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Ann. Geophys., 24, 1639-1648, 2006
www.ann-geophys.net/24/1639/2006/
© European Geosciences Union 2006
ULF hydromagnetic oscillations with the discrete spectrum as eigenmodes of MHD-resonator in the near-Earth part of the plasma sheet
V. A. Mazur and A. S. Leonovich
Institute of Solar-Terrestrial Physics (ISTP), Russian Academy of Science, Siberian Branch, Irkutsk 33, 664033, Russia
Abstract. A new concept is proposed for the emergence of ULF geomagnetic oscillations
with a discrete spectrum of frequencies (0.8, 1.3, 1.9, 2.6 ...mHz)
registered in the magnetosphere's midnight-morning sector. The concept relies
on the assumption that these oscillations are MHD-resonator eigenmodes in the
near-Earth plasma sheet. This magnetospheric area is where conditions are met
for fast magnetosonic waves to be confined. The confinement is a result of
the velocity values of fast magnetosonic waves in the near-Earth plasma sheet
which differ greatly from those in the magnetotail lobes, leading to
turning points forming in the tailward direction for the waves under study.
To compute the eigenfrequency spectrum of such a resonator, we used a model
magnetosphere with parabolic geometry. The fundamental harmonics of this
resonator's eigenfrequencies are shown to be capable of being clustered into
groups with average frequencies matching, with good accuracy, the
frequencies of the observed oscillations. A possible explanation for the
stability of the observed oscillation frequencies is that such a resonator
might only form when the magnetosphere is in a certain unperturbed state.
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