Signatures of mode conversion and kinetic Alfven waves at the magnetopause Page: 4 of 10
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JOHNSON & CHENG: Signatures of Mode Conversion ...
background magnetic field undergoes a rotation of
approximately 80 during the crossing of the mag-
netopause. While in the magnetosheath (prior to
19:30 UT), the wave power is mainly in the compres-
sional component as evidenced in the large fluctua-
tions of Btot. The plasma j for this period is mod-
erate (around 2) and the pressure anisotropy is not
very large [see Fig. 2 from Phan et al., 1997], sugges-
tive that they are compressional Alfv6n waves rather
than mirror modes. The compressional wave activ-
ity continues through the magnetopause even though
the plasma j drops below unity in a plasma depletion
layer (between 19:25 and 20:00 UT). As the density
gradually decreases and the magnetic field increases,
the transverse component of the magnetic field fluc-
tuations becomes dominant, although the frequency
range remains roughly the same. There is a marked
increase in wave activity in the individual GSE Carte-
sian components of the magnetic field, but the fluctu-
ations of Bot remain roughly the same. The enhanced
transverse wave activity persists all the way into the
low-latitude boundary layer from (20:00 UT to 20:45
UT). This magnetopause crossing is a typical exam-
ple of situations where compressional wave activity is
found in the magnetosheath.
The change in wave polarization from the mag-
netosheath to the magnetosphere is clearly seen in
the power spectrogram for the crossing. The spec-
trum is computed in a sliding Hanning window ev-
ery 30 seconds and includes 4 minutes of data. The
power spectral densities are obtained consistently us-
ing either the Fourier Transform or the Thompson
Multitaper Method (with 128 modes). Power spec-
tra are obtained for all magnetic field components
and the magnetic field magnitude and are used to ob-
tain the compressional and transverse magnetic field
spectrum. In Fig. 2 we show the wave power spec-
tral density of compressional, PIS, and transverse, Pi,
magnetic fluctuations as well as the fraction of wave
power in the transverse magnetic field component. A
broadband of waves is found in the 10 100 mHz fre-
quency range. In the magnetosheath, the waves are
primarily compressional, but at the magnetopause,
where the density and magnetic field gradients are
found, the transverse component is dominant in the
same frequency range. The compressional magnetic
field spectra remain approximately the same from the
magnetosheath into the magnetopause with an even-
tual cutoff as WIND moved through the low-latitude
boundary layer into the plasma sheet at 20:30 UT.
In the magnetosheath, the transverse component ap-
17:00 18:00 19:00 20:00 21:00
Figure 1. Density and magnetic field data from a
magnetopause crossing by the WIND satellite on Jan
12, 1996 at approximately 19:30 UT as indicated by
the vertical reference line. Note that in the magne-
tosheath (prior to 19:30 UT) wave power is primarily
compressional. Coincident with the strong density
gradients at the magnetopause there is a strong en-
hancement in the transverse components of the mag-
netic field fluctuations, but the compressional ampli-
tude remains approximately the same as in the mag-
netosheath. The magnetic field angle relative to the
sheath field rotates by about BOs 80 during the
crossing. Data courtesy of R. Lepping and R. Lin.
pears in the same frequency range and is well corre-
lated with the compressional component but appears
to fall off slightly faster with frequency suggestive that
the wavevector is more oblique for higher frequency
waves. However, at the magnetopause there is a dra-
matic increase in the transverse power spectrum as
evidenced in the lower panel of Fig. 2. Notice also
that the amplification (ratio of spectral density at the
magnetopause to that of the magnetosheath) of the
transverse spectrum falls off as frequency increases.
In order to understand the wave activity better, we
have examined 13 cases of ISEE1, ISEE2, and WIND
data as a function of magnetic shear, 0sh, which is
defined as the angle between the magnetic field in the
magnetosheath and the magnetic field on the mag-
netospheric side of the magnetopause. The crossings
are listed in Table 1. In these cases compressional
waves were found in the magnetosheath, the space-
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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/m1/4/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.