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  • GEOPHYSICS  (5)
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  • 1
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    Is the magnetosphere a lens for MHD waves? (1993)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: A viewpoint of the magnetosphere as a lens for MHD waves is presented. Using a simple model of the variation of the Alfven speed as proportional to the local magnetic value given by the Earth's dipole field and that due to the magnetopause currents represented by a current loop, it is found that the near-Earth magnetotail, in the range 8-16 R(sub E), is the focus of the magnetospheric lens. This location is found to be quite insensitive to a wide variation of parameters. By using simple diffraction theory analysis it is found that the focal region extends about 1 R(sub E) about the neutral sheet in the north-south plane and 0.2 - 0.5 R(sub E) along the Sun-Earth line. Compressive MHD waves carried by the solar wind or created by the interaction of the wind with the magnetopause can be amplified by a factor of about 100 in the focal region and this has potentially important implications to substorm activity.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 20; 24; p. 2809-2812
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  • 2
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    Reconstruction of low-dimensional magnetospheric dynamics by singular spectrum analysis (1993)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Singular spectrum analysis was used to study global magnetospheric dynamics using time series auroral electrojet (AE) data. The calculated correlation dimension of approximately 2.5 confirms the low dimensionality of earlier results. This technique indicated that global magnetospheric dynamics can be described by three variables whose dynamical features were obtained from the AE data.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 20; 5; p. 335-338.
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  • 3
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    Lyapunov exponent of magnetospheric activity from AL time series (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A correlation dimension analysis of the AE index indicates that the magnetosphere behaves as a low-dimensional chaotic system with a dimension close to 4. Similar techniques are used to determine if the system's behavior is due to an intrinsic sensitivity to initial conditions and thus is truly chaotic. The quantity used to measure the sensitivity to initial conditions is the Liapunov exponent. Its calculation for AL shows that it is nonzero (0.11 + or - 0.05/min). This gives the exponential rate at which initially similar configurations of the magnetosphere evolve into completely different states. Also, predictions of deterministic nonlinear models are expected to deviate from the observed behavior at the same rate.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 18; 1643-164
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  • 4
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    Low-dimensional chaos in magnetospheric activity from AE time series (1990)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The magnetospheric response to the solar-wind input, as represented by the time-series measurements of the auroral electrojet (AE) index, has been examined using phase-space reconstruction techniques. The system was found to behave as a low-dimensional chaotic system with a fractal dimension of 3.6 and has Kolmogorov entropy less than 0.2/min. These indicate that the dynamics of the system can be adequately described by four independent variables, and that the corresponding intrinsic time scale is of the order of 5 min. The relevance of the results to magnetospheric modeling is discussed.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 17; 1841-184
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  • 5
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    An empirical model relating the auroral geomagnetic activity to the Interplanetary Magnetic Field (1993)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: A damped linear oscillator (LRC circuit) predicts a significant amount of the AL index variations when it is driven by the IMF's B(South) component. The model parameters are determined from fitting the AL time series and show a small, apparently nonsystematic variation with the activity level. Following an isolated external disturbance the LRC model's response peaks at 15 min - 0.5 h and decays after 1-2 more hours. The time scales and the correlation between observed AL and the circuit output (60-80 percent) are in good agreement with earlier linear prediction filter results. The results support the modeling of the global magnetospheric behavior with dynamical systems of a few degrees of freedom.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 20; 16; p. 1731-1734.
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