One-Dimensional Transient Response of the Inner Magnetosphere at the Magnetic Equator, 2. Analysis of Waveforms.

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  • Itonaga M.
    Computation Center, Kyushu University
  • Yoshikawa A.
    Department of Earth and Planetary Sciences, Kyushu University
  • Yumoto K.
    Department of Earth and Planetary Sciences, Kyushu University

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  • One-Dimensional Transient Response of t

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Abstract

Under a model of altitude distribution of the Alfvén speed VA, one-dimensional transient response of the inner magnetosphere at the magnetic equator to earthward propagating impulse- and step-like MHD disturbances is considered. The waveforms of transient compressional oscillations due to these disturbances at some L shells are directly simulated by a numerical inversion of the Laplace transform with orthonormal Laguerre functions. The present paper concentrates on the analysis of waveforms. Then, it is verified that the compressional oscillations are due to the poles of the system under consideration. The oscillation arising from the cavity resonance all over the inner magnetosphere is most dominant. However, its amplitude becomes smaller as the characteristic time scale T of an incident disturbance grows large, and it is negligibly small for T greater than several times of eigenperiod of the resonance. On the other hand, when T is relatively small (e.g., T ≤ 10 s), the oscillations due to the cavity resonances trapped around the trough in VA are outstanding. It is also found that the relative phase between the cavity-mode oscillations all over the inner magnetosphere at the earth's surface and another L shell increases monotonically with L when the inner magnetosphere has no strong gradient or a strong positive gradient of VA at its outer boundary. However, the relative phase is nearly zero and nearly 180° inside and outside a specific L shell, respectively, when the inner magnetosphere has a strong negative gradient at its outer boundary. The one-dimensional cavity-mode type resonance of the inner magnetosphere is certainly a cause of equatorial Pit pulsations. However, some constituents of the Pi2's may be not cavity-mode oscillations but quasi-steady-state oscillations forced by some damped sinusoidal waves incident on the outer boundary of the inner magnetosphere.

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