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| Titel |
Properties and geoeffectiveness of magnetic clouds in the rising, maximum and early declining phases of solar cycle 23 |
| VerfasserIn |
K. E. J. Huttunen, R. Schwenn, V. Bothmer, H. E. J. Koskinen |
| 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 ; 23, no. 2 ; Nr. 23, no. 2 (2005-02-28), S.625-641 |
| Datensatznummer |
250015165
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| Publikation (Nr.) |
 copernicus.org/angeo-23-625-2005.pdf |
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| Zusammenfassung |
| The magnetic structure and geomagnetic response of 73 magnetic clouds (MC)
observed by the WIND and ACE satellites in solar cycle 23 are examined. The
results have been compared with the surveys from the previous solar cycles.
The preselected candidate MC events were investigated using the minimum
variance analysis to determine if they have a flux-rope structure and to
obtain the estimation for the axial orientation (θC, φC).
Depending on the calculated inclination relative to the ecliptic we divided
MCs into "bipolar" (θC<45°) and "unipolar"
(θC>45°). The number of observed MCs was largest in the
early rising phase, although the halo CME rate was still low. It is likely
that near solar maximum we did not identify all MCs at 1AU, as they were
crossed far from the axis or they had interacted strongly with the ambient
solar wind or with other CMEs. The occurrence rate of MCs at 1AU is also
modified by the migration of the filament sites on the Sun towards the poles
near solar maximum and by the deflection of CMEs towards the equator due to
the fast solar wind flow from large polar coronal holes near solar minimum.
In the rising phase nearly all bipolar MCs were associated with the rotation
of the magnetic field from the south at the leading edge to the north at the
trailing edge. The results for solar cycles 21-22 showed that the direction
of the magnetic field in the leading portion of the MC starts to reverse at
solar maximum. At solar maximum and in the declining phase (2000-2003) we
observed several MCs with the rotation from the north to the south. We
observed unipolar (i.e. highly inclined) MCs frequently during the whole
investigated period. For solar cycles 21-22 the majority of MCs identified
in the rising phase were bipolar while in the declining phase most MCs were
unipolar. The geomagnetic response of a given MC depends greatly on its
magnetic structure and the orientation of the sheath fields. For each event
we distinguished the effect of the sheath fields and the MC fields. All
unipolar MCs with magnetic field southward at the axis were geoeffective
(Dst<-50nT) while those with the field pointing northward did not
cause magnetic storms at all. About half of the all identified MCs were not
geoffective or the sheath fields preceding the MC caused the storm. MCs
caused more intense magnetic storms (Dst<-100nT) than moderate
magnetic storms (-50nT ≥Dst≥-100nT). |
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