External and internal sources of low-frequency MHD waves in the magnetosphere - A review.

Abstract

Theoretical studies concerning MHD source waves of long-period (T=10-600s) magnetic pulsations are reviewed. The source waves can be classified into two groups: One is external origins, that are upstream waves driven by the ion beam instabilities in the earth's foreshock, surface waves excited by the Kelvin-Helmholtz instabilities in the magnetospheric boundary, and sudden impulses caused by an interplanetary shock and dayside reconnection. The other is internal origins , e.g., drift mirror instabilities in ring current hot ions and sudden changes during substorm onset in the nightside magnetosphere. The upstream waves, the surface waves, and the drift waves associated with a finite energy distribution and a finite thickness of layered plasma show a narrow-band frequency, while the sudden impulses have a broad-band frequency. These source waves with a nearly monochromatic frequency and a broadband frequency can couple into standing field-line oscillations in the inner magnetosphere through the field-line resonance mechanism.

Initial Results from the 210.DEG. Magnetic Meridian Project-Review.

“…At the onset of magnetospheric substorms, transient hydromagnetic oscillations with periods of 40-150 s, called Pi 2 magnetic pulsations, are excited globally in the magnetosphere (Baumjohann and Glassmeier, 1984;Yumoto, 1986Yumoto, , 1988. One possible source of nighttime Pi 2 pulsations is a sudden change in magnetospheric convection or configuration during the substorm expansive phase (Akasofu, 1980), which would be caused by a plasma flow from the reconnection (or current disruption) region or by the formation of a substorm current wedge (McPherron et al, 1973).…”

Spectral power of low-latitude Pi 2 pulsations at the 210° magnetic meridian stations and plasmaspheric cavity resonances

“…This type of pulsations is predominantly a nighttime phenomenon (Yeoman et al, 1994) and known to be associated with substorm onset (Saito et al, 1976). Hence, Pi 2 pulsations are considered to result from hydromagnetic disturbances driven by a sudden change in magnetospheric convection or reconfiguration in the magnetotail during the substorm expansive phase (see review by Yumoto, 1986Yumoto, , 1988. However, it is not fully certain how Pi 2 pulsations propagate in the inner magnetosphere and how their frequencies are selected.…”

Correlation of High- and Low-Law-Latitude Pi 2 Magnetic Pulsations Observed at 210.DEG. Magnetic Meridian Chain Stations.

“…Recently, however, evidence of global cavity-mode oscillations has been demonstrated by many workers (Yeoman and On, 1989;Yumoto, 1990;Yumoto et al, 1990;Yumoto, 1989, 1991;Lin et al, 1991). Yumoto et al (1989) proposed a possible scenario for the excitation and propagation mechanisms of the substorm-associated global Pi 2 pulsations.…”

“…At the magnetospheric substorm onset, transient hydromagnetic oscillations with periods of 40-150 s, called Pi 2 magnetic pulsations, are excited globally in the magnetosphere (Baumjohann and Glassmeier, 1984;Yumoto, 1986Yumoto, , 1988. One possible source of nighttime Pi 2 pulsations is a sudden change in magnetospheric convection or configuration during the substorm expansive phase (Akasofu, 1980), which would be caused by a plasma flow from the reconnection (or current disruption) region or by the formation of a substorm current wedge (McPherron et al, 1973).…”