Comparison of magnetic field observations of an average magnetic cloud with a simple force free model: the importance of field compression and expansion
We investigate the ability of the cylindrically symmetric force-free magnetic cloud (MC) fitting model of Lepping et al. (1990) to faithfully reproduce actual magnetic field observations by examining two quantities: (1) a <i>difference angle</i>, called β, i.e., the angle between the...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2007-01-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/25/2641/2007/angeo-25-2641-2007.pdf |
| Summary: | We investigate the ability of the cylindrically symmetric
force-free magnetic cloud (MC) fitting model of Lepping et al. (1990) to
faithfully reproduce actual magnetic field observations by examining two
quantities: (1) a <i>difference angle</i>, called β, i.e., the angle between the direction
of the observed magnetic field (<i>B<sub>obs</sub></i>) and the derived force
free model field (<i>B<sub>mod</sub></i>) and (2) the <i>difference in magnitudes<i> between the observed and
modeled fields, i.e., Δ<i>B</i>(=|<i>B</i><sub>obs</sub>|−|</i>B</i><sub>mod</sub>|),
and a <i>normalized</i> Δ<i>B</i> (i.e., Δ<i>B</i>/<<i>B</i>>) is also examined, all for a judiciously chosen set of 50 WIND
interplanetary MCs, based on quality considerations. These three quantities
are developed as a percent of MC duration and averaged over this set of MCs
to obtain average profiles. It is found that, although <Δ<i>B</i>> and
its normalize version are significantly enhanced (from a broad central
average value) early in an average MC (and to a lesser extent also late in
the MC), the angle <β> is small (less than 8°) and
<i>approximately constant</i> all throughout the MC. The field intensity enhancements are due mainly to
interaction of the MC with the surrounding solar wind plasma causing field
<i>compression</i> at front and rear. For example, for a typical MC, Δ<i>B</i>/<<i>B</i>> is:
0.21±0.27 very early in the MC, −0.11±0.10 at the center
(and −0.085±0.12 averaged over the full "central region," i.e.,
for 30% to 80% of duration), and 0.05±0.29 very late in the
MC, showing a double sign change as we travel from front to center to back,
in the MC. When individual MCs are examined we find that over 80% of them
possess field enhancements within several to many hours of the front
boundary, but only about 30% show such enhancements at their rear
portions. The enhancement of the MC's front field is also due to MC
expansion, but this is usually a lesser effect compared to compression. It
is expected that this compression is manifested as significant distortion to
the MC's cross-section from the ideal circle, first suggested by Crooker et
al. (1990), into a more elliptical/oval shape, as some global MC studies
seem to confirm (e.g., Riley and Crooker, 2004) and apparently also as
confirmed for local studies of MCs (e.g., Hidalgo et al., 2002;
Nieves-Chinchilla et al., 2005). |
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| ISSN: | 0992-7689 1432-0576 |