulations will be superimposed on the Birkeland currents, which close via these ionospheric currents. Modulated region 2 Birkeland currents will in turn provide a narrow-band source of wave energy to a wide range of dayside local times in the outer magnetosphere.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 1527-154
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  • 4
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    Comparison of VLBI and SLR geocentric site coordinates (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Results are reported from a systematic comparison of the geocentric coordinates determined for 18 pairs of VLBI and satellite laser ranging (SLR) sites. The data and results are presented in tables and briefly characterized. The rms differences in the X, Y, and Z coordinates are found, after a 7-parameter frame adjustment, to be 15, 22, and 22 mm, respectively. The potential usefulness of a combined VLBI-SLR reference frame for spacecraft tracking and similar applications is indicated.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 18; 231-234
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  • 5
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    Model and observation comparison of the universal time and IMF by dependence of the ionospheric polar hole (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The polar ionospheric F-region often exhibits regions of marked density depletion. These depletions have been observed by a variety of polar orbiting ionospheric satellites over a full range of solar cycle, season, magnetic activity, and universal time (UT). An empirical model of these observations has recently been developed to describe the polar depletion dependence on these parameters. Specifically, the dependence has been defined as a function of F10.7 (solar), summer or winter, Kp (magnetic), and UT. Polar cap depletions have also been predicted /1, 2/ and are, hence, present in physical models of the high latitude ionosphere. Using the Utah State University Time Dependent Ionospheric Model (TDIM) the predicted polar depletion characteristics are compared with those described by the above empirical model. In addition, the TDIM is used to predict the IMF By dependence of the polar hole feature.
    Keywords: GEOPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 11; 10 1
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  • 6
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    Measurements of ClO and O3 from 21 deg N to 61 deg N in the lower stratosphere during February 1988 - Implications for heterogeneous chemistry (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The decline in stratospheric ozone at northern midlatitudes in wintertime may be caused by chlorine photochemistry that has been enhanced by heterogeneous reactions. The possibility that the heterogeneous reaction of N2O5 on sulfate aerosols is the cause of this decadal ozone decline is examined by comparing ClO and O3 measurements made in the lower stratosphere during February, 1988, with results from a 2D model. At midlatitudes, the abundances, latitudinal, and seasonal gradients of the observed ClO are similar to the results of a model with heterogeneous chemistry, but are in strong disagreement with the results from the model with only gas-phase chemistry. At low latitudes, agreement is best with the results of the model with only gas-phase chemistry. Limited observations indicate that the amount of reactive chlorine is being enhanced, and that heterogeneous chemistry is a likely cause.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 18; 2273-227
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  • 7
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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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  • 8
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    An upper limit for the rate coefficient of the reaction of NH2 radicals with O2 using FTIR product analysis (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Fourier transform infrared spectrometry has been used to study the products of the photooxidation of ammonia in the presence of oxygen at 296 K. The data have been used to derive an upper limit of 6 x 10 exp -21/cu cm molecule s for the reaction of NH2 radicals with O2 to produce NO(x) at 296 K. This upper limit, which is three orders of magnitude lower than previous estimates based on the kinetics of NH2 loss, rules out the importance of this reaction in the atmosphere and suggests that NH2 will be oxidized by O3 or NO2. The effect on the NO(x) and N2O budgets depends critically on the products of the NH2 + O3 reaction. Simulations of the experimental product yields also allow an evaluation of possible product channels for the reaction of NH2 with HO2.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 20
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  • 9
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    Ion cyclotron waves near L = 4.6 - A ground-satellite correlation study (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Results are presented of the first ground-satellite correlation study of ion cyclotron wave events inside geosynchronous orbit. Several intervals are found during which waves at the same frequency were observed simultaneously by the DE 1 Goddard Space Flight Center Fluxgate Magnetometer and by the Air Force Geophysics Laboratory Magnetometer Network. This represents about 33 percent of the intervals during which waves were seen on the ground and DE 1 was in a favorable location (near the geomagnetic equator around L = 4.6) to observe the wave generation region. Three of seven simultaneous events showed a positive correlation when a detailed correlation analysis was performed. Peaks in the the cross-correlation function occurred at delays of 30-100 s and are interpreted as the group delay time of signals from space to the ground station. The small number of events seen in space during conjunction intervals results from the satellite's path missing the source region.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 1451-146
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  • 10
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    The BLISS measurements of NO2 - Some new insights (1991)
    In:  Other Sources
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    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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