| Summary: | 博士 === 國立中央大學 === 大氣物理研究所 === 85 === ABSTRACTThe terrestrial magnetosheath is a region rich in ultra-
low frequency waves due to interactions of solar wind with the
bow shock and magnetopause. The most often observed and studied
wave mode in the magnetosheath is the mirror wave.
Identification of mirror waves is an important step to further
investigate their physical behaviors in the magnetosheath. A new
approach is proposed to identify mirror mode based on the 90
degree phase difference between the magnetic field and plasma
velocity fluctuations. Magnetosheath observations, linear MHD
theory and one-dimensional hybrid simulations support that the
90 degree phase difference is a special character for mirror
waves. From this phase difference, it is useful to distinguish
mirror waves from slow waves which both have an anti-correlation
relationship between field and plasma density fluctuations in an
isotropic plasma. The property of 90 degree phase difference is
unaffected by the Doppler shift and therefore is more accurate
for analysis of satellite observations. The components of the
perturbed dB and dv perpendicular to the plane containing the
background field B0 and the wave vector k are found both in
observations and hybrid simulations. These components are not
allowed in the linear MHD theory. However, the 90 degree phase
difference between the perturbed dB and dv indicates they are
associated with mirror waves. It shows that the wave
identification based on linear MHD theory may not be correct.
The generation mechanism of mirror waves is one of the
controversial issues in space physics. It is often found that
the mirror waves in the magnetosheath was stable. In order to
excite mirror waves in the magnetosheath, it is proposed that
helium ions in the solar wind when enter the magnetosheath can
de-stabilize the plasma. The moderate growth rate can explain
why do the mirror waves not usually seen in the immediate
downstream of the bow shock and why do the observed wave
amplitudes become larger and larger when satellites move toward
the magnetopause. Observations and calculations from the Vlasov
dispersion equations support very well.
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