Signatures of mode conversion and kinetic Alfven waves at the magnetopause Page: 3 of 10
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Submitted to Geophysical Research Letters, 2000.
Signatures of Mode Conversion and Kinetic Alfv6n
Waves at the Magnetopause
Jay R. Johnson and C. Z. Cheng
Princeton University, Plasma Physics Laboratory, Princeton, NJ
Abstract. It has been suggested that resonant mode conversion of com-
pressional MHD waves into kinetic Alfven waves at the magnetopause can
explain the abrupt transition in wave polarization from compressional to
transverse commonly observed during magnetopause crossings [Johnson and
Cheng, 1997b]. We analyze magnetic field data for magnetopause crossings as
a function of magnetic shear angle (defined as the angle between the magnetic
fields in the magnetosheath and magnetosphere) and compare with the the-
ory of resonant mode conversion. The data suggest that amplification in the
transverse magnetic field component at the magnetopause is not significant
up to a threshold magnetic shear angle. Above the threshold angle significant
amplification results, but with weak dependence on magnetic shear angle.
Waves with higher frequency are less amplified and have a higher threshold
angle. These observations are qualitatively consistent with theoretical results
obtained from the kinetic-fluid wave equations.
Ultra-low frequency (ULF) waves (with frequencies
below 500 mHz) dominate the spectrum of nearly ev-
ery magnetopause crossing [Rezeau et al., 1993; Song,
1994; Song et al., 1993; Phan and Paschmann, 1996,
and references therein]. It has been suggested that
these waves are associated mode conversion of MHD
waves in the magnetosheath to kinetic Alfv6n waves
at the magnetopause which occurs near a field line
resonance location Lee et al. ; Belmont et al.
. The mode conversion process can explain
(a) a shift in wave polarization at the magnetopause
and (b) the amplification of the transverse magnetic
field component by an order of magnitude [Johnson
and Cheng, 1997b]. However, comparative studies
between theory and observation have not been per-
formed. In this work, we present evidence that ULF
waves at the magnetopause are the result of a mode
conversion process which transforms compressional
MHD waves that originate in the magnetosheath into
transverse shear/kinetic Alfv6n waves at the magne-
topause. Based on a data survey of ISEE1, ISEE2,
and WIND magnetopause crossings, we examined the
dependence of the mode conversion process on the
magnetic shear angle (defined to be the angle between
the magnetic field in the magnetosheath and magne-
tosphere on each side of the magnetopause). The re-
sults of the survey compare favorably with theoretical
predictions based on the model developed by Johnson
and Cheng [1997b].
Magnetosheath/Magnetopause Wave Ac-
tivity and Magnetic Shear
The most striking wave feature associated with
magnetopause crossings is the sharp transition in
the polarization of low frequency wave power from
compressional (SB > SB_) in the magnetosheath
to transverse (SB1 SB i) at the magnetopause
[Rezeau et al., 1993; Song, 1994; Song et al., 1993;
Phan and Paschmann, 1996, and references therein].
A good example of such a transition is illustrated
by the WIND crossing of the magnetopause on De-
cember 12, 1996 which is shown in Fig. 1. In this
figure, WIND is passing from the magnetosheath into
the magnetosphere on the flank of the magnetosphere
at roughly XgSe 0, YgSe 15RE, Zgse 0. The
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Johnson, Jay R. & Cheng, C. Z. Signatures of mode conversion and kinetic Alfven waves at the magnetopause, report, July 21, 2000; Princeton, New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc703047/m1/3/: accessed April 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.