Observational evidence of cavity modes in the earth's magnetosphere

Details Observational evidence of cavity modes in the earth's magnetosphere Observational evidence of cavity modes in the earth's magnetosphere

Nov 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987jgr....9212233c&link_type=abstract

Journal of Geophysical Research (ISSN 0148-0227), vol. 92, Nov. 1, 1987, p. 12233-12240. SERC-supported research.

Physics

50

Damping, Earth Ionosphere, Earth Magnetosphere, Energy Dissipation, Geomagnetic Pulsations, Magnetic Variations, Magnetometers, Perturbation Theory

Scientific paper

On November 30, 1982 two large-amplitude Pc 5 pulsations were observed at Tromso, Norway, by both EISCAT radar and ground-based magnetometers. The pulsations were excited by a sudden impulse. Their amplitude subsequently decreased, allowing a damping rate to be determined. Estimates of the height-integrated Pedersen conductivity obtained from the EISCAT data permit theoretical pulsation damping rates to be predicted. However, the theoretical damping rates are much greater than those actually measured, which indicates that energy was continuously fed into the field line resonance after its onset. The most likely source of this energy is coupling from a hydromagnetic cavity mode. If this is the case, the measured pulsation damping rate is controlled by the damping of the cavity mode rather than energy dissipation from the field-line resonance. An upper limit of gamma/omega = 0.08 is obtained from the coupling efficiency between the two modes. The energy dissipated in the ionosphere by these pulsations was about 4 TJ. To store this energy in the cavity mode requires an initial cavity mode amplitude of only about 400 pT. Long-period magnetic perturbations of this amplitude would be difficult to detect in existing spaceborne magnetometer data sets.

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