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| Titel |
The structure of an earthward propagating magnetic flux rope early in its evolution: comparison of methods |
| VerfasserIn |
C. Möstl, C. J. Farrugia, H. K. Biernat, S. A. Kiehas, R. Nakamura, V. V. Ivanova, Y. Khotyaintsev |
| 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. 5 ; Nr. 27, no. 5 (2009-05-15), S.2215-2224 |
| Datensatznummer |
250016538
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| Publikation (Nr.) |
 copernicus.org/angeo-27-2215-2009.pdf |
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| Zusammenfassung |
| We analyze a magnetic signature associated with the leading edge of a bursty
bulk flow observed by Cluster at −19 RE downtail on 22 August 2001. A
distinct rotation of the magnetic field was seen by all four spacecraft. This
event was previously examined by Slavin et al. (2003b) using both
linear force-free modeling as well as a curlometer technique. Extending this
work, we apply here single- and multi-spacecraft Grad-Shafranov (GS)
reconstruction techniques to the Cluster observations and find good evidence
that the structure encountered is indeed a magnetic flux rope and contains
helical magnetic field lines. We find that the flux rope has a diameter of
approximately 1 RE, an axial field of 26.4 nT, a velocity of
≈650 km/s, a total axial current of 0.16 MA and magnetic fluxes of
order 105 Wb. The field line twist is estimated as half a turn per
RE. The invariant axis is inclined at 40° to the ecliptic plane and 10° to the GSM equatorial plane.
The flux rope has a force-free core and non-force-free boundaries. When we
compare and contrast our results with those obtained from minimum variance,
single-spacecraft force-free fitting and curlometer techniques, we find in
general fair agreement, but also clear differences such as a higher
inclination of the axis to the ecliptic. We further conclude that
single-spacecraft methods have limitations which should be kept in mind when
applied to THEMIS observations, and that non-force-free GS and curlometer
techniques are to be preferred in their analysis. Some properties we derived
for this earthward– moving structure are similar to those inferred by
Lui et al. (2007), using a different approach, for a tailward-moving flux
rope observed during the expansion phase of the same substorm. |
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