Selection effects in identifying magnetic clouds and the importance of the closest approach parameter
This study is motivated by the unusually low number of magnetic clouds (MCs) that are strictly identified within interplanetary coronal mass ejections (ICMEs), as observed at 1 AU; this is usually estimated to be around 30% or lower. But a looser definition of MCs may significantly increase this...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2010-08-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/28/1539/2010/angeo-28-1539-2010.pdf |
| Summary: | This study is motivated by the unusually low number of magnetic clouds (MCs)
that are strictly identified within interplanetary coronal mass ejections (ICMEs),
as observed at 1 AU; this is usually estimated to be around 30% or lower.
But a looser definition of MCs may significantly increase this percentage.
Another motivation is the unexpected shape of the occurrence distribution of
the observers' "closest approach distances" (measured from a MC's axis,
and called CA) which drops off somewhat rapidly as |CA| (in
% of MC radius) approaches 100%, based on earlier studies. We suggest,
for various geometrical and physical reasons, that the |CA|-distribution
should be somewhere between a uniform one and the one
actually observed, and therefore the 30% estimate should be higher. So we
ask, When there is a failure to identify a MC within an ICME, is it
occasionally due to a large |CA| passage, making MC
identification more difficult, i.e., is it due to an event selection effect? In attempting to
answer this question we examine WIND data to obtain an accurate distribution
of the number of MCs vs. |CA| distance, whether the event is
ICME-related or not, where initially a large number of cases (<I>N</I>=98) are
considered. This gives a frequence distribution that is far from uniform,
confirming earlier studies. This along with the fact that there are many
ICME identification-parameters that do not depend on |CA|
suggest that, indeed an <I>MC event selection effect</I> may explain at least part of the low ratio of
(No. MCs)/(No. ICMEs). We also show that there is an acceptable geometrical
and physical consistency in the relationships for both average
"normalized" magnetic field intensity change and field direction change
vs. |CA| within a MC, suggesting that our estimates of |CA|,
<I>B<sub>O</sub></I> (magnetic field intensity on the axis), and choice of a
proper "cloud coordinate" system (all needed in the analysis) are
acceptably accurate. Therefore, the MC fitting model (Lepping et al., 1990)
is adequate, on average, for our analysis. However, this selection effect is
not likely to completely answer our original question, on the unexpected
ratio of MCs to ICMEs, so we must look for other factors, such as
peculiarities of CME birth conditions. As a by-product of this analysis, we
determine that the first order structural effects within a MC due to its
interaction with the solar wind, plus the MC's usual expansion at 1 AU
(i.e., the non-force free components of the MC's field) are, on average,
weakly dependent on radial distance from the MC's axis; that is, in the
outer reaches of a typical MC the non-force free effects show up, but even
there they are rather weak. Finally, we show that it is not likely that a
MC's size distribution statistically controls the occurrence distribution of the
estimated |CA|s. |
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| ISSN: | 0992-7689 1432-0576 |