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| Titel |
A scheme for finding the front boundary of an interplanetary magnetic cloud |
| VerfasserIn |
R. P. Lepping, T. W. Narock, C.-C. Wu |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 27, no. 3 ; Nr. 27, no. 3 (2009-03-13), S.1295-1311 |
| Datensatznummer |
250016456
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| Publikation (Nr.) |
 copernicus.org/angeo-27-1295-2009.pdf |
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| Zusammenfassung |
| We develop a scheme for finding a "refined" front
boundary-time (tB*) of an interplanetary magnetic cloud (MC) based on
criteria that depend on the possible existence of any one or more of four
specific solar wind features. The features that the program looks for,
within ±2 h (i.e., the initial uncertainty interval) of a
preliminarily estimated front boundary time, are: (1) a sufficiently large
directional discontinuity in the interplanetary magnetic field (IMF), (2) a
significant proton plasma beta (βP) drop, (3) a significant
proton temperature drop, and (4) a marked increase in the IMF's intensity.
Also we examine to see if the "MC-side" of the boundary has a MC-like
value of βP. The scheme was tested using 5, 10, 15, and 20 min
averages of the relevant physical quantities from WIND data, in order
to find the optimum average to use. The 5 min average, initially based on
analysis of N=26 carefully chosen MCs, turned out to be marginally the best
average to use for our purposes. Other criteria, besides the four described
above, such as the existence of a magnetic hole, plasma speed change, and/or
field fluctuation level change, were examined and dismissed as not reliable
enough, or usually associated with physical quantities that change too
slowly around the boundary to be useful. The preliminarily estimated front
boundary time, tB, and its initial ±2-h uncertainty interval
are determined by either an automatic MC identification scheme or by visual
inspection. The boundary-scheme was developed specifically for aiding in
forecasting the strength and timing of a geomagnetic storm due to the
passage of a MC in real-time, but can be used in post ground-data collection
for imposing consistency when choosing front boundaries of MCs. This scheme
has been extensively tested, first using 81 bona fide MCs, collected over about 8.6
years of WIND data (at 1 AU), and also by using 122 MC-like structures as
defined by Lepping et al. (2005) over about the same period. Final
statistical testing of the 81 MCs to see how close the refined boundary-time
tB* lies with respect to a preliminary time tB(VI) was carried
out, i.e., to find Δt1=(tB*–tB(VI)), for the full
set of MCs, where tB(VI) is usually a very accurate time previously
determined from visual inspection, This testing showed that 59 Δt1s
(i.e., 73%) lie within ±30 min, 71 Δt1s (i.e., 88%) lie within ±45 min, and only 5 cases
lie outside a |Δt1| of 1.0 h, which is only 6% of the
full 81, and these 6% would be considered unsatisfactory.
Since MC parameter fitting is usually done on the basis of 30 or 60 min
averages, these results seem quite satisfactory. The program for this front
boundary estimation scheme is located at the Website:
http://wind.nasa.gov/mc/boundary.php. |
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